sha1_name.c: restructure disambiguation of short names

We try to find zero, one or more matches from loose objects and
packed objects independently and then decide if the given short
object name is unique across them.

Instead, introduce a "struct disambiguate_state" that keeps track of
what we have found so far, that can be one of:

 - We have seen one object that _could_ be what we are looking for;
 - We have also checked that object for additional constraints (if any),
   and found that the object satisfies it;
 - We have also checked that object for additional constraints (if any),
   and found that the object does not satisfy it; or
 - We have seen more than one objects that satisfy the constraints.

and pass it to the enumeration functions for loose and packed
objects.  The disambiguation state can optionally take a callback
function that takes a candidate object name and reports if the
object satisifies additional criteria (e.g. when the caller knows
that the short name must refer to a commit, this mechanism can be
used to check the type of the given object).

Compared to the earlier attempt, this round avoids the optional
check if there is only one candidate that matches the short name in
the first place.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -9,11 +9,67 @@
 
 static int get_sha1_oneline(const char *, unsigned char *, struct commit_list *);
 
-static int find_short_object_filename(int len, const char *hex_pfx, unsigned char *sha1)
+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;
+};
+
+static void update_candidates(struct disambiguate_state *ds, const unsigned char *current)
+{
+	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];
-	int found = 0;
 	static struct alternate_object_database *fakeent;
 
 	if (!fakeent) {
@@ -35,32 +91,27 @@ static int find_short_object_filename(int len, const char *hex_pfx, unsigned cha
 	fakeent->next = alt_odb_list;
 
 	sprintf(hex, "%.2s", hex_pfx);
-	for (alt = fakeent; alt && found < 2; alt = alt->next) {
+	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 ((de = readdir(dir)) != NULL) {
+
+		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;
-			if (!found) {
 			memcpy(hex + 2, de->d_name, 38);
-				found++;
-			}
-			else if (memcmp(hex + 2, de->d_name, 38)) {
-				found = 2;
-				break;
-			}
+			if (!get_sha1_hex(hex, sha1))
+				update_candidates(ds, sha1);
 		}
 		closedir(dir);
 	}
-	if (found == 1)
-		return get_sha1_hex(hex, sha1) == 0;
-	return found;
 }
 
 static int match_sha(unsigned len, const unsigned char *a, const unsigned char *b)
@@ -78,11 +129,10 @@ static int match_sha(unsigned len, const unsigned char *a, const unsigned char *
 	return 1;
 }
 
-static int unique_in_pack(int len,
+static void unique_in_pack(int len,
 			  const unsigned char *bin_pfx,
 			   struct packed_git *p,
-			  const unsigned char **found_sha1,
-			  int seen_so_far)
+			   struct disambiguate_state *ds)
 {
 	uint32_t num, last, i, first = 0;
 	const unsigned char *current = NULL;
@@ -113,63 +163,58 @@ static int unique_in_pack(int len,
 	 * 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; i++) {
-		current = nth_packed_object_sha1(p, first);
+	for (i = first; i < num && !ds->ambiguous; i++) {
+		current = nth_packed_object_sha1(p, i);
 		if (!match_sha(len, bin_pfx, current))
 			break;
-
-		/* current matches */
-		if (!seen_so_far) {
-			*found_sha1 = current;
-			seen_so_far++;
-		} else if (seen_so_far) {
-			/* is it the same as the one previously found elsewhere? */
-			if (hashcmp(*found_sha1, current))
-				return 2; /* definitely not unique */
+		update_candidates(ds, current);
 	}
 }
-	return seen_so_far;
-}
 
-static int find_short_packed_object(int len, const unsigned char *bin_pfx, unsigned char *sha1)
+static void find_short_packed_object(int len, const unsigned char *bin_pfx,
+				     struct disambiguate_state *ds)
 {
 	struct packed_git *p;
-	const unsigned char *found_sha1 = NULL;
-	int found = 0;
 
 	prepare_packed_git();
-	for (p = packed_git; p && found < 2; p = p->next)
-		found = unique_in_pack(len, bin_pfx, p, &found_sha1, found);
-
-	if (found == 1)
-		hashcpy(sha1, found_sha1);
-	return found;
+	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 find_unique_short_object(int len, char *hex_pfx,
-				    unsigned char *bin_pfx, unsigned char *sha1)
+static int finish_object_disambiguation(struct disambiguate_state *ds,
+					unsigned char *sha1)
 {
-	int has_unpacked, has_packed;
-	unsigned char unpacked_sha1[20], packed_sha1[20];
+	if (ds->ambiguous)
+		return SHORT_NAME_AMBIGUOUS;
 
-	prepare_alt_odb();
-	has_unpacked = find_short_object_filename(len, hex_pfx, unpacked_sha1);
-	has_packed = find_short_packed_object(len, bin_pfx, packed_sha1);
-	if (!has_unpacked && !has_packed)
+	if (!ds->candidate_exists)
 		return SHORT_NAME_NOT_FOUND;
-	if (1 < has_unpacked || 1 < has_packed)
-		return SHORT_NAME_AMBIGUOUS;
-	if (has_unpacked != has_packed) {
-		hashcpy(sha1, (has_packed ? packed_sha1 : unpacked_sha1));
-		return 0;
-	}
-	/* Both have unique ones -- do they match? */
-	if (hashcmp(packed_sha1, unpacked_sha1))
-		return SHORT_NAME_AMBIGUOUS;
-	hashcpy(sha1, packed_sha1);
+
+	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_NOT_FOUND;
+
+	hashcpy(sha1, ds->candidate);
 	return 0;
 }
 
@@ -179,6 +224,7 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 	int i, status;
 	char hex_pfx[40];
 	unsigned char bin_pfx[20];
+	struct disambiguate_state ds;
 
 	if (len < MINIMUM_ABBREV || len > 40)
 		return -1;
@@ -203,7 +249,13 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 		bin_pfx[i >> 1] |= val;
 	}
 
-	status = find_unique_short_object(i, hex_pfx, bin_pfx, sha1);
+	prepare_alt_odb();
+
+	memset(&ds, 0, sizeof(ds));
+	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;