[PATCH 9/9] read-cache.c: write index v4 format

[Junio C Hamano <gitster@pobox.com>]

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 builtin/update-index.c |    2 ++
 cache.h                |    4 ++++
 config.c               |   11 ++++++++++
 environment.c          |    1 +
 read-cache.c           |   56 ++++++++++++++++++++++++++++++++++++++++--------
 5 files changed, 65 insertions(+), 9 deletions(-)

diff --git a/builtin/update-index.c b/builtin/update-index.c
index a6a23fa..b663f45 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -791,6 +791,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			"(for porcelains) forget saved unresolved conflicts",
 			PARSE_OPT_NOARG | PARSE_OPT_NONEG,
 			resolve_undo_clear_callback},
+		OPT_INTEGER(0, "index-format", &preferred_index_format,
+			    "write index in this format"),
 		OPT_END()
 	};
 
diff --git a/cache.h b/cache.h
index 65a7aba..bdec32c 100644
--- a/cache.h
+++ b/cache.h
@@ -105,6 +105,9 @@ struct cache_header {
 	unsigned int hdr_entries;
 };
 
+#define SUPPORTED_INDEX_FORMAT_LB 2
+#define SUPPORTED_INDEX_FORMAT_UB 4
+
 /*
  * The "cache_time" is just the low 32 bits of the
  * time. It doesn't matter if it overflows - we only
@@ -537,6 +540,7 @@ extern int has_symlinks;
 extern int minimum_abbrev, default_abbrev;
 extern int ignore_case;
 extern int assume_unchanged;
+extern int preferred_index_format;
 extern int prefer_symlink_refs;
 extern int log_all_ref_updates;
 extern int warn_ambiguous_refs;
diff --git a/config.c b/config.c
index 68d3294..0244a85 100644
--- a/config.c
+++ b/config.c
@@ -564,6 +564,17 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.indexformat")) {
+		int val = git_config_int(var, value);
+		if (val < SUPPORTED_INDEX_FORMAT_LB ||
+		    SUPPORTED_INDEX_FORMAT_UB < val)
+			return error("%s not in supported range: %d..%d",
+				     var, SUPPORTED_INDEX_FORMAT_LB,
+				     SUPPORTED_INDEX_FORMAT_UB);
+		preferred_index_format = val;
+		return 0;
+	}
+
 	if (!strcmp(var, "core.quotepath")) {
 		quote_path_fully = git_config_bool(var, value);
 		return 0;
diff --git a/environment.c b/environment.c
index c93b8f4..4ecf8e6 100644
--- a/environment.c
+++ b/environment.c
@@ -20,6 +20,7 @@ int has_symlinks = 1;
 int minimum_abbrev = 4, default_abbrev = 7;
 int ignore_case;
 int assume_unchanged;
+int preferred_index_format;
 int prefer_symlink_refs;
 int is_bare_repository_cfg = -1; /* unspecified */
 int log_all_ref_updates = -1; /* unspecified */
diff --git a/read-cache.c b/read-cache.c
index 1c173f7..fdac89a 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1676,15 +1676,45 @@ static char *copy_cache_entry_to_ondisk(struct ondisk_cache_entry *ondisk,
 	}
 }
 
-static int ce_write_entry(git_SHA_CTX *c, int fd, struct cache_entry *ce)
+static int ce_write_entry(git_SHA_CTX *c, int fd, struct cache_entry *ce,
+			  struct strbuf *previous_name)
 {
-	int size = ondisk_ce_size(ce);
-	struct ondisk_cache_entry *ondisk = xcalloc(1, size);
+	int size;
+	struct ondisk_cache_entry *ondisk;
 	char *name;
 	int result;
 
-	name = copy_cache_entry_to_ondisk(ondisk, ce);
-	memcpy(name, ce->name, ce_namelen(ce));
+	if (!previous_name) {
+		size = ondisk_ce_size(ce);
+		ondisk = xcalloc(1, size);
+		name = copy_cache_entry_to_ondisk(ondisk, ce);
+		memcpy(name, ce->name, ce_namelen(ce));
+	} else {
+		int common, to_remove, prefix_size;
+		unsigned char to_remove_vi[16];
+		for (common = 0;
+		     (ce->name[common] &&
+		      common < previous_name->len &&
+		      ce->name[common] == previous_name->buf[common]);
+		     common++)
+			; /* still matching */
+		to_remove = previous_name->len - common;
+		prefix_size = encode_varint(to_remove, to_remove_vi);
+
+		if (ce->ce_flags & CE_EXTENDED)
+			size = offsetof(struct ondisk_cache_entry_extended, name);
+		else
+			size = offsetof(struct ondisk_cache_entry, name);
+		size += prefix_size + (ce_namelen(ce) - common + 1);
+
+		ondisk = xcalloc(1, size);
+		name = copy_cache_entry_to_ondisk(ondisk, ce);
+		memcpy(name, to_remove_vi, prefix_size);
+		memcpy(name + prefix_size, ce->name + common, ce_namelen(ce) - common);
+
+		strbuf_splice(previous_name, common, to_remove,
+			      ce->name + common, ce_namelen(ce) - common);
+	}
 
 	result = ce_write(c, fd, ondisk, size);
 	free(ondisk);
@@ -1720,10 +1750,11 @@ int write_index(struct index_state *istate, int newfd)
 {
 	git_SHA_CTX c;
 	struct cache_header hdr;
-	int i, err, removed, extended;
+	int i, err, removed, extended, hdr_version;
 	struct cache_entry **cache = istate->cache;
 	int entries = istate->cache_nr;
 	struct stat st;
+	struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
 
 	for (i = removed = extended = 0; i < entries; i++) {
 		if (cache[i]->ce_flags & CE_REMOVE)
@@ -1737,24 +1768,31 @@ int write_index(struct index_state *istate, int newfd)
 		}
 	}
 
+	if (preferred_index_format)
+		hdr_version = preferred_index_format;
+	else
+		hdr_version = extended ? 3 : 2;
+
+
 	hdr.hdr_signature = htonl(CACHE_SIGNATURE);
-	/* for extended format, increase version so older git won't try to read it */
-	hdr.hdr_version = htonl(extended ? 3 : 2);
+	hdr.hdr_version = htonl(hdr_version);
 	hdr.hdr_entries = htonl(entries - removed);
 
 	git_SHA1_Init(&c);
 	if (ce_write(&c, newfd, &hdr, sizeof(hdr)) < 0)
 		return -1;
 
+	previous_name = (hdr_version == 4) ? &previous_name_buf : NULL;
 	for (i = 0; i < entries; i++) {
 		struct cache_entry *ce = cache[i];
 		if (ce->ce_flags & CE_REMOVE)
 			continue;
 		if (!ce_uptodate(ce) && is_racy_timestamp(istate, ce))
 			ce_smudge_racily_clean_entry(ce);
-		if (ce_write_entry(&c, newfd, ce) < 0)
+		if (ce_write_entry(&c, newfd, ce, previous_name) < 0)
 			return -1;
 	}
+	strbuf_release(&previous_name_buf);
 
 	/* Write extension data here */
 	if (istate->cache_tree) {
-- 
1.7.10.rc4.54.g1d5dd3