Introduce git_path_make_relative

src/path.c

@@ -750,6 +750,61 @@ int git_path_cmp(
 	return (c1 < c2) ? -1 : (c1 > c2) ? 1 : 0;
 }
 
+int git_path_make_relative(git_buf *path, const char *parent)
+{
+	const char *p, *q, *p_dirsep, *q_dirsep;
+	size_t plen = path->size, newlen, depth = 1, i;
+
+	for (p_dirsep = p = path->ptr, q_dirsep = q = parent; *p && *q; p++, q++) {
+		if (*p == '/' && *q == '/') {
+			p_dirsep = p;
+			q_dirsep = q;
+		}
+		else if (*p != *q)
+			break;
+	}
+
+	/* need at least 1 common path segment */
+	if ((p_dirsep == path->ptr || q_dirsep == parent) &&
+		(*p_dirsep != '/' || *q_dirsep != '/')) {
+		giterr_set(GITERR_INVALID,
+			"%s is not a parent of %s", parent, path->ptr);
+		return GIT_ENOTFOUND;
+	}
+
+	if (*p == '/' && !*q)
+		p++;
+	else if (!*p && *q == '/')
+		q++;
+	else if (!*p && !*q)
+		return git_buf_clear(path), 0;
+	else {
+		p = p_dirsep + 1;
+		q = q_dirsep + 1;
+	}
+
+	plen -= (p - path->ptr);
+
+	if (!*q)
+		return git_buf_set(path, p, plen);
+
+	for (; (q = strchr(q, '/')) && *(q + 1); q++)
+		depth++;
+
+	newlen = (depth * 3) + plen;
+
+	if (git_buf_try_grow(path, newlen + 1, 1, 0) < 0)
+		return -1;
+
+	memmove(path->ptr + (depth * 3), p, plen + 1);
+
+	for (i = 0; i < depth; i++)
+		memcpy(path->ptr + (i * 3), "../", 3);
+
+	path->size = newlen;
+	return 0;
+}
+
 bool git_path_has_non_ascii(const char *path, size_t pathlen)
 {
 	const uint8_t *scan = (const uint8_t *)path, *end;

src/path.h

@@ -196,6 +196,17 @@ extern bool git_path_contains(git_buf *dir, const char *item);
  */
 extern bool git_path_contains_dir(git_buf *parent, const char *subdir);
 
+/**
+ * Make the path relative to the given parent path.
+ *
+ * @param path The path to make relative
+ * @param parent The parent path to make path relative to
+ * @return 0 if path was made relative, GIT_ENOTFOUND
+ *         if there was not common root between the paths,
+ *         or <0.
+ */
+extern int git_path_make_relative(git_buf *path, const char *parent);
+
 /**
  * Check if the given path contains the given file.
  *

tests/path/core.c

@@ -0,0 +1,55 @@
+#include "clar_libgit2.h"
+#include "path.h"
+
+static void test_make_relative(
+	const char *expected_path,
+	const char *path,
+	const char *parent,
+	int expected_status)
+{
+	git_buf buf = GIT_BUF_INIT;
+	git_buf_puts(&buf, path);
+	cl_assert_equal_i(expected_status, git_path_make_relative(&buf, parent));
+	cl_assert_equal_s(expected_path, buf.ptr);
+	git_buf_free(&buf);
+}
+
+void test_path_core__make_relative(void)
+{
+	git_buf buf = GIT_BUF_INIT;
+
+	test_make_relative("foo.c", "/path/to/foo.c", "/path/to", 0);
+	test_make_relative("bar/foo.c", "/path/to/bar/foo.c", "/path/to", 0);
+	test_make_relative("foo.c", "/path/to/foo.c", "/path/to/", 0);
+
+	test_make_relative("", "/path/to", "/path/to", 0);
+	test_make_relative("", "/path/to", "/path/to/", 0);
+
+	test_make_relative("../", "/path/to", "/path/to/foo", 0);
+
+	test_make_relative("../foo.c", "/path/to/foo.c", "/path/to/bar", 0);
+	test_make_relative("../bar/foo.c", "/path/to/bar/foo.c", "/path/to/baz", 0);
+
+	test_make_relative("../../foo.c", "/path/to/foo.c", "/path/to/foo/bar", 0);
+	test_make_relative("../../foo/bar.c", "/path/to/foo/bar.c", "/path/to/bar/foo", 0);
+
+	test_make_relative("../../foo.c", "/foo.c", "/bar/foo", 0);
+
+	test_make_relative("foo.c", "/path/to/foo.c", "/path/to/", 0);
+	test_make_relative("../foo.c", "/path/to/foo.c", "/path/to/bar/", 0);
+
+	test_make_relative("foo.c", "d:/path/to/foo.c", "d:/path/to", 0);
+
+	test_make_relative("../foo", "/foo", "/bar", 0);
+	test_make_relative("path/to/foo.c", "/path/to/foo.c", "/", 0);
+	test_make_relative("../foo", "path/to/foo", "path/to/bar", 0);
+
+	test_make_relative("/path/to/foo.c", "/path/to/foo.c", "d:/path/to", GIT_ENOTFOUND);
+	test_make_relative("d:/path/to/foo.c", "d:/path/to/foo.c", "/path/to", GIT_ENOTFOUND);
+	
+	test_make_relative("/path/to/foo.c", "/path/to/foo.c", "not-a-rooted-path", GIT_ENOTFOUND);
+	test_make_relative("not-a-rooted-path", "not-a-rooted-path", "/path/to", GIT_ENOTFOUND);
+	
+	test_make_relative("/path", "/path", "pathtofoo", GIT_ENOTFOUND);
+	test_make_relative("path", "path", "pathtofoo", GIT_ENOTFOUND);
+}