merge-base.c

   1 #include <stdlib.h>
   2 #include "cache.h"
   3 #include "commit.h"
   4
   5 #define PARENT1 1
   6 #define PARENT2 2
   7 #define UNINTERESTING 4
   8
   9 static struct commit *interesting(struct commit_list *list)
  10 {
  11         while (list) {
  12                 struct commit *commit = list->item;
  13                 list = list->next;
  14                 if (commit->object.flags & UNINTERESTING)
  15                         continue;
  16                 return commit;
  17         }
  18         return NULL;
  19 }
  20
  21 /*
  22  * A pathological example of how this thing works.
  23  *
  24  * Suppose we had this commit graph, where chronologically
  25  * the timestamp on the commit are A <= B <= C <= D <= E <= F
  26  * and we are trying to figure out the merge base for E and F
  27  * commits.
  28  *
  29  *                  F
  30  *                 / \
  31  *            E   A   D
  32  *             \ /   /
  33  *              B   /
  34  *               \ /
  35  *                C
  36  *
  37  * First we push E and F to list to be processed.  E gets bit 1
  38  * and F gets bit 2.  The list becomes:
  39  *
  40  *     list=F(2) E(1), result=empty
  41  *
  42  * Then we pop F, the newest commit, from the list.  Its flag is 2.
  43  * We scan its parents, mark them reachable from the side that F is
  44  * reachable from, and push them to the list:
  45  *
  46  *     list=E(1) D(2) A(2), result=empty
  47  *
  48  * Next pop E and do the same.
  49  *
  50  *     list=D(2) B(1) A(2), result=empty
  51  *
  52  * Next pop D and do the same.
  53  *
  54  *     list=C(2) B(1) A(2), result=empty
  55  *
  56  * Next pop C and do the same.
  57  *
  58  *     list=B(1) A(2), result=empty
  59  *
  60  * Now it is B's turn.  We mark its parent, C, reachable from B's side,
  61  * and push it to the list:
  62  *
  63  *     list=C(3) A(2), result=empty
  64  *
  65  * Now pop C and notice it has flags==3.  It is placed on the result list,
  66  * and the list now contains:
  67  *
  68  *     list=A(2), result=C(3)
  69  *
  70  * We pop A and do the same.
  71  *
  72  *     list=B(3), result=C(3)
  73  *
  74  * Next, we pop B and something very interesting happens.  It has flags==3
  75  * so it is also placed on the result list, and its parents are marked
  76  * uninteresting, retroactively, and placed back on the list:
  77  *
  78  *    list=C(7), result=C(7) B(3)
  79  *
  80  * Now, list does not have any interesting commit.  So we find the newest
  81  * commit from the result list that is not marked uninteresting.  Which is
  82  * commit B.
  83  *
  84  *
  85  * Another pathological example how this thing can fail to mark an ancestor
  86  * of a merge base as UNINTERESTING without the postprocessing phase.
  87  *
  88  *                2
  89  *                H
  90  *          1    / \
  91  *          G   A   \
  92  *          |\ /     \
  93  *          | B       \
  94  *          |  \       \
  95  *           \  C       F
  96  *            \  \     /
  97  *             \  D   /
  98  *              \ |  /
  99  *               \| /
 100  *                E
 101  *
 102  *       list                   A B C D E F G H
 103  *       G1 H2                  - - - - - - 1 2
 104  *       H2 E1 B1               - 1 - - 1 - 1 2
 105  *       F2 E1 B1 A2            2 1 - - 1 2 1 2
 106  *       E3 B1 A2               2 1 - - 3 2 1 2
 107  *       B1 A2                  2 1 - - 3 2 1 2
 108  *       C1 A2                  2 1 1 - 3 2 1 2
 109  *       D1 A2                  2 1 1 1 3 2 1 2
 110  *       A2                     2 1 1 1 3 2 1 2
 111  *       B3                     2 3 1 1 3 2 1 2
 112  *       C7                     2 3 7 1 3 2 1 2
 113  *
 114  * At this point, unfortunately, everybody in the list is
 115  * uninteresting, so we fail to complete the following two
 116  * steps to fully marking uninteresting commits.
 117  *
 118  *       D7                     2 3 7 7 3 2 1 2
 119  *       E7                     2 3 7 7 7 2 1 2
 120  *
 121  * and we end up showing E as an interesting merge base.
 122  */
 123
 124 static int show_all = 0;
 125
 126 static void mark_reachable_commits(struct commit_list *result,
 127                                    struct commit_list *list)
 128 {
 129         struct commit_list *tmp;
 130
 131         /*
 132          * Postprocess to fully contaminate the well.
 133          */
 134         for (tmp = result; tmp; tmp = tmp->next) {
 135                 struct commit *c = tmp->item;
 136                 /* Reinject uninteresting ones to list,
 137                  * so we can scan their parents.
 138                  */
 139                 if (c->object.flags & UNINTERESTING)
 140                         commit_list_insert(c, &list);
 141         }
 142         while (list) {
 143                 struct commit *c = list->item;
 144                 struct commit_list *parents;
 145
 146                 tmp = list;
 147                 list = list->next;
 148                 free(tmp);
 149
 150                 /* Anything taken out of the list is uninteresting, so
 151                  * mark all its parents uninteresting.  We do not
 152                  * parse new ones (we already parsed all the relevant
 153                  * ones).
 154                  */
 155                 parents = c->parents;
 156                 while (parents) {
 157                         struct commit *p = parents->item;
 158                         parents = parents->next;
 159                         if (!(p->object.flags & UNINTERESTING)) {
 160                                 p->object.flags |= UNINTERESTING;
 161                                 commit_list_insert(p, &list);
 162                         }
 163                 }
 164         }
 165 }
 166
 167 static int merge_base(struct commit *rev1, struct commit *rev2)
 168 {
 169         struct commit_list *list = NULL;
 170         struct commit_list *result = NULL;
 171         struct commit_list *tmp = NULL;
 172
 173         if (rev1 == rev2) {
 174                 printf("%s\n", sha1_to_hex(rev1->object.sha1));
 175                 return 0;
 176         }
 177
 178         parse_commit(rev1);
 179         parse_commit(rev2);
 180
 181         rev1->object.flags |= 1;
 182         rev2->object.flags |= 2;
 183         insert_by_date(rev1, &list);
 184         insert_by_date(rev2, &list);
 185
 186         while (interesting(list)) {
 187                 struct commit *commit = list->item;
 188                 struct commit_list *parents;
 189                 int flags = commit->object.flags & 7;
 190
 191                 tmp = list;
 192                 list = list->next;
 193                 free(tmp);
 194                 if (flags == 3) {
 195                         insert_by_date(commit, &result);
 196
 197                         /* Mark parents of a found merge uninteresting */
 198                         flags |= UNINTERESTING;
 199                 }
 200                 parents = commit->parents;
 201                 while (parents) {
 202                         struct commit *p = parents->item;
 203                         parents = parents->next;
 204                         if ((p->object.flags & flags) == flags)
 205                                 continue;
 206                         parse_commit(p);
 207                         p->object.flags |= flags;
 208                         insert_by_date(p, &list);
 209                 }
 210         }
 211
 212         if (!result)
 213                 return 1;
 214
 215         if (result->next && list)
 216                 mark_reachable_commits(result, list);
 217
 218         while (result) {
 219                 struct commit *commit = result->item;
 220                 result = result->next;
 221                 if (commit->object.flags & UNINTERESTING)
 222                         continue;
 223                 printf("%s\n", sha1_to_hex(commit->object.sha1));
 224                 if (!show_all)
 225                         return 0;
 226                 commit->object.flags |= UNINTERESTING;
 227         }
 228         return 0;
 229 }
 230
 231 static const char merge_base_usage[] =
 232 "git-merge-base [--all] <commit-id> <commit-id>";
 233
 234 int main(int argc, char **argv)
 235 {
 236         struct commit *rev1, *rev2;
 237         unsigned char rev1key[20], rev2key[20];
 238
 239         while (1 < argc && argv[1][0] == '-') {
 240                 char *arg = argv[1];
 241                 if (!strcmp(arg, "-a") || !strcmp(arg, "--all"))
 242                         show_all = 1;
 243                 else
 244                         usage(merge_base_usage);
 245                 argc--; argv++;
 246         }
 247         if (argc != 3 ||
 248             get_sha1(argv[1], rev1key) ||
 249             get_sha1(argv[2], rev2key))
 250                 usage(merge_base_usage);
 251         rev1 = lookup_commit_reference(rev1key);
 252         rev2 = lookup_commit_reference(rev2key);
 253         if (!rev1 || !rev2)
 254                 return 1;
 255         return merge_base(rev1, rev2);
 256 }