[RFC/PATCHv2 4/6] pretty: support %G to show the generation number of a commit

[Jeff King <peff@peff.net>]

This might be useful for external programs doing topological
sorting or other graph analysis. It's also handy for testing
the generation calculation code.

Signed-off-by: Jeff King <peff@peff.net>
---
This now includes some basic tests.

 Documentation/pretty-formats.txt |    1 +
 pretty.c                         |    3 ++
 t/t6070-commit-generations.sh    |   41 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 45 insertions(+), 0 deletions(-)
 create mode 100755 t/t6070-commit-generations.sh

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 561cc9f..c58ab52 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -133,6 +133,7 @@ The placeholders are:
 - '%gD': reflog selector, e.g., `refs/stash@\{1\}`
 - '%gd': shortened reflog selector, e.g., `stash@\{1\}`
 - '%gs': reflog subject
+- '%G': generation number (i.e., distance of path to farthest root ancestor)
 - '%Cred': switch color to red
 - '%Cgreen': switch color to green
 - '%Cblue': switch color to blue
diff --git a/pretty.c b/pretty.c
index f45eb54..8f1b321 100644
--- a/pretty.c
+++ b/pretty.c
@@ -965,6 +965,9 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 			return 2;
 		}
 		return 0;	/* unknown %g placeholder */
+	case 'G':
+		strbuf_addf(sb, "%lu", commit_generation(commit));
+		return 1;
 	case 'N':
 		if (c->pretty_ctx->show_notes) {
 			format_display_notes(commit->object.sha1, sb,
diff --git a/t/t6070-commit-generations.sh b/t/t6070-commit-generations.sh
new file mode 100755
index 0000000..3e0f2ad
--- /dev/null
+++ b/t/t6070-commit-generations.sh
@@ -0,0 +1,41 @@
+#!/bin/sh
+
+test_description='calculate and cache commit generations'
+. ./test-lib.sh
+
+test_expect_success 'setup history' '
+	test_commit one &&
+	test_commit two &&
+	test_commit three &&
+	test_commit four &&
+	git checkout -b other two &&
+	test_commit five &&
+	git checkout master &&
+	git merge other &&
+	test_commit six
+'
+
+cat >expect <<'EOF'
+5 six
+4 Merge branch 'other'
+2 five
+3 four
+2 three
+1 two
+0 one
+EOF
+test_expect_success 'check commit generations' '
+	git log --format="%G %s" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'cache file was created' '
+	test_path_is_file .git/cache/generations
+'
+
+test_expect_success 'cached values are the same' '
+	git log --format="%G %s" >actual &&
+	test_cmp expect actual
+'
+
+test_done
-- 
1.7.6.37.g989c6