[PATCH v2 4/7] midx: refactor permutation logic

["Derrick Stolee via GitGitGadget" <gitgitgadget@xxxxxxxxx>]

From: Derrick Stolee <dstolee@xxxxxxxxxxxxx>

When writing a multi-pack-index, we keep track of an integer
permutation, tracking the list of pack-files that we know about
(both from the existing multi-pack-index and the new pack-files
being introduced) and converting them into a sorted order for
the new multi-pack-index.

In anticipation of dropping pack-files from the existing multi-
pack-index, refactor the logic around how we track this permutation.

First, insert the permutation into the pack_list structure. This
allows us to grow the permutation dynamically as we add packs.

Second, fill the permutation with values corresponding to their
position in the list of pack-files, sorted as follows:

  1. The pack-files in the existing multi-pack-index,
     sorted lexicographically.

  2. The pack-files not in the existing multi-pack-index,
     sorted as discovered from the filesystem.

There is a subtle thing in how we initialize this permutation,
specifically how we use 'i' for the initial value. This will
matter more when we implement the logic for dropping existing
packs, as we will create holes in the ordering.

Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
---
 midx.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/midx.c b/midx.c
index bb825ef816..3bd7183a53 100644
--- a/midx.c
+++ b/midx.c
@@ -380,9 +380,11 @@ static size_t write_midx_header(struct hashfile *f,
 struct pack_list {
 	struct packed_git **list;
 	char **names;
+	uint32_t *perm;
 	uint32_t nr;
 	uint32_t alloc_list;
 	uint32_t alloc_names;
+	uint32_t alloc_perm;
 	size_t pack_name_concat_len;
 	struct multi_pack_index *m;
 };
@@ -398,6 +400,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 
 		ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
 		ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);
+		ALLOC_GROW(packs->perm, packs->nr + 1, packs->alloc_perm);
 
 		packs->list[packs->nr] = add_packed_git(full_path,
 							full_path_len,
@@ -417,6 +420,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 			return;
 		}
 
+		packs->perm[packs->nr] = packs->nr;
 		packs->names[packs->nr] = xstrdup(file_name);
 		packs->pack_name_concat_len += strlen(file_name) + 1;
 		packs->nr++;
@@ -443,7 +447,7 @@ static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *p
 	ALLOC_ARRAY(pairs, nr_packs);
 
 	for (i = 0; i < nr_packs; i++) {
-		pairs[i].pack_int_id = i;
+		pairs[i].pack_int_id = perm[i];
 		pairs[i].pack_name = pack_names[i];
 	}
 
@@ -755,7 +759,6 @@ int write_midx_file(const char *object_dir)
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct pack_list packs;
-	uint32_t *pack_perm = NULL;
 	uint64_t written = 0;
 	uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
 	uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
@@ -774,18 +777,22 @@ int write_midx_file(const char *object_dir)
 
 	packs.nr = 0;
 	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
-	packs.alloc_names = packs.alloc_list;
+	packs.alloc_perm = packs.alloc_names = packs.alloc_list;
 	packs.list = NULL;
 	packs.names = NULL;
+	packs.perm = NULL;
 	packs.pack_name_concat_len = 0;
 	ALLOC_ARRAY(packs.list, packs.alloc_list);
 	ALLOC_ARRAY(packs.names, packs.alloc_names);
+	ALLOC_ARRAY(packs.perm, packs.alloc_perm);
 
 	if (packs.m) {
 		for (i = 0; i < packs.m->num_packs; i++) {
 			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
 			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
+			ALLOC_GROW(packs.perm, packs.nr + 1, packs.alloc_perm);
 
+			packs.perm[packs.nr] = i;
 			packs.list[packs.nr] = NULL;
 			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
 			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
@@ -802,10 +809,9 @@ int write_midx_file(const char *object_dir)
 		packs.pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
 					      (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
 
-	ALLOC_ARRAY(pack_perm, packs.nr);
-	sort_packs_by_name(packs.names, packs.nr, pack_perm);
+	sort_packs_by_name(packs.names, packs.nr, packs.perm);
 
-	entries = get_sorted_entries(packs.m, packs.list, pack_perm, packs.nr, &nr_entries);
+	entries = get_sorted_entries(packs.m, packs.list, packs.perm, packs.nr, &nr_entries);
 
 	for (i = 0; i < nr_entries; i++) {
 		if (entries[i].offset > 0x7fffffff)
@@ -923,8 +929,8 @@ int write_midx_file(const char *object_dir)
 
 	free(packs.list);
 	free(packs.names);
+	free(packs.perm);
 	free(entries);
-	free(pack_perm);
 	free(midx_name);
 	return 0;
 }
-- 
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