Refactor notes concatenation into a flexible interface for combining notes

[git/mjg.git] / notes.c

#include "cache.h"
#include "notes.h"
#include "blob.h"
#include "tree.h"
#include "utf8.h"
#include "strbuf.h"
#include "tree-walk.h"

/*
 * Use a non-balancing simple 16-tree structure with struct int_node as
 * internal nodes, and struct leaf_node as leaf nodes. Each int_node has a
 * 16-array of pointers to its children.
 * The bottom 2 bits of each pointer is used to identify the pointer type
 * - ptr & 3 == 0 - NULL pointer, assert(ptr == NULL)
 * - ptr & 3 == 1 - pointer to next internal node - cast to struct int_node *
 * - ptr & 3 == 2 - pointer to note entry - cast to struct leaf_node *
 * - ptr & 3 == 3 - pointer to subtree entry - cast to struct leaf_node *
 *
 * The root node is a statically allocated struct int_node.
 */
struct int_node {
        void *a[16];
};

/*
 * Leaf nodes come in two variants, note entries and subtree entries,
 * distinguished by the LSb of the leaf node pointer (see above).
 * As a note entry, the key is the SHA1 of the referenced object, and the
 * value is the SHA1 of the note object.
 * As a subtree entry, the key is the prefix SHA1 (w/trailing NULs) of the
 * referenced object, using the last byte of the key to store the length of
 * the prefix. The value is the SHA1 of the tree object containing the notes
 * subtree.
 */
struct leaf_node {
        unsigned char key_sha1[20];
        unsigned char val_sha1[20];
};

#define PTR_TYPE_NULL     0
#define PTR_TYPE_INTERNAL 1
#define PTR_TYPE_NOTE     2
#define PTR_TYPE_SUBTREE  3

#define GET_PTR_TYPE(ptr)       ((uintptr_t) (ptr) & 3)
#define CLR_PTR_TYPE(ptr)       ((void *) ((uintptr_t) (ptr) & ~3))
#define SET_PTR_TYPE(ptr, type) ((void *) ((uintptr_t) (ptr) | (type)))

#define GET_NIBBLE(n, sha1) (((sha1[(n) >> 1]) >> ((~(n) & 0x01) << 2)) & 0x0f)

#define SUBTREE_SHA1_PREFIXCMP(key_sha1, subtree_sha1) \
        (memcmp(key_sha1, subtree_sha1, subtree_sha1[19]))

struct notes_tree default_notes_tree;

static void load_subtree(struct leaf_node *subtree, struct int_node *node,
                unsigned int n);

/*
 * Search the tree until the appropriate location for the given key is found:
 * 1. Start at the root node, with n = 0
 * 2. If a[0] at the current level is a matching subtree entry, unpack that
 *    subtree entry and remove it; restart search at the current level.
 * 3. Use the nth nibble of the key as an index into a:
 *    - If a[n] is an int_node, recurse from #2 into that node and increment n
 *    - If a matching subtree entry, unpack that subtree entry (and remove it);
 *      restart search at the current level.
 *    - Otherwise, we have found one of the following:
 *      - a subtree entry which does not match the key
 *      - a note entry which may or may not match the key
 *      - an unused leaf node (NULL)
 *      In any case, set *tree and *n, and return pointer to the tree location.
 */
static void **note_tree_search(struct int_node **tree,
                unsigned char *n, const unsigned char *key_sha1)
{
        struct leaf_node *l;
        unsigned char i;
        void *p = (*tree)->a[0];

        if (GET_PTR_TYPE(p) == PTR_TYPE_SUBTREE) {
                l = (struct leaf_node *) CLR_PTR_TYPE(p);
                if (!SUBTREE_SHA1_PREFIXCMP(key_sha1, l->key_sha1)) {
                        /* unpack tree and resume search */
                        (*tree)->a[0] = NULL;
                        load_subtree(l, *tree, *n);
                        free(l);
                        return note_tree_search(tree, n, key_sha1);
                }
        }

        i = GET_NIBBLE(*n, key_sha1);
        p = (*tree)->a[i];
        switch (GET_PTR_TYPE(p)) {
        case PTR_TYPE_INTERNAL:
                *tree = CLR_PTR_TYPE(p);
                (*n)++;
                return note_tree_search(tree, n, key_sha1);
        case PTR_TYPE_SUBTREE:
                l = (struct leaf_node *) CLR_PTR_TYPE(p);
                if (!SUBTREE_SHA1_PREFIXCMP(key_sha1, l->key_sha1)) {
                        /* unpack tree and resume search */
                        (*tree)->a[i] = NULL;
                        load_subtree(l, *tree, *n);
                        free(l);
                        return note_tree_search(tree, n, key_sha1);
                }
                /* fall through */
        default:
                return &((*tree)->a[i]);
        }
}

/*
 * To find a leaf_node:
 * Search to the tree location appropriate for the given key:
 * If a note entry with matching key, return the note entry, else return NULL.
 */
static struct leaf_node *note_tree_find(struct int_node *tree, unsigned char n,
                const unsigned char *key_sha1)
{
        void **p = note_tree_search(&tree, &n, key_sha1);
        if (GET_PTR_TYPE(*p) == PTR_TYPE_NOTE) {
                struct leaf_node *l = (struct leaf_node *) CLR_PTR_TYPE(*p);
                if (!hashcmp(key_sha1, l->key_sha1))
                        return l;
        }
        return NULL;
}

/*
 * To insert a leaf_node:
 * Search to the tree location appropriate for the given leaf_node's key:
 * - If location is unused (NULL), store the tweaked pointer directly there
 * - If location holds a note entry that matches the note-to-be-inserted, then
 *   combine the two notes (by calling the given combine_notes function).
 * - If location holds a note entry that matches the subtree-to-be-inserted,
 *   then unpack the subtree-to-be-inserted into the location.
 * - If location holds a matching subtree entry, unpack the subtree at that
 *   location, and restart the insert operation from that level.
 * - Else, create a new int_node, holding both the node-at-location and the
 *   node-to-be-inserted, and store the new int_node into the location.
 */
static void note_tree_insert(struct int_node *tree, unsigned char n,
                struct leaf_node *entry, unsigned char type,
                combine_notes_fn combine_notes)
{
        struct int_node *new_node;
        struct leaf_node *l;
        void **p = note_tree_search(&tree, &n, entry->key_sha1);

        assert(GET_PTR_TYPE(entry) == 0); /* no type bits set */
        l = (struct leaf_node *) CLR_PTR_TYPE(*p);
        switch (GET_PTR_TYPE(*p)) {
        case PTR_TYPE_NULL:
                assert(!*p);
                *p = SET_PTR_TYPE(entry, type);
                return;
        case PTR_TYPE_NOTE:
                switch (type) {
                case PTR_TYPE_NOTE:
                        if (!hashcmp(l->key_sha1, entry->key_sha1)) {
                                /* skip concatenation if l == entry */
                                if (!hashcmp(l->val_sha1, entry->val_sha1))
                                        return;

                                if (combine_notes(l->val_sha1, entry->val_sha1))
                                        die("failed to combine notes %s and %s"
                                            " for object %s",
                                            sha1_to_hex(l->val_sha1),
                                            sha1_to_hex(entry->val_sha1),
                                            sha1_to_hex(l->key_sha1));
                                free(entry);
                                return;
                        }
                        break;
                case PTR_TYPE_SUBTREE:
                        if (!SUBTREE_SHA1_PREFIXCMP(l->key_sha1,
                                                    entry->key_sha1)) {
                                /* unpack 'entry' */
                                load_subtree(entry, tree, n);
                                free(entry);
                                return;
                        }
                        break;
                }
                break;
        case PTR_TYPE_SUBTREE:
                if (!SUBTREE_SHA1_PREFIXCMP(entry->key_sha1, l->key_sha1)) {
                        /* unpack 'l' and restart insert */
                        *p = NULL;
                        load_subtree(l, tree, n);
                        free(l);
                        note_tree_insert(tree, n, entry, type, combine_notes);
                        return;
                }
                break;
        }

        /* non-matching leaf_node */
        assert(GET_PTR_TYPE(*p) == PTR_TYPE_NOTE ||
               GET_PTR_TYPE(*p) == PTR_TYPE_SUBTREE);
        new_node = (struct int_node *) xcalloc(sizeof(struct int_node), 1);
        note_tree_insert(new_node, n + 1, l, GET_PTR_TYPE(*p), combine_notes);
        *p = SET_PTR_TYPE(new_node, PTR_TYPE_INTERNAL);
        note_tree_insert(new_node, n + 1, entry, type, combine_notes);
}

/*
 * How to consolidate an int_node:
 * If there are > 1 non-NULL entries, give up and return non-zero.
 * Otherwise replace the int_node at the given index in the given parent node
 * with the only entry (or a NULL entry if no entries) from the given tree,
 * and return 0.
 */
static int note_tree_consolidate(struct int_node *tree,
        struct int_node *parent, unsigned char index)
{
        unsigned int i;
        void *p = NULL;

        assert(tree && parent);
        assert(CLR_PTR_TYPE(parent->a[index]) == tree);

        for (i = 0; i < 16; i++) {
                if (GET_PTR_TYPE(tree->a[i]) != PTR_TYPE_NULL) {
                        if (p) /* more than one entry */
                                return -2;
                        p = tree->a[i];
                }
        }

        /* replace tree with p in parent[index] */
        parent->a[index] = p;
        free(tree);
        return 0;
}

/*
 * To remove a leaf_node:
 * Search to the tree location appropriate for the given leaf_node's key:
 * - If location does not hold a matching entry, abort and do nothing.
 * - Replace the matching leaf_node with a NULL entry (and free the leaf_node).
 * - Consolidate int_nodes repeatedly, while walking up the tree towards root.
 */
static void note_tree_remove(struct notes_tree *t, struct int_node *tree,
                unsigned char n, struct leaf_node *entry)
{
        struct leaf_node *l;
        struct int_node *parent_stack[20];
        unsigned char i, j;
        void **p = note_tree_search(&tree, &n, entry->key_sha1);

        assert(GET_PTR_TYPE(entry) == 0); /* no type bits set */
        if (GET_PTR_TYPE(*p) != PTR_TYPE_NOTE)
                return; /* type mismatch, nothing to remove */
        l = (struct leaf_node *) CLR_PTR_TYPE(*p);
        if (hashcmp(l->key_sha1, entry->key_sha1))
                return; /* key mismatch, nothing to remove */

        /* we have found a matching entry */
        free(l);
        *p = SET_PTR_TYPE(NULL, PTR_TYPE_NULL);

        /* consolidate this tree level, and parent levels, if possible */
        if (!n)
                return; /* cannot consolidate top level */
        /* first, build stack of ancestors between root and current node */
        parent_stack[0] = t->root;
        for (i = 0; i < n; i++) {
                j = GET_NIBBLE(i, entry->key_sha1);
                parent_stack[i + 1] = CLR_PTR_TYPE(parent_stack[i]->a[j]);
        }
        assert(i == n && parent_stack[i] == tree);
        /* next, unwind stack until note_tree_consolidate() is done */
        while (i > 0 &&
               !note_tree_consolidate(parent_stack[i], parent_stack[i - 1],
                                      GET_NIBBLE(i - 1, entry->key_sha1)))
                i--;
}

/* Free the entire notes data contained in the given tree */
static void note_tree_free(struct int_node *tree)
{
        unsigned int i;
        for (i = 0; i < 16; i++) {
                void *p = tree->a[i];
                switch (GET_PTR_TYPE(p)) {
                case PTR_TYPE_INTERNAL:
                        note_tree_free(CLR_PTR_TYPE(p));
                        /* fall through */
                case PTR_TYPE_NOTE:
                case PTR_TYPE_SUBTREE:
                        free(CLR_PTR_TYPE(p));
                }
        }
}

/*
 * Convert a partial SHA1 hex string to the corresponding partial SHA1 value.
 * - hex      - Partial SHA1 segment in ASCII hex format
 * - hex_len  - Length of above segment. Must be multiple of 2 between 0 and 40
 * - sha1     - Partial SHA1 value is written here
 * - sha1_len - Max #bytes to store in sha1, Must be >= hex_len / 2, and < 20
 * Returns -1 on error (invalid arguments or invalid SHA1 (not in hex format)).
 * Otherwise, returns number of bytes written to sha1 (i.e. hex_len / 2).
 * Pads sha1 with NULs up to sha1_len (not included in returned length).
 */
static int get_sha1_hex_segment(const char *hex, unsigned int hex_len,
                unsigned char *sha1, unsigned int sha1_len)
{
        unsigned int i, len = hex_len >> 1;
        if (hex_len % 2 != 0 || len > sha1_len)
                return -1;
        for (i = 0; i < len; i++) {
                unsigned int val = (hexval(hex[0]) << 4) | hexval(hex[1]);
                if (val & ~0xff)
                        return -1;
                *sha1++ = val;
                hex += 2;
        }
        for (; i < sha1_len; i++)
                *sha1++ = 0;
        return len;
}

static void load_subtree(struct leaf_node *subtree, struct int_node *node,
                unsigned int n)
{
        unsigned char object_sha1[20];
        unsigned int prefix_len;
        void *buf;
        struct tree_desc desc;
        struct name_entry entry;

        buf = fill_tree_descriptor(&desc, subtree->val_sha1);
        if (!buf)
                die("Could not read %s for notes-index",
                     sha1_to_hex(subtree->val_sha1));

        prefix_len = subtree->key_sha1[19];
        assert(prefix_len * 2 >= n);
        memcpy(object_sha1, subtree->key_sha1, prefix_len);
        while (tree_entry(&desc, &entry)) {
                int len = get_sha1_hex_segment(entry.path, strlen(entry.path),
                                object_sha1 + prefix_len, 20 - prefix_len);
                if (len < 0)
                        continue; /* entry.path is not a SHA1 sum. Skip */
                len += prefix_len;

                /*
                 * If object SHA1 is complete (len == 20), assume note object
                 * If object SHA1 is incomplete (len < 20), assume note subtree
                 */
                if (len <= 20) {
                        unsigned char type = PTR_TYPE_NOTE;
                        struct leaf_node *l = (struct leaf_node *)
                                xcalloc(sizeof(struct leaf_node), 1);
                        hashcpy(l->key_sha1, object_sha1);
                        hashcpy(l->val_sha1, entry.sha1);
                        if (len < 20) {
                                if (!S_ISDIR(entry.mode))
                                        continue; /* entry cannot be subtree */
                                l->key_sha1[19] = (unsigned char) len;
                                type = PTR_TYPE_SUBTREE;
                        }
                        note_tree_insert(node, n, l, type,
                                         combine_notes_concatenate);
                }
        }
        free(buf);
}

/*
 * Determine optimal on-disk fanout for this part of the notes tree
 *
 * Given a (sub)tree and the level in the internal tree structure, determine
 * whether or not the given existing fanout should be expanded for this
 * (sub)tree.
 *
 * Values of the 'fanout' variable:
 * - 0: No fanout (all notes are stored directly in the root notes tree)
 * - 1: 2/38 fanout
 * - 2: 2/2/36 fanout
 * - 3: 2/2/2/34 fanout
 * etc.
 */
static unsigned char determine_fanout(struct int_node *tree, unsigned char n,
                unsigned char fanout)
{
        /*
         * The following is a simple heuristic that works well in practice:
         * For each even-numbered 16-tree level (remember that each on-disk
         * fanout level corresponds to _two_ 16-tree levels), peek at all 16
         * entries at that tree level. If all of them are either int_nodes or
         * subtree entries, then there are likely plenty of notes below this
         * level, so we return an incremented fanout.
         */
        unsigned int i;
        if ((n % 2) || (n > 2 * fanout))
                return fanout;
        for (i = 0; i < 16; i++) {
                switch (GET_PTR_TYPE(tree->a[i])) {
                case PTR_TYPE_SUBTREE:
                case PTR_TYPE_INTERNAL:
                        continue;
                default:
                        return fanout;
                }
        }
        return fanout + 1;
}

static void construct_path_with_fanout(const unsigned char *sha1,
                unsigned char fanout, char *path)
{
        unsigned int i = 0, j = 0;
        const char *hex_sha1 = sha1_to_hex(sha1);
        assert(fanout < 20);
        while (fanout) {
                path[i++] = hex_sha1[j++];
                path[i++] = hex_sha1[j++];
                path[i++] = '/';
                fanout--;
        }
        strcpy(path + i, hex_sha1 + j);
}

static int for_each_note_helper(struct int_node *tree, unsigned char n,
                unsigned char fanout, int flags, each_note_fn fn,
                void *cb_data)
{
        unsigned int i;
        void *p;
        int ret = 0;
        struct leaf_node *l;
        static char path[40 + 19 + 1];  /* hex SHA1 + 19 * '/' + NUL */

        fanout = determine_fanout(tree, n, fanout);
        for (i = 0; i < 16; i++) {
redo:
                p = tree->a[i];
                switch (GET_PTR_TYPE(p)) {
                case PTR_TYPE_INTERNAL:
                        /* recurse into int_node */
                        ret = for_each_note_helper(CLR_PTR_TYPE(p), n + 1,
                                fanout, flags, fn, cb_data);
                        break;
                case PTR_TYPE_SUBTREE:
                        l = (struct leaf_node *) CLR_PTR_TYPE(p);
                        /*
                         * Subtree entries in the note tree represent parts of
                         * the note tree that have not yet been explored. There
                         * is a direct relationship between subtree entries at
                         * level 'n' in the tree, and the 'fanout' variable:
                         * Subtree entries at level 'n <= 2 * fanout' should be
                         * preserved, since they correspond exactly to a fanout
                         * directory in the on-disk structure. However, subtree
                         * entries at level 'n > 2 * fanout' should NOT be
                         * preserved, but rather consolidated into the above
                         * notes tree level. We achieve this by unconditionally
                         * unpacking subtree entries that exist below the
                         * threshold level at 'n = 2 * fanout'.
                         */
                        if (n <= 2 * fanout &&
                            flags & FOR_EACH_NOTE_YIELD_SUBTREES) {
                                /* invoke callback with subtree */
                                unsigned int path_len =
                                        l->key_sha1[19] * 2 + fanout;
                                assert(path_len < 40 + 19);
                                construct_path_with_fanout(l->key_sha1, fanout,
                                                           path);
                                /* Create trailing slash, if needed */
                                if (path[path_len - 1] != '/')
                                        path[path_len++] = '/';
                                path[path_len] = '\0';
                                ret = fn(l->key_sha1, l->val_sha1, path,
                                         cb_data);
                        }
                        if (n > fanout * 2 ||
                            !(flags & FOR_EACH_NOTE_DONT_UNPACK_SUBTREES)) {
                                /* unpack subtree and resume traversal */
                                tree->a[i] = NULL;
                                load_subtree(l, tree, n);
                                free(l);
                                goto redo;
                        }
                        break;
                case PTR_TYPE_NOTE:
                        l = (struct leaf_node *) CLR_PTR_TYPE(p);
                        construct_path_with_fanout(l->key_sha1, fanout, path);
                        ret = fn(l->key_sha1, l->val_sha1, path, cb_data);
                        break;
                }
                if (ret)
                        return ret;
        }
        return 0;
}

struct tree_write_stack {
        struct tree_write_stack *next;
        struct strbuf buf;
        char path[2]; /* path to subtree in next, if any */
};

static inline int matches_tree_write_stack(struct tree_write_stack *tws,
                const char *full_path)
{
        return  full_path[0] == tws->path[0] &&
                full_path[1] == tws->path[1] &&
                full_path[2] == '/';
}

static void write_tree_entry(struct strbuf *buf, unsigned int mode,
                const char *path, unsigned int path_len, const
                unsigned char *sha1)
{
                strbuf_addf(buf, "%06o %.*s%c", mode, path_len, path, '\0');
                strbuf_add(buf, sha1, 20);
}

static void tree_write_stack_init_subtree(struct tree_write_stack *tws,
                const char *path)
{
        struct tree_write_stack *n;
        assert(!tws->next);
        assert(tws->path[0] == '\0' && tws->path[1] == '\0');
        n = (struct tree_write_stack *)
                xmalloc(sizeof(struct tree_write_stack));
        n->next = NULL;
        strbuf_init(&n->buf, 256 * (32 + 40)); /* assume 256 entries per tree */
        n->path[0] = n->path[1] = '\0';
        tws->next = n;
        tws->path[0] = path[0];
        tws->path[1] = path[1];
}

static int tree_write_stack_finish_subtree(struct tree_write_stack *tws)
{
        int ret;
        struct tree_write_stack *n = tws->next;
        unsigned char s[20];
        if (n) {
                ret = tree_write_stack_finish_subtree(n);
                if (ret)
                        return ret;
                ret = write_sha1_file(n->buf.buf, n->buf.len, tree_type, s);
                if (ret)
                        return ret;
                strbuf_release(&n->buf);
                free(n);
                tws->next = NULL;
                write_tree_entry(&tws->buf, 040000, tws->path, 2, s);
                tws->path[0] = tws->path[1] = '\0';
        }
        return 0;
}

static int write_each_note_helper(struct tree_write_stack *tws,
                const char *path, unsigned int mode,
                const unsigned char *sha1)
{
        size_t path_len = strlen(path);
        unsigned int n = 0;
        int ret;

        /* Determine common part of tree write stack */
        while (tws && 3 * n < path_len &&
               matches_tree_write_stack(tws, path + 3 * n)) {
                n++;
                tws = tws->next;
        }

        /* tws point to last matching tree_write_stack entry */
        ret = tree_write_stack_finish_subtree(tws);
        if (ret)
                return ret;

        /* Start subtrees needed to satisfy path */
        while (3 * n + 2 < path_len && path[3 * n + 2] == '/') {
                tree_write_stack_init_subtree(tws, path + 3 * n);
                n++;
                tws = tws->next;
        }

        /* There should be no more directory components in the given path */
        assert(memchr(path + 3 * n, '/', path_len - (3 * n)) == NULL);

        /* Finally add given entry to the current tree object */
        write_tree_entry(&tws->buf, mode, path + 3 * n, path_len - (3 * n),
                         sha1);

        return 0;
}

struct write_each_note_data {
        struct tree_write_stack *root;
};

static int write_each_note(const unsigned char *object_sha1,
                const unsigned char *note_sha1, char *note_path,
                void *cb_data)
{
        struct write_each_note_data *d =
                (struct write_each_note_data *) cb_data;
        size_t note_path_len = strlen(note_path);
        unsigned int mode = 0100644;

        if (note_path[note_path_len - 1] == '/') {
                /* subtree entry */
                note_path_len--;
                note_path[note_path_len] = '\0';
                mode = 040000;
        }
        assert(note_path_len <= 40 + 19);

        return write_each_note_helper(d->root, note_path, mode, note_sha1);
}

int combine_notes_concatenate(unsigned char *cur_sha1,
                const unsigned char *new_sha1)
{
        char *cur_msg = NULL, *new_msg = NULL, *buf;
        unsigned long cur_len, new_len, buf_len;
        enum object_type cur_type, new_type;
        int ret;

        /* read in both note blob objects */
        if (!is_null_sha1(new_sha1))
                new_msg = read_sha1_file(new_sha1, &new_type, &new_len);
        if (!new_msg || !new_len || new_type != OBJ_BLOB) {
                free(new_msg);
                return 0;
        }
        if (!is_null_sha1(cur_sha1))
                cur_msg = read_sha1_file(cur_sha1, &cur_type, &cur_len);
        if (!cur_msg || !cur_len || cur_type != OBJ_BLOB) {
                free(cur_msg);
                free(new_msg);
                hashcpy(cur_sha1, new_sha1);
                return 0;
        }

        /* we will separate the notes by a newline anyway */
        if (cur_msg[cur_len - 1] == '\n')
                cur_len--;

        /* concatenate cur_msg and new_msg into buf */
        buf_len = cur_len + 1 + new_len;
        buf = (char *) xmalloc(buf_len);
        memcpy(buf, cur_msg, cur_len);
        buf[cur_len] = '\n';
        memcpy(buf + cur_len + 1, new_msg, new_len);
        free(cur_msg);
        free(new_msg);

        /* create a new blob object from buf */
        ret = write_sha1_file(buf, buf_len, blob_type, cur_sha1);
        free(buf);
        return ret;
}

int combine_notes_overwrite(unsigned char *cur_sha1,
                const unsigned char *new_sha1)
{
        hashcpy(cur_sha1, new_sha1);
        return 0;
}

int combine_notes_ignore(unsigned char *cur_sha1,
                const unsigned char *new_sha1)
{
        return 0;
}

void init_notes(struct notes_tree *t, const char *notes_ref,
                combine_notes_fn combine_notes, int flags)
{
        unsigned char sha1[20], object_sha1[20];
        unsigned mode;
        struct leaf_node root_tree;

        if (!t)
                t = &default_notes_tree;
        assert(!t->initialized);

        if (!notes_ref)
                notes_ref = getenv(GIT_NOTES_REF_ENVIRONMENT);
        if (!notes_ref)
                notes_ref = notes_ref_name; /* value of core.notesRef config */
        if (!notes_ref)
                notes_ref = GIT_NOTES_DEFAULT_REF;

        if (!combine_notes)
                combine_notes = combine_notes_concatenate;

        t->root = (struct int_node *) xcalloc(sizeof(struct int_node), 1);
        t->ref = notes_ref ? xstrdup(notes_ref) : NULL;
        t->combine_notes = combine_notes;
        t->initialized = 1;

        if (flags & NOTES_INIT_EMPTY || !notes_ref ||
            read_ref(notes_ref, object_sha1))
                return;
        if (get_tree_entry(object_sha1, "", sha1, &mode))
                die("Failed to read notes tree referenced by %s (%s)",
                    notes_ref, object_sha1);

        hashclr(root_tree.key_sha1);
        hashcpy(root_tree.val_sha1, sha1);
        load_subtree(&root_tree, t->root, 0);
}

void add_note(struct notes_tree *t, const unsigned char *object_sha1,
                const unsigned char *note_sha1, combine_notes_fn combine_notes)
{
        struct leaf_node *l;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);
        if (!combine_notes)
                combine_notes = t->combine_notes;
        l = (struct leaf_node *) xmalloc(sizeof(struct leaf_node));
        hashcpy(l->key_sha1, object_sha1);
        hashcpy(l->val_sha1, note_sha1);
        note_tree_insert(t->root, 0, l, PTR_TYPE_NOTE, combine_notes);
}

void remove_note(struct notes_tree *t, const unsigned char *object_sha1)
{
        struct leaf_node l;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);
        hashcpy(l.key_sha1, object_sha1);
        hashclr(l.val_sha1);
        return note_tree_remove(t, t->root, 0, &l);
}

const unsigned char *get_note(struct notes_tree *t,
                const unsigned char *object_sha1)
{
        struct leaf_node *found;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);
        found = note_tree_find(t->root, 0, object_sha1);
        return found ? found->val_sha1 : NULL;
}

int for_each_note(struct notes_tree *t, int flags, each_note_fn fn,
                void *cb_data)
{
        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);
        return for_each_note_helper(t->root, 0, 0, flags, fn, cb_data);
}

int write_notes_tree(struct notes_tree *t, unsigned char *result)
{
        struct tree_write_stack root;
        struct write_each_note_data cb_data;
        int ret;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);

        /* Prepare for traversal of current notes tree */
        root.next = NULL; /* last forward entry in list is grounded */
        strbuf_init(&root.buf, 256 * (32 + 40)); /* assume 256 entries */
        root.path[0] = root.path[1] = '\0';
        cb_data.root = &root;

        /* Write tree objects representing current notes tree */
        ret = for_each_note(t, FOR_EACH_NOTE_DONT_UNPACK_SUBTREES |
                                FOR_EACH_NOTE_YIELD_SUBTREES,
                        write_each_note, &cb_data) ||
                tree_write_stack_finish_subtree(&root) ||
                write_sha1_file(root.buf.buf, root.buf.len, tree_type, result);
        strbuf_release(&root.buf);
        return ret;
}

void free_notes(struct notes_tree *t)
{
        if (!t)
                t = &default_notes_tree;
        if (t->root)
                note_tree_free(t->root);
        free(t->root);
        free(t->ref);
        memset(t, 0, sizeof(struct notes_tree));
}

void format_note(struct notes_tree *t, const unsigned char *object_sha1,
                struct strbuf *sb, const char *output_encoding, int flags)
{
        static const char utf8[] = "utf-8";
        const unsigned char *sha1;
        char *msg, *msg_p;
        unsigned long linelen, msglen;
        enum object_type type;

        if (!t)
                t = &default_notes_tree;
        if (!t->initialized)
                init_notes(t, NULL, NULL, 0);

        sha1 = get_note(t, object_sha1);
        if (!sha1)
                return;

        if (!(msg = read_sha1_file(sha1, &type, &msglen)) || !msglen ||
                        type != OBJ_BLOB) {
                free(msg);
                return;
        }

        if (output_encoding && *output_encoding &&
                        strcmp(utf8, output_encoding)) {
                char *reencoded = reencode_string(msg, output_encoding, utf8);
                if (reencoded) {
                        free(msg);
                        msg = reencoded;
                        msglen = strlen(msg);
                }
        }

        /* we will end the annotation by a newline anyway */
        if (msglen && msg[msglen - 1] == '\n')
                msglen--;

        if (flags & NOTES_SHOW_HEADER)
                strbuf_addstr(sb, "\nNotes:\n");

        for (msg_p = msg; msg_p < msg + msglen; msg_p += linelen + 1) {
                linelen = strchrnul(msg_p, '\n') - msg_p;

                if (flags & NOTES_INDENT)
                        strbuf_addstr(sb, "    ");
                strbuf_add(sb, msg_p, linelen);
                strbuf_addch(sb, '\n');
        }

        free(msg);
}