[PATCH 28/30] test-name-hash: add helper to compute name-hash functions

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <stolee@gmail.com>

Using this tool, we can count how many distinct name-hash values exist
within a list of paths. Examples include

 git ls-tree -r --name-only HEAD | \
	     test-tool name-hash | \
  	      awk "{print \$1;}" | \
  		 sort -ns | uniq | wc -l

which outputs the number of distinct name-hash values that appear at
HEAD. Or, the following which presents the resulting name-hash values of
maximum multiplicity:

 git ls-tree -r --name-only HEAD | \
	     test-tool name-hash | \
	      awk "{print \$1;}" | \
	       sort -n | uniq -c | sort -nr | head -n 25

For an internal monorepo with around a quarter million paths at HEAD,
the highest multiplicity for the standard name-hash function was 14,424
while the full name-hash algorithm had only seven hash values with any
collision, with a maximum multiplicity of two.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
 Makefile                  |  1 +
 t/helper/test-name-hash.c | 23 +++++++++++++++++++++++
 t/helper/test-tool.c      |  1 +
 t/helper/test-tool.h      |  1 +
 4 files changed, 26 insertions(+)
 create mode 100644 t/helper/test-name-hash.c

diff --git a/Makefile b/Makefile
index 154de6e01d0..462aff65a50 100644
--- a/Makefile
+++ b/Makefile
@@ -808,6 +808,7 @@ TEST_BUILTINS_OBJS += test-lazy-init-name-hash.o
 TEST_BUILTINS_OBJS += test-match-trees.o
 TEST_BUILTINS_OBJS += test-mergesort.o
 TEST_BUILTINS_OBJS += test-mktemp.o
+TEST_BUILTINS_OBJS += test-name-hash.o
 TEST_BUILTINS_OBJS += test-oid-array.o
 TEST_BUILTINS_OBJS += test-online-cpus.o
 TEST_BUILTINS_OBJS += test-pack-mtimes.o
diff --git a/t/helper/test-name-hash.c b/t/helper/test-name-hash.c
new file mode 100644
index 00000000000..c82ccd7cefd
--- /dev/null
+++ b/t/helper/test-name-hash.c
@@ -0,0 +1,23 @@
+/*
+ * test-name-hash.c: Read a list of paths over stdin and report on their
+ * name-hash and full name-hash.
+ */
+
+#include "test-tool.h"
+#include "git-compat-util.h"
+#include "pack-objects.h"
+#include "strbuf.h"
+
+int cmd__name_hash(int argc, const char **argv)
+{
+	struct strbuf line = STRBUF_INIT;
+
+	while (!strbuf_getline(&line, stdin)) {
+		uint32_t name_hash = pack_name_hash(line.buf);
+		uint32_t full_hash = pack_full_name_hash(line.buf);
+
+		printf("%10"PRIu32"\t%10"PRIu32"\t%s\n", name_hash, full_hash, line.buf);
+	}
+
+	return 0;
+}
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index f8a67df7de9..4a603921002 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -43,6 +43,7 @@ static struct test_cmd cmds[] = {
 	{ "match-trees", cmd__match_trees },
 	{ "mergesort", cmd__mergesort },
 	{ "mktemp", cmd__mktemp },
+	{ "name-hash", cmd__name_hash },
 	{ "oid-array", cmd__oid_array },
 	{ "online-cpus", cmd__online_cpus },
 	{ "pack-mtimes", cmd__pack_mtimes },
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index e74bc0ffd41..56a83bf3aac 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -37,6 +37,7 @@ int cmd__lazy_init_name_hash(int argc, const char **argv);
 int cmd__match_trees(int argc, const char **argv);
 int cmd__mergesort(int argc, const char **argv);
 int cmd__mktemp(int argc, const char **argv);
+int cmd__name_hash(int argc, const char **argv);
 int cmd__online_cpus(int argc, const char **argv);
 int cmd__pack_mtimes(int argc, const char **argv);
 int cmd__parse_options(int argc, const char **argv);
-- 
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