peel_onion: disambiguate to favor tree-ish when we know we want a tree-ish

[alt-git.git] / sha1_name.c

#include "cache.h"
#include "tag.h"
#include "commit.h"
#include "tree.h"
#include "blob.h"
#include "tree-walk.h"
#include "refs.h"
#include "remote.h"

static int get_sha1_oneline(const char *, unsigned char *, struct commit_list *);

typedef int (*disambiguate_hint_fn)(const unsigned char *, void *);

struct disambiguate_state {
        disambiguate_hint_fn fn;
        void *cb_data;
        unsigned char candidate[20];
        unsigned candidate_exists:1;
        unsigned candidate_checked:1;
        unsigned candidate_ok:1;
        unsigned disambiguate_fn_used:1;
        unsigned ambiguous:1;
        unsigned always_call_fn:1;
};

static void update_candidates(struct disambiguate_state *ds, const unsigned char *current)
{
        if (ds->always_call_fn) {
                ds->ambiguous = ds->fn(current, ds->cb_data) ? 1 : 0;
                return;
        }
        if (!ds->candidate_exists) {
                /* this is the first candidate */
                hashcpy(ds->candidate, current);
                ds->candidate_exists = 1;
                return;
        } else if (!hashcmp(ds->candidate, current)) {
                /* the same as what we already have seen */
                return;
        }

        if (!ds->fn) {
                /* cannot disambiguate between ds->candidate and current */
                ds->ambiguous = 1;
                return;
        }

        if (!ds->candidate_checked) {
                ds->candidate_ok = ds->fn(ds->candidate, ds->cb_data);
                ds->disambiguate_fn_used = 1;
                ds->candidate_checked = 1;
        }

        if (!ds->candidate_ok) {
                /* discard the candidate; we know it does not satisify fn */
                hashcpy(ds->candidate, current);
                ds->candidate_checked = 0;
                return;
        }

        /* if we reach this point, we know ds->candidate satisfies fn */
        if (ds->fn(current, ds->cb_data)) {
                /*
                 * if both current and candidate satisfy fn, we cannot
                 * disambiguate.
                 */
                ds->candidate_ok = 0;
                ds->ambiguous = 1;
        }

        /* otherwise, current can be discarded and candidate is still good */
}

static void find_short_object_filename(int len, const char *hex_pfx, struct disambiguate_state *ds)
{
        struct alternate_object_database *alt;
        char hex[40];
        static struct alternate_object_database *fakeent;

        if (!fakeent) {
                /*
                 * Create a "fake" alternate object database that
                 * points to our own object database, to make it
                 * easier to get a temporary working space in
                 * alt->name/alt->base while iterating over the
                 * object databases including our own.
                 */
                const char *objdir = get_object_directory();
                int objdir_len = strlen(objdir);
                int entlen = objdir_len + 43;
                fakeent = xmalloc(sizeof(*fakeent) + entlen);
                memcpy(fakeent->base, objdir, objdir_len);
                fakeent->name = fakeent->base + objdir_len + 1;
                fakeent->name[-1] = '/';
        }
        fakeent->next = alt_odb_list;

        sprintf(hex, "%.2s", hex_pfx);
        for (alt = fakeent; alt && !ds->ambiguous; alt = alt->next) {
                struct dirent *de;
                DIR *dir;
                sprintf(alt->name, "%.2s/", hex_pfx);
                dir = opendir(alt->base);
                if (!dir)
                        continue;

                while (!ds->ambiguous && (de = readdir(dir)) != NULL) {
                        unsigned char sha1[20];

                        if (strlen(de->d_name) != 38)
                                continue;
                        if (memcmp(de->d_name, hex_pfx + 2, len - 2))
                                continue;
                        memcpy(hex + 2, de->d_name, 38);
                        if (!get_sha1_hex(hex, sha1))
                                update_candidates(ds, sha1);
                }
                closedir(dir);
        }
}

static int match_sha(unsigned len, const unsigned char *a, const unsigned char *b)
{
        do {
                if (*a != *b)
                        return 0;
                a++;
                b++;
                len -= 2;
        } while (len > 1);
        if (len)
                if ((*a ^ *b) & 0xf0)
                        return 0;
        return 1;
}

static void unique_in_pack(int len,
                          const unsigned char *bin_pfx,
                           struct packed_git *p,
                           struct disambiguate_state *ds)
{
        uint32_t num, last, i, first = 0;
        const unsigned char *current = NULL;

        open_pack_index(p);
        num = p->num_objects;
        last = num;
        while (first < last) {
                uint32_t mid = (first + last) / 2;
                const unsigned char *current;
                int cmp;

                current = nth_packed_object_sha1(p, mid);
                cmp = hashcmp(bin_pfx, current);
                if (!cmp) {
                        first = mid;
                        break;
                }
                if (cmp > 0) {
                        first = mid+1;
                        continue;
                }
                last = mid;
        }

        /*
         * At this point, "first" is the location of the lowest object
         * with an object name that could match "bin_pfx".  See if we have
         * 0, 1 or more objects that actually match(es).
         */
        for (i = first; i < num && !ds->ambiguous; i++) {
                current = nth_packed_object_sha1(p, i);
                if (!match_sha(len, bin_pfx, current))
                        break;
                update_candidates(ds, current);
        }
}

static void find_short_packed_object(int len, const unsigned char *bin_pfx,
                                     struct disambiguate_state *ds)
{
        struct packed_git *p;

        prepare_packed_git();
        for (p = packed_git; p && !ds->ambiguous; p = p->next)
                unique_in_pack(len, bin_pfx, p, ds);
}

#define SHORT_NAME_NOT_FOUND (-1)
#define SHORT_NAME_AMBIGUOUS (-2)

static int finish_object_disambiguation(struct disambiguate_state *ds,
                                        unsigned char *sha1)
{
        if (ds->ambiguous)
                return SHORT_NAME_AMBIGUOUS;

        if (!ds->candidate_exists)
                return SHORT_NAME_NOT_FOUND;

        if (!ds->candidate_checked)
                /*
                 * If this is the only candidate, there is no point
                 * calling the disambiguation hint callback.
                 *
                 * On the other hand, if the current candidate
                 * replaced an earlier candidate that did _not_ pass
                 * the disambiguation hint callback, then we do have
                 * more than one objects that match the short name
                 * given, so we should make sure this one matches;
                 * otherwise, if we discovered this one and the one
                 * that we previously discarded in the reverse order,
                 * we would end up showing different results in the
                 * same repository!
                 */
                ds->candidate_ok = (!ds->disambiguate_fn_used ||
                                    ds->fn(ds->candidate, ds->cb_data));

        if (!ds->candidate_ok)
                return SHORT_NAME_AMBIGUOUS;

        hashcpy(sha1, ds->candidate);
        return 0;
}

static int disambiguate_commit_only(const unsigned char *sha1, void *cb_data_unused)
{
        int kind = sha1_object_info(sha1, NULL);
        return kind == OBJ_COMMIT;
}

static int disambiguate_committish_only(const unsigned char *sha1, void *cb_data_unused)
{
        struct object *obj;
        int kind;

        kind = sha1_object_info(sha1, NULL);
        if (kind == OBJ_COMMIT)
                return 1;
        if (kind != OBJ_TAG)
                return 0;

        /* We need to do this the hard way... */
        obj = deref_tag(lookup_object(sha1), NULL, 0);
        if (obj && obj->type == OBJ_COMMIT)
                return 1;
        return 0;
}

static int disambiguate_tree_only(const unsigned char *sha1, void *cb_data_unused)
{
        int kind = sha1_object_info(sha1, NULL);
        return kind == OBJ_TREE;
}

static int disambiguate_treeish_only(const unsigned char *sha1, void *cb_data_unused)
{
        struct object *obj;
        int kind;

        kind = sha1_object_info(sha1, NULL);
        if (kind == OBJ_TREE || kind == OBJ_COMMIT)
                return 1;
        if (kind != OBJ_TAG)
                return 0;

        /* We need to do this the hard way... */
        obj = deref_tag(lookup_object(sha1), NULL, 0);
        if (obj && (obj->type == OBJ_TREE || obj->type == OBJ_COMMIT))
                return 1;
        return 0;
}

static int disambiguate_blob_only(const unsigned char *sha1, void *cb_data_unused)
{
        int kind = sha1_object_info(sha1, NULL);
        return kind == OBJ_BLOB;
}

static int prepare_prefixes(const char *name, int len,
                            unsigned char *bin_pfx,
                            char *hex_pfx)
{
        int i;

        hashclr(bin_pfx);
        memset(hex_pfx, 'x', 40);
        for (i = 0; i < len ;i++) {
                unsigned char c = name[i];
                unsigned char val;
                if (c >= '0' && c <= '9')
                        val = c - '0';
                else if (c >= 'a' && c <= 'f')
                        val = c - 'a' + 10;
                else if (c >= 'A' && c <='F') {
                        val = c - 'A' + 10;
                        c -= 'A' - 'a';
                }
                else
                        return -1;
                hex_pfx[i] = c;
                if (!(i & 1))
                        val <<= 4;
                bin_pfx[i >> 1] |= val;
        }
        return 0;
}

static int get_short_sha1(const char *name, int len, unsigned char *sha1,
                          unsigned flags)
{
        int status;
        char hex_pfx[40];
        unsigned char bin_pfx[20];
        struct disambiguate_state ds;
        int quietly = !!(flags & GET_SHA1_QUIETLY);

        if (len < MINIMUM_ABBREV || len > 40)
                return -1;
        if (prepare_prefixes(name, len, bin_pfx, hex_pfx) < 0)
                return -1;

        prepare_alt_odb();

        memset(&ds, 0, sizeof(ds));
        if (flags & GET_SHA1_COMMIT)
                ds.fn = disambiguate_commit_only;
        else if (flags & GET_SHA1_COMMITTISH)
                ds.fn = disambiguate_committish_only;
        else if (flags & GET_SHA1_TREE)
                ds.fn = disambiguate_tree_only;
        else if (flags & GET_SHA1_TREEISH)
                ds.fn = disambiguate_treeish_only;
        else if (flags & GET_SHA1_BLOB)
                ds.fn = disambiguate_blob_only;

        find_short_object_filename(len, hex_pfx, &ds);
        find_short_packed_object(len, bin_pfx, &ds);
        status = finish_object_disambiguation(&ds, sha1);

        if (!quietly && (status == SHORT_NAME_AMBIGUOUS))
                return error("short SHA1 %.*s is ambiguous.", len, hex_pfx);
        return status;
}


int for_each_abbrev(const char *prefix, each_abbrev_fn fn, void *cb_data)
{
        char hex_pfx[40];
        unsigned char bin_pfx[20];
        struct disambiguate_state ds;
        int len = strlen(prefix);

        if (len < MINIMUM_ABBREV || len > 40)
                return -1;
        if (prepare_prefixes(prefix, len, bin_pfx, hex_pfx) < 0)
                return -1;

        prepare_alt_odb();

        memset(&ds, 0, sizeof(ds));
        ds.always_call_fn = 1;
        ds.cb_data = cb_data;
        ds.fn = fn;

        find_short_object_filename(len, hex_pfx, &ds);
        find_short_packed_object(len, bin_pfx, &ds);
        return ds.ambiguous;
}

const char *find_unique_abbrev(const unsigned char *sha1, int len)
{
        int status, exists;
        static char hex[41];

        exists = has_sha1_file(sha1);
        memcpy(hex, sha1_to_hex(sha1), 40);
        if (len == 40 || !len)
                return hex;
        while (len < 40) {
                unsigned char sha1_ret[20];
                status = get_short_sha1(hex, len, sha1_ret, GET_SHA1_QUIETLY);
                if (exists
                    ? !status
                    : status == SHORT_NAME_NOT_FOUND) {
                        hex[len] = 0;
                        return hex;
                }
                len++;
        }
        return hex;
}

static int ambiguous_path(const char *path, int len)
{
        int slash = 1;
        int cnt;

        for (cnt = 0; cnt < len; cnt++) {
                switch (*path++) {
                case '\0':
                        break;
                case '/':
                        if (slash)
                                break;
                        slash = 1;
                        continue;
                case '.':
                        continue;
                default:
                        slash = 0;
                        continue;
                }
                break;
        }
        return slash;
}

static inline int upstream_mark(const char *string, int len)
{
        const char *suffix[] = { "@{upstream}", "@{u}" };
        int i;

        for (i = 0; i < ARRAY_SIZE(suffix); i++) {
                int suffix_len = strlen(suffix[i]);
                if (suffix_len <= len
                    && !memcmp(string, suffix[i], suffix_len))
                        return suffix_len;
        }
        return 0;
}

static int get_sha1_1(const char *name, int len, unsigned char *sha1, unsigned lookup_flags);

static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
{
        static const char *warn_msg = "refname '%.*s' is ambiguous.";
        char *real_ref = NULL;
        int refs_found = 0;
        int at, reflog_len;

        if (len == 40 && !get_sha1_hex(str, sha1))
                return 0;

        /* basic@{time or number or -number} format to query ref-log */
        reflog_len = at = 0;
        if (len && str[len-1] == '}') {
                for (at = len-2; at >= 0; at--) {
                        if (str[at] == '@' && str[at+1] == '{') {
                                if (!upstream_mark(str + at, len - at)) {
                                        reflog_len = (len-1) - (at+2);
                                        len = at;
                                }
                                break;
                        }
                }
        }

        /* Accept only unambiguous ref paths. */
        if (len && ambiguous_path(str, len))
                return -1;

        if (!len && reflog_len) {
                struct strbuf buf = STRBUF_INIT;
                int ret;
                /* try the @{-N} syntax for n-th checkout */
                ret = interpret_branch_name(str+at, &buf);
                if (ret > 0) {
                        /* substitute this branch name and restart */
                        return get_sha1_1(buf.buf, buf.len, sha1, 0);
                } else if (ret == 0) {
                        return -1;
                }
                /* allow "@{...}" to mean the current branch reflog */
                refs_found = dwim_ref("HEAD", 4, sha1, &real_ref);
        } else if (reflog_len)
                refs_found = dwim_log(str, len, sha1, &real_ref);
        else
                refs_found = dwim_ref(str, len, sha1, &real_ref);

        if (!refs_found)
                return -1;

        if (warn_ambiguous_refs && refs_found > 1)
                warning(warn_msg, len, str);

        if (reflog_len) {
                int nth, i;
                unsigned long at_time;
                unsigned long co_time;
                int co_tz, co_cnt;

                /* a @{-N} placed anywhere except the start is an error */
                if (str[at+2] == '-')
                        return -1;

                /* Is it asking for N-th entry, or approxidate? */
                for (i = nth = 0; 0 <= nth && i < reflog_len; i++) {
                        char ch = str[at+2+i];
                        if ('0' <= ch && ch <= '9')
                                nth = nth * 10 + ch - '0';
                        else
                                nth = -1;
                }
                if (100000000 <= nth) {
                        at_time = nth;
                        nth = -1;
                } else if (0 <= nth)
                        at_time = 0;
                else {
                        int errors = 0;
                        char *tmp = xstrndup(str + at + 2, reflog_len);
                        at_time = approxidate_careful(tmp, &errors);
                        free(tmp);
                        if (errors)
                                return -1;
                }
                if (read_ref_at(real_ref, at_time, nth, sha1, NULL,
                                &co_time, &co_tz, &co_cnt)) {
                        if (at_time)
                                warning("Log for '%.*s' only goes "
                                        "back to %s.", len, str,
                                        show_date(co_time, co_tz, DATE_RFC2822));
                        else {
                                free(real_ref);
                                die("Log for '%.*s' only has %d entries.",
                                    len, str, co_cnt);
                        }
                }
        }

        free(real_ref);
        return 0;
}

static int get_parent(const char *name, int len,
                      unsigned char *result, int idx)
{
        unsigned char sha1[20];
        int ret = get_sha1_1(name, len, sha1, GET_SHA1_COMMITTISH);
        struct commit *commit;
        struct commit_list *p;

        if (ret)
                return ret;
        commit = lookup_commit_reference(sha1);
        if (!commit)
                return -1;
        if (parse_commit(commit))
                return -1;
        if (!idx) {
                hashcpy(result, commit->object.sha1);
                return 0;
        }
        p = commit->parents;
        while (p) {
                if (!--idx) {
                        hashcpy(result, p->item->object.sha1);
                        return 0;
                }
                p = p->next;
        }
        return -1;
}

static int get_nth_ancestor(const char *name, int len,
                            unsigned char *result, int generation)
{
        unsigned char sha1[20];
        struct commit *commit;
        int ret;

        ret = get_sha1_1(name, len, sha1, GET_SHA1_COMMITTISH);
        if (ret)
                return ret;
        commit = lookup_commit_reference(sha1);
        if (!commit)
                return -1;

        while (generation--) {
                if (parse_commit(commit) || !commit->parents)
                        return -1;
                commit = commit->parents->item;
        }
        hashcpy(result, commit->object.sha1);
        return 0;
}

struct object *peel_to_type(const char *name, int namelen,
                            struct object *o, enum object_type expected_type)
{
        if (name && !namelen)
                namelen = strlen(name);
        while (1) {
                if (!o || (!o->parsed && !parse_object(o->sha1)))
                        return NULL;
                if (o->type == expected_type)
                        return o;
                if (o->type == OBJ_TAG)
                        o = ((struct tag*) o)->tagged;
                else if (o->type == OBJ_COMMIT)
                        o = &(((struct commit *) o)->tree->object);
                else {
                        if (name)
                                error("%.*s: expected %s type, but the object "
                                      "dereferences to %s type",
                                      namelen, name, typename(expected_type),
                                      typename(o->type));
                        return NULL;
                }
        }
}

static int peel_onion(const char *name, int len, unsigned char *sha1)
{
        unsigned char outer[20];
        const char *sp;
        unsigned int expected_type = 0;
        unsigned lookup_flags = 0;
        struct object *o;

        /*
         * "ref^{type}" dereferences ref repeatedly until you cannot
         * dereference anymore, or you get an object of given type,
         * whichever comes first.  "ref^{}" means just dereference
         * tags until you get a non-tag.  "ref^0" is a shorthand for
         * "ref^{commit}".  "commit^{tree}" could be used to find the
         * top-level tree of the given commit.
         */
        if (len < 4 || name[len-1] != '}')
                return -1;

        for (sp = name + len - 1; name <= sp; sp--) {
                int ch = *sp;
                if (ch == '{' && name < sp && sp[-1] == '^')
                        break;
        }
        if (sp <= name)
                return -1;

        sp++; /* beginning of type name, or closing brace for empty */
        if (!strncmp(commit_type, sp, 6) && sp[6] == '}')
                expected_type = OBJ_COMMIT;
        else if (!strncmp(tree_type, sp, 4) && sp[4] == '}')
                expected_type = OBJ_TREE;
        else if (!strncmp(blob_type, sp, 4) && sp[4] == '}')
                expected_type = OBJ_BLOB;
        else if (sp[0] == '}')
                expected_type = OBJ_NONE;
        else if (sp[0] == '/')
                expected_type = OBJ_COMMIT;
        else
                return -1;

        if (expected_type == OBJ_COMMIT)
                lookup_flags = GET_SHA1_COMMITTISH;
        else if (expected_type == OBJ_TREE)
                lookup_flags = GET_SHA1_TREEISH;

        if (get_sha1_1(name, sp - name - 2, outer, lookup_flags))
                return -1;

        o = parse_object(outer);
        if (!o)
                return -1;
        if (!expected_type) {
                o = deref_tag(o, name, sp - name - 2);
                if (!o || (!o->parsed && !parse_object(o->sha1)))
                        return -1;
                hashcpy(sha1, o->sha1);
                return 0;
        }

        /*
         * At this point, the syntax look correct, so
         * if we do not get the needed object, we should
         * barf.
         */
        o = peel_to_type(name, len, o, expected_type);
        if (!o)
                return -1;

        hashcpy(sha1, o->sha1);
        if (sp[0] == '/') {
                /* "$commit^{/foo}" */
                char *prefix;
                int ret;
                struct commit_list *list = NULL;

                /*
                 * $commit^{/}. Some regex implementation may reject.
                 * We don't need regex anyway. '' pattern always matches.
                 */
                if (sp[1] == '}')
                        return 0;

                prefix = xstrndup(sp + 1, name + len - 1 - (sp + 1));
                commit_list_insert((struct commit *)o, &list);
                ret = get_sha1_oneline(prefix, sha1, list);
                free(prefix);
                return ret;
        }
        return 0;
}

static int get_describe_name(const char *name, int len, unsigned char *sha1)
{
        const char *cp;
        unsigned flags = GET_SHA1_QUIETLY | GET_SHA1_COMMIT;

        for (cp = name + len - 1; name + 2 <= cp; cp--) {
                char ch = *cp;
                if (hexval(ch) & ~0377) {
                        /* We must be looking at g in "SOMETHING-g"
                         * for it to be describe output.
                         */
                        if (ch == 'g' && cp[-1] == '-') {
                                cp++;
                                len -= cp - name;
                                return get_short_sha1(cp, len, sha1, flags);
                        }
                }
        }
        return -1;
}

static int get_sha1_1(const char *name, int len, unsigned char *sha1, unsigned lookup_flags)
{
        int ret, has_suffix;
        const char *cp;

        /*
         * "name~3" is "name^^^", "name~" is "name~1", and "name^" is "name^1".
         */
        has_suffix = 0;
        for (cp = name + len - 1; name <= cp; cp--) {
                int ch = *cp;
                if ('0' <= ch && ch <= '9')
                        continue;
                if (ch == '~' || ch == '^')
                        has_suffix = ch;
                break;
        }

        if (has_suffix) {
                int num = 0;
                int len1 = cp - name;
                cp++;
                while (cp < name + len)
                        num = num * 10 + *cp++ - '0';
                if (!num && len1 == len - 1)
                        num = 1;
                if (has_suffix == '^')
                        return get_parent(name, len1, sha1, num);
                /* else if (has_suffix == '~') -- goes without saying */
                return get_nth_ancestor(name, len1, sha1, num);
        }

        ret = peel_onion(name, len, sha1);
        if (!ret)
                return 0;

        ret = get_sha1_basic(name, len, sha1);
        if (!ret)
                return 0;

        /* It could be describe output that is "SOMETHING-gXXXX" */
        ret = get_describe_name(name, len, sha1);
        if (!ret)
                return 0;

        return get_short_sha1(name, len, sha1, lookup_flags);
}

/*
 * This interprets names like ':/Initial revision of "git"' by searching
 * through history and returning the first commit whose message starts
 * the given regular expression.
 *
 * For future extension, ':/!' is reserved. If you want to match a message
 * beginning with a '!', you have to repeat the exclamation mark.
 */
#define ONELINE_SEEN (1u<<20)

static int handle_one_ref(const char *path,
                const unsigned char *sha1, int flag, void *cb_data)
{
        struct commit_list **list = cb_data;
        struct object *object = parse_object(sha1);
        if (!object)
                return 0;
        if (object->type == OBJ_TAG) {
                object = deref_tag(object, path, strlen(path));
                if (!object)
                        return 0;
        }
        if (object->type != OBJ_COMMIT)
                return 0;
        commit_list_insert_by_date((struct commit *)object, list);
        return 0;
}

static int get_sha1_oneline(const char *prefix, unsigned char *sha1,
                            struct commit_list *list)
{
        struct commit_list *backup = NULL, *l;
        int found = 0;
        regex_t regex;

        if (prefix[0] == '!') {
                if (prefix[1] != '!')
                        die ("Invalid search pattern: %s", prefix);
                prefix++;
        }

        if (regcomp(&regex, prefix, REG_EXTENDED))
                die("Invalid search pattern: %s", prefix);

        for (l = list; l; l = l->next) {
                l->item->object.flags |= ONELINE_SEEN;
                commit_list_insert(l->item, &backup);
        }
        while (list) {
                char *p, *to_free = NULL;
                struct commit *commit;
                enum object_type type;
                unsigned long size;
                int matches;

                commit = pop_most_recent_commit(&list, ONELINE_SEEN);
                if (!parse_object(commit->object.sha1))
                        continue;
                if (commit->buffer)
                        p = commit->buffer;
                else {
                        p = read_sha1_file(commit->object.sha1, &type, &size);
                        if (!p)
                                continue;
                        to_free = p;
                }

                p = strstr(p, "\n\n");
                matches = p && !regexec(&regex, p + 2, 0, NULL, 0);
                free(to_free);

                if (matches) {
                        hashcpy(sha1, commit->object.sha1);
                        found = 1;
                        break;
                }
        }
        regfree(&regex);
        free_commit_list(list);
        for (l = backup; l; l = l->next)
                clear_commit_marks(l->item, ONELINE_SEEN);
        free_commit_list(backup);
        return found ? 0 : -1;
}

struct grab_nth_branch_switch_cbdata {
        long cnt, alloc;
        struct strbuf *buf;
};

static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
                                  const char *email, unsigned long timestamp, int tz,
                                  const char *message, void *cb_data)
{
        struct grab_nth_branch_switch_cbdata *cb = cb_data;
        const char *match = NULL, *target = NULL;
        size_t len;
        int nth;

        if (!prefixcmp(message, "checkout: moving from ")) {
                match = message + strlen("checkout: moving from ");
                target = strstr(match, " to ");
        }

        if (!match || !target)
                return 0;

        len = target - match;
        nth = cb->cnt++ % cb->alloc;
        strbuf_reset(&cb->buf[nth]);
        strbuf_add(&cb->buf[nth], match, len);
        return 0;
}

/*
 * Parse @{-N} syntax, return the number of characters parsed
 * if successful; otherwise signal an error with negative value.
 */
static int interpret_nth_prior_checkout(const char *name, struct strbuf *buf)
{
        long nth;
        int i, retval;
        struct grab_nth_branch_switch_cbdata cb;
        const char *brace;
        char *num_end;

        if (name[0] != '@' || name[1] != '{' || name[2] != '-')
                return -1;
        brace = strchr(name, '}');
        if (!brace)
                return -1;
        nth = strtol(name+3, &num_end, 10);
        if (num_end != brace)
                return -1;
        if (nth <= 0)
                return -1;
        cb.alloc = nth;
        cb.buf = xmalloc(nth * sizeof(struct strbuf));
        for (i = 0; i < nth; i++)
                strbuf_init(&cb.buf[i], 20);
        cb.cnt = 0;
        retval = 0;
        for_each_recent_reflog_ent("HEAD", grab_nth_branch_switch, 40960, &cb);
        if (cb.cnt < nth) {
                cb.cnt = 0;
                for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
        }
        if (cb.cnt < nth)
                goto release_return;
        i = cb.cnt % nth;
        strbuf_reset(buf);
        strbuf_add(buf, cb.buf[i].buf, cb.buf[i].len);
        retval = brace-name+1;

release_return:
        for (i = 0; i < nth; i++)
                strbuf_release(&cb.buf[i]);
        free(cb.buf);

        return retval;
}

int get_sha1_mb(const char *name, unsigned char *sha1)
{
        struct commit *one, *two;
        struct commit_list *mbs;
        unsigned char sha1_tmp[20];
        const char *dots;
        int st;

        dots = strstr(name, "...");
        if (!dots)
                return get_sha1(name, sha1);
        if (dots == name)
                st = get_sha1("HEAD", sha1_tmp);
        else {
                struct strbuf sb;
                strbuf_init(&sb, dots - name);
                strbuf_add(&sb, name, dots - name);
                st = get_sha1_committish(sb.buf, sha1_tmp);
                strbuf_release(&sb);
        }
        if (st)
                return st;
        one = lookup_commit_reference_gently(sha1_tmp, 0);
        if (!one)
                return -1;

        if (get_sha1_committish(dots[3] ? (dots + 3) : "HEAD", sha1_tmp))
                return -1;
        two = lookup_commit_reference_gently(sha1_tmp, 0);
        if (!two)
                return -1;
        mbs = get_merge_bases(one, two, 1);
        if (!mbs || mbs->next)
                st = -1;
        else {
                st = 0;
                hashcpy(sha1, mbs->item->object.sha1);
        }
        free_commit_list(mbs);
        return st;
}

/*
 * This reads short-hand syntax that not only evaluates to a commit
 * object name, but also can act as if the end user spelled the name
 * of the branch from the command line.
 *
 * - "@{-N}" finds the name of the Nth previous branch we were on, and
 *   places the name of the branch in the given buf and returns the
 *   number of characters parsed if successful.
 *
 * - "<branch>@{upstream}" finds the name of the other ref that
 *   <branch> is configured to merge with (missing <branch> defaults
 *   to the current branch), and places the name of the branch in the
 *   given buf and returns the number of characters parsed if
 *   successful.
 *
 * If the input is not of the accepted format, it returns a negative
 * number to signal an error.
 *
 * If the input was ok but there are not N branch switches in the
 * reflog, it returns 0.
 */
int interpret_branch_name(const char *name, struct strbuf *buf)
{
        char *cp;
        struct branch *upstream;
        int namelen = strlen(name);
        int len = interpret_nth_prior_checkout(name, buf);
        int tmp_len;

        if (!len)
                return len; /* syntax Ok, not enough switches */
        if (0 < len && len == namelen)
                return len; /* consumed all */
        else if (0 < len) {
                /* we have extra data, which might need further processing */
                struct strbuf tmp = STRBUF_INIT;
                int used = buf->len;
                int ret;

                strbuf_add(buf, name + len, namelen - len);
                ret = interpret_branch_name(buf->buf, &tmp);
                /* that data was not interpreted, remove our cruft */
                if (ret < 0) {
                        strbuf_setlen(buf, used);
                        return len;
                }
                strbuf_reset(buf);
                strbuf_addbuf(buf, &tmp);
                strbuf_release(&tmp);
                /* tweak for size of {-N} versus expanded ref name */
                return ret - used + len;
        }

        cp = strchr(name, '@');
        if (!cp)
                return -1;
        tmp_len = upstream_mark(cp, namelen - (cp - name));
        if (!tmp_len)
                return -1;
        len = cp + tmp_len - name;
        cp = xstrndup(name, cp - name);
        upstream = branch_get(*cp ? cp : NULL);
        /*
         * Upstream can be NULL only if cp refers to HEAD and HEAD
         * points to something different than a branch.
         */
        if (!upstream)
                return error(_("HEAD does not point to a branch"));
        if (!upstream->merge || !upstream->merge[0]->dst) {
                if (!ref_exists(upstream->refname))
                        return error(_("No such branch: '%s'"), cp);
                if (!upstream->merge)
                        return error(_("No upstream configured for branch '%s'"),
                                     upstream->name);
                return error(
                        _("Upstream branch '%s' not stored as a remote-tracking branch"),
                        upstream->merge[0]->src);
        }
        free(cp);
        cp = shorten_unambiguous_ref(upstream->merge[0]->dst, 0);
        strbuf_reset(buf);
        strbuf_addstr(buf, cp);
        free(cp);
        return len;
}

int strbuf_branchname(struct strbuf *sb, const char *name)
{
        int len = strlen(name);
        if (interpret_branch_name(name, sb) == len)
                return 0;
        strbuf_add(sb, name, len);
        return len;
}

int strbuf_check_branch_ref(struct strbuf *sb, const char *name)
{
        strbuf_branchname(sb, name);
        if (name[0] == '-')
                return -1;
        strbuf_splice(sb, 0, 0, "refs/heads/", 11);
        return check_refname_format(sb->buf, 0);
}

/*
 * This is like "get_sha1_basic()", except it allows "sha1 expressions",
 * notably "xyz^" for "parent of xyz"
 */
int get_sha1(const char *name, unsigned char *sha1)
{
        struct object_context unused;
        return get_sha1_with_context(name, 0, sha1, &unused);
}

/*
 * Many callers know that the user meant to name a committish by
 * syntactical positions where the object name appears.  Calling this
 * function allows the machinery to disambiguate shorter-than-unique
 * abbreviated object names between committish and others.
 *
 * Note that this does NOT error out when the named object is not a
 * committish. It is merely to give a hint to the disambiguation
 * machinery.
 */
int get_sha1_committish(const char *name, unsigned char *sha1)
{
        struct object_context unused;
        return get_sha1_with_context(name, GET_SHA1_COMMITTISH,
                                     sha1, &unused);
}

int get_sha1_treeish(const char *name, unsigned char *sha1)
{
        struct object_context unused;
        return get_sha1_with_context(name, GET_SHA1_TREEISH,
                                     sha1, &unused);
}

int get_sha1_commit(const char *name, unsigned char *sha1)
{
        struct object_context unused;
        return get_sha1_with_context(name, GET_SHA1_COMMIT,
                                     sha1, &unused);
}

int get_sha1_tree(const char *name, unsigned char *sha1)
{
        struct object_context unused;
        return get_sha1_with_context(name, GET_SHA1_TREE,
                                     sha1, &unused);
}

int get_sha1_blob(const char *name, unsigned char *sha1)
{
        struct object_context unused;
        return get_sha1_with_context(name, GET_SHA1_BLOB,
                                     sha1, &unused);
}

/* Must be called only when object_name:filename doesn't exist. */
static void diagnose_invalid_sha1_path(const char *prefix,
                                       const char *filename,
                                       const unsigned char *tree_sha1,
                                       const char *object_name,
                                       int object_name_len)
{
        struct stat st;
        unsigned char sha1[20];
        unsigned mode;

        if (!prefix)
                prefix = "";

        if (!lstat(filename, &st))
                die("Path '%s' exists on disk, but not in '%.*s'.",
                    filename, object_name_len, object_name);
        if (errno == ENOENT || errno == ENOTDIR) {
                char *fullname = xmalloc(strlen(filename)
                                             + strlen(prefix) + 1);
                strcpy(fullname, prefix);
                strcat(fullname, filename);

                if (!get_tree_entry(tree_sha1, fullname,
                                    sha1, &mode)) {
                        die("Path '%s' exists, but not '%s'.\n"
                            "Did you mean '%.*s:%s' aka '%.*s:./%s'?",
                            fullname,
                            filename,
                            object_name_len, object_name,
                            fullname,
                            object_name_len, object_name,
                            filename);
                }
                die("Path '%s' does not exist in '%.*s'",
                    filename, object_name_len, object_name);
        }
}

/* Must be called only when :stage:filename doesn't exist. */
static void diagnose_invalid_index_path(int stage,
                                        const char *prefix,
                                        const char *filename)
{
        struct stat st;
        struct cache_entry *ce;
        int pos;
        unsigned namelen = strlen(filename);
        unsigned fullnamelen;
        char *fullname;

        if (!prefix)
                prefix = "";

        /* Wrong stage number? */
        pos = cache_name_pos(filename, namelen);
        if (pos < 0)
                pos = -pos - 1;
        if (pos < active_nr) {
                ce = active_cache[pos];
                if (ce_namelen(ce) == namelen &&
                    !memcmp(ce->name, filename, namelen))
                        die("Path '%s' is in the index, but not at stage %d.\n"
                            "Did you mean ':%d:%s'?",
                            filename, stage,
                            ce_stage(ce), filename);
        }

        /* Confusion between relative and absolute filenames? */
        fullnamelen = namelen + strlen(prefix);
        fullname = xmalloc(fullnamelen + 1);
        strcpy(fullname, prefix);
        strcat(fullname, filename);
        pos = cache_name_pos(fullname, fullnamelen);
        if (pos < 0)
                pos = -pos - 1;
        if (pos < active_nr) {
                ce = active_cache[pos];
                if (ce_namelen(ce) == fullnamelen &&
                    !memcmp(ce->name, fullname, fullnamelen))
                        die("Path '%s' is in the index, but not '%s'.\n"
                            "Did you mean ':%d:%s' aka ':%d:./%s'?",
                            fullname, filename,
                            ce_stage(ce), fullname,
                            ce_stage(ce), filename);
        }

        if (!lstat(filename, &st))
                die("Path '%s' exists on disk, but not in the index.", filename);
        if (errno == ENOENT || errno == ENOTDIR)
                die("Path '%s' does not exist (neither on disk nor in the index).",
                    filename);

        free(fullname);
}


static char *resolve_relative_path(const char *rel)
{
        if (prefixcmp(rel, "./") && prefixcmp(rel, "../"))
                return NULL;

        if (!startup_info)
                die("BUG: startup_info struct is not initialized.");

        if (!is_inside_work_tree())
                die("relative path syntax can't be used outside working tree.");

        /* die() inside prefix_path() if resolved path is outside worktree */
        return prefix_path(startup_info->prefix,
                           startup_info->prefix ? strlen(startup_info->prefix) : 0,
                           rel);
}

static int get_sha1_with_context_1(const char *name,
                                   unsigned flags,
                                   const char *prefix,
                                   unsigned char *sha1,
                                   struct object_context *oc)
{
        int ret, bracket_depth;
        int namelen = strlen(name);
        const char *cp;
        int only_to_die = flags & GET_SHA1_ONLY_TO_DIE;

        memset(oc, 0, sizeof(*oc));
        oc->mode = S_IFINVALID;
        ret = get_sha1_1(name, namelen, sha1, flags);
        if (!ret)
                return ret;
        /*
         * sha1:path --> object name of path in ent sha1
         * :path -> object name of absolute path in index
         * :./path -> object name of path relative to cwd in index
         * :[0-3]:path -> object name of path in index at stage
         * :/foo -> recent commit matching foo
         */
        if (name[0] == ':') {
                int stage = 0;
                struct cache_entry *ce;
                char *new_path = NULL;
                int pos;
                if (!only_to_die && namelen > 2 && name[1] == '/') {
                        struct commit_list *list = NULL;
                        for_each_ref(handle_one_ref, &list);
                        return get_sha1_oneline(name + 2, sha1, list);
                }
                if (namelen < 3 ||
                    name[2] != ':' ||
                    name[1] < '0' || '3' < name[1])
                        cp = name + 1;
                else {
                        stage = name[1] - '0';
                        cp = name + 3;
                }
                new_path = resolve_relative_path(cp);
                if (!new_path) {
                        namelen = namelen - (cp - name);
                } else {
                        cp = new_path;
                        namelen = strlen(cp);
                }

                strncpy(oc->path, cp,
                        sizeof(oc->path));
                oc->path[sizeof(oc->path)-1] = '\0';

                if (!active_cache)
                        read_cache();
                pos = cache_name_pos(cp, namelen);
                if (pos < 0)
                        pos = -pos - 1;
                while (pos < active_nr) {
                        ce = active_cache[pos];
                        if (ce_namelen(ce) != namelen ||
                            memcmp(ce->name, cp, namelen))
                                break;
                        if (ce_stage(ce) == stage) {
                                hashcpy(sha1, ce->sha1);
                                oc->mode = ce->ce_mode;
                                free(new_path);
                                return 0;
                        }
                        pos++;
                }
                if (only_to_die && name[1] && name[1] != '/')
                        diagnose_invalid_index_path(stage, prefix, cp);
                free(new_path);
                return -1;
        }
        for (cp = name, bracket_depth = 0; *cp; cp++) {
                if (*cp == '{')
                        bracket_depth++;
                else if (bracket_depth && *cp == '}')
                        bracket_depth--;
                else if (!bracket_depth && *cp == ':')
                        break;
        }
        if (*cp == ':') {
                unsigned char tree_sha1[20];
                int len = cp - name;
                if (!get_sha1_1(name, len, tree_sha1, GET_SHA1_TREEISH)) {
                        const char *filename = cp+1;
                        char *new_filename = NULL;

                        new_filename = resolve_relative_path(filename);
                        if (new_filename)
                                filename = new_filename;
                        ret = get_tree_entry(tree_sha1, filename, sha1, &oc->mode);
                        if (ret && only_to_die) {
                                diagnose_invalid_sha1_path(prefix, filename,
                                                           tree_sha1,
                                                           name, len);
                        }
                        hashcpy(oc->tree, tree_sha1);
                        strncpy(oc->path, filename,
                                sizeof(oc->path));
                        oc->path[sizeof(oc->path)-1] = '\0';

                        free(new_filename);
                        return ret;
                } else {
                        if (only_to_die)
                                die("Invalid object name '%.*s'.", len, name);
                }
        }
        return ret;
}

/*
 * Call this function when you know "name" given by the end user must
 * name an object but it doesn't; the function _may_ die with a better
 * diagnostic message than "no such object 'name'", e.g. "Path 'doc' does not
 * exist in 'HEAD'" when given "HEAD:doc", or it may return in which case
 * you have a chance to diagnose the error further.
 */
void maybe_die_on_misspelt_object_name(const char *name, const char *prefix)
{
        struct object_context oc;
        unsigned char sha1[20];
        get_sha1_with_context_1(name, GET_SHA1_ONLY_TO_DIE, prefix, sha1, &oc);
}

int get_sha1_with_context(const char *str, unsigned flags, unsigned char *sha1, struct object_context *orc)
{
        return get_sha1_with_context_1(str, flags, NULL, sha1, orc);
}