[PATCH 5.5/22] Add documentation for the index api

[Thomas Gummerer <t.gummerer@gmail.com>]

Document the new index api and add examples of how it should be used
instead of the old functions directly accessing the index.

Helped-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---

Duy Nguyen <pclouds@gmail.com> writes:

> Hmm.. I was confused actually (documentation on the api would help
> greatly).

As promised, a draft for a documentation for the index api as it is in
this series.

Documentation/technical/api-in-core-index.txt | 108 +++++++++++++++++++++++++-
 1 file changed, 106 insertions(+), 2 deletions(-)

diff --git a/Documentation/technical/api-in-core-index.txt b/Documentation/technical/api-in-core-index.txt
index adbdbf5..5269bb1 100644
--- a/Documentation/technical/api-in-core-index.txt
+++ b/Documentation/technical/api-in-core-index.txt
@@ -1,14 +1,116 @@
 in-core index API
 =================

+Reading API
+-----------
+
+`read_index()`::
+	Read the whole index file from disk.
+
+`index_name_pos(name, namelen)`::
+	Find a cache_entry with name in the index.  Returns pos if an
+	entry is matched exactly and -pos-1 if an entry is matched
+	partially.
+	e.g.
+	index:
+	file1
+	file2
+	path/file1
+	zzz
+
+	index_name_pos("path/file1", 10) returns 2, while
+	index_name_pos("path", 4) returns -1
+
+`read_index_filtered(opts)`::
+	This method behaves differently for index-v2 and index-v5.
+
+	For index-v2 it simply reads the whole index as read_index()
+	does, so we are sure we don't have to reload anything if the
+	user wants a different filter.  It also sets the filter_opts
+	in the index_state, which is used to limit the results when
+	iterating over the index with for_each_index_entry().
+
+	The whole index is read to avoid the need to eventually
+	re-read the index later, because the performance is no
+	different when reading it partially.
+
+	For index-v5 it creates an adjusted_pathspec to filter the
+	reading.  First all the directory entries are read and then
+	the cache_entries in the directories that match the adjusted
+	pathspec are read.  The filter_opts in the index_state are set
+	to filter out the rest of the cache_entries that are matched
+	by the adjusted pathspec but not by the pathspec given.  The
+	rest of the index entries are filtered out when iterating over
+	the cache with for_each_index_entries.
+
+`get_index_entry_by_name(name, namelen, &ce)`::
+	Returns a cache_entry matched by the name, returned via the
+	&ce parameter.  If a cache entry is matched exactly, 1 is
+	returned, otherwise 0.  For an example see index_name_pos().
+	This function should be used instead of the index_name_pos()
+	function to retrieve cache entries.
+
+`for_each_index_entry(fn, cb_data)`::
+	Iterates over all cache_entries in the index filtered by
+	filter_opts in the index_stat.  For each cache entry fn is
+	executed with cb_data as callback data.  From within the loop
+	do `return 0` to continue, or `return 1` to break the loop.
+
+`next_index_entry(ce)`::
+	Returns the cache_entry that follows after ce
+
+`index_change_filter_opts(opts)`::
+	This function again has a slightly different functionality for
+	index-v2 and index-v5.
+
+	For index-v2 it simply changes the filter_opts, so
+	for_each_index_entry uses the changed index_opts, to iterate
+	over a different set of cache entries.
+
+	For index-v5 it refreshes the index if the filter_opts have
+	changed and sets the new filter_opts in the index state, again
+	to iterate over a different set of cache entries as with
+	index-v2.
+
+	This has some optimization potential, in the case that the
+	opts get stricter (less of the index should be read) it
+	doesn't have to reload anything, but currently does.
+
+Using the new index api
+-----------------------
+
+Currently loops over a specific set of index entry were written as:
+  i = start_index;
+  while (i < active_nr) { ce = active_cache[i]; do(something); i++; }
+
+they should be rewritten to:
+  ce = start;
+  while (ce) { do(something); ce = next_cache_entry(ce); }
+
+which is the equivalent operation but hides the in-memory format of
+the index from the user.
+
+For getting a cache entry get_cache_entry_by_name() should be used
+instead of cache_name_pos(). e.g.:
+  int pos = cache_name_pos(name, namelen);
+  struct cache_entry *ce = active_cache[pos];
+  if (pos < 0) { do(something) }
+  else { do(somethingelse) }
+
+should be written as:
+  struct cache_entry *ce;
+  int ret = get_cache_entry_by_name(name, namelen, &ce);
+  if (!ret) { do(something) }
+  else { do(somethingelse) }
+
+TODO
+----
 Talk about <read-cache.c> and <cache-tree.c>, things like:

 * cache -> the_index macros
-* read_index()
 * write_index()
 * ie_match_stat() and ie_modified(); how they are different and when to
   use which.
-* index_name_pos()
 * remove_index_entry_at()
 * remove_file_from_index()
 * add_file_to_index()
@@ -18,4 +120,6 @@ Talk about <read-cache.c> and <cache-tree.c>, things like:
 * cache_tree_invalidate_path()
 * cache_tree_update()

+
+
 (JC, Linus)
--
1.8.3.453.g1dfc63d