Merge branch 'gs/commit-graph-path-filter'

Introduce an extension to the commit-graph to make it efficient to
check for the paths that were modified at each commit using Bloom
filters.

* gs/commit-graph-path-filter:
  bloom: ignore renames when computing changed paths
  commit-graph: add GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS test flag
  t4216: add end to end tests for git log with Bloom filters
  revision.c: add trace2 stats around Bloom filter usage
  revision.c: use Bloom filters to speed up path based revision walks
  commit-graph: add --changed-paths option to write subcommand
  commit-graph: reuse existing Bloom filters during write
  commit-graph: write Bloom filters to commit graph file
  commit-graph: examine commits by generation number
  commit-graph: examine changed-path objects in pack order
  commit-graph: compute Bloom filters for changed paths
  diff: halt tree-diff early after max_changes
  bloom.c: core Bloom filter implementation for changed paths.
  bloom.c: introduce core Bloom filter constructs
  bloom.c: add the murmur3 hash implementation
  commit-graph: define and use MAX_NUM_CHUNKS

2 files changed, 35 insertions, 0 deletions

diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
index 46f7f7c..53a6502 100644
--- a/Documentation/git-commit-graph.txt
+++ b/Documentation/git-commit-graph.txt
@@ -57,6 +57,11 @@ or `--stdin-packs`.)
 With the `--append` option, include all commits that are present in the
 existing commit-graph file.
 +
+With the `--changed-paths` option, compute and write information about the
+paths changed between a commit and it's first parent. This operation can
+take a while on large repositories. It provides significant performance gains
+for getting history of a directory or a file with `git log -- <path>`.
++
 With the `--split[=<strategy>]` option, write the commit-graph as a
 chain of multiple commit-graph files stored in
 `<dir>/info/commit-graphs`. Commit-graph layers are merged based on the
diff --git a/Documentation/technical/commit-graph-format.txt b/Documentation/technical/commit-graph-format.txt
index a4f1744..de56f9f 100644
--- a/Documentation/technical/commit-graph-format.txt
+++ b/Documentation/technical/commit-graph-format.txt
@@ -17,6 +17,9 @@ metadata, including:
 - The parents of the commit, stored using positional references within
   the graph file.
 
+- The Bloom filter of the commit carrying the paths that were changed between
+  the commit and its first parent, if requested.
+
 These positional references are stored as unsigned 32-bit integers
 corresponding to the array position within the list of commit OIDs. Due
 to some special constants we use to track parents, we can store at most
@@ -93,6 +96,33 @@ CHUNK DATA:
       positions for the parents until reaching a value with the most-significant
       bit on. The other bits correspond to the position of the last parent.
 
+  Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]
+    * The ith entry, BIDX[i], stores the number of 8-byte word blocks in all
+      Bloom filters from commit 0 to commit i (inclusive) in lexicographic
+      order. The Bloom filter for the i-th commit spans from BIDX[i-1] to
+      BIDX[i] (plus header length), where BIDX[-1] is 0.
+    * The BIDX chunk is ignored if the BDAT chunk is not present.
+
+  Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
+    * It starts with header consisting of three unsigned 32-bit integers:
+      - Version of the hash algorithm being used. We currently only support
+	value 1 which corresponds to the 32-bit version of the murmur3 hash
+	implemented exactly as described in
+	https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double
+	hashing technique using seed values 0x293ae76f and 0x7e646e2 as
+	described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters
+	in Probabilistic Verification"
+      - The number of times a path is hashed and hence the number of bit positions
+	      that cumulatively determine whether a file is present in the commit.
+      - The minimum number of bits 'b' per entry in the Bloom filter. If the filter
+	      contains 'n' entries, then the filter size is the minimum number of 64-bit
+	      words that contain n*b bits.
+    * The rest of the chunk is the concatenation of all the computed Bloom
+      filters for the commits in lexicographic order.
+    * Note: Commits with no changes or more than 512 changes have Bloom filters
+      of length zero.
+    * The BDAT chunk is present if and only if BIDX is present.
+
   Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional]
       This list of H-byte hashes describe a set of B commit-graph files that
       form a commit-graph chain. The graph position for the ith commit in this