[PATCH 3/6] pack-bitmap-write.c: write lookup table extension

["Abhradeep Chakraborty via GitGitGadget" <gitgitgadget@gmail.com>]

From: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>

Teach git to write bitmap lookup table extension. The table has the
following information:

    - `N` no of Object ids of each bitmapped commits

    - A list of offset, xor-offset pair; the i'th pair denotes the
      offsets and xor-offsets of i'th commit in the previous list.

    - 4-byte integer denoting the flags

Co-authored-by: Taylor Blau <ttaylorr@github.com>
Mentored-by: Taylor Blau <ttaylorr@github.com>
Co-mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
---
 pack-bitmap-write.c | 59 +++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 57 insertions(+), 2 deletions(-)

diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index c43375bd344..9e88a64dd65 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -650,7 +650,8 @@ static const struct object_id *oid_access(size_t pos, const void *table)
 
 static void write_selected_commits_v1(struct hashfile *f,
 				      struct pack_idx_entry **index,
-				      uint32_t index_nr)
+				      uint32_t index_nr,
+				      off_t *offsets)
 {
 	int i;
 
@@ -663,6 +664,9 @@ static void write_selected_commits_v1(struct hashfile *f,
 		if (commit_pos < 0)
 			BUG("trying to write commit not in index");
 
+		if (offsets)
+			offsets[i] = hashfile_total(f);
+
 		hashwrite_be32(f, commit_pos);
 		hashwrite_u8(f, stored->xor_offset);
 		hashwrite_u8(f, stored->flags);
@@ -671,6 +675,49 @@ static void write_selected_commits_v1(struct hashfile *f,
 	}
 }
 
+static int table_cmp(const void *_va, const void *_vb)
+{
+	return oidcmp(&writer.selected[*(uint32_t*)_va].commit->object.oid,
+		      &writer.selected[*(uint32_t*)_vb].commit->object.oid);
+}
+
+static void write_lookup_table(struct hashfile *f,
+			       off_t *offsets)
+{
+	uint32_t i;
+	uint32_t flags = 0;
+	uint32_t *table, *table_inv;
+
+	ALLOC_ARRAY(table, writer.selected_nr);
+	ALLOC_ARRAY(table_inv, writer.selected_nr);
+
+	for (i = 0; i < writer.selected_nr; i++)
+		table[i] = i;
+	QSORT(table, writer.selected_nr, table_cmp);
+	for (i = 0; i < writer.selected_nr; i++)
+		table_inv[table[i]] = i;
+
+	for (i = 0; i < writer.selected_nr; i++) {
+		struct bitmapped_commit *selected = &writer.selected[table[i]];
+		struct object_id *oid = &selected->commit->object.oid;
+
+		hashwrite(f, oid->hash, the_hash_algo->rawsz);
+	}
+	for (i = 0; i < writer.selected_nr; i++) {
+		struct bitmapped_commit *selected = &writer.selected[table[i]];
+
+		hashwrite_be32(f, offsets[table[i]]);
+		hashwrite_be32(f, selected->xor_offset
+			       ? table_inv[table[i] - selected->xor_offset]
+			       : 0xffffffff);
+	}
+
+	hashwrite_be32(f, flags);
+
+	free(table);
+	free(table_inv);
+}
+
 static void write_hash_cache(struct hashfile *f,
 			     struct pack_idx_entry **index,
 			     uint32_t index_nr)
@@ -695,6 +742,7 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 {
 	static uint16_t default_version = 1;
 	static uint16_t flags = BITMAP_OPT_FULL_DAG;
+	off_t *offsets = NULL;
 	struct strbuf tmp_file = STRBUF_INIT;
 	struct hashfile *f;
 
@@ -715,8 +763,14 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 	dump_bitmap(f, writer.trees);
 	dump_bitmap(f, writer.blobs);
 	dump_bitmap(f, writer.tags);
-	write_selected_commits_v1(f, index, index_nr);
 
+	if (options & BITMAP_OPT_LOOKUP_TABLE)
+		CALLOC_ARRAY(offsets, index_nr);
+
+	write_selected_commits_v1(f, index, index_nr, offsets);
+
+	if (options & BITMAP_OPT_LOOKUP_TABLE)
+		write_lookup_table(f, offsets);
 	if (options & BITMAP_OPT_HASH_CACHE)
 		write_hash_cache(f, index, index_nr);
 
@@ -730,4 +784,5 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 		die_errno("unable to rename temporary bitmap file to '%s'", filename);
 
 	strbuf_release(&tmp_file);
+	free(offsets);
 }
-- 
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