s3: tests: Add new test_stream_dir_rename.sh test.

[Samba.git] / lib / compression / lzxpress_huffman.c

/*
 * Samba compression library - LGPLv3
 *
 * Copyright © Catalyst IT 2022
 *
 * Written by Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
 *        and Joseph Sutton   <josephsutton@catalyst.net.nz>
 *
 *  ** NOTE! The following LGPL license applies to this file.
 *  ** It does NOT imply that all of Samba is released under the LGPL
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 3 of the License, or (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, see <http://www.gnu.org/licenses/>.
 */

#include <talloc.h>

#include "replace.h"
#include "lzxpress_huffman.h"
#include "lib/util/stable_sort.h"
#include "lib/util/debug.h"
#include "lib/util/byteorder.h"
#include "lib/util/bytearray.h"

/*
 * DEBUG_NO_LZ77_MATCHES toggles the encoding of matches as matches. If it is
 * false the potential match is written as a series of literals, which is a
 * valid but usually inefficient encoding. This is useful for isolating a
 * problem to either the LZ77 or the Huffman stage.
 */
#ifndef DEBUG_NO_LZ77_MATCHES
#define DEBUG_NO_LZ77_MATCHES false
#endif

/*
 * DEBUG_HUFFMAN_TREE forces the drawing of ascii art huffman trees during
 * compression and decompression.
 *
 * These trees will also be drawn at DEBUG level 10, but that doesn't work
 * with cmocka tests.
 */
#ifndef DEBUG_HUFFMAN_TREE
#define DEBUG_HUFFMAN_TREE false
#endif

#if DEBUG_HUFFMAN_TREE
#define DBG(...) fprintf(stderr, __VA_ARGS__)
#else
#define DBG(...) DBG_INFO(__VA_ARGS__)
#endif


#define LZXPRESS_ERROR -1LL

/*
 * We won't encode a match length longer than MAX_MATCH_LENGTH.
 *
 * Reports are that Windows has a limit at 64M.
 */
#define MAX_MATCH_LENGTH (64 * 1024 * 1024)


struct bitstream {
        const uint8_t *bytes;
        size_t byte_pos;
        size_t byte_size;
        uint32_t bits;
        int remaining_bits;
        uint16_t *table;
};


#if ! defined __has_builtin
#define __has_builtin(x) 0
#endif

/*
 * bitlen_nonzero_16() returns the bit number of the most significant bit, or
 * put another way, the integer log base 2. Log(0) is undefined; the argument
 * has to be non-zero!
 * 1     -> 0
 * 2,3   -> 1
 * 4-7   -> 2
 * 1024  -> 10, etc
 *
 * Probably this is handled by a compiler intrinsic function that maps to a
 * dedicated machine instruction.
 */

static inline int bitlen_nonzero_16(uint16_t x)
{
#if  __has_builtin(__builtin_clz)

        /* __builtin_clz returns the number of leading zeros */
        return (sizeof(unsigned int) * CHAR_BIT) - 1
                - __builtin_clz((unsigned int) x);

#else

        int count = -1;
        while(x) {
                x >>= 1;
                count++;
        }
        return count;

#endif
}


struct lzxhuff_compressor_context {
        const uint8_t *input_bytes;
        size_t input_size;
        size_t input_pos;
        size_t prev_block_pos;
        uint8_t *output;
        size_t available_size;
        size_t output_pos;
};

static int compare_huffman_node_count(struct huffman_node *a,
                                      struct huffman_node *b)
{
        return a->count - b->count;
}

static int compare_huffman_node_depth(struct huffman_node *a,
                                      struct huffman_node *b)
{
        int c = a->depth - b->depth;
        if (c != 0) {
                return c;
        }
        return (int)a->symbol - (int)b->symbol;
}


#define HASH_MASK ((1 << LZX_HUFF_COMP_HASH_BITS) - 1)

static inline uint16_t three_byte_hash(const uint8_t *bytes)
{
        /*
         * MS-XCA says "three byte hash", but does not specify it.
         *
         * This one is just cobbled together, but has quite good distribution
         * in the 12-14 bit forms, which is what we care about most.
         * e.g: 13 bit: median 2048, min 2022, max 2074, stddev 6.0
         */
        uint16_t a = bytes[0];
        uint16_t b = bytes[1] ^ 0x2e;
        uint16_t c = bytes[2] ^ 0x55;
        uint16_t ca = c - a;
        uint16_t d = ((a + b) << 8) ^ (ca << 5) ^ (c + b) ^ (0xcab + a);
        return d & HASH_MASK;
}


static inline uint16_t encode_match(size_t len, size_t offset)
{
        uint16_t code = 256;
        code |= MIN(len - 3, 15);
        code |= bitlen_nonzero_16(offset) << 4;
        return code;
}

/*
 * debug_huffman_tree() uses debug_huffman_tree_print() to draw the Huffman
 * tree in ascii art.
 *
 * Note that the Huffman tree is probably not the same as that implied by the
 * canonical Huffman encoding that is finally used. That tree would be the
 * same shape, but with the left and right toggled to sort the branches by
 * length, after which the symbols for each length sorted by value.
 */

static void debug_huffman_tree_print(struct huffman_node *node,
                                     int *trail, int depth)
{
        if (node->left == NULL) {
                /* time to print a row */
                int j;
                bool branched = false;
                int row[17];
                char c[100];
                int s = node->symbol;
                char code[17];
                if (depth > 15) {
                        fprintf(stderr,
                                " \033[1;31m Max depth exceeded! (%d)\033[0m "
                                " symbol %#3x claimed depth %d count %d\n",
                                depth, node->symbol, node->depth, node->count);
                        return;
                }
                for (j = depth - 1; j >= 0; j--) {
                        if (branched) {
                                if (trail[j] == -1) {
                                        row[j] = -3;
                                } else {
                                        row[j] = -2;
                                }
                        } else if (trail[j] == -1) {
                                row[j] = -1;
                                branched = true;
                        } else {
                                row[j] = trail[j];
                        }
                }
                for (j = 0; j < depth; j++) {
                        switch (row[j]) {
                        case -3:
                                code[j] = '1';
                                fprintf(stderr, "        ");
                                break;
                        case -2:
                                code[j] = '0';
                                fprintf(stderr, "      │ ");
                                break;
                        case -1:
                                code[j] = '1';
                                fprintf(stderr, "      ╰─");
                                break;
                        default:
                                code[j] = '0';
                                fprintf(stderr, "%5d─┬─", row[j]);
                                break;
                        }
                }
                code[depth] = 0;
                if (s < 32) {
                        snprintf(c, sizeof(c),
                                "\033[1;32m%02x\033[0m \033[1;33m%c%c%c\033[0m",
                                 s,
                                 0xE2, 0x90, 0x80 + s); /* utf-8 for symbol */
                }  else if (s < 127) {
                        snprintf(c, sizeof(c),
                                 "\033[1;32m%2x\033[0m '\033[10;32m%c\033[0m'",
                                 s, s);
                } else if (s < 256) {
                        snprintf(c, sizeof(c), "\033[1;32m%2x\033[0m", s);
                } else {
                        uint16_t len = (s & 15) + 3;
                        uint16_t dbits = ((s >> 4) & 15) + 1;
                        snprintf(c, sizeof(c),
                                 " \033[0;33mlen:%2d%s, "
                                 "dist:%d-%d \033[0m \033[1;32m%3x\033[0m%s",
                                 len,
                                 len == 18 ? "+" : "",
                                 1 << (dbits - 1),
                                 (1 << dbits) - 1,
                                 s,
                                 s == 256 ? " \033[1;31mEOF\033[0m" : "");

                }

                fprintf(stderr, "──%5d %s \033[2;37m%s\033[0m\n",
                        node->count, c, code);
                return;
        }
        trail[depth] = node->count;
        debug_huffman_tree_print(node->left, trail, depth + 1);
        trail[depth] = -1;
        debug_huffman_tree_print(node->right, trail, depth + 1);
}


/*
 * If DEBUG_HUFFMAN_TREE is defined true, debug_huffman_tree()
 * will print a tree looking something like this:
 *
 *     7─┬───    3  len:18+, dist:1-1  10f 0
 *       ╰─    4─┬─    2─┬───    1 61 'a' 100
 *               │       ╰───    1 62 'b' 101
 *               ╰─    2─┬───    1 63 'c' 110
 *                       ╰───    1  len: 3, dist:1-1  100 EOF 111
 *
 * This is based off a Huffman root node, and the tree may not be the same as
 * the canonical tree.
 */
static void debug_huffman_tree(struct huffman_node *root)
{
        int trail[17];
        debug_huffman_tree_print(root, trail, 0);
}


/*
 * If DEBUG_HUFFMAN_TREE is defined true, debug_huffman_tree_from_table()
 * will print something like this based on a decoding symbol table.
 *
 *  Tree from decoding table 9 nodes → 5 codes
 * 10000─┬─── 5000  len:18+, dist:1-1  10f 0
 *       ╰─ 5000─┬─ 2500─┬─── 1250 61 'a' 100
 *               │       ╰─── 1250 62 'b' 101
 *               ╰─ 2500─┬─── 1250 63 'c' 110
 *                       ╰─── 1250  len: 3, dist:1-1  100 EOF 111
 *
 * This is the canonical form of the Huffman tree where the actual counts
 * aren't known (we use "10000" to help indicate relative frequencies).
 */
static void debug_huffman_tree_from_table(uint16_t *table)
{
        int trail[17];
        struct huffman_node nodes[1024] = {{0}};
        uint16_t codes[1024];
        size_t n = 1;
        size_t i = 0;
        codes[0] = 0;
        nodes[0].count = 10000;

        while (i < n) {
                uint16_t index = codes[i];
                struct huffman_node *node = &nodes[i];
                if (table[index] == 0xffff) {
                        /* internal node */
                        index <<= 1;
                        /* left */
                        index++;
                        codes[n] = index;
                        node->left = nodes + n;
                        nodes[n].count = node->count >> 1;
                        n++;
                        /*right*/
                        index++;
                        codes[n] = index;
                        node->right = nodes + n;
                        nodes[n].count = node->count >> 1;
                        n++;
                } else {
                        /* leaf node */
                        node->symbol = table[index] & 511;
                }
                i++;
        }

        fprintf(stderr,
                "\033[1;34m Tree from decoding table\033[0m "
                "%zu nodes → %zu codes\n",
                n, (n + 1) / 2);
        debug_huffman_tree_print(nodes, trail, 0);
}


static bool depth_walk(struct huffman_node *n, uint32_t depth)
{
        bool ok;
        if (n->left == NULL) {
                /* this is a leaf, record the depth */
                n->depth = depth;
                return true;
        }
        if (depth > 14) {
                return false;
        }
        ok = (depth_walk(n->left, depth + 1) &&
              depth_walk(n->right, depth + 1));

        return ok;
}


static bool check_and_record_depths(struct huffman_node *root)
{
        return depth_walk(root, 0);
}


static bool encode_values(struct huffman_node *leaves,
                          size_t n_leaves,
                          uint16_t symbol_values[512])
{
        size_t i;
        /*
         * See, we have a leading 1 in our internal code representation, which
         * indicates the code length.
         */
        uint32_t code = 1;
        uint32_t code_len = 0;
        memset(symbol_values, 0, sizeof(uint16_t) * 512);
        for (i = 0; i < n_leaves; i++) {
                code <<= leaves[i].depth - code_len;
                code_len = leaves[i].depth;

                symbol_values[leaves[i].symbol] = code;
                code++;
        }
        /*
         * The last code should be 11111... with code_len + 1 ones. The final
         * code++ will wrap this round to 1000... with code_len + 1 zeroes.
         */

        if (code != 2 << code_len) {
                return false;
        }
        return true;
}


static int generate_huffman_codes(struct huffman_node *leaf_nodes,
                                  struct huffman_node *internal_nodes,
                                  uint16_t symbol_values[512])
{
        size_t head_leaf = 0;
        size_t head_branch = 0;
        size_t tail_branch = 0;
        struct huffman_node *huffman_root = NULL;
        size_t i, j;
        size_t n_leaves = 0;

        /*
         * Before we sort the nodes, we can eliminate the unused ones.
         */
        for (i = 0; i < 512; i++) {
                if (leaf_nodes[i].count) {
                        leaf_nodes[n_leaves] = leaf_nodes[i];
                        n_leaves++;
                }
        }
        if (n_leaves == 0) {
                return LZXPRESS_ERROR;
        }
        if (n_leaves == 1) {
                /*
                 * There is *almost* no way this should happen, and it would
                 * ruin the tree (because the shortest possible codes are 1
                 * bit long, and there are two of them).
                 *
                 * The only way to get here is in an internal block in a
                 * 3-or-more block message (i.e. > 128k), which consists
                 * entirely of a match starting in the previous block (if it
                 * was the end block, it would have the EOF symbol).
                 *
                 * What we do is add a dummy symbol which is this one XOR 256.
                 * It won't be used in the stream but will balance the tree.
                 */
                leaf_nodes[1] = leaf_nodes[0];
                leaf_nodes[1].symbol ^= 0x100;
                n_leaves = 2;
        }

        /* note, in sort we're using internal_nodes as auxillary space */
        stable_sort(leaf_nodes,
                    internal_nodes,
                    n_leaves,
                    sizeof(struct huffman_node),
                    (samba_compare_fn_t)compare_huffman_node_count);

        /*
         * This outer loop is for re-quantizing the counts if the tree is too
         * tall (>15), which we need to do because the final encoding can't
         * express a tree that deep.
         *
         * In theory, this should be a 'while (true)' loop, but we chicken
         * out with 10 iterations, just in case.
         *
         * In practice it will almost always resolve in the first round; if
         * not then, in the second or third. Remember we'll looking at 64k or
         * less, so the rarest we can have is 1 in 64k; each round of
         * quantization effecively doubles its frequency to 1 in 32k, 1 in
         * 16k, etc, until we're treating the rare symbol as actually quite
         * common.
         */
        for (j = 0; j < 10; j++) {
                bool less_than_15_bits;
                while (true) {
                        struct huffman_node *a = NULL;
                        struct huffman_node *b = NULL;
                        size_t leaf_len = n_leaves - head_leaf;
                        size_t internal_len = tail_branch - head_branch;

                        if (leaf_len + internal_len == 1) {
                                /*
                                 * We have the complete tree. The root will be
                                 * an internal node unless there is just one
                                 * symbol, which is already impossible.
                                 */
                                if (unlikely(leaf_len == 1)) {
                                        return LZXPRESS_ERROR;
                                } else {
                                        huffman_root = \
                                                &internal_nodes[head_branch];
                                }
                                break;
                        }
                        /*
                         * We know here we have at least two nodes, and we
                         * want to select the two lowest scoring ones. Those
                         * have to be either a) the head of each queue, or b)
                         * the first two nodes of either queue.
                         *
                         * The complicating factors are: a) we need to check
                         * the length of each queue, and b) in the case of
                         * ties, we prefer to pair leaves with leaves.
                         *
                         * Note a complication we don't have: the leaf node
                         * queue never grows, and the subtree queue starts
                         * empty and cannot grow beyond n - 1. It feeds on
                         * itself. We don't need to think about overflow.
                         */
                        if (leaf_len == 0) {
                                /* two from subtrees */
                                a = &internal_nodes[head_branch];
                                b = &internal_nodes[head_branch + 1];
                                head_branch += 2;
                        } else if (internal_len == 0) {
                                /* two from nodes */
                                a = &leaf_nodes[head_leaf];
                                b = &leaf_nodes[head_leaf + 1];
                                head_leaf += 2;
                        } else if (leaf_len == 1 && internal_len == 1) {
                                /* one of each */
                                a = &leaf_nodes[head_leaf];
                                b = &internal_nodes[head_branch];
                                head_branch++;
                                head_leaf++;
                        } else {
                                /*
                                 * Take the lowest head, twice, checking for
                                 * length after taking the first one.
                                 */
                                if (leaf_nodes[head_leaf].count >
                                    internal_nodes[head_branch].count) {
                                        a = &internal_nodes[head_branch];
                                        head_branch++;
                                        if (internal_len == 1) {
                                                b = &leaf_nodes[head_leaf];
                                                head_leaf++;
                                                goto done;
                                        }
                                } else {
                                        a = &leaf_nodes[head_leaf];
                                        head_leaf++;
                                        if (leaf_len == 1) {
                                                b = &internal_nodes[head_branch];
                                                head_branch++;
                                                goto done;
                                        }
                                }
                                /* the other node */
                                if (leaf_nodes[head_leaf].count >
                                    internal_nodes[head_branch].count) {
                                        b = &internal_nodes[head_branch];
                                        head_branch++;
                                } else {
                                        b = &leaf_nodes[head_leaf];
                                        head_leaf++;
                                }
                        }
                done:
                        /*
                         * Now we add a new node to the subtrees list that
                         * combines the score of node_a and node_b, and points
                         * to them as children.
                         */
                        internal_nodes[tail_branch].count = a->count + b->count;
                        internal_nodes[tail_branch].left = a;
                        internal_nodes[tail_branch].right = b;
                        tail_branch++;
                        if (tail_branch == n_leaves) {
                                /*
                                 * We're not getting here, no way, never ever.
                                 * Unless we made a terible mistake.
                                 *
                                 * That is, in a binary tree with n leaves,
                                 * there are ALWAYS n-1 internal nodes.
                                 */
                                return LZXPRESS_ERROR;
                        }
                }
                if (CHECK_DEBUGLVL(10) || DEBUG_HUFFMAN_TREE) {
                        debug_huffman_tree(huffman_root);
                }
                /*
                 * We have a tree, and need to turn it into a lookup table,
                 * and see if it is shallow enough (<= 15).
                 */
                less_than_15_bits = check_and_record_depths(huffman_root);
                if (less_than_15_bits) {
                        /*
                         * Now the leaf nodes know how deep they are, and we
                         * no longer need the internal nodes.
                         *
                         * We need to sort the nodes of equal depth, so that
                         * they are sorted by depth first, and symbol value
                         * second. The internal_nodes can again be auxillary
                         * memory.
                         */
                        stable_sort(
                                leaf_nodes,
                                internal_nodes,
                                n_leaves,
                                sizeof(struct huffman_node),
                                (samba_compare_fn_t)compare_huffman_node_depth);

                        encode_values(leaf_nodes, n_leaves, symbol_values);

                        return n_leaves;
                }

                /*
                 * requantize by halfing and rounding up, so that small counts
                 * become relatively bigger. This will lead to a flatter tree.
                 */
                for (i = 0; i < n_leaves; i++) {
                        leaf_nodes[i].count >>= 1;
                        leaf_nodes[i].count += 1;
                }
                head_leaf = 0;
                head_branch = 0;
                tail_branch = 0;
        }
        return LZXPRESS_ERROR;
}

/*
 * LZX_HUFF_COMP_HASH_SEARCH_ATTEMPTS is how far ahead to search in the
 * circular hash table for a match, before we give up. A bigger number will
 * generally lead to better but slower compression, but a stupidly big number
 * will just be worse.
 *
 * If you're fiddling with this, consider also fiddling with
 * LZX_HUFF_COMP_HASH_BITS.
 */
#define LZX_HUFF_COMP_HASH_SEARCH_ATTEMPTS 5

static inline void store_match(uint16_t *hash_table,
                               uint16_t h,
                               uint16_t offset)
{
        int i;
        uint16_t o = hash_table[h];
        uint16_t h2;
        uint16_t worst_h;
        int worst_score;

        if (o == 0xffff) {
                /* there is nothing there yet */
                hash_table[h] = offset;
                return;
        }
        for (i = 1; i < LZX_HUFF_COMP_HASH_SEARCH_ATTEMPTS; i++) {
                h2 = (h + i) & HASH_MASK;
                if (hash_table[h2] == 0xffff) {
                        hash_table[h2] = offset;
                        return;
                }
        }
        /*
         * There are no slots, but we really want to store this, so we'll kick
         * out the one with the longest distance.
         */
        worst_h = h;
        worst_score = offset - o;
        for (i = 1; i < LZX_HUFF_COMP_HASH_SEARCH_ATTEMPTS; i++) {
                int score;
                h2 = (h + i) & HASH_MASK;
                o = hash_table[h2];
                score = offset - o;
                if (score > worst_score) {
                        worst_score = score;
                        worst_h = h2;
                }
        }
        hash_table[worst_h] = offset;
}


/*
 * Yes, struct match looks a lot like a DATA_BLOB.
 */
struct match {
        const uint8_t *there;
        size_t length;
};


static inline struct match lookup_match(uint16_t *hash_table,
                                        uint16_t h,
                                        const uint8_t *data,
                                        const uint8_t *here,
                                        size_t max_len)
{
        int i;
        uint16_t o = hash_table[h];
        uint16_t h2;
        size_t len;
        const uint8_t *there = NULL;
        struct match best = {0};

        for (i = 0; i < LZX_HUFF_COMP_HASH_SEARCH_ATTEMPTS; i++) {
                h2 = (h + i) & HASH_MASK;
                o = hash_table[h2];
                if (o == 0xffff) {
                        /*
                         * in setting this, we would never have stepped over
                         * an 0xffff, so we won't now.
                         */
                        break;
                }
                there = data + o;
                if (here - there > 65534 || there > here) {
                        continue;
                }

                /*
                 * When we already have a long match, we can try to avoid
                 * measuring out another long, but shorter match.
                 */
                if (best.length > 1000 &&
                    there[best.length - 1] != best.there[best.length - 1]) {
                        continue;
                }

                for (len = 0;
                     len < max_len && here[len] == there[len];
                     len++) {
                        /* counting */
                }
                if (len > 2) {
                        /*
                         * As a tiebreaker, we prefer the closer match which
                         * is likely to encode smaller (and certainly no worse).
                         */
                        if (len > best.length ||
                            (len == best.length && there > best.there)) {
                                best.length = len;
                                best.there = there;
                        }
                }
        }
        return best;
}



static ssize_t lz77_encode_block(struct lzxhuff_compressor_context *cmp_ctx,
                                 struct lzxhuff_compressor_mem *cmp_mem,
                                 uint16_t *hash_table,
                                 uint16_t *prev_hash_table)
{
        uint16_t *intermediate = cmp_mem->intermediate;
        struct huffman_node *leaf_nodes = cmp_mem->leaf_nodes;
        uint16_t *symbol_values = cmp_mem->symbol_values;
        size_t i, j, intermediate_len;
        const uint8_t *data = cmp_ctx->input_bytes + cmp_ctx->input_pos;
        const uint8_t *prev_block = NULL;
        size_t remaining_size = cmp_ctx->input_size - cmp_ctx->input_pos;
        size_t block_end = MIN(65536, remaining_size);
        struct match match;
        int n_symbols;

        if (cmp_ctx->input_size < cmp_ctx->input_pos) {
                return LZXPRESS_ERROR;
        }

        if (cmp_ctx->prev_block_pos != cmp_ctx->input_pos) {
                prev_block = cmp_ctx->input_bytes + cmp_ctx->prev_block_pos;
        } else if (prev_hash_table != NULL) {
                /* we've got confused! hash and block should go together */
                return LZXPRESS_ERROR;
        }

        /*
         * leaf_nodes is used to count the symbols seen, for later Huffman
         * encoding.
         */
        for (i = 0; i < 512; i++) {
                leaf_nodes[i] = (struct huffman_node) {
                        .symbol = i
                };
        }

        j = 0;

        if (remaining_size < 41 || DEBUG_NO_LZ77_MATCHES) {
                /*
                 * There is no point doing a hash table and looking for
                 * matches in this tiny block (remembering we are committed to
                 * using 32 bits, so there's a good chance we wouldn't even
                 * save a byte). The threshold of 41 matches Windows.
                 * If remaining_size < 3, we *can't* do the hash.
                 */
                i = 0;
        } else {
                /*
                 * We use 0xffff as the unset value for table, because it is
                 * not a valid match offset (and 0x0 is).
                 */
                memset(hash_table, 0xff, sizeof(cmp_mem->hash_table1));

                for (i = 0; i <= block_end - 3; i++) {
                        uint16_t code;
                        const uint8_t *here = data + i;
                        uint16_t h = three_byte_hash(here);
                        size_t max_len = MIN(remaining_size - i, MAX_MATCH_LENGTH);
                        match = lookup_match(hash_table,
                                             h,
                                             data,
                                             here,
                                             max_len);

                        if (match.there == NULL && prev_hash_table != NULL) {
                                /*
                                 * If this is not the first block,
                                 * backreferences can look into the previous
                                 * block (but only as far as 65535 bytes, so
                                 * the end of this block cannot see the start
                                 * of the last one).
                                 */
                                match = lookup_match(prev_hash_table,
                                                     h,
                                                     prev_block,
                                                     here,
                                                     remaining_size - i);
                        }

                        store_match(hash_table, h, i);

                        if (match.there == NULL) {
                                /* add a literal and move on. */
                                uint8_t c = data[i];
                                leaf_nodes[c].count++;
                                intermediate[j] = c;
                                j++;
                                continue;
                        }

                        /* a real match */
                        if (match.length <= 65538) {
                                intermediate[j] = 0xffff;
                                intermediate[j + 1] = match.length - 3;
                                intermediate[j + 2] = here - match.there;
                                j += 3;
                        } else {
                                size_t m = match.length - 3;
                                intermediate[j] = 0xfffe;
                                intermediate[j + 1] = m & 0xffff;
                                intermediate[j + 2] = m >> 16;
                                intermediate[j + 3] = here - match.there;
                                j += 4;
                        }
                        code = encode_match(match.length, here - match.there);
                        leaf_nodes[code].count++;
                        i += match.length - 1; /* `- 1` for the loop i++ */
                        /*
                         * A match can take us past the intended block length,
                         * extending the block. We don't need to do anything
                         * special for this case -- the loops will naturally
                         * do the right thing.
                         */
                }
        }

        /*
         * There might be some bytes at the end.
         */
        for (; i < block_end; i++) {
                leaf_nodes[data[i]].count++;
                intermediate[j] = data[i];
                j++;
        }

        if (i == remaining_size) {
                /* add a trailing EOF marker (256) */
                intermediate[j] = 0xffff;
                intermediate[j + 1] = 0;
                intermediate[j + 2] = 1;
                j += 3;
                leaf_nodes[256].count++;
        }

        intermediate_len = j;

        cmp_ctx->prev_block_pos = cmp_ctx->input_pos;
        cmp_ctx->input_pos += i;

        /* fill in the symbols table */
        n_symbols = generate_huffman_codes(leaf_nodes,
                                           cmp_mem->internal_nodes,
                                           symbol_values);
        if (n_symbols < 0) {
                return n_symbols;
        }

        return intermediate_len;
}



static ssize_t write_huffman_table(uint16_t symbol_values[512],
                                   uint8_t *output,
                                   size_t available_size)
{
        size_t i;

        if (available_size < 256) {
                return LZXPRESS_ERROR;
        }

        for (i = 0; i < 256; i++) {
                uint8_t b = 0;
                uint16_t even = symbol_values[i * 2];
                uint16_t odd = symbol_values[i * 2 + 1];
                if (even != 0) {
                        b = bitlen_nonzero_16(even);
                }
                if (odd != 0) {
                        b |= bitlen_nonzero_16(odd) << 4;
                }
                output[i] = b;
        }
        return i;
}


struct write_context {
        uint8_t *dest;
        size_t dest_len;
        size_t head;                 /* where lengths go */
        size_t next_code;            /* where symbol stream goes */
        size_t pending_next_code;    /* will be next_code */
        unsigned bit_len;
        uint32_t bits;
};

/*
 * Write out 16 bits, little-endian, for write_huffman_codes()
 *
 * As you'll notice, there's a bit to do.
 *
 * We are collecting up bits in a uint32_t, then when there are 16 of them we
 * write out a word into the stream, using a trio of offsets (wc->next_code,
 * wc->pending_next_code, and wc->head) which dance around ensuring that the
 * bitstream and the interspersed lengths are in the right places relative to
 * each other.
 */

static inline bool write_bits(struct write_context *wc,
                              uint16_t code, uint16_t length)
{
        wc->bits <<= length;
        wc->bits |= code;
        wc->bit_len += length;
        if (wc->bit_len > 16) {
                uint32_t w = wc->bits >> (wc->bit_len - 16);
                wc->bit_len -= 16;
                if (wc->next_code + 2 > wc->dest_len ||
                    unlikely(wc->bit_len > 16)) {
                        return false;
                }
                wc->dest[wc->next_code] = w & 0xff;
                wc->dest[wc->next_code + 1] = (w >> 8) & 0xff;
                wc->next_code = wc->pending_next_code;
                wc->pending_next_code = wc->head;
                wc->head += 2;
        }
        return true;
}


static inline bool write_code(struct write_context *wc, uint16_t code)
{
        int code_bit_len = bitlen_nonzero_16(code);
        if (unlikely(code == 0)) {
                return false;
        }
        code &= (1 << code_bit_len) - 1;
        return  write_bits(wc, code, code_bit_len);
}

static inline bool write_byte(struct write_context *wc, uint8_t byte)
{
        if (wc->head + 1 > wc->dest_len) {
                return false;
        }
        wc->dest[wc->head] = byte;
        wc->head++;
        return true;
}


static inline bool write_long_len(struct write_context *wc, size_t len)
{
        if (len < 65535) {
                if (wc->head + 3 > wc->dest_len) {
                        return false;
                }
                wc->dest[wc->head] = 255;
                wc->dest[wc->head + 1] = len & 255;
                wc->dest[wc->head + 2] = len >> 8;
                wc->head += 3;
        } else {
                if (wc->head + 7 > wc->dest_len) {
                        return false;
                }
                wc->dest[wc->head] = 255;
                wc->dest[wc->head + 1] = 0;
                wc->dest[wc->head + 2] = 0;
                wc->dest[wc->head + 3] = len & 255;
                wc->dest[wc->head + 4] = (len >> 8) & 255;
                wc->dest[wc->head + 5] = (len >> 16) & 255;
                wc->dest[wc->head + 6] = (len >> 24) & 255;
                wc->head += 7;
        }
        return true;
}

static ssize_t write_compressed_bytes(uint16_t symbol_values[512],
                                      uint16_t *intermediate,
                                      size_t intermediate_len,
                                      uint8_t *dest,
                                      size_t dest_len)
{
        bool ok;
        size_t i;
        size_t end;
        struct write_context wc = {
                .head = 4,
                .pending_next_code = 2,
                .dest = dest,
                .dest_len = dest_len
        };
        for (i = 0; i < intermediate_len; i++) {
                uint16_t c = intermediate[i];
                size_t len;
                uint16_t distance;
                uint16_t code_len = 0;
                uint16_t code_dist = 0;
                if (c < 256) {
                        ok = write_code(&wc, symbol_values[c]);
                        if (!ok) {
                                return LZXPRESS_ERROR;
                        }
                        continue;
                }

                if (c == 0xfffe) {
                        if (i > intermediate_len - 4) {
                                return LZXPRESS_ERROR;
                        }

                        len = intermediate[i + 1];
                        len |= intermediate[i + 2] << 16U;
                        distance = intermediate[i + 3];
                        i += 3;
                } else if (c == 0xffff) {
                        if (i > intermediate_len - 3) {
                                return LZXPRESS_ERROR;
                        }
                        len = intermediate[i + 1];
                        distance = intermediate[i + 2];
                        i += 2;
                } else {
                        return LZXPRESS_ERROR;
                }
                if (unlikely(distance == 0)) {
                        return LZXPRESS_ERROR;
                }
                /* len has already had 3 subtracted */
                if (len >= 15) {
                        /*
                         * We are going to need to write extra length
                         * bytes into the stream, but we don't do it
                         * now, we do it after the code has been
                         * written (and before the distance bits).
                         */
                        code_len = 15;
                } else {
                        code_len = len;
                }
                code_dist = bitlen_nonzero_16(distance);
                c = 256 | (code_dist << 4) | code_len;
                if (c > 511) {
                        return LZXPRESS_ERROR;
                }

                ok = write_code(&wc, symbol_values[c]);
                if (!ok) {
                        return LZXPRESS_ERROR;
                }

                if (code_len == 15) {
                        if (len >= 270) {
                                ok = write_long_len(&wc, len);
                        } else {
                                ok = write_byte(&wc, len - 15);
                        }
                        if (! ok) {
                                return LZXPRESS_ERROR;
                        }
                }
                if (code_dist != 0) {
                        uint16_t dist_bits = distance - (1 << code_dist);
                        ok = write_bits(&wc, dist_bits, code_dist);
                        if (!ok) {
                                return LZXPRESS_ERROR;
                        }
                }
        }
        /*
         * There are some intricacies around flushing the bits and returning
         * the length.
         *
         * If the returned length is not exactly right and there is another
         * block, that block will read its huffman table from the wrong place,
         * and have all the symbol codes out by a multiple of 4.
         */
        end = wc.head;
        if (wc.bit_len == 0) {
                end -= 2;
        }
        ok = write_bits(&wc, 0, 16 - wc.bit_len);
        if (!ok) {
                return LZXPRESS_ERROR;
        }
        for (i = 0; i < 2; i++) {
                /*
                 * Flush out the bits with zeroes. It doesn't matter if we do
                 * a round too many, as we have buffer space, and have already
                 * determined the returned length (end).
                 */
                ok = write_bits(&wc, 0, 16);
                if (!ok) {
                        return LZXPRESS_ERROR;
                }
        }
        return end;
}


static ssize_t lzx_huffman_compress_block(struct lzxhuff_compressor_context *cmp_ctx,
                                          struct lzxhuff_compressor_mem *cmp_mem,
                                          size_t block_no)
{
        ssize_t intermediate_size;
        uint16_t *hash_table = NULL;
        uint16_t *back_window_hash_table = NULL;
        ssize_t bytes_written;

        if (cmp_ctx->available_size - cmp_ctx->output_pos < 260) {
                /* huffman block + 4 bytes */
                return LZXPRESS_ERROR;
        }

        /*
         * For LZ77 compression, we keep a hash table for the previous block,
         * via alternation after the first block.
         *
         * LZ77 writes into the intermediate buffer in the cmp_mem context.
         */
        if (block_no == 0) {
                hash_table = cmp_mem->hash_table1;
                back_window_hash_table = NULL;
        } else if (block_no & 1) {
                hash_table = cmp_mem->hash_table2;
                back_window_hash_table = cmp_mem->hash_table1;
        } else {
                hash_table = cmp_mem->hash_table1;
                back_window_hash_table = cmp_mem->hash_table2;
        }

        intermediate_size = lz77_encode_block(cmp_ctx,
                                              cmp_mem,
                                              hash_table,
                                              back_window_hash_table);

        if (intermediate_size < 0) {
                return intermediate_size;
        }

        /*
         * Write the 256 byte Huffman table, based on the counts gained in
         * LZ77 phase.
         */
        bytes_written = write_huffman_table(
                cmp_mem->symbol_values,
                cmp_ctx->output + cmp_ctx->output_pos,
                cmp_ctx->available_size - cmp_ctx->output_pos);

        if (bytes_written != 256) {
                return LZXPRESS_ERROR;
        }
        cmp_ctx->output_pos += 256;

        /*
         * Write the compressed bytes using the LZ77 matches and Huffman codes
         * worked out in the previous steps.
         */
        bytes_written = write_compressed_bytes(
                cmp_mem->symbol_values,
                cmp_mem->intermediate,
                intermediate_size,
                cmp_ctx->output + cmp_ctx->output_pos,
                cmp_ctx->available_size - cmp_ctx->output_pos);

        if (bytes_written < 0) {
                return bytes_written;
        }

        cmp_ctx->output_pos += bytes_written;
        return bytes_written;
}


/*
 * lzxpress_huffman_compress_talloc()
 *
 * This is the convenience function that allocates the compressor context and
 * output memory for you. The return value is the number of bytes written to
 * the location indicated by the output pointer.
 *
 * The maximum input_size is effectively around 227MB due to the need to guess
 * an upper bound on the output size that hits an internal limitation in
 * talloc.
 *
 * @param mem_ctx      TALLOC_CTX parent for the compressed buffer.
 * @param input_bytes  memory to be compressed.
 * @param input_size   length of the input buffer.
 * @param output       destination pointer for the compressed data.
 *
 * @return the number of bytes written or -1 on error.
 */

ssize_t lzxpress_huffman_compress_talloc(TALLOC_CTX *mem_ctx,
                                         const uint8_t *input_bytes,
                                         size_t input_size,
                                         uint8_t **output)
{
        struct lzxhuff_compressor_mem *cmp = NULL;
        /*
         * In the worst case, the output size should be about the same as the
         * input size, plus the 256 byte header per 64k block. We aim for
         * ample, but within the order of magnitude.
         */
        size_t alloc_size = input_size + (input_size / 8) + 270;
        ssize_t output_size;

        *output = talloc_array(mem_ctx, uint8_t, alloc_size);
        if (*output == NULL) {
                return LZXPRESS_ERROR;
        }

        cmp = talloc(mem_ctx, struct lzxhuff_compressor_mem);
        if (cmp == NULL) {
                TALLOC_FREE(*output);
                return LZXPRESS_ERROR;
        }

        output_size = lzxpress_huffman_compress(cmp,
                                                input_bytes,
                                                input_size,
                                                *output,
                                                alloc_size);

        talloc_free(cmp);

        if (output_size < 0) {
                TALLOC_FREE(*output);
                return LZXPRESS_ERROR;
        }

        *output = talloc_realloc(mem_ctx, *output, uint8_t, output_size);
        if (*output == NULL) {
                return LZXPRESS_ERROR;
        }

        return output_size;
}

/*
 * lzxpress_huffman_compress()
 *
 * This is the inconvenience function, slightly faster and fiddlier than
 * lzxpress_huffman_compress_talloc().
 *
 * To use this, you need to have allocated (but not initialised) a `struct
 * lzxhuff_compressor_context`, and an output buffer. If the buffer is not big
 * enough (per `output_size`), you'll get a negative return value, otherwise
 * the number of bytes actually consumed, which will always be at least 260.
 *
 * The `struct lzxhuff_compressor_context` is reusable -- it is basically a
 * collection of uninitialised memory buffers. The total size is less than
 * 150k, so stack allocation is plausible.
 *
 * input_size and available_size are limited to the minimum of UINT32_MAX and
 * SSIZE_MAX. On 64 bit machines that will be UINT32_MAX, or 4GB.
 *
 * @param cmp_mem         a struct lzxhuff_compressor_mem.
 * @param input_bytes     memory to be compressed.
 * @param input_size      length of the input buffer.
 * @param output          destination for the compressed data.
 * @param available_size  allocated output bytes.
 *
 * @return the number of bytes written or -1 on error.
 */
ssize_t lzxpress_huffman_compress(struct lzxhuff_compressor_mem *cmp_mem,
                                  const uint8_t *input_bytes,
                                  size_t input_size,
                                  uint8_t *output,
                                  size_t available_size)
{
        size_t i = 0;
        struct lzxhuff_compressor_context cmp_ctx = {
                .input_bytes = input_bytes,
                .input_size = input_size,
                .input_pos = 0,
                .prev_block_pos = 0,
                .output = output,
                .available_size = available_size,
                .output_pos = 0
        };

        if (input_size == 0) {
                /*
                 * We can't deal with this for a number of reasons (e.g. it
                 * breaks the Huffman tree), and the output will be infinitely
                 * bigger than the input. The caller needs to go and think
                 * about what they're trying to do here.
                 */
                return LZXPRESS_ERROR;
        }

        if (input_size > SSIZE_MAX ||
            input_size > UINT32_MAX ||
            available_size > SSIZE_MAX ||
            available_size > UINT32_MAX ||
            available_size == 0) {
                /*
                 * We use negative ssize_t to return errors, which is limiting
                 * on 32 bit machines; otherwise we adhere to Microsoft's 4GB
                 * limit.
                 *
                 * lzxpress_huffman_compress_talloc() will not get this far,
                 * having already have failed on talloc's 256 MB limit.
                 */
                return LZXPRESS_ERROR;
        }

        if (cmp_mem == NULL ||
            output == NULL ||
            input_bytes == NULL) {
                return LZXPRESS_ERROR;
        }

        while (cmp_ctx.input_pos < cmp_ctx.input_size) {
                ssize_t ret;
                ret = lzx_huffman_compress_block(&cmp_ctx,
                                                 cmp_mem,
                                                 i);
                if (ret < 0) {
                        return ret;
                }
                i++;
        }

        return cmp_ctx.output_pos;
}

static void debug_tree_codes(struct bitstream *input)
{
        /*
         */
        size_t head = 0;
        size_t tail = 2;
        size_t ffff_count = 0;
        struct q {
                uint16_t tree_code;
                uint16_t code_code;
        };
        struct q queue[65536];
        char bits[17];
        uint16_t *t = input->table;
        queue[0].tree_code = 1;
        queue[0].code_code = 2;
        queue[1].tree_code = 2;
        queue[1].code_code = 3;
        while (head < tail) {
                struct q q = queue[head];
                uint16_t x = t[q.tree_code];
                if (x != 0xffff) {
                        int k;
                        uint16_t j = q.code_code;
                        size_t offset = bitlen_nonzero_16(j) - 1;
                        if (unlikely(j == 0)) {
                                DBG("BROKEN code is 0!\n");
                                return;
                        }

                        for (k = 0; k <= offset; k++) {
                                bool b = (j >> (offset - k)) & 1;
                                bits[k] = b ? '1' : '0';
                        }
                        bits[k] = 0;
                        DBG("%03x   %s\n", x & 511, bits);
                        head++;
                        continue;
                }
                ffff_count++;
                queue[tail].tree_code = q.tree_code * 2 + 1;
                queue[tail].code_code = q.code_code * 2;
                tail++;
                queue[tail].tree_code = q.tree_code * 2 + 1 + 1;
                queue[tail].code_code = q.code_code * 2 + 1;
                tail++;
                head++;
        }
        DBG("0xffff count: %zu\n", ffff_count);
}

/**
 * Determines the sort order of one prefix_code_symbol relative to another
 */
static int compare_uint16(const uint16_t *a, const uint16_t *b)
{
        if (*a < *b) {
                return -1;
        }
        if (*a > *b) {
                return 1;
        }
        return 0;
}


static bool fill_decomp_table(struct bitstream *input)
{
        /*
         * There are 512 symbols, each encoded in 4 bits, which indicates
         * their depth in the Huffman tree. The even numbers get the lower
         * nibble of each byte, so that the byte hex values look backwards
         * (i.e. 0xab encodes b then a). These are allocated Huffman codes in
         * order of appearance, per depth.
         *
         * For example, if the first two bytes were:
         *
         * 0x23 0x53
         *
         * the first four codes have the lengths 3, 2, 3, 5.
         * Let's call them A, B, C, D.
         *
         * Suppose there is no other codeword with length 1 (which is
         * necessarily true in this example) or 2, but there might be others
         * of length 3 or 4. Then we can say this about the codes:
         *
         *        _ --*--_
         *      /          \
         *     0           1
         *    / \         / \
         *   0   1       0   1
         *  B    |\     / \  |\
         *       0 1   0   1 0 1
         *       A C   |\ /| | |\
         *
         * pos bits  code
         * A    3    010
         * B    2    00
         * C    3    011
         * D    5    1????
         *
         * B has the shortest code, so takes the leftmost branch, 00. That
         * ends the branch -- nothing else can start with 00. There are no
         * more 2s, so we look at the 3s, starting as far left as possible. So
         * A takes 010 and C takes 011. That means everything else has to
         * start with 1xx. We don't know how many codewords of length 3 or 4
         * there are; if there are none, D would end up with 10000, the
         * leftmost available code of length 5. If the compressor is any good,
         * there should be no unused leaf nodes left dangling at the end.
         *
         * (this is "Canonical Huffman Coding").
         *
         *
         * But what symbols do these codes actually stand for?
         * --------------------------------------------------
         *
         * Good question. The first 256 codes stand for the corresponding
         * literal bytes. The codes from 256 to 511 stand for LZ77 matches,
         * which have a distance and a length, encoded in a strange way that
         * isn't entirely the purview of this function.
         *
         * What does the value 0 mean?
         * ---------------------------
         *
         * The code does not occur. For example, if the next byte in the
         * example above was 0x07, that would give the byte 0x04 a 7-long
         * code, and no code to the 0x05 byte, which means we there is no way
         * we going to see a 5 in the decoded stream.
         *
         * Isn't LZ77 + Huffman what zip/gzip/zlib do?
         * -------------------------------------------
         *
         * Yes, DEFLATE is LZ77 + Huffman, but the details are quite different.
         */
        uint16_t symbols[512];
        uint16_t sort_mem[512];
        size_t i, n_symbols;
        ssize_t code;
        uint16_t len, prev_len;
        const uint8_t *table_bytes = input->bytes + input->byte_pos;

        if (input->byte_pos + 260 > input->byte_size) {
                return false;
        }

        n_symbols = 0;
        for (i = 0; i < 256; i++) {
                uint16_t even = table_bytes[i] & 15;
                uint16_t odd = table_bytes[i] >> 4;
                if (even != 0) {
                        symbols[n_symbols] = (even << 9) + i * 2;
                        n_symbols++;
                }
                if (odd != 0) {
                        symbols[n_symbols] = (odd << 9) + i * 2 + 1;
                        n_symbols++;
                }
        }
        input->byte_pos += 256;
        if (n_symbols == 0) {
                return false;
        }

        stable_sort(symbols, sort_mem, n_symbols, sizeof(uint16_t),
                    (samba_compare_fn_t)compare_uint16);

        /*
         * we're using an implicit binary tree, as you'd see in a heap.
         * table[0] = unused
         * table[1] = '0'
         * table[2] = '1'
         * table[3] = '00'     <-- '00' and '01' are children of '0'
         * table[4] = '01'     <-- '0' is [0], children are [0 * 2 + {1,2}]
         * table[5] = '10'
         * table[6] = '11'
         * table[7] = '000'
         * table[8] = '001'
         * table[9] = '010'
         * table[10]= '011'
         * table[11]= '100
         *'
         * table[1 << n - 1] = '0' * n
         * table[1 << n - 1 + x] = n-bit wide x (left padded with '0')
         * table[1 << n - 2] = '1' * (n - 1)
         *
         * table[i]->left =  table[i*2 + 1]
         * table[i]->right = table[i*2 + 2]
         * table[0xffff] = unused (16 '0's, max len is 15)
         *
         * therefore e.g. table[70] = table[64     - 1 + 7]
         *                          = table[1 << 6 - 1 + 7]
         *                          = '000111' (binary 7, widened to 6 bits)
         *
         *   and if '000111' is a code,
         *   '00011', '0001', '000', '00', '0' are unavailable prefixes.
         *       34      16      7     3    1  are their indices
         *   and (i - 1) >> 1 is the path back from 70 through these.
         *
         * the lookup is
         *
         * 1 start with i = 0
         * 2 extract a symbol bit (i = (i << 1) + bit + 1)
         * 3 is table[i] == 0xffff?
         * 4  yes -- goto 2
         * 4  table[i] & 511 is the symbol, stop
         *
         * and the construction (here) is sort of the reverse.
         *
         * Most of this table is free space that can never be reached, and
         * most of the activity is at the beginning (since all codes start
         * there, and by design the shortest codes are the most common).
         */
        for (i = 0; i < 32; i++) {
                /* prefill the table head */
                input->table[i] = 0xffff;
        }
        code = -1;
        prev_len = 0;
        for (i = 0; i < n_symbols; i++) {
                uint16_t s = symbols[i];
                uint16_t prefix;
                len = (s >> 9) & 15;
                s &= 511;
                code++;
                while (len != prev_len) {
                        code <<= 1;
                        code++;
                        prev_len++;
                }

                if (code >= 65535) {
                        return false;
                }
                input->table[code] = s;
                for(prefix = (code - 1) >> 1;
                    prefix > 31;
                    prefix = (prefix - 1) >> 1) {
                        input->table[prefix] = 0xffff;
                }
        }
        if (CHECK_DEBUGLVL(10)) {
                debug_tree_codes(input);
        }

        /*
         * check that the last code encodes 11111..., with right number of
         * ones, pointing to the right symbol -- otherwise we have a dangling
         * uninitialised symbol.
         */
        if (code != (1 << (len + 1)) - 2) {
                return false;
        }
        return true;
}


#define CHECK_READ_32(dest)                                       \
        do {                                                      \
                if (input->byte_pos + 4 > input->byte_size) {     \
                        return LZXPRESS_ERROR;                     \
                }                                                  \
                dest = PULL_LE_U32(input->bytes, input->byte_pos); \
                input->byte_pos += 4;                              \
        } while (0)

#define CHECK_READ_16(dest)                                       \
        do {                                                      \
                if (input->byte_pos + 2 > input->byte_size) {     \
                        return LZXPRESS_ERROR;                     \
                }                                                  \
                dest = PULL_LE_U16(input->bytes, input->byte_pos); \
                input->byte_pos += 2;                              \
        } while (0)

#define CHECK_READ_8(dest) \
        do {                                                            \
                if (input->byte_pos >= input->byte_size) {              \
                        return LZXPRESS_ERROR;                          \
                }                                                       \
                dest = PULL_LE_U8(input->bytes, input->byte_pos);       \
                input->byte_pos++;                                      \
        } while(0)


static inline ssize_t pull_bits(struct bitstream *input)
{
        if (input->byte_pos + 1 < input->byte_size) {
                uint16_t tmp;
                CHECK_READ_16(tmp);
                input->remaining_bits += 16;
                input->bits <<= 16;
                input->bits |= tmp;
        } else if (input->byte_pos < input->byte_size) {
                uint8_t tmp;
                CHECK_READ_8(tmp);
                input->remaining_bits += 8;
                input->bits <<= 8;
                input->bits |= tmp;
        } else {
                return LZXPRESS_ERROR;
        }
        return 0;
}


/*
 * Decompress a block. The actual decompressed size is returned (or -1 on
 * error). The putative block length is 64k (or shorter, if the message ends
 * first), but a match can run over the end, extending the block. That's why
 * we need the overall output size as well as the block size. A match encoded
 * in this block can point back to previous blocks, but not before the
 * beginning of the message, so we also need the previously decoded size.
 *
 * The compressed block will have 256 bytes for the Huffman table, and at
 * least 4 bytes of (possibly padded) encoded values.
 */
static ssize_t lzx_huffman_decompress_block(struct bitstream *input,
                                            uint8_t *output,
                                            size_t block_size,
                                            size_t output_size,
                                            size_t previous_size)
{
        size_t output_pos = 0;
        uint16_t symbol;
        size_t index;
        uint16_t distance_bits_wanted = 0;
        size_t distance = 0;
        size_t length = 0;
        bool ok;
        uint32_t tmp;
        bool seen_eof_marker = false;

        ok = fill_decomp_table(input);
        if (! ok) {
                return LZXPRESS_ERROR;
        }
        if (CHECK_DEBUGLVL(10) || DEBUG_HUFFMAN_TREE) {
                debug_huffman_tree_from_table(input->table);
        }
        /*
         * Always read 32 bits at the start, even if we don't need them.
         */
        CHECK_READ_16(tmp);
        CHECK_READ_16(input->bits);
        input->bits |= tmp << 16;
        input->remaining_bits = 32;

        /*
         * This loop iterates over individual *bits*. These are read from
         * little-endian 16 bit words, most significant bit first.
         *
         * At points in the bitstream, the following are possible:
         *
         * # the source word is empty and needs to be refilled from the input
         *    stream.
         * # an incomplete codeword is being extended.
         * # a codeword is resolved, either as a literal or a match.
         * # a literal is written.
         * # a match is collecting distance bits.
         * # the output stream is copied, as specified by a match.
         * # input bytes are read for match lengths.
         *
         * Note that we *don't* specifically check for the EOF marker (symbol
         * 256) in this loop, because the a precondition for stopping for the
         * EOF marker is that the output buffer is full (otherwise, you
         * wouldn't know which 256 is EOF, rather than an actual symbol), and
         * we *always* want to stop when the buffer is full. So we work out if
         * there is an EOF in in another loop after we stop writing.
         */

        index = 0;
        while (output_pos < block_size) {
                uint16_t b;
                if (input->remaining_bits == 16) {
                        ssize_t ret = pull_bits(input);
                        if (ret) {
                                return ret;
                        }
                }
                input->remaining_bits--;

                b = (input->bits >> input->remaining_bits) & 1;
                if (length == 0) {
                        /* not in a match; pulling a codeword */
                        index <<= 1;
                        index += b + 1;
                        if (input->table[index] == 0xffff) {
                                /* incomplete codeword, the common case */
                                continue;
                        }
                        /* found the symbol, reset the code string */
                        symbol = input->table[index] & 511;
                        index = 0;
                        if (symbol < 256) {
                                /* a literal, the easy case */
                                output[output_pos] = symbol;
                                output_pos++;
                                continue;
                        }

                        /* the beginning of a match */
                        distance_bits_wanted = (symbol >> 4) & 15;
                        distance = 1 << distance_bits_wanted;
                        length = symbol & 15;
                        if (length == 15) {
                                CHECK_READ_8(tmp);
                                length += tmp;
                                if (length == 255 + 15) {
                                        /*
                                         * note, we discard (don't add) the
                                         * length so far.
                                         */
                                        CHECK_READ_16(length);
                                        if (length == 0) {
                                                CHECK_READ_32(length);
                                        }
                                }
                        }
                        length += 3;
                } else {
                        /* we are pulling extra distance bits */
                        distance_bits_wanted--;
                        distance |= b << distance_bits_wanted;
                }

                if (distance_bits_wanted == 0) {
                        /*
                         * We have a complete match, and it is time to do the
                         * copy (byte by byte, because the ranges can overlap,
                         * and we might need to copy bytes we just copied in).
                         *
                         * It is possible that this match will extend beyond
                         * the end of the expected block. That's fine, so long
                         * as it doesn't extend past the total output size.
                         */
                        size_t i;
                        size_t end = output_pos + length;
                        uint8_t *here = output + output_pos;
                        uint8_t *there = here - distance;
                        if (end > output_size ||
                            previous_size + output_pos < distance ||
                            unlikely(end < output_pos || there > here)) {
                                return LZXPRESS_ERROR;
                        }
                        for (i = 0; i < length; i++) {
                                here[i] = there[i];
                        }
                        output_pos += length;
                        distance = 0;
                        length = 0;
                }
        }

        if (length != 0 || index != 0) {
                /* it seems like we've hit an early end, mid-code */
                return LZXPRESS_ERROR;
        }

        if (input->byte_pos + 256 < input->byte_size) {
                /*
                 * This block is over, but it clearly isn't the last block, so
                 * we don't want to look for the EOF.
                 */
                return output_pos;
        }
        /*
         * We won't write any more, but we try to read some more to make sure
         * we're finishing in a good place. That means we want to see a 256
         * symbol and then some number of zeroes, possibly zero, but as many
         * as 32.
         *
         * In this we are perhaps a bit stricter than Windows, which
         * apparently does not insist on the EOF marker, nor on a lack of
         * trailing bytes.
         */
        while (true) {
                uint16_t b;
                if (input->remaining_bits == 16) {
                        ssize_t ret;
                        if (input->byte_pos == input->byte_size) {
                                /* FIN */
                                break;
                        }
                        ret = pull_bits(input);
                        if (ret) {
                                return ret;
                        }
                }
                input->remaining_bits--;
                b = (input->bits >> input->remaining_bits) & 1;
                if (seen_eof_marker) {
                        /*
                         * we have read an EOF symbols. Now we just want to
                         * see zeroes.
                         */
                        if (b != 0) {
                                return LZXPRESS_ERROR;
                        }
                        continue;
                }

                /* we're pulling in a symbol, which had better be 256 */
                index <<= 1;
                index += b + 1;
                if (input->table[index] == 0xffff) {
                        continue;
                }

                symbol = input->table[index] & 511;
                if (symbol != 256) {
                        return LZXPRESS_ERROR;
                }
                seen_eof_marker = true;
                continue;
        }

        if (! seen_eof_marker) {
                return LZXPRESS_ERROR;
        }

        return output_pos;
}

static ssize_t lzxpress_huffman_decompress_internal(struct bitstream *input,
                                                    uint8_t *output,
                                                    size_t output_size)
{
        size_t output_pos = 0;

        if (input->byte_size < 260) {
                return LZXPRESS_ERROR;
        }

        while (input->byte_pos < input->byte_size) {
                ssize_t block_output_pos;
                ssize_t block_output_size;
                size_t remaining_output_size = output_size - output_pos;

                block_output_size = MIN(65536, remaining_output_size);

                block_output_pos = lzx_huffman_decompress_block(
                        input,
                        output + output_pos,
                        block_output_size,
                        remaining_output_size,
                        output_pos);

                if (block_output_pos < block_output_size) {
                        return LZXPRESS_ERROR;
                }
                output_pos += block_output_pos;
                if (output_pos > output_size) {
                        /* not expecting to get here. */
                        return LZXPRESS_ERROR;
                }
        }

        if (input->byte_pos != input->byte_size) {
                return LZXPRESS_ERROR;
        }

        return output_pos;
}


/*
 * lzxpress_huffman_decompress()
 *
 * output_size must be the expected length of the decompressed data.
 * input_size and output_size are limited to the minimum of UINT32_MAX and
 * SSIZE_MAX. On 64 bit machines that will be UINT32_MAX, or 4GB.
 *
 * @param input_bytes  memory to be decompressed.
 * @param input_size   length of the compressed buffer.
 * @param output       destination for the decompressed data.
 * @param output_size  exact expected length of the decompressed data.
 *
 * @return the number of bytes written or -1 on error.
 */

ssize_t lzxpress_huffman_decompress(const uint8_t *input_bytes,
                                    size_t input_size,
                                    uint8_t *output,
                                    size_t output_size)
{
        uint16_t table[65536];
        struct bitstream input = {
                .bytes = input_bytes,
                .byte_size = input_size,
                .byte_pos = 0,
                .bits = 0,
                .remaining_bits = 0,
                .table = table
        };

        if (input_size > SSIZE_MAX ||
            input_size > UINT32_MAX ||
            output_size > SSIZE_MAX ||
            output_size > UINT32_MAX ||
            input_size == 0 ||
            output_size == 0 ||
            input_bytes == NULL ||
            output == NULL) {
                /*
                 * We use negative ssize_t to return errors, which is limiting
                 * on 32 bit machines, and the 4GB limit exists on Windows.
                 */
                return  LZXPRESS_ERROR;
        }

        return lzxpress_huffman_decompress_internal(&input,
                                                    output,
                                                    output_size);
}


/**
 * lzxpress_huffman_decompress_talloc()
 *
 * The caller must provide the exact size of the expected output.
 *
 * The input_size is limited to the minimum of UINT32_MAX and SSIZE_MAX, but
 * output_size is limited to 256MB due to a limit in talloc. This effectively
 * limits input_size too, as non-crafted compressed data will not exceed the
 * decompressed size by very much.
 *
 * @param mem_ctx      TALLOC_CTX parent for the decompressed buffer.
 * @param input_bytes  memory to be decompressed.
 * @param input_size   length of the compressed buffer.
 * @param output_size  expected decompressed size.
 *
 * @return a talloc'ed buffer exactly output_size in length, or NULL.
 */

uint8_t *lzxpress_huffman_decompress_talloc(TALLOC_CTX *mem_ctx,
                                            const uint8_t *input_bytes,
                                            size_t input_size,
                                            size_t output_size)
{
        ssize_t result;
        uint8_t *output = NULL;
        struct bitstream input = {
                .bytes = input_bytes,
                .byte_size = input_size
        };

        output = talloc_array(mem_ctx, uint8_t, output_size);
        if (output == NULL) {
                return NULL;
        }

        input.table = talloc_array(mem_ctx, uint16_t, 65536);
        if (input.table == NULL) {
                talloc_free(output);
                return NULL;
        }
        result = lzxpress_huffman_decompress_internal(&input,
                                                      output,
                                                      output_size);
        talloc_free(input.table);

        if (result != output_size) {
                talloc_free(output);
                return NULL;
        }
        return output;
}