epoch.c

   1 /*
   2  * Copyright (c) 2005, Jon Seymour
   3  *
   4  * For more information about epoch theory on which this module is based,
   5  * refer to http://blackcubes.dyndns.org/epoch/. That web page defines
   6  * terms such as "epoch" and "minimal, non-linear epoch" and provides rationales
   7  * for some of the algorithms used here.
   8  *
   9  */
  10 #include <stdlib.h>
  11
  12 /* Provides arbitrary precision integers required to accurately represent
  13  * fractional mass: */
  14 #include <openssl/bn.h>
  15
  16 #include "cache.h"
  17 #include "commit.h"
  18 #include "epoch.h"
  19
  20 struct fraction {
  21         BIGNUM numerator;
  22         BIGNUM denominator;
  23 };
  24
  25 #define HAS_EXACTLY_ONE_PARENT(n) ((n)->parents && !(n)->parents->next)
  26
  27 static BN_CTX *context = NULL;
  28 static struct fraction *one = NULL;
  29 static struct fraction *zero = NULL;
  30
  31 static BN_CTX *get_BN_CTX(void)
  32 {
  33         if (!context) {
  34                 context = BN_CTX_new();
  35         }
  36         return context;
  37 }
  38
  39 static struct fraction *new_zero(void)
  40 {
  41         struct fraction *result = xmalloc(sizeof(*result));
  42         BN_init(&result->numerator);
  43         BN_init(&result->denominator);
  44         BN_zero(&result->numerator);
  45         BN_one(&result->denominator);
  46         return result;
  47 }
  48
  49 static void clear_fraction(struct fraction *fraction)
  50 {
  51         BN_clear(&fraction->numerator);
  52         BN_clear(&fraction->denominator);
  53 }
  54
  55 static struct fraction *divide(struct fraction *result, struct fraction *fraction, int divisor)
  56 {
  57         BIGNUM bn_divisor;
  58
  59         BN_init(&bn_divisor);
  60         BN_set_word(&bn_divisor, divisor);
  61
  62         BN_copy(&result->numerator, &fraction->numerator);
  63         BN_mul(&result->denominator, &fraction->denominator, &bn_divisor, get_BN_CTX());
  64
  65         BN_clear(&bn_divisor);
  66         return result;
  67 }
  68
  69 static struct fraction *init_fraction(struct fraction *fraction)
  70 {
  71         BN_init(&fraction->numerator);
  72         BN_init(&fraction->denominator);
  73         BN_zero(&fraction->numerator);
  74         BN_one(&fraction->denominator);
  75         return fraction;
  76 }
  77
  78 static struct fraction *get_one(void)
  79 {
  80         if (!one) {
  81                 one = new_zero();
  82                 BN_one(&one->numerator);
  83         }
  84         return one;
  85 }
  86
  87 static struct fraction *get_zero(void)
  88 {
  89         if (!zero) {
  90                 zero = new_zero();
  91         }
  92         return zero;
  93 }
  94
  95 static struct fraction *copy(struct fraction *to, struct fraction *from)
  96 {
  97         BN_copy(&to->numerator, &from->numerator);
  98         BN_copy(&to->denominator, &from->denominator);
  99         return to;
 100 }
 101
 102 static struct fraction *add(struct fraction *result, struct fraction *left, struct fraction *right)
 103 {
 104         BIGNUM a, b, gcd;
 105
 106         BN_init(&a);
 107         BN_init(&b);
 108         BN_init(&gcd);
 109
 110         BN_mul(&a, &left->numerator, &right->denominator, get_BN_CTX());
 111         BN_mul(&b, &left->denominator, &right->numerator, get_BN_CTX());
 112         BN_mul(&result->denominator, &left->denominator, &right->denominator, get_BN_CTX());
 113         BN_add(&result->numerator, &a, &b);
 114
 115         BN_gcd(&gcd, &result->denominator, &result->numerator, get_BN_CTX());
 116         BN_div(&result->denominator, NULL, &result->denominator, &gcd, get_BN_CTX());
 117         BN_div(&result->numerator, NULL, &result->numerator, &gcd, get_BN_CTX());
 118
 119         BN_clear(&a);
 120         BN_clear(&b);
 121         BN_clear(&gcd);
 122
 123         return result;
 124 }
 125
 126 static int compare(struct fraction *left, struct fraction *right)
 127 {
 128         BIGNUM a, b;
 129         int result;
 130
 131         BN_init(&a);
 132         BN_init(&b);
 133
 134         BN_mul(&a, &left->numerator, &right->denominator, get_BN_CTX());
 135         BN_mul(&b, &left->denominator, &right->numerator, get_BN_CTX());
 136
 137         result = BN_cmp(&a, &b);
 138
 139         BN_clear(&a);
 140         BN_clear(&b);
 141
 142         return result;
 143 }
 144
 145 struct mass_counter {
 146         struct fraction seen;
 147         struct fraction pending;
 148 };
 149
 150 static struct mass_counter *new_mass_counter(struct commit *commit, struct fraction *pending)
 151 {
 152         struct mass_counter *mass_counter = xmalloc(sizeof(*mass_counter));
 153         memset(mass_counter, 0, sizeof(*mass_counter));
 154
 155         init_fraction(&mass_counter->seen);
 156         init_fraction(&mass_counter->pending);
 157
 158         copy(&mass_counter->pending, pending);
 159         copy(&mass_counter->seen, get_zero());
 160
 161         if (commit->object.util) {
 162                 die("multiple attempts to initialize mass counter for %s",
 163                     sha1_to_hex(commit->object.sha1));
 164         }
 165
 166         commit->object.util = mass_counter;
 167
 168         return mass_counter;
 169 }
 170
 171 static void free_mass_counter(struct mass_counter *counter)
 172 {
 173         clear_fraction(&counter->seen);
 174         clear_fraction(&counter->pending);
 175         free(counter);
 176 }
 177
 178 /*
 179  * Finds the base commit of a list of commits.
 180  *
 181  * One property of the commit being searched for is that every commit reachable
 182  * from the base commit is reachable from the commits in the starting list only
 183  * via paths that include the base commit.
 184  *
 185  * This algorithm uses a conservation of mass approach to find the base commit.
 186  *
 187  * We start by injecting one unit of mass into the graph at each
 188  * of the commits in the starting list. Injecting mass into a commit
 189  * is achieved by adding to its pending mass counter and, if it is not already
 190  * enqueued, enqueuing the commit in a list of pending commits, in latest
 191  * commit date first order.
 192  *
 193  * The algorithm then proceeds to visit each commit in the pending queue.
 194  * Upon each visit, the pending mass is added to the mass already seen for that
 195  * commit and then divided into N equal portions, where N is the number of
 196  * parents of the commit being visited. The divided portions are then injected
 197  * into each of the parents.
 198  *
 199  * The algorithm continues until we discover a commit which has seen all the
 200  * mass originally injected or until we run out of things to do.
 201  *
 202  * If we find a commit that has seen all the original mass, we have found
 203  * the common base of all the commits in the starting list.
 204  *
 205  * The algorithm does _not_ depend on accurate timestamps for correct operation.
 206  * However, reasonably sane (e.g. non-random) timestamps are required in order
 207  * to prevent an exponential performance characteristic. The occasional
 208  * timestamp inaccuracy will not dramatically affect performance but may
 209  * result in more nodes being processed than strictly necessary.
 210  *
 211  * This procedure sets *boundary to the address of the base commit. It returns
 212  * non-zero if, and only if, there was a problem parsing one of the
 213  * commits discovered during the traversal.
 214  */
 215 static int find_base_for_list(struct commit_list *list, struct commit **boundary)
 216 {
 217         int ret = 0;
 218         struct commit_list *cleaner = NULL;
 219         struct commit_list *pending = NULL;
 220         struct fraction injected;
 221         init_fraction(&injected);
 222         *boundary = NULL;
 223
 224         for (; list; list = list->next) {
 225                 struct commit *item = list->item;
 226
 227                 if (!item->object.util) {
 228                         new_mass_counter(list->item, get_one());
 229                         add(&injected, &injected, get_one());
 230
 231                         commit_list_insert(list->item, &cleaner);
 232                         commit_list_insert(list->item, &pending);
 233                 }
 234         }
 235
 236         while (!*boundary && pending && !ret) {
 237                 struct commit *latest = pop_commit(&pending);
 238                 struct mass_counter *latest_node = (struct mass_counter *) latest->object.util;
 239                 int num_parents;
 240
 241                 if ((ret = parse_commit(latest)))
 242                         continue;
 243                 add(&latest_node->seen, &latest_node->seen, &latest_node->pending);
 244
 245                 num_parents = count_parents(latest);
 246                 if (num_parents) {
 247                         struct fraction distribution;
 248                         struct commit_list *parents;
 249
 250                         divide(init_fraction(&distribution), &latest_node->pending, num_parents);
 251
 252                         for (parents = latest->parents; parents; parents = parents->next) {
 253                                 struct commit *parent = parents->item;
 254                                 struct mass_counter *parent_node = (struct mass_counter *) parent->object.util;
 255
 256                                 if (!parent_node) {
 257                                         parent_node = new_mass_counter(parent, &distribution);
 258                                         insert_by_date(parent, &pending);
 259                                         commit_list_insert(parent, &cleaner);
 260                                 } else {
 261                                         if (!compare(&parent_node->pending, get_zero()))
 262                                                 insert_by_date(parent, &pending);
 263                                         add(&parent_node->pending, &parent_node->pending, &distribution);
 264                                 }
 265                         }
 266
 267                         clear_fraction(&distribution);
 268                 }
 269
 270                 if (!compare(&latest_node->seen, &injected))
 271                         *boundary = latest;
 272                 copy(&latest_node->pending, get_zero());
 273         }
 274
 275         while (cleaner) {
 276                 struct commit *next = pop_commit(&cleaner);
 277                 free_mass_counter((struct mass_counter *) next->object.util);
 278                 next->object.util = NULL;
 279         }
 280
 281         if (pending)
 282                 free_commit_list(pending);
 283
 284         clear_fraction(&injected);
 285         return ret;
 286 }
 287
 288
 289 /*
 290  * Finds the base of an minimal, non-linear epoch, headed at head, by
 291  * applying the find_base_for_list to a list consisting of the parents
 292  */
 293 static int find_base(struct commit *head, struct commit **boundary)
 294 {
 295         int ret = 0;
 296         struct commit_list *pending = NULL;
 297         struct commit_list *next;
 298
 299         for (next = head->parents; next; next = next->next) {
 300                 commit_list_insert(next->item, &pending);
 301         }
 302         ret = find_base_for_list(pending, boundary);
 303         free_commit_list(pending);
 304
 305         return ret;
 306 }
 307
 308 /*
 309  * This procedure traverses to the boundary of the first epoch in the epoch
 310  * sequence of the epoch headed at head_of_epoch. This is either the end of
 311  * the maximal linear epoch or the base of a minimal non-linear epoch.
 312  *
 313  * The queue of pending nodes is sorted in reverse date order and each node
 314  * is currently in the queue at most once.
 315  */
 316 static int find_next_epoch_boundary(struct commit *head_of_epoch, struct commit **boundary)
 317 {
 318         int ret;
 319         struct commit *item = head_of_epoch;
 320
 321         ret = parse_commit(item);
 322         if (ret)
 323                 return ret;
 324
 325         if (HAS_EXACTLY_ONE_PARENT(item)) {
 326                 /*
 327                  * We are at the start of a maximimal linear epoch.
 328                  * Traverse to the end.
 329                  */
 330                 while (HAS_EXACTLY_ONE_PARENT(item) && !ret) {
 331                         item = item->parents->item;
 332                         ret = parse_commit(item);
 333                 }
 334                 *boundary = item;
 335
 336         } else {
 337                 /*
 338                  * Otherwise, we are at the start of a minimal, non-linear
 339                  * epoch - find the common base of all parents.
 340                  */
 341                 ret = find_base(item, boundary);
 342         }
 343
 344         return ret;
 345 }
 346
 347 /*
 348  * Returns non-zero if parent is known to be a parent of child.
 349  */
 350 static int is_parent_of(struct commit *parent, struct commit *child)
 351 {
 352         struct commit_list *parents;
 353         for (parents = child->parents; parents; parents = parents->next) {
 354                 if (!memcmp(parent->object.sha1, parents->item->object.sha1,
 355                             sizeof(parents->item->object.sha1)))
 356                         return 1;
 357         }
 358         return 0;
 359 }
 360
 361 /*
 362  * Pushes an item onto the merge order stack. If the top of the stack is
 363  * marked as being a possible "break", we check to see whether it actually
 364  * is a break.
 365  */
 366 static void push_onto_merge_order_stack(struct commit_list **stack, struct commit *item)
 367 {
 368         struct commit_list *top = *stack;
 369         if (top && (top->item->object.flags & DISCONTINUITY)) {
 370                 if (is_parent_of(top->item, item)) {
 371                         top->item->object.flags &= ~DISCONTINUITY;
 372                 }
 373         }
 374         commit_list_insert(item, stack);
 375 }
 376
 377 /*
 378  * Marks all interesting, visited commits reachable from this commit
 379  * as uninteresting. We stop recursing when we reach the epoch boundary,
 380  * an unvisited node or a node that has already been marking uninteresting.
 381  *
 382  * This doesn't actually mark all ancestors between the start node and the
 383  * epoch boundary uninteresting, but does ensure that they will eventually
 384  * be marked uninteresting when the main sort_first_epoch() traversal
 385  * eventually reaches them.
 386  */
 387 static void mark_ancestors_uninteresting(struct commit *commit)
 388 {
 389         unsigned int flags = commit->object.flags;
 390         int visited = flags & VISITED;
 391         int boundary = flags & BOUNDARY;
 392         int uninteresting = flags & UNINTERESTING;
 393         struct commit_list *next;
 394
 395         commit->object.flags |= UNINTERESTING;
 396
 397         /*
 398          * We only need to recurse if
 399          *      we are not on the boundary and
 400          *      we have not already been marked uninteresting and
 401          *      we have already been visited.
 402          *
 403          * The main sort_first_epoch traverse will mark unreachable
 404          * all uninteresting, unvisited parents as they are visited
 405          * so there is no need to duplicate that traversal here.
 406          *
 407          * Similarly, if we are already marked uninteresting
 408          * then either all ancestors have already been marked
 409          * uninteresting or will be once the sort_first_epoch
 410          * traverse reaches them.
 411          */
 412
 413         if (uninteresting || boundary || !visited)
 414                 return;
 415
 416         for (next = commit->parents; next; next = next->next)
 417                 mark_ancestors_uninteresting(next->item);
 418 }
 419
 420 /*
 421  * Sorts the nodes of the first epoch of the epoch sequence of the epoch headed at head
 422  * into merge order.
 423  */
 424 static void sort_first_epoch(struct commit *head, struct commit_list **stack)
 425 {
 426         struct commit_list *parents;
 427
 428         head->object.flags |= VISITED;
 429
 430         /*
 431          * TODO: By sorting the parents in a different order, we can alter the
 432          * merge order to show contemporaneous changes in parallel branches
 433          * occurring after "local" changes. This is useful for a developer
 434          * when a developer wants to see all changes that were incorporated
 435          * into the same merge as her own changes occur after her own
 436          * changes.
 437          */
 438
 439         for (parents = head->parents; parents; parents = parents->next) {
 440                 struct commit *parent = parents->item;
 441
 442                 if (head->object.flags & UNINTERESTING) {
 443                         /*
 444                          * Propagates the uninteresting bit to all parents.
 445                          * if we have already visited this parent, then
 446                          * the uninteresting bit will be propagated to each
 447                          * reachable commit that is still not marked
 448                          * uninteresting and won't otherwise be reached.
 449                          */
 450                         mark_ancestors_uninteresting(parent);
 451                 }
 452
 453                 if (!(parent->object.flags & VISITED)) {
 454                         if (parent->object.flags & BOUNDARY) {
 455                                 if (*stack) {
 456                                         die("something else is on the stack - %s",
 457                                             sha1_to_hex((*stack)->item->object.sha1));
 458                                 }
 459                                 push_onto_merge_order_stack(stack, parent);
 460                                 parent->object.flags |= VISITED;
 461
 462                         } else {
 463                                 sort_first_epoch(parent, stack);
 464                                 if (parents) {
 465                                         /*
 466                                          * This indicates a possible
 467                                          * discontinuity it may not be be
 468                                          * actual discontinuity if the head
 469                                          * of parent N happens to be the tail
 470                                          * of parent N+1.
 471                                          *
 472                                          * The next push onto the stack will
 473                                          * resolve the question.
 474                                          */
 475                                         (*stack)->item->object.flags |= DISCONTINUITY;
 476                                 }
 477                         }
 478                 }
 479         }
 480
 481         push_onto_merge_order_stack(stack, head);
 482 }
 483
 484 /*
 485  * Emit the contents of the stack.
 486  *
 487  * The stack is freed and replaced by NULL.
 488  *
 489  * Sets the return value to STOP if no further output should be generated.
 490  */
 491 static int emit_stack(struct commit_list **stack, emitter_func emitter, int include_last)
 492 {
 493         unsigned int seen = 0;
 494         int action = CONTINUE;
 495
 496         while (*stack && (action != STOP)) {
 497                 struct commit *next = pop_commit(stack);
 498                 seen |= next->object.flags;
 499                 if (*stack || include_last) {
 500                         if (!*stack)
 501                                 next->object.flags |= BOUNDARY;
 502                         action = emitter(next);
 503                 }
 504         }
 505
 506         if (*stack) {
 507                 free_commit_list(*stack);
 508                 *stack = NULL;
 509         }
 510
 511         return (action == STOP || (seen & UNINTERESTING)) ? STOP : CONTINUE;
 512 }
 513
 514 /*
 515  * Sorts an arbitrary epoch into merge order by sorting each epoch
 516  * of its epoch sequence into order.
 517  *
 518  * Note: this algorithm currently leaves traces of its execution in the
 519  * object flags of nodes it discovers. This should probably be fixed.
 520  */
 521 static int sort_in_merge_order(struct commit *head_of_epoch, emitter_func emitter)
 522 {
 523         struct commit *next = head_of_epoch;
 524         int ret = 0;
 525         int action = CONTINUE;
 526
 527         ret = parse_commit(head_of_epoch);
 528
 529         next->object.flags |= BOUNDARY;
 530
 531         while (next && next->parents && !ret && (action != STOP)) {
 532                 struct commit *base = NULL;
 533
 534                 ret = find_next_epoch_boundary(next, &base);
 535                 if (ret)
 536                         return ret;
 537                 next->object.flags |= BOUNDARY;
 538                 if (base)
 539                         base->object.flags |= BOUNDARY;
 540
 541                 if (HAS_EXACTLY_ONE_PARENT(next)) {
 542                         while (HAS_EXACTLY_ONE_PARENT(next)
 543                                && (action != STOP)
 544                                && !ret) {
 545                                 if (next->object.flags & UNINTERESTING) {
 546                                         action = STOP;
 547                                 } else {
 548                                         action = emitter(next);
 549                                 }
 550                                 if (action != STOP) {
 551                                         next = next->parents->item;
 552                                         ret = parse_commit(next);
 553                                 }
 554                         }
 555
 556                 } else {
 557                         struct commit_list *stack = NULL;
 558                         sort_first_epoch(next, &stack);
 559                         action = emit_stack(&stack, emitter, (base == NULL));
 560                         next = base;
 561                 }
 562         }
 563
 564         if (next && (action != STOP) && !ret) {
 565                 emitter(next);
 566         }
 567
 568         return ret;
 569 }
 570
 571 /*
 572  * Sorts the nodes reachable from a starting list in merge order, we
 573  * first find the base for the starting list and then sort all nodes
 574  * in this subgraph using the sort_first_epoch algorithm. Once we have
 575  * reached the base we can continue sorting using sort_in_merge_order.
 576  */
 577 int sort_list_in_merge_order(struct commit_list *list, emitter_func emitter)
 578 {
 579         struct commit_list *stack = NULL;
 580         struct commit *base;
 581         int ret = 0;
 582         int action = CONTINUE;
 583         struct commit_list *reversed = NULL;
 584
 585         for (; list; list = list->next)
 586                 commit_list_insert(list->item, &reversed);
 587
 588         if (!reversed)
 589                 return ret;
 590         else if (!reversed->next) {
 591                 /*
 592                  * If there is only one element in the list, we can sort it
 593                  * using sort_in_merge_order.
 594                  */
 595                 base = reversed->item;
 596         } else {
 597                 /*
 598                  * Otherwise, we search for the base of the list.
 599                  */
 600                 ret = find_base_for_list(reversed, &base);
 601                 if (ret)
 602                         return ret;
 603                 if (base)
 604                         base->object.flags |= BOUNDARY;
 605
 606                 while (reversed) {
 607                         struct commit * next = pop_commit(&reversed);
 608
 609                         if (!(next->object.flags & VISITED) && next!=base) {
 610                                 sort_first_epoch(next, &stack);
 611                                 if (reversed) {
 612                                         /*
 613                                          * If we have more commits
 614                                          * to push, then the first
 615                                          * push for the next parent may
 616                                          * (or may * not) represent a
 617                                          * discontinuity with respect
 618                                          * to the parent currently on
 619                                          * the top of the stack.
 620                                          *
 621                                          * Mark it for checking here,
 622                                          * and check it with the next
 623                                          * push. See sort_first_epoch()
 624                                          * for more details.
 625                                          */
 626                                         stack->item->object.flags |= DISCONTINUITY;
 627                                 }
 628                         }
 629                 }
 630
 631                 action = emit_stack(&stack, emitter, (base==NULL));
 632         }
 633
 634         if (base && (action != STOP)) {
 635                 ret = sort_in_merge_order(base, emitter);
 636         }
 637
 638         return ret;
 639 }