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![Derrick Stolee](/assets/img/avatar_default.png)
The current --topo-order algorithm requires walking all reachable commits up front, topo-sorting them, all before outputting the first value. This patch introduces a new algorithm which uses stored generation numbers to incrementally walk in topo-order, outputting commits as we go. This can dramatically reduce the computation time to write a fixed number of commits, such as when limiting with "-n <N>" or filling the first page of a pager. When running a command like 'git rev-list --topo-order HEAD', Git performed the following steps: 1. Run limit_list(), which parses all reachable commits, adds them to a linked list, and distributes UNINTERESTING flags. If all unprocessed commits are UNINTERESTING, then it may terminate without walking all reachable commits. This does not occur if we do not specify UNINTERESTING commits. 2. Run sort_in_topological_order(), which is an implementation of Kahn's algorithm. It first iterates through the entire set of important commits and computes the in-degree of each (plus one, as we use 'zero' as a special value here). Then, we walk the commits in priority order, adding them to the priority queue if and only if their in-degree is one. As we remove commits from this priority queue, we decrement the in-degree of their parents. 3. While we are peeling commits for output, get_revision_1() uses pop_commit on the full list of commits computed by sort_in_topological_order(). In the new algorithm, these three steps correspond to three different commit walks. We run these walks simultaneously, and advance each only as far as necessary to satisfy the requirements of the 'higher order' walk. We know when we can pause each walk by using generation numbers from the commit- graph feature. Recall that the generation number of a commit satisfies: * If the commit has at least one parent, then the generation number is one more than the maximum generation number among its parents. * If the commit has no parent, then the generation number is one. There are two special generation numbers: * GENERATION_NUMBER_INFINITY: this value is 0xffffffff and indicates that the commit is not stored in the commit-graph and the generation number was not previously calculated. * GENERATION_NUMBER_ZERO: this value (0) is a special indicator to say that the commit-graph was generated by a version of Git that does not compute generation numbers (such as v2.18.0). Since we use generation_numbers_enabled() before using the new algorithm, we do not need to worry about GENERATION_NUMBER_ZERO. However, the existence of GENERATION_NUMBER_INFINITY implies the following weaker statement than the usual we expect from generation numbers: If A and B are commits with generation numbers gen(A) and gen(B) and gen(A) < gen(B), then A cannot reach B. Thus, we will walk in each of our stages until the "maximum unexpanded generation number" is strictly lower than the generation number of a commit we are about to use. The walks are as follows: 1. EXPLORE: using the explore_queue priority queue (ordered by maximizing the generation number), parse each reachable commit until all commits in the queue have generation number strictly lower than needed. During this walk, update the UNINTERESTING flags as necessary. 2. INDEGREE: using the indegree_queue priority queue (ordered by maximizing the generation number), add one to the in- degree of each parent for each commit that is walked. Since we walk in order of decreasing generation number, we know that discovering an in-degree value of 0 means the value for that commit was not initialized, so should be initialized to two. (Recall that in-degree value "1" is what we use to say a commit is ready for output.) As we iterate the parents of a commit during this walk, ensure the EXPLORE walk has walked beyond their generation numbers. 3. TOPO: using the topo_queue priority queue (ordered based on the sort_order given, which could be commit-date, author- date, or typical topo-order which treats the queue as a LIFO stack), remove a commit from the queue and decrement the in-degree of each parent. If a parent has an in-degree of one, then we add it to the topo_queue. Before we decrement the in-degree, however, ensure the INDEGREE walk has walked beyond that generation number. The implementations of these walks are in the following methods: * explore_walk_step and explore_to_depth * indegree_walk_step and compute_indegrees_to_depth * next_topo_commit and expand_topo_walk These methods have some patterns that may seem strange at first, but they are probably carry-overs from their equivalents in limit_list and sort_in_topological_order. One thing that is missing from this implementation is a proper way to stop walking when the entire queue is UNINTERESTING, so this implementation is not enabled by comparisions, such as in 'git rev-list --topo-order A..B'. This can be updated in the future. In my local testing, I used the following Git commands on the Linux repository in three modes: HEAD~1 with no commit-graph, HEAD~1 with a commit-graph, and HEAD with a commit-graph. This allows comparing the benefits we get from parsing commits from the commit-graph and then again the benefits we get by restricting the set of commits we walk. Test: git rev-list --topo-order -100 HEAD HEAD~1, no commit-graph: 6.80 s HEAD~1, w/ commit-graph: 0.77 s HEAD, w/ commit-graph: 0.02 s Test: git rev-list --topo-order -100 HEAD -- tools HEAD~1, no commit-graph: 9.63 s HEAD~1, w/ commit-graph: 6.06 s HEAD, w/ commit-graph: 0.06 s This speedup is due to a few things. First, the new generation- number-enabled algorithm walks commits on order of the number of results output (subject to some branching structure expectations). Since we limit to 100 results, we are running a query similar to filling a single page of results. Second, when specifying a path, we must parse the root tree object for each commit we walk. The previous benefits from the commit-graph are entirely from reading the commit-graph instead of parsing commits. Since we need to parse trees for the same number of commits as before, we slow down significantly from the non-path-based query. For the test above, I specifically selected a path that is changed frequently, including by merge commits. A less-frequently-changed path (such as 'README') has similar end-to-end time since we need to walk the same number of commits (before determining we do not have 100 hits). However, get the benefit that the output is presented to the user as it is discovered, much the same as a normal 'git log' command (no '--topo-order'). This is an improved user experience, even if the command has the same runtime. Helped-by: Jeff King <peff@peff.net> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
195 lines
6.3 KiB
C
195 lines
6.3 KiB
C
#ifndef OBJECT_H
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#define OBJECT_H
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#include "cache.h"
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struct buffer_slab;
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struct parsed_object_pool {
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struct object **obj_hash;
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int nr_objs, obj_hash_size;
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/* TODO: migrate alloc_states to mem-pool? */
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struct alloc_state *blob_state;
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struct alloc_state *tree_state;
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struct alloc_state *commit_state;
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struct alloc_state *tag_state;
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struct alloc_state *object_state;
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unsigned commit_count;
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/* parent substitutions from .git/info/grafts and .git/shallow */
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struct commit_graft **grafts;
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int grafts_alloc, grafts_nr;
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int is_shallow;
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struct stat_validity *shallow_stat;
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char *alternate_shallow_file;
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int commit_graft_prepared;
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struct buffer_slab *buffer_slab;
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};
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struct parsed_object_pool *parsed_object_pool_new(void);
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void parsed_object_pool_clear(struct parsed_object_pool *o);
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struct object_list {
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struct object *item;
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struct object_list *next;
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};
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struct object_array {
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unsigned int nr;
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unsigned int alloc;
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struct object_array_entry {
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struct object *item;
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/*
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* name or NULL. If non-NULL, the memory pointed to
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* is owned by this object *except* if it points at
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* object_array_slopbuf, which is a static copy of the
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* empty string.
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*/
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char *name;
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char *path;
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unsigned mode;
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} *objects;
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};
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#define OBJECT_ARRAY_INIT { 0, 0, NULL }
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/*
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* object flag allocation:
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* revision.h: 0---------10 25----28
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* fetch-pack.c: 01
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* negotiator/default.c: 2--5
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* walker.c: 0-2
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* upload-pack.c: 4 11-----14 16-----19
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* builtin/blame.c: 12-13
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* bisect.c: 16
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* bundle.c: 16
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* http-push.c: 16-----19
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* commit-reach.c: 15-------19
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* sha1-name.c: 20
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* list-objects-filter.c: 21
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* builtin/fsck.c: 0--3
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* builtin/index-pack.c: 2021
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* builtin/pack-objects.c: 20
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* builtin/reflog.c: 10--12
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* builtin/show-branch.c: 0-------------------------------------------26
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* builtin/unpack-objects.c: 2021
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*/
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#define FLAG_BITS 29
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/*
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* The object type is stored in 3 bits.
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*/
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struct object {
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unsigned parsed : 1;
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unsigned type : TYPE_BITS;
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unsigned flags : FLAG_BITS;
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struct object_id oid;
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};
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extern const char *type_name(unsigned int type);
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extern int type_from_string_gently(const char *str, ssize_t, int gentle);
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#define type_from_string(str) type_from_string_gently(str, -1, 0)
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/*
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* Return the current number of buckets in the object hashmap.
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*/
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extern unsigned int get_max_object_index(void);
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/*
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* Return the object from the specified bucket in the object hashmap.
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*/
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extern struct object *get_indexed_object(unsigned int);
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/*
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* This can be used to see if we have heard of the object before, but
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* it can return "yes we have, and here is a half-initialised object"
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* for an object that we haven't loaded/parsed yet.
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*
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* When parsing a commit to create an in-core commit object, its
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* parents list holds commit objects that represent its parents, but
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* they are expected to be lazily initialized and do not know what
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* their trees or parents are yet. When this function returns such a
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* half-initialised objects, the caller is expected to initialize them
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* by calling parse_object() on them.
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*/
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struct object *lookup_object(struct repository *r, const unsigned char *sha1);
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extern void *create_object(struct repository *r, const unsigned char *sha1, void *obj);
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void *object_as_type(struct repository *r, struct object *obj, enum object_type type, int quiet);
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/*
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* Returns the object, having parsed it to find out what it is.
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*
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* Returns NULL if the object is missing or corrupt.
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*/
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struct object *parse_object(struct repository *r, const struct object_id *oid);
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/*
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* Like parse_object, but will die() instead of returning NULL. If the
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* "name" parameter is not NULL, it is included in the error message
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* (otherwise, the hex object ID is given).
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*/
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struct object *parse_object_or_die(const struct object_id *oid, const char *name);
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/* Given the result of read_sha1_file(), returns the object after
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* parsing it. eaten_p indicates if the object has a borrowed copy
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* of buffer and the caller should not free() it.
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*/
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struct object *parse_object_buffer(struct repository *r, const struct object_id *oid, enum object_type type, unsigned long size, void *buffer, int *eaten_p);
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/** Returns the object, with potentially excess memory allocated. **/
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struct object *lookup_unknown_object(const unsigned char *sha1);
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struct object_list *object_list_insert(struct object *item,
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struct object_list **list_p);
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int object_list_contains(struct object_list *list, struct object *obj);
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/* Object array handling .. */
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void add_object_array(struct object *obj, const char *name, struct object_array *array);
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void add_object_array_with_path(struct object *obj, const char *name, struct object_array *array, unsigned mode, const char *path);
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/*
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* Returns NULL if the array is empty. Otherwise, returns the last object
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* after removing its entry from the array. Other resources associated
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* with that object are left in an unspecified state and should not be
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* examined.
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*/
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struct object *object_array_pop(struct object_array *array);
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typedef int (*object_array_each_func_t)(struct object_array_entry *, void *);
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/*
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* Apply want to each entry in array, retaining only the entries for
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* which the function returns true. Preserve the order of the entries
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* that are retained.
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*/
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void object_array_filter(struct object_array *array,
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object_array_each_func_t want, void *cb_data);
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/*
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* Remove from array all but the first entry with a given name.
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* Warning: this function uses an O(N^2) algorithm.
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*/
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void object_array_remove_duplicates(struct object_array *array);
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/*
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* Remove any objects from the array, freeing all used memory; afterwards
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* the array is ready to store more objects with add_object_array().
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*/
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void object_array_clear(struct object_array *array);
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void clear_object_flags(unsigned flags);
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/*
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* Clear the specified object flags from all in-core commit objects.
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*/
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extern void clear_commit_marks_all(unsigned int flags);
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#endif /* OBJECT_H */
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