git-read-tree: simplify merge loops enormously

Stop trying to haev this stateful thing that keeps track of what it has
seen, and use a much simpler "gather all the different stages with the
same name together and just merge them in one go" approach.

Makes it a lot more understandable, and allows the different merge
algorithms to share the basic merge loop.

diff --git a/read-tree.c b/read-tree.c
index 63a1eb543fef2b84f2ff7d295610c09b6c2ab2da..4acbb6b3f25cd5cb50dda3ac34b1b37edad639b1 100644 (file)
--- a/read-tree.c
+++ b/read-tree.c
@@ -39,27 +39,11 @@ static int same(struct cache_entry *a, struct cache_entry *b)
 /*
  * This removes all trivial merges that don't change the tree
  * and collapses them to state 0.
- *
- * _Any_ other merge is left to user policy.  That includes "both
- * created the same file", and "both removed the same file" - which are
- * trivial, but the user might still want to _note_ it. 
  */
 static struct cache_entry *merge_entries(struct cache_entry *a,
                                         struct cache_entry *b,
                                         struct cache_entry *c)
 {
-       int len = ce_namelen(a);
-
-       /*
-        * Are they all the same filename? We won't do
-        * any name merging
-        */
-       if (ce_namelen(b) != len ||
-           ce_namelen(c) != len ||
-           memcmp(a->name, b->name, len) ||
-           memcmp(a->name, c->name, len))
-               return NULL;
-
        /*
         * Ok, all three entries describe the same
         * filename, but maybe the contents or file
@@ -77,12 +61,14 @@ static struct cache_entry *merge_entries(struct cache_entry *a,
         * Here "a" is "original", and "b" and "c" are the two
         * trees we are merging.
         */
-       if (same(b,c))
-               return c;
-       if (same(a,b))
-               return c;
-       if (same(a,c))
-               return b;
+       if (a && b && c) {
+               if (same(b,c))
+                       return c;
+               if (same(a,b))
+                       return c;
+               if (same(a,c))
+                       return b;
+       }
        return NULL;
 }
 
@@ -116,88 +102,54 @@ static void reject_merge(struct cache_entry *ce)
        die("Entry '%s' would be overwritten by merge. Cannot merge.", ce->name);
 }
 
-#define CHECK_OLD(ce) if (old && same(old, ce)) { verify_uptodate(old); old = NULL; }
-
-static void trivially_merge_cache(struct cache_entry **src, int nr)
+static int merged_entry(struct cache_entry *merge, struct cache_entry *old, struct cache_entry **dst)
 {
-       struct cache_entry **dst = src;
-       struct cache_entry *old = NULL;
-
-       while (nr--) {
-               struct cache_entry *ce, *result;
-
-               ce = *src++;
-
-               /* We throw away original cache entries except for the stat information */
-               if (!ce_stage(ce)) {
-                       if (old)
-                               reject_merge(old);
-                       old = ce;
-                       active_nr--;
-                       continue;
-               }
-               if (old && !path_matches(old, ce))
-                       reject_merge(old);
-               if (nr > 1 && (result = merge_entries(ce, src[0], src[1])) != NULL) {
-                       result->ce_flags |= htons(CE_UPDATE);
-                       /*
-                        * See if we can re-use the old CE directly?
-                        * That way we get the uptodate stat info.
-                        *
-                        * This also removes the UPDATE flag on
-                        * a match.
-                        */
-                       if (old && same(old, result)) {
-                               *result = *old;
-                               old = NULL;
-                       }
-                       CHECK_OLD(ce);
-                       CHECK_OLD(src[0]);
-                       CHECK_OLD(src[1]);
-                       ce = result;
-                       ce->ce_flags &= ~htons(CE_STAGEMASK);
-                       src += 2;
-                       nr -= 2;
-                       active_nr -= 2;
-               }
-
+       merge->ce_flags |= htons(CE_UPDATE);
+       if (old) {
                /*
-                * If we had an old entry that we now effectively
-                * overwrite, make sure it wasn't dirty.
+                * See if we can re-use the old CE directly?
+                * That way we get the uptodate stat info.
+                *
+                * This also removes the UPDATE flag on
+                * a match.
                 */
-               CHECK_OLD(ce);
-               *dst++ = ce;
+               if (same(old, merge)) {
+                       *merge = *old;
+               } else {
+                       verify_uptodate(old);
+               }
        }
-       if (old)
-               reject_merge(old);
+       merge->ce_flags &= ~htons(CE_STAGEMASK);
+       *dst++ = merge;
+       return 1;
 }
 
-/*
- * When we find a "stage2" entry in the two-way merge, that's
- * the one that will remain. If we have an exact old match,
- * we don't care whether the file is up-to-date or not, we just
- * re-use the thing directly.
- *
- * If we didn't have an exact match, then we want to make sure
- * that we've seen a stage1 that matched the old, and that the
- * old file was up-to-date. Because it will be gone after this
- * merge..
- */
-static void twoway_check(struct cache_entry *old, int seen_stage1, struct cache_entry *ce)
+static int threeway_merge(struct cache_entry *stages[4], struct cache_entry **dst)
 {
-       if (path_matches(old, ce)) {
-               /*
-                * This also removes the UPDATE flag on
-                * a match
-                */
-               if (same(old, ce)) {
-                       *ce = *old;
-                       return;
-               }
-               if (!seen_stage1)
-                       reject_merge(old);
+       struct cache_entry *old = stages[0];
+       struct cache_entry *a = stages[1], *b = stages[2], *c = stages[3];
+       struct cache_entry *merge;
+       int count;
+
+       /*
+        * If we have an entry in the index cache ("old"), then we want
+        * to make sure that it matches any entries in stage 2 ("first
+        * branch", aka "b").
+        */
+       if (old) {
+               if (!b || !same(old, b))
+                       return -1;
        }
-       verify_uptodate(old);
+       merge = merge_entries(a, b, c);
+       if (merge)
+               return merged_entry(merge, old, dst);
+       if (old)
+               verify_uptodate(old);
+       count = 0;
+       if (a) { *dst++ = a; count++; }
+       if (b) { *dst++ = b; count++; }
+       if (c) { *dst++ = c; count++; }
+       return count;
 }
 
 /*
@@ -208,79 +160,46 @@ static void twoway_check(struct cache_entry *old, int seen_stage1, struct cache_
  *  - if the current entry matches the new tree, we leave it
  *    as-is. Otherwise we require that it be up-to-date.
  */
-static void twoway_merge(struct cache_entry **src, int nr)
+static int twoway_merge(struct cache_entry **src, struct cache_entry **dst)
 {
-       int seen_stage1 = 0;
-       struct cache_entry *old = NULL;
-       struct cache_entry **dst = src;
-
-       while (nr--) {
-               struct cache_entry *ce = *src++;
-               int stage = ce_stage(ce);
+       struct cache_entry *old = src[0];
+       struct cache_entry *a = src[1], *b = src[2];
 
-               switch (stage) {
-               case 0:
-                       if (old)
-                               reject_merge(old);
-                       old = ce;
-                       seen_stage1 = 0;
-                       active_nr--;
-                       continue;
-
-               case 1:
-                       active_nr--;
-                       if (!old)
-                               continue;
-                       if (!path_matches(old, ce) || !same(old, ce))
-                               reject_merge(old);
-                       seen_stage1 = 1;
-                       continue;
-
-               case 2:
-                       ce->ce_flags |= htons(CE_UPDATE);
-                       if (old) {
-                               twoway_check(old, seen_stage1, ce);
-                               old = NULL;
-                       }
-                       ce->ce_flags &= ~htons(CE_STAGEMASK);
-                       *dst++ = ce;
-                       continue;
-               }
-               die("impossible two-way stage");
-       }
+       if (src[3])
+               return -1;
 
-       /*
-        * Unmatched with a new entry? Make sure it was
-        * at least uptodate in the working directory _and_
-        * the original tree..
-        */
        if (old) {
-               if (!seen_stage1)
-                       reject_merge(old);
-               verify_uptodate(old);
+               if (!a || !same(old, a))
+                       return -1;
        }
+       if (b)
+               return merged_entry(b, old, dst);
+       if (old)
+               verify_uptodate(old);
+       return 0;
 }
 
-static void merge_stat_info(struct cache_entry **src, int nr)
+/*
+ * One-way merge.
+ *
+ * The rule is:
+ * - take the stat information from stage0, take the data from stage1
+ */
+static int oneway_merge(struct cache_entry **src, struct cache_entry **dst)
 {
-       static struct cache_entry null_entry;
-       struct cache_entry **dst = src;
-       struct cache_entry *stat = &null_entry;
+       struct cache_entry *old = src[0];
+       struct cache_entry *a = src[1];
 
-       while (nr--) {
-               struct cache_entry *ce = *src++;
+       if (src[2] || src[3])
+               return -1;
 
-               /* We throw away original cache entries except for the stat information */
-               if (!ce_stage(ce)) {
-                       stat = ce;
-                       active_nr--;
-                       continue;
-               }
-               if (path_matches(ce, stat) && same(ce, stat))
-                       *ce = *stat;
-               ce->ce_flags &= ~htons(CE_STAGEMASK);
-               *dst++ = ce;
-       }
+       if (!a)
+               return 0;
+       if (old && same(old, a))
+               *a = *old;
+       a->ce_flags &= ~htons(CE_STAGEMASK);
+       *dst++ = a;
+       return 1;
 }
 
 static void check_updates(struct cache_entry **src, int nr)
@@ -302,6 +221,35 @@ static void check_updates(struct cache_entry **src, int nr)
        }
 }
 
+static void merge_cache(struct cache_entry **src, int nr, int (*fn)(struct cache_entry **, struct cache_entry **))
+{
+       struct cache_entry **dst = src;
+
+       while (nr) {
+               int entries;
+               struct cache_entry *name, *ce, *stages[4] = { NULL, };
+
+               name = ce = *src;
+               for (;;) {
+                       int stage = ce_stage(ce);
+                       stages[stage] = ce;
+                       ce = *++src;
+                       active_nr--;
+                       if (!--nr)
+                               break;
+                       if (!path_matches(ce, name))
+                               break;
+               }
+
+               entries = fn(stages, dst);
+               if (entries < 0)
+                       reject_merge(name);
+               dst += entries;
+               active_nr += entries;
+       }
+       check_updates(active_cache, active_nr);
+}
+
 static char *read_tree_usage = "git-read-tree (<sha> | -m <sha1> [<sha2> [<sha3>]])";
 
 static struct cache_file cache_file;
@@ -350,15 +298,14 @@ int main(int argc, char **argv)
        if (merge) {
                switch (stage) {
                case 4: /* Three-way merge */
-                       trivially_merge_cache(active_cache, active_nr);
-                       check_updates(active_cache, active_nr);
+                       merge_cache(active_cache, active_nr, threeway_merge);
                        break;
                case 3: /* Update from one tree to another */
-                       twoway_merge(active_cache, active_nr);
+                       merge_cache(active_cache, active_nr, twoway_merge);
                        check_updates(active_cache, active_nr);
                        break;
                case 2: /* Just read a tree, merge with old cache contents */
-                       merge_stat_info(active_cache, active_nr);
+                       merge_cache(active_cache, active_nr, oneway_merge);
                        break;
                default:
                        die("just how do you expect me to merge %d trees?", stage-1);