Re: [PATCH 0/6] Compute and consume generation numbers

[Jeff King]

On Tue, Apr 03, 2018 at 02:29:01PM -0400, Derrick Stolee wrote:

> If we have generic "can X reach Y?" queries, then we can also use generation
> numbers there to great effect (by not walking commits Z with gen(Z) <=
> gen(Y)). Perhaps I should look at that "git branch --contains" thread for
> ideas.

I think the gist of it is the patch below. Which I hastily adapted from
the patch we run at GitHub that uses timestamps as a proxy. So it's
possible I completely flubbed the logic. I'm assuming unavailable
generation numbers are set to 0; the logic is actually a bit simpler if
they end up as (uint32_t)-1.

Assuming it works, that would cover for-each-ref and tag. You'd probably
want to drop the "with_commit_tag_algo" flag in ref-filter.h, and just
use always use it by default (and that would cover "git branch").

---
diff --git a/ref-filter.c b/ref-filter.c
index 45fc56216a..6bea6173d1 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -1584,7 +1584,8 @@ static int in_commit_list(const struct commit_list *want, 
struct commit *c)
  */
 static enum contains_result contains_test(struct commit *candidate,
                                          const struct commit_list *want,
-                                         struct contains_cache *cache)
+                                         struct contains_cache *cache,
+                                         uint32_t cutoff)
 {
        enum contains_result *cached = contains_cache_at(cache, candidate);
 
@@ -1598,8 +1599,11 @@ static enum contains_result contains_test(struct commit 
*candidate,
                return CONTAINS_YES;
        }
 
-       /* Otherwise, we don't know; prepare to recurse */
        parse_commit_or_die(candidate);
+
+       if (candidate->generation && candidate->generation < cutoff)
+               return CONTAINS_NO;
+
        return CONTAINS_UNKNOWN;
 }
 
@@ -1615,8 +1619,20 @@ static enum contains_result contains_tag_algo(struct 
commit *candidate,
                                              struct contains_cache *cache)
 {
        struct contains_stack contains_stack = { 0, 0, NULL };
-       enum contains_result result = contains_test(candidate, want, cache);
+       enum contains_result result;
+       uint32_t cutoff = -1;
+       const struct commit_list *p;
+
+       for (p = want; p; p = p->next) {
+               struct commit *c = p->item;
+               parse_commit_or_die(c);
+               if (c->generation && c->generation < cutoff )
+                       cutoff = c->generation;
+       }
+       if (cutoff == -1)
+               cutoff = 0;
 
+       result = contains_test(candidate, want, cache, cutoff);
        if (result != CONTAINS_UNKNOWN)
                return result;
 
@@ -1634,7 +1650,7 @@ static enum contains_result contains_tag_algo(struct 
commit *candidate,
                 * If we just popped the stack, parents->item has been marked,
                 * therefore contains_test will return a meaningful yes/no.
                 */
-               else switch (contains_test(parents->item, want, cache)) {
+               else switch (contains_test(parents->item, want, cache, cutoff)) 
{
                case CONTAINS_YES:
                        *contains_cache_at(cache, commit) = CONTAINS_YES;
                        contains_stack.nr--;
@@ -1648,7 +1664,7 @@ static enum contains_result contains_tag_algo(struct 
commit *candidate,
                }
        }
        free(contains_stack.contains_stack);
-       return contains_test(candidate, want, cache);
+       return contains_test(candidate, want, cache, cutoff);
 }
 
 static int commit_contains(struct ref_filter *filter, struct commit *commit,