Before any code is introduced the full documentation is put forth. This
provides a man page for the porcelain, and a technical doc in technical/. The
latter describes the API, and discusses rev-cache's design, file format and
mechanics.
Signed-off-by: Nick Edelen <sirnot@xxxxxxxxx>
---
Slight clean-up of man page.
Documentation/git-rev-cache.txt | 190 ++++++++++
Documentation/technical/rev-cache.txt | 634 +++++++++++++++++++++++++++++++++
2 files changed, 824 insertions(+), 0 deletions(-)
diff --git a/Documentation/git-rev-cache.txt b/Documentation/git-rev-cache.txt
new file mode 100644
index 0000000..5a713ad
--- /dev/null
+++ b/Documentation/git-rev-cache.txt
@@ -0,0 +1,190 @@
+git-rev-cache(1)
+================
+
+NAME
+----
+git-rev-cache - Add, walk and maintain revision cache slices
+
+SYNOPSIS
+--------
+'git-rev-cache' COMMAND [options] [<commit>...]
+
+DESCRIPTION
+-----------
+The revision cache ('rev-cache') provides a mechanism for significantly
+speeding up revision traversals. It does this by creating an efficient
+database (cache) of commits, their related objects and topological relations.
+Independant of packs and the object store, this database is composed of
+rev-cache "slices" -- each a different file storing a given segment of commit
+history. To map commits to their respective slices, a single index file is
+kept for the rev-cache.
+
+'git-rev-cache' provides a front-end for the rev-cache mechanism, intended for
+updating and maintaining rev-cache slices in the current repository. New cache
+slice files can be 'add'ed, to keep the cache up-to-date; individual slices can
+be traversed; smaller slices can be 'fuse'd into a larger slice; and the
+rev-cache index can be regenerated.
+
+COMMANDS
+--------
+
+add
+~~~
+Add revisions to the cache by creating a new cache slice. Reads a revision
+list from the command line, formatted as: `START START ... \--not END END ...`
+
+Options
+^^^^^^^
+
+\--all::
+ Include all refs in the new cache slice, like the \--all option in
+ 'rev-list'.
+
+\--fresh/\--incremental::
+ Exclude everything already in the revision cache, analogous to
+ \--incremental in 'pack-objects'.
+
+\--stdin::
+ Read newline-seperated revisions from the standard input. Use \--not
+ to exclude commits, as on the command line.
+
+\--legs::
+ Ensure newly-generated cache slice has no partial ends. This means that
+ no commit has partially cached parents, in that all its parents are
+ cached or none of them are. 99.9% of users can ignore this command.
++
+\--legs will cause 'rev-cache' to expand potential slice end-points (creating
+"legs") until this condition is met, simplifying the cache slice structure.
+'rev-cache' itself does not care if a slice has legs or not, but the condition
+may reduce the required complexity of other applications that might use the
+revision cache.
+
+\--no-objects::
+ Non-commit objects are normally included along with the commit with
+ which they were introduced. This is obviously very benificial, but can
+ take longer in cache slice generation. Using this option will disable
+ non-commit object caching.
++
+\--no-objects is mainly intended for debugging or development purposes, but may
+find use in special situations (e.g. common traversal of only commits).
+
+Output
+^^^^^^
+
+On `stderr` 'add' outputs general information about the generated slice,
+including the number of objects and paths, and the start/end commits (prefix S
+indicates start, E an end). Through `stdout` it emits only the SHA-1 of the
+slice.
+
+walk
+~~~~
+Analogous to a slice-oriented 'rev-list', 'walk' will traverse a region in a
+particular cache slice. Interesting and uninteresting (delimited, as with
+'rev-list', with \--not) are specified on the command line, and output is the
+same as vanilla 'rev-list'.
+
+Options
+^^^^^^^
+
+\--objects::
+ Like 'rev-list', 'walk' will normally only list commits. Use this
+ option to list non-commit objects as well, if they are present in the
+ cache slice.
+
+Output
+^^^^^^
+
+'walk' will simply dump the contents of the output commit list, work list, and
+pending object array. The headers are outputed on `stderr`, the object hashes
+and names on `stdout`.
+
+fuse
+~~~~
+Merge several cache slices into a single large slice, like 'repack' for
+'rev-cache'. On each invocation of 'add' a new file ("slice") is added to the
+revision cache directory, and after several additions the directory may become
+populated with many, relatively small slices. Numerous smaller slices will
+yield poorer performance than a one or two large ones, because of the overhead
+of loading new slices into memory.
+
+Running 'fuse' every once in a while will solve this problem by coalescing all
+the cache slices into one larger slice. For very large projects, using
+\--ignore-size is advisable to prevent overly large cache slices. This can be
+set to run on garbage collection; see 'Automation' for more info.
+
+Note that 'fuse' uses the internal revision walker, so the options used in
+fusion override those of the cache slices upon which it operates. For example,
+if some slices were generated with \--no-objects, yet 'fuse' was performed with
+non-commit objects, the resulting slice would still contain objects but would
+take longer to generate.
+
+Options
+^^^^^^^
+
+\--all::
+ Normally fuse will only include everything that's already in the
+ revision cache. \--all tells it to start walking from the branch
+ heads, effectively a `add --all --fresh; fuse`
+ (pseudo-revcache-command).
+
+\--no-objects::
+ As in 'add', this option disables inclusion of non-commit objects. If
+ some cache slices do contain such objects, the information will be lost.
+
+\--ignore-size[=N]::
+ Do not merge cache slices of size >=N (be aware that slices must be
+ mapped to memory). N can have a suffix of "k" or "m", denoting N as
+ kilobytes and megabytes, respectively. If N is not provided 'fuse'
+ will default to a size specified in `revcache.ignoresize`, or ~25MB if
+ the config var is not set.
+
+Output
+^^^^^^
+
+This command prints the SHA-1 of the new slice on `stdout`, and information
+about its work on `stderr` -- specifically which files it's removing.
+
+Automation
+^^^^^^^^^^
+
+Set the git configuration variable `gc.revcache` to run 'fuse' on garbage
+collection. The arguments passed are `fuse \--all \--ignore-size`; i.e. 'gc'
+will keep everything cached into size-regulated slices.
+
+index
+~~~~~
+Regenerate the revision cache index. If the rev-cache index file associating
+objects with cache slices gets corrupted, lost, or otherwise becomes unusable,
+'index' will quickly regenerate the file. It's most likely that this won't be
+needed in every day use, as it is targeted towards debugging and development.
+
+alt
+~~~
+Create a cache slice pointer to another slice, identified by its full path:
+`fuse path/to/other/slice`
+
+This command is useful if you have several repositories sharing a common
+history. Although space requirements for rev-cache are slim anyway, you can in
+this situation reduce it further by using slice pointers, pointing to relavant
+slices in other repositories. Note that only one level of redirection is
+allowed, and the slice pointer will break if the original slice is removed.
+'fuse' will not touch slice pointers.
+
+NOTES
+-----
+In certain circumstances there may be some inconsistencies with object names
+between cached and non-cached walks. Specifically, if two objects in a commit
+tree have the same content (= same SHA-1); or if objects of the same SHA-1 are
+introduced independantly in parallel branches.
+
+In the first case rev-cache will use the name of the youngest file, while
+vanilla rev-list will return the name of the entry first encountered in walking
+the tree. The latter case is a result of rev-cache's internal topological
+ordering: the difference is the same between sorted and unsorted revision walks.
+
+See 'Discussion' for the underlying reasons for the discrepencies.
+
+DISCUSSION
+----------
+For an explanation of the API and its inner workings, see
+link:technical/rev-cache.txt[technical info on rev-cache].
diff --git a/Documentation/technical/rev-cache.txt b/Documentation/technical/rev-cache.txt
new file mode 100644
index 0000000..91fce8b
--- /dev/null
+++ b/Documentation/technical/rev-cache.txt
@@ -0,0 +1,634 @@
+rev-cache
+=========
+
+The revision cache API ('rev-cache') provides a method for efficiently storing
+and accessing commit branch sections. Such branch slices are defined by a
+series of start/top (interesting) and end/bottom (uninteresting) commits. Each
+slice contains information on commits in topological order. Recorded with each
+commit is:
+
+* All intra-slice topological relations, encoded into path "channels" (see
+ 'Mechanics' for full explanation).
+* Object meta-data: type, SHA-1, size, date (for commits).
+* Objects introduced by that commit, not present in the its cached parents.
+
+In addition to the API, basic structures are exported for the possibility of
+direct access.
+
+The API
+-------
+You can find the function prototypes in `revision.h`.
+
+Data Structures
+~~~~~~~~~~~~~~~
+The `rev_cache_info` struct holds all the options and flags for the API.
+
+----
+struct rev_cache_info {
+ /* generation flags */
+ unsigned objects : 1,
+ legs : 1,
+ make_index : 1,
+ fuse_me : 1;
+
+ /* index inclusion */
+ unsigned overwrite_all : 1;
+
+ /* traversal flags */
+ unsigned add_to_pending : 1;
+
+ /* fuse options */
+ unsigned int ignore_size;
+
+ /* reserved */
+ struct rev_cache_slice_map *maps,
+ *last_map;
+};
+----
+
+The fields:
+
+`objects`::
+ Add non-commit objects to slice.
+
+`legs`::
+ Ensure end/bottom commits have no children.
+
+`make_index`::
+ Integrate newly-made slice into index.
+
+`fuse_me`::
+ This is specified if a fuse is occuring, and slices are to be reused.
+ This option requires `maps` and `last_maps` to be initialized.
+
+`overwrite_all`::
+ When a cache slice is added to the index, sometimes overlap occures
+ between it and other slices. Normally, original index entries are kept
+ unless the new entry represents a start commit (older entries are more
+ likely to lead to greater in-slice traversals). This options overrides
+ that, and updates all entries of the new slice.
+
+`add_to_pending`::
+ Append unique non-commit objects to the `pending` object list in the
+ passed `rev_info` instance.
+
+`add_names`::
+ Include non-commit object names in the pending object entries if
+ `add_to_pending` is set.
+
+`ignore_size`::
+ If non-zero, ignore slices with size greater or equal to this during
+fusion.
+
+`maps`/`last_map`::
+ An array of slice mappings, indexed by their id in the slice index
+ header, to be re-used with `fuse_me`. `last_map` points to the last
+ mapping used, and should be initialized to 0.
+
+Functions
+~~~~~~~~~
+
+init_rev_cache
+^^^^^^^^^^^^^^
+----
+void init_rev_cache_info(
+ struct rev_cache_info *rci OUT
+)
+----
+
+Initialize `rci` to default options.
+
+make_cache_slice
+^^^^^^^^^^^^^^^^
+----
+int make_cache_slice(
+ struct rev_cache_info *rci IN,
+ struct rev_info *revs IN,
+ struct commit_list **starts IN/OUT,
+ struct commit_list **ends IN/OUT,
+ unsigned char *cache_sha1 OUT
+)
+----
+
+Create a cache slice based on either `revs` (if non-NULL) *or* the `starts` and
+`ends` lists. The actual list of start and end commits of the slice may be
+different from the parameters, based on what defines the branch segment, and
+this actual list is passed back through `starts` and `ends`.
+
+The cache slice is identified via a SHA-1 generated from the actual start/end
+commit lists. `cache_sha1`, if non-NULL, can recieve the cache slice name.
+`rci` is used to specify generation options, but can be NULL if you want
+`make_cache_slice` to fall back on defaults. Returns 0 on success, non-zero on
+failure.
+
+make_cache_index
+^^^^^^^^^^^^^^^^
+----
+int make_cache_index(
+ struct rev_cache_info *rci IN,
+ unsigned char *cache_sha1 IN,
+ int fd IN,
+ unsigned int size IN
+)
+----
+
+Add a slice to the rev-cache index. `cache_sha1` is the identity hash of the
+cache slice; `fd` is a file descriptor of the cache slice opened with
+read/write privileges (the slice is not actually modified); `size` is the size
+of the cache slice. Although there are currently no options for index
+updating, `rci` is a placeholder in case of future options. Note that this
+function is normally called by `make_cache_slice`. Returns 0 on success,
+non-zero on failure.
+
+open_cache_slice
+^^^^^^^^^^^^^^^^
+----
+int open_cache_slice(
+ unsigned char *sha1 IN,
+ int flags IN
+)
+----
+
+Returns a file descriptor to a cache slice described by `sha1` hash, using
+`flags` as the access mode. This will follow cache slice pointers to one level
+of indirection.
+
+get_cache_slice
+^^^^^^^^^^^^^^^
+----
+unsigned char *get_cache_slice(
+ struct commit *commit IN
+)
+----
+
+Given a commit object `get_cache_slice` will search the revision cache index
+and return, if found, the cache slice SHA-1.
+
+traverse_cache_slice
+^^^^^^^^^^^^^^^^^^^^
+----
+int traverse_cache_slice(
+ struct rev_info *revs IN/OUT,
+ unsigned char *cache_sha1 IN,
+ struct commit *commit IN,
+ unsigned long *date_so_far IN/OUT,
+ int *slop_so_far IN/OUT,
+ struct commit_list ***queue OUT,
+ struct commit_list **work IN/OUT
+)
+----
+
+Traverse a specified cache slice. An explanation of the each field:
+
+`revs`::
+ The revision walk instance. `traverse_cache_slice` uses this for
+ general options (e.g. which objects are included) and slice traversal
+ options (in the `rev_cache_info` field). If the `add_to_pending`
+ option is specified, non-commit objects are appended to the `pending`
+ object list field.
+
+`cache_sha1`::
+ SHA-1 identifying the cache slice to use. This can be taken directly
+ from `get_cache_slice`.
+
+`commit`::
+ The current commit object in the revision walk, i.e. the commit which
+ inspired this slice traversal. Although theoretically redundant in
+ view of the `work` list, this simplifies interaction with normal
+ revision walks, which pop commits from `work` before analyzing them.
+
+`date_so_far`::
+ The date of the oldest encountered interesting commit. Passing NULL
+ will let `traverse_cache_slice` use defaults.
+
+`slop_so_far`::
+ The `slop` value, a la revision.c. This is a counter used to determine
+ when to stop traversing, based on how many extra uninteresting commits
+ should be encountered. NULL will enable defaults, as above.
+
+`queue`::
+ Refers to a pointer to the head of a FIFO commit list, recieving the
+ commits we've seen and added.
+
+`work`::
+ A date-ordered list of commits that have yet to be processed (i.e. seen
+ but not added). Commits from here present in the slice are removed
+ (and, obviously, used as starting places for traversal), and any end
+ commits encountered are inserted.
+
+starts_from_slices
+^^^^^^^^^^^^^^^^^^
+----
+void starts_from_slices(
+ struct rev_info *revs OUT,
+ unsigned int flags IN,
+ unsigned char *which IN,
+ int n IN
+)
+----
+
+Will mark start-commits in certain rev-cache slices with `flag`, and added them
+to the pending list of `revs`. If `n` is zero, `starts_from_slices` will use
+all slices. Otherwise `which` will specify an *unseperated* list of cache
+SHA-1s to use (20 bytes each), and `n` will contain the number of slices (i.e.
+20 * `n` = size of `which`).
+
+fuse_cache_slices
+^^^^^^^^^^^^^^^^^
+----
+int fuse_cache_slices(
+ struct rev_cache_info *rci IN,
+ struct rev_info *revs IN
+)
+----
+
+Generate a slice based on `revs`, replacing all encountered slices with one
+(larger) slice. The `ignore_size` field in `rci`, if non-zero, will dictate
+which cache slice sizes to ignore in both traversal and replacement.
+
+regenerate_cache_index
+^^^^^^^^^^^^^^^^^^^^^^
+----
+int regenerate_cache_index(
+ struct rev_cache_info *rci IN
+)
+----
+
+Remake the revision cache index, including all the slices. Currently no
+options in `rci` exist for index (re)generation, but some may develop in the
+future.
+
+to/from_disked_rc_object/index_entry
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+----
+struct rc_object/index_entry *from_disked_rc_object/index_entry(
+ struct rc_object/index_entry_ondisk *src IN,
+ struct rc_object/index_entry *dst OUT
+)
+
+struct rc_object/index_entry_ondisk *to_disked_rc_object/index_entry(
+ struct rc_object/index_entry *src IN,
+ struct rc_object/index_entry_ondisk *dst OUT
+)
+----
+
+Functions to convert between the internal and storage (`_ondisk`) versions of
+object and index entry structures. These are necessary for direct access to
+the cache slices. If NULL is provided for `dst` a statically allocated
+structure is used, and a pointer to the struct is returned. Otherwise the
+functions return `dst`.
+
+Example Usage
+-------------
+
+A few examples to demonstrate usage:
+
+.Creating a slice
+----
+/* pretend you're a porcelain for rev-cache reading from the command line */
+struct rev_info revs;
+struct rev_cache_info rci;
+
+init_revisions(&revs, 0);
+init_rci(&rci);
+
+flags = 0;
+for (i = 1; i < argc; i++) {
+ if (!strcmp(argv[i], "--not"))
+ flags ^= UNINTERESTING;
+ else if(!strcmp(argv[i], "--fresh"))
+ starts_from_slices(&revs, UNINTERESTING, 0, 0);
+ else
+ handle_revision_arg(argv[i], &revs, flags, 1);
+}
+
+/* we want to explicitly set certain options */
+rci.objects = 0;
+
+if (!make_cache_slice(&rci, &revs, 0, 0, cache_sha1))
+ printf("made slice! it's called %s\n", sha1_to_hex(cache_sha1));
+----
+
+.Traversing a slice
+----
+/* let's say you're walking the tree with a 'work' list of current heads and a
+ * FILO output list 'out' */
+out = 0;
+outp = &out;
+
+while (work) {
+ struct commit *commit = pop_commit(&work);
+ struct object *object = &commit->object;
+ unsigned char *cache_sha1;
+
+ if (cache_sha1 = get_cache_slice(object->sha1)) {
+ /* note that this will instatiate any topo-relations
+ * as it goes */
+ if (traverse_cache_slice(&revs, cache_sha1,
+ commit, 0, 0, /* use defaults */
+ &outp, &work) < 0)
+ die("I'm overreacting to a non-fatal cache error");
+ } else {
+ struct commit_list *parents = commit->parents;
+
+ while (parents) {
+ struct commit *p = parents->item;
+ struct object *po = &p->object;
+
+ parents = parents->next;
+ if (po->flags & UNINTERESTING)
+ continue;
+
+ if (object->flags & UNINTERESTING)
+ po->flags |= UNINTERESTING;
+ else if (po->flags & SEEN)
+ continue;
+
+ if (!po->parsed)
+ parse_commit(p);
+ insert_by_date(p, &work);
+ }
+
+ if (object->flags & (SEEN | UNINTERESTING) == 0)
+ outp = &commit_list_insert(commit, outp)->next;
+ object->flags |= SEEN;
+ }
+}
+----
+
+Some Internals
+--------------
+For more advanced usage, the slice and index file(s) may be accessed directly.
+Relavant structures are availabe in `rev-cache.h`.
+
+File Formats
+~~~~~~~~~~~~
+
+Cache Slices
+^^^^^^^^^^^^
+A slice has a basic fixed-size header, followed by a certain number of object
+entries, then a NULL-seperated list of object names. Commits are sorted in
+topo-order, and each commit entry is followed by the objects added in that
+commit.
+
+----
+ -- +--------------------------------+
+header | object number, etc... |
+ -- +--------------------------------+
+commit | commit info |
+entry | path data |
+ +--------------------------------+
+ | tree/blob info |
+ +--------------------------------+
+ | tree/blob info |
+ +--------------------------------+
+ | ... |
+ -- +--------------------------------+
+commit | commit info |
+entry | path data |
+ +--------------------------------+
+ | tree/blob info |
+ +--------------------------------+
+ | ... |
+ -- +--------------------------------+
+... ...
+ -- +--------------------------------+
+name list | \0some_file_name\0 |
+(note +--------------------------------+
+preceeding | another_file\0 |
+null) ... |
+ +--------------------------------+
+----
+
+Here is the header:
+
+----
+struct rc_cache_slice_header {
+ char signature[8]; /* REVCACHE */
+ unsigned char version;
+ uint32_t ofs_objects;
+
+ uint32_t object_nr;
+ uint16_t path_nr;
+ uint32_t size;
+
+ unsigned char sha1[20];
+
+ uint32_t names_size;
+};
+----
+
+Explanations:
+
+`signature`::
+ The identifying signature of cache slice file. Always "REVCACHE".
+`version`::
+ The version number, currently 1.
+`ofs_objects`::
+ The byte offset at which the commit/object listing starts. Always
+ present at the 10th byte, regardless of file version.
+`object_nr`::
+ The total number of objects (commit + non-commit objects) present in
+ the slice.
+`path_nr`::
+ The total number of paths/channels used in encoding the topological
+ data. Note that paths are reused (see 'Mechanics'), so there will
+ never be more than a few hundred paths (if that) used.
+`size`::
+ The size of the slice *excluding* the name list. In other words, the
+ size of the portion mapped to memory.
+`sha1`::
+ The cache slice SHA-1.
+`names_size`::
+ The size of the name list. `size` + `names_size` = size of slice
+
+Revision Cache Index
+^^^^^^^^^^^^^^^^^^^^
+The index is a single file that associates SHA-1s with cache slices and file
+positions. It is somewhat similar to pack-file indexes, containing a fanout
+table and a list of index entries sorted by hash.
+
+----
+ -- +--------------------------------+
+header | object #, cache #, etc. |
+ -- +--------------------------------+
+sha1s of | SHA-1 |
+slices | ... |
+ -- +--------------------------------+
+fanout | fanout[0x00] |
+table ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ | fanout[0xff] |
+ -- +--------------------------------+
+index | SHA-1 of object |
+entries | index of cache slice SHA-1 |
+ | position in cache slice |
+ +--------------------------------+
+ | |
+ ...
+ +--------------------------------+
+----
+
+The header:
+
+----
+struct rc_index_header {
+ char signature[8]; /* REVINDEX */
+ unsigned char version;
+ uint32_t ofs_objects;
+
+ uint32_t object_nr;
+ unsigned char cache_nr;
+
+ uint32_t max_date;
+};
+----
+
+Explanations:
+
+`signature`::
+ Always "REVINDEX".
+`version`::
+ Version number, currently 1.
+`ofs_objects`::
+ Offset at which the entry objects begin. This is more obviously useful
+ in the index because the list of slice SHA-1s is variably-sized.
+`object_nr`::
+ Number of index entry objects present.
+`cache_nr`::
+ Number of cache slices to which the index maps, and hence the number of
+slice SHA-1s listed.
+`max_date`::
+ The oldest commit represented in the index. This is used to help speed
+up lookup times by knowing what range of commits we definitely don't have
+cached. Normal usage of 'rev-cache' would leave no "holes" in its coverage of
+commit history -- once a commit is cached, everything reachable from it should
+be cached as well. Most of the time refs are added to rev-cache simultaneous
+as well. This means that in most situations almost everything <= `max_date`
+will be cached.
+
+Mechanics
+~~~~~~~~~
+
+The most important part of rev-cache is its method of encoding topological
+relations. To ensure fluid traversal and reconstruction, commits are related
+through high-level "streams"/"channels" rather than individual
+interconnections. Intuitively, rev-cache stores history the way gitk shows it:
+commits strung up on lines, which interconnect at merges and branches.
+
+Each commit is associated to a given channel/path via a 'path id', and
+variable-length fields govern which paths (if any) are closed or opened at that
+object. This means that topo-data can be preserved in only a few bytes extra
+per object entry. Other information stored per entry is the sha-1 hash, type,
+date, size, name, and status in cache slice. Here is format of an object
+entry, both on-disk and in-memory:
+
+----
+struct object_entry {
+ unsigned type : 3;
+ unsigned is_end : 1;
+ unsigned is_start : 1;
+ unsigned uninteresting : 1;
+ unsigned include : 1;
+ unsigned flags : 1;
+ unsigned char sha1[20];
+
+ unsigned char merge_nr;
+ unsigned char split_nr;
+ unsigned size_size : 3;
+ unsigned name_size : 3;
+
+ uint32_t date;
+ uint16_t path;
+
+ /* merge paths */
+ /* split paths */
+ /* size */
+ /* name index */
+};
+----
+
+An explanation of each field:
+
+`type`::
+ Object type
+`is_end`::
+ The commit has some parents outside the cache slice (all if slice has
+ legs)
+`is_start`::
+ The commit has no children in cache slice
+`uninteresting`::
+ Run-time flag, used in traversal
+`include`::
+ Run-time flag, used in traversal (initialization)
+`flags`::
+ Currently unused, extra bit
+`sha1`::
+ Object SHA-1 hash
+
+`merge_nr`::
+ The number of paths the current channel diverges into; the current path
+ ends upon any merge.
+`split_nr`::
+ The number of paths this commit ends; used on both merging and
+ branching.
+`size_size`::
+ Number of bytes the object size takes up.
+`name_size`::
+ Number of bytes the name index takes up.
+
+`date`::
+ The date of the commit.
+`path`::
+ The path ID of the channel with which this commit is associated.
+
+merge paths::
+ The path IDs (16-bit) that are to be created. Overflow is not a
+ problem as path IDs are reused, leaving even complicated projects to
+ consume no more than a few hundred IDs.
+split paths::
+ The path IDs (16-bit) that are to be ended.
+size::
+ The size split into the minimum number of bytes. That is, 1-8 bytes
+ representing the size, least-significant byte first.
+name index::
+ An offset for the null-seperated, object name list at the end of the
+ cache slice. Also split into the minimum number of bytes.
+
+Each path ID refers to an index in a 'path array', which stores the current
+status (eg. active, interestingness) of each channel.
+
+Due to topo-relations and boundary tracking, all of a commit's parents must be
+encountered before the path is reallocated. This is achieved by using a
+counter system per merge: starting at the parent number, the counter is
+decremented as each parent is encountered (dictated by 'split paths'); at 0 the
+path is cleared.
+
+Boundary tracking is necessary because non-commits are stored relative to the
+commit in which they were introduced. If a series of commits is not included
+in the output, the last interesting commit must be parsed manually to ensure
+all objects are accounted for.
+
+To prevent list-objects from recursing into trees that we've already taken care
+of, the flag `FACE_VALUE` is introduced. An object with this flag is not
+explored (= "taken at face value"), significantly reducing I/O and processing
+time.
+
+Notes
+~~~~~
+
+Due to rev-cache's internal storage format, walking may lead to some
+discrepencies between cached and uncached repositories. Although noticeable to
+users directly calling rev-list, these are unused or corner cases and
+internally a non-issue.
+
+First note that rev-cache records commits in topological order. Large portions
+of commit history will already be sorted topologically in the revision walk,
+yielding a different output for unsorted calls to rev-list. A more obscure
+consquence occurs when two objects of the same SHA-1, but different name, are
+introduced seperately in parallel branches: different names might be shown for
+that object depending on which object entry was encountered first.
+
+A similar disparity arises when two objects of same SHA-1/different name are
+present in the same tree structure. rev-cache, walking objects as they were
+introduced, lists the youngest file's name; rev-list, walking the full trees
+each commit, shows the first file encountered.
--
tg: (0130fb5..) t/revcache/docs (depends on: t/revcache/integration)
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