unit tests: add a project plan document

[git.git] / tree-diff.c

/*
 * Helper functions for tree diff generation
 */
#include "git-compat-util.h"
#include "diff.h"
#include "diffcore.h"
#include "tree.h"
#include "tree-walk.h"

/*
 * Some mode bits are also used internally for computations.
 *
 * They *must* not overlap with any valid modes, and they *must* not be emitted
 * to outside world - i.e. appear on disk or network. In other words, it's just
 * temporary fields, which we internally use, but they have to stay in-house.
 *
 * ( such approach is valid, as standard S_IF* fits into 16 bits, and in Git
 *   codebase mode is `unsigned int` which is assumed to be at least 32 bits )
 */

#define S_DIFFTREE_IFXMIN_NEQ   0x80000000

/*
 * internal mode marker, saying a tree entry != entry of tp[imin]
 * (see ll_diff_tree_paths for what it means there)
 *
 * we will update/use/emit entry for diff only with it unset.
 */
#define S_IFXMIN_NEQ    S_DIFFTREE_IFXMIN_NEQ

#define FAST_ARRAY_ALLOC(x, nr) do { \
        if ((nr) <= 2) \
                (x) = xalloca((nr) * sizeof(*(x))); \
        else \
                ALLOC_ARRAY((x), nr); \
} while(0)
#define FAST_ARRAY_FREE(x, nr) do { \
        if ((nr) <= 2) \
                xalloca_free((x)); \
        else \
                free((x)); \
} while(0)

static struct combine_diff_path *ll_diff_tree_paths(
        struct combine_diff_path *p, const struct object_id *oid,
        const struct object_id **parents_oid, int nparent,
        struct strbuf *base, struct diff_options *opt);
static void ll_diff_tree_oid(const struct object_id *old_oid,
                             const struct object_id *new_oid,
                             struct strbuf *base, struct diff_options *opt);

/*
 * Compare two tree entries, taking into account only path/S_ISDIR(mode),
 * but not their sha1's.
 *
 * NOTE files and directories *always* compare differently, even when having
 *      the same name - thanks to base_name_compare().
 *
 * NOTE empty (=invalid) descriptor(s) take part in comparison as +infty,
 *      so that they sort *after* valid tree entries.
 *
 *      Due to this convention, if trees are scanned in sorted order, all
 *      non-empty descriptors will be processed first.
 */
static int tree_entry_pathcmp(struct tree_desc *t1, struct tree_desc *t2)
{
        struct name_entry *e1, *e2;
        int cmp;

        /* empty descriptors sort after valid tree entries */
        if (!t1->size)
                return t2->size ? 1 : 0;
        else if (!t2->size)
                return -1;

        e1 = &t1->entry;
        e2 = &t2->entry;
        cmp = base_name_compare(e1->path, tree_entry_len(e1), e1->mode,
                                e2->path, tree_entry_len(e2), e2->mode);
        return cmp;
}


/*
 * convert path -> opt->diff_*() callbacks
 *
 * emits diff to first parent only, and tells diff tree-walker that we are done
 * with p and it can be freed.
 */
static int emit_diff_first_parent_only(struct diff_options *opt, struct combine_diff_path *p)
{
        struct combine_diff_parent *p0 = &p->parent[0];
        if (p->mode && p0->mode) {
                opt->change(opt, p0->mode, p->mode, &p0->oid, &p->oid,
                        1, 1, p->path, 0, 0);
        }
        else {
                const struct object_id *oid;
                unsigned int mode;
                int addremove;

                if (p->mode) {
                        addremove = '+';
                        oid = &p->oid;
                        mode = p->mode;
                } else {
                        addremove = '-';
                        oid = &p0->oid;
                        mode = p0->mode;
                }

                opt->add_remove(opt, addremove, mode, oid, 1, p->path, 0);
        }

        return 0;       /* we are done with p */
}


/*
 * Make a new combine_diff_path from path/mode/sha1
 * and append it to paths list tail.
 *
 * Memory for created elements could be reused:
 *
 *      - if last->next == NULL, the memory is allocated;
 *
 *      - if last->next != NULL, it is assumed that p=last->next was returned
 *        earlier by this function, and p->next was *not* modified.
 *        The memory is then reused from p.
 *
 * so for clients,
 *
 * - if you do need to keep the element
 *
 *      p = path_appendnew(p, ...);
 *      process(p);
 *      p->next = NULL;
 *
 * - if you don't need to keep the element after processing
 *
 *      pprev = p;
 *      p = path_appendnew(p, ...);
 *      process(p);
 *      p = pprev;
 *      ; don't forget to free tail->next in the end
 *
 * p->parent[] remains uninitialized.
 */
static struct combine_diff_path *path_appendnew(struct combine_diff_path *last,
        int nparent, const struct strbuf *base, const char *path, int pathlen,
        unsigned mode, const struct object_id *oid)
{
        struct combine_diff_path *p;
        size_t len = st_add(base->len, pathlen);
        size_t alloclen = combine_diff_path_size(nparent, len);

        /* if last->next is !NULL - it is a pre-allocated memory, we can reuse */
        p = last->next;
        if (p && (alloclen > (intptr_t)p->next)) {
                FREE_AND_NULL(p);
        }

        if (!p) {
                p = xmalloc(alloclen);

                /*
                 * until we go to it next round, .next holds how many bytes we
                 * allocated (for faster realloc - we don't need copying old data).
                 */
                p->next = (struct combine_diff_path *)(intptr_t)alloclen;
        }

        last->next = p;

        p->path = (char *)&(p->parent[nparent]);
        memcpy(p->path, base->buf, base->len);
        memcpy(p->path + base->len, path, pathlen);
        p->path[len] = 0;
        p->mode = mode;
        oidcpy(&p->oid, oid ? oid : null_oid());

        return p;
}

/*
 * new path should be added to combine diff
 *
 * 3 cases on how/when it should be called and behaves:
 *
 *       t, !tp         -> path added, all parents lack it
 *      !t,  tp         -> path removed from all parents
 *       t,  tp         -> path modified/added
 *                         (M for tp[i]=tp[imin], A otherwise)
 */
static struct combine_diff_path *emit_path(struct combine_diff_path *p,
        struct strbuf *base, struct diff_options *opt, int nparent,
        struct tree_desc *t, struct tree_desc *tp,
        int imin)
{
        unsigned short mode;
        const char *path;
        const struct object_id *oid;
        int pathlen;
        int old_baselen = base->len;
        int i, isdir, recurse = 0, emitthis = 1;

        /* at least something has to be valid */
        assert(t || tp);

        if (t) {
                /* path present in resulting tree */
                oid = tree_entry_extract(t, &path, &mode);
                pathlen = tree_entry_len(&t->entry);
                isdir = S_ISDIR(mode);
        } else {
                /*
                 * a path was removed - take path from imin parent. Also take
                 * mode from that parent, to decide on recursion(1).
                 *
                 * 1) all modes for tp[i]=tp[imin] should be the same wrt
                 *    S_ISDIR, thanks to base_name_compare().
                 */
                tree_entry_extract(&tp[imin], &path, &mode);
                pathlen = tree_entry_len(&tp[imin].entry);

                isdir = S_ISDIR(mode);
                oid = NULL;
                mode = 0;
        }

        if (opt->flags.recursive && isdir) {
                recurse = 1;
                emitthis = opt->flags.tree_in_recursive;
        }

        if (emitthis) {
                int keep;
                struct combine_diff_path *pprev = p;
                p = path_appendnew(p, nparent, base, path, pathlen, mode, oid);

                for (i = 0; i < nparent; ++i) {
                        /*
                         * tp[i] is valid, if present and if tp[i]==tp[imin] -
                         * otherwise, we should ignore it.
                         */
                        int tpi_valid = tp && !(tp[i].entry.mode & S_IFXMIN_NEQ);

                        const struct object_id *oid_i;
                        unsigned mode_i;

                        p->parent[i].status =
                                !t ? DIFF_STATUS_DELETED :
                                        tpi_valid ?
                                                DIFF_STATUS_MODIFIED :
                                                DIFF_STATUS_ADDED;

                        if (tpi_valid) {
                                oid_i = &tp[i].entry.oid;
                                mode_i = tp[i].entry.mode;
                        }
                        else {
                                oid_i = null_oid();
                                mode_i = 0;
                        }

                        p->parent[i].mode = mode_i;
                        oidcpy(&p->parent[i].oid, oid_i);
                }

                keep = 1;
                if (opt->pathchange)
                        keep = opt->pathchange(opt, p);

                /*
                 * If a path was filtered or consumed - we don't need to add it
                 * to the list and can reuse its memory, leaving it as
                 * pre-allocated element on the tail.
                 *
                 * On the other hand, if path needs to be kept, we need to
                 * correct its .next to NULL, as it was pre-initialized to how
                 * much memory was allocated.
                 *
                 * see path_appendnew() for details.
                 */
                if (!keep)
                        p = pprev;
                else
                        p->next = NULL;
        }

        if (recurse) {
                const struct object_id **parents_oid;

                FAST_ARRAY_ALLOC(parents_oid, nparent);
                for (i = 0; i < nparent; ++i) {
                        /* same rule as in emitthis */
                        int tpi_valid = tp && !(tp[i].entry.mode & S_IFXMIN_NEQ);

                        parents_oid[i] = tpi_valid ? &tp[i].entry.oid : NULL;
                }

                strbuf_add(base, path, pathlen);
                strbuf_addch(base, '/');
                p = ll_diff_tree_paths(p, oid, parents_oid, nparent, base, opt);
                FAST_ARRAY_FREE(parents_oid, nparent);
        }

        strbuf_setlen(base, old_baselen);
        return p;
}

static void skip_uninteresting(struct tree_desc *t, struct strbuf *base,
                               struct diff_options *opt)
{
        enum interesting match;

        while (t->size) {
                match = tree_entry_interesting(opt->repo->index, &t->entry,
                                               base, 0, &opt->pathspec);
                if (match) {
                        if (match == all_entries_not_interesting)
                                t->size = 0;
                        break;
                }
                update_tree_entry(t);
        }
}


/*
 * generate paths for combined diff D(sha1,parents_oid[])
 *
 * Resulting paths are appended to combine_diff_path linked list, and also, are
 * emitted on the go via opt->pathchange() callback, so it is possible to
 * process the result as batch or incrementally.
 *
 * The paths are generated scanning new tree and all parents trees
 * simultaneously, similarly to what diff_tree() was doing for 2 trees.
 * The theory behind such scan is as follows:
 *
 *
 * D(T,P1...Pn) calculation scheme
 * -------------------------------
 *
 * D(T,P1...Pn) = D(T,P1) ^ ... ^ D(T,Pn)       (regarding resulting paths set)
 *
 *      D(T,Pj)         - diff between T..Pj
 *      D(T,P1...Pn)    - combined diff from T to parents P1,...,Pn
 *
 *
 * We start from all trees, which are sorted, and compare their entries in
 * lock-step:
 *
 *       T     P1       Pn
 *       -     -        -
 *      |t|   |p1|     |pn|
 *      |-|   |--| ... |--|      imin = argmin(p1...pn)
 *      | |   |  |     |  |
 *      |-|   |--|     |--|
 *      |.|   |. |     |. |
 *       .     .        .
 *       .     .        .
 *
 * at any time there could be 3 cases:
 *
 *      1)  t < p[imin];
 *      2)  t > p[imin];
 *      3)  t = p[imin].
 *
 * Schematic deduction of what every case means, and what to do, follows:
 *
 * 1)  t < p[imin]  ->  ∀j t ∉ Pj  ->  "+t" ∈ D(T,Pj)  ->  D += "+t";  t↓
 *
 * 2)  t > p[imin]
 *
 *     2.1) ∃j: pj > p[imin]  ->  "-p[imin]" ∉ D(T,Pj)  ->  D += ø;  ∀ pi=p[imin]  pi↓
 *     2.2) ∀i  pi = p[imin]  ->  pi ∉ T  ->  "-pi" ∈ D(T,Pi)  ->  D += "-p[imin]";  ∀i pi↓
 *
 * 3)  t = p[imin]
 *
 *     3.1) ∃j: pj > p[imin]  ->  "+t" ∈ D(T,Pj)  ->  only pi=p[imin] remains to investigate
 *     3.2) pi = p[imin]  ->  investigate δ(t,pi)
 *      |
 *      |
 *      v
 *
 *     3.1+3.2) looking at δ(t,pi) ∀i: pi=p[imin] - if all != ø  ->
 *
 *                       ⎧δ(t,pi)  - if pi=p[imin]
 *              ->  D += ⎨
 *                       ⎩"+t"     - if pi>p[imin]
 *
 *
 *     in any case t↓  ∀ pi=p[imin]  pi↓
 *
 *
 * ~~~~~~~~
 *
 * NOTE
 *
 *      Usual diff D(A,B) is by definition the same as combined diff D(A,[B]),
 *      so this diff paths generator can, and is used, for plain diffs
 *      generation too.
 *
 *      Please keep attention to the common D(A,[B]) case when working on the
 *      code, in order not to slow it down.
 *
 * NOTE
 *      nparent must be > 0.
 */


/* ∀ pi=p[imin]  pi↓ */
static inline void update_tp_entries(struct tree_desc *tp, int nparent)
{
        int i;
        for (i = 0; i < nparent; ++i)
                if (!(tp[i].entry.mode & S_IFXMIN_NEQ))
                        update_tree_entry(&tp[i]);
}

static struct combine_diff_path *ll_diff_tree_paths(
        struct combine_diff_path *p, const struct object_id *oid,
        const struct object_id **parents_oid, int nparent,
        struct strbuf *base, struct diff_options *opt)
{
        struct tree_desc t, *tp;
        void *ttree, **tptree;
        int i;

        FAST_ARRAY_ALLOC(tp, nparent);
        FAST_ARRAY_ALLOC(tptree, nparent);

        /*
         * load parents first, as they are probably already cached.
         *
         * ( log_tree_diff() parses commit->parent before calling here via
         *   diff_tree_oid(parent, commit) )
         */
        for (i = 0; i < nparent; ++i)
                tptree[i] = fill_tree_descriptor(opt->repo, &tp[i], parents_oid[i]);
        ttree = fill_tree_descriptor(opt->repo, &t, oid);

        /* Enable recursion indefinitely */
        opt->pathspec.recursive = opt->flags.recursive;

        for (;;) {
                int imin, cmp;

                if (diff_can_quit_early(opt))
                        break;

                if (opt->max_changes && diff_queued_diff.nr > opt->max_changes)
                        break;

                if (opt->pathspec.nr) {
                        skip_uninteresting(&t, base, opt);
                        for (i = 0; i < nparent; i++)
                                skip_uninteresting(&tp[i], base, opt);
                }

                /* comparing is finished when all trees are done */
                if (!t.size) {
                        int done = 1;
                        for (i = 0; i < nparent; ++i)
                                if (tp[i].size) {
                                        done = 0;
                                        break;
                                }
                        if (done)
                                break;
                }

                /*
                 * lookup imin = argmin(p1...pn),
                 * mark entries whether they =p[imin] along the way
                 */
                imin = 0;
                tp[0].entry.mode &= ~S_IFXMIN_NEQ;

                for (i = 1; i < nparent; ++i) {
                        cmp = tree_entry_pathcmp(&tp[i], &tp[imin]);
                        if (cmp < 0) {
                                imin = i;
                                tp[i].entry.mode &= ~S_IFXMIN_NEQ;
                        }
                        else if (cmp == 0) {
                                tp[i].entry.mode &= ~S_IFXMIN_NEQ;
                        }
                        else {
                                tp[i].entry.mode |= S_IFXMIN_NEQ;
                        }
                }

                /* fixup markings for entries before imin */
                for (i = 0; i < imin; ++i)
                        tp[i].entry.mode |= S_IFXMIN_NEQ;       /* pi > p[imin] */



                /* compare t vs p[imin] */
                cmp = tree_entry_pathcmp(&t, &tp[imin]);

                /* t = p[imin] */
                if (cmp == 0) {
                        /* are either pi > p[imin] or diff(t,pi) != ø ? */
                        if (!opt->flags.find_copies_harder) {
                                for (i = 0; i < nparent; ++i) {
                                        /* p[i] > p[imin] */
                                        if (tp[i].entry.mode & S_IFXMIN_NEQ)
                                                continue;

                                        /* diff(t,pi) != ø */
                                        if (!oideq(&t.entry.oid, &tp[i].entry.oid) ||
                                            (t.entry.mode != tp[i].entry.mode))
                                                continue;

                                        goto skip_emit_t_tp;
                                }
                        }

                        /* D += {δ(t,pi) if pi=p[imin];  "+a" if pi > p[imin]} */
                        p = emit_path(p, base, opt, nparent,
                                        &t, tp, imin);

                skip_emit_t_tp:
                        /* t↓,  ∀ pi=p[imin]  pi↓ */
                        update_tree_entry(&t);
                        update_tp_entries(tp, nparent);
                }

                /* t < p[imin] */
                else if (cmp < 0) {
                        /* D += "+t" */
                        p = emit_path(p, base, opt, nparent,
                                        &t, /*tp=*/NULL, -1);

                        /* t↓ */
                        update_tree_entry(&t);
                }

                /* t > p[imin] */
                else {
                        /* ∀i pi=p[imin] -> D += "-p[imin]" */
                        if (!opt->flags.find_copies_harder) {
                                for (i = 0; i < nparent; ++i)
                                        if (tp[i].entry.mode & S_IFXMIN_NEQ)
                                                goto skip_emit_tp;
                        }

                        p = emit_path(p, base, opt, nparent,
                                        /*t=*/NULL, tp, imin);

                skip_emit_tp:
                        /* ∀ pi=p[imin]  pi↓ */
                        update_tp_entries(tp, nparent);
                }
        }

        free(ttree);
        for (i = nparent-1; i >= 0; i--)
                free(tptree[i]);
        FAST_ARRAY_FREE(tptree, nparent);
        FAST_ARRAY_FREE(tp, nparent);

        return p;
}

struct combine_diff_path *diff_tree_paths(
        struct combine_diff_path *p, const struct object_id *oid,
        const struct object_id **parents_oid, int nparent,
        struct strbuf *base, struct diff_options *opt)
{
        p = ll_diff_tree_paths(p, oid, parents_oid, nparent, base, opt);

        /*
         * free pre-allocated last element, if any
         * (see path_appendnew() for details about why)
         */
        FREE_AND_NULL(p->next);

        return p;
}

/*
 * Does it look like the resulting diff might be due to a rename?
 *  - single entry
 *  - not a valid previous file
 */
static inline int diff_might_be_rename(void)
{
        return diff_queued_diff.nr == 1 &&
                !DIFF_FILE_VALID(diff_queued_diff.queue[0]->one);
}

static void try_to_follow_renames(const struct object_id *old_oid,
                                  const struct object_id *new_oid,
                                  struct strbuf *base, struct diff_options *opt)
{
        struct diff_options diff_opts;
        struct diff_queue_struct *q = &diff_queued_diff;
        struct diff_filepair *choice;
        int i;

        /*
         * follow-rename code is very specific, we need exactly one
         * path. Magic that matches more than one path is not
         * supported.
         */
        GUARD_PATHSPEC(&opt->pathspec, PATHSPEC_FROMTOP | PATHSPEC_LITERAL);
#if 0
        /*
         * We should reject wildcards as well. Unfortunately we
         * haven't got a reliable way to detect that 'foo\*bar' in
         * fact has no wildcards. nowildcard_len is merely a hint for
         * optimization. Let it slip for now until wildmatch is taught
         * about dry-run mode and returns wildcard info.
         */
        if (opt->pathspec.has_wildcard)
                BUG("wildcards are not supported");
#endif

        /* Remove the file creation entry from the diff queue, and remember it */
        choice = q->queue[0];
        q->nr = 0;

        repo_diff_setup(opt->repo, &diff_opts);
        diff_opts.flags.recursive = 1;
        diff_opts.flags.find_copies_harder = 1;
        diff_opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        diff_opts.single_follow = opt->pathspec.items[0].match;
        diff_opts.break_opt = opt->break_opt;
        diff_opts.rename_score = opt->rename_score;
        diff_setup_done(&diff_opts);
        ll_diff_tree_oid(old_oid, new_oid, base, &diff_opts);
        diffcore_std(&diff_opts);
        clear_pathspec(&diff_opts.pathspec);

        /* Go through the new set of filepairing, and see if we find a more interesting one */
        opt->found_follow = 0;
        for (i = 0; i < q->nr; i++) {
                struct diff_filepair *p = q->queue[i];

                /*
                 * Found a source? Not only do we use that for the new
                 * diff_queued_diff, we will also use that as the path in
                 * the future!
                 */
                if ((p->status == 'R' || p->status == 'C') &&
                    !strcmp(p->two->path, opt->pathspec.items[0].match)) {
                        const char *path[2];

                        /* Switch the file-pairs around */
                        q->queue[i] = choice;
                        choice = p;

                        /* Update the path we use from now on.. */
                        path[0] = p->one->path;
                        path[1] = NULL;
                        clear_pathspec(&opt->pathspec);
                        parse_pathspec(&opt->pathspec,
                                       PATHSPEC_ALL_MAGIC & ~PATHSPEC_LITERAL,
                                       PATHSPEC_LITERAL_PATH, "", path);

                        /*
                         * The caller expects us to return a set of vanilla
                         * filepairs to let a later call to diffcore_std()
                         * it makes to sort the renames out (among other
                         * things), but we already have found renames
                         * ourselves; signal diffcore_std() not to muck with
                         * rename information.
                         */
                        opt->found_follow = 1;
                        break;
                }
        }

        /*
         * Then, discard all the non-relevant file pairs...
         */
        for (i = 0; i < q->nr; i++) {
                struct diff_filepair *p = q->queue[i];
                diff_free_filepair(p);
        }

        /*
         * .. and re-instate the one we want (which might be either the
         * original one, or the rename/copy we found)
         */
        q->queue[0] = choice;
        q->nr = 1;
}

static void ll_diff_tree_oid(const struct object_id *old_oid,
                             const struct object_id *new_oid,
                             struct strbuf *base, struct diff_options *opt)
{
        struct combine_diff_path phead, *p;
        pathchange_fn_t pathchange_old = opt->pathchange;

        phead.next = NULL;
        opt->pathchange = emit_diff_first_parent_only;
        diff_tree_paths(&phead, new_oid, &old_oid, 1, base, opt);

        for (p = phead.next; p;) {
                struct combine_diff_path *pprev = p;
                p = p->next;
                free(pprev);
        }

        opt->pathchange = pathchange_old;
}

void diff_tree_oid(const struct object_id *old_oid,
                   const struct object_id *new_oid,
                   const char *base_str, struct diff_options *opt)
{
        struct strbuf base;

        strbuf_init(&base, PATH_MAX);
        strbuf_addstr(&base, base_str);

        ll_diff_tree_oid(old_oid, new_oid, &base, opt);
        if (!*base_str && opt->flags.follow_renames && diff_might_be_rename())
                try_to_follow_renames(old_oid, new_oid, &base, opt);

        strbuf_release(&base);
}

void diff_root_tree_oid(const struct object_id *new_oid,
                        const char *base,
                        struct diff_options *opt)
{
        diff_tree_oid(NULL, new_oid, base, opt);
}