path: introduce append_formatted_path() for shared path formatting

The path-formatting logic in builtin/rev-parse.c is tightly coupled
to that command and writes directly to stdout, making it impossible
for other builtins to reuse.

Extract the core algorithm into append_formatted_path() in path.c
and expose a path_format enum in path.h so that any builtin can
format paths consistently without duplicating logic.

Mentored-by: Justin Tobler <jltobler@gmail.com>
Mentored-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -1579,6 +1579,76 @@ char *xdg_cache_home(const char *filename)
 	return NULL;
 }
 
+void append_formatted_path(struct strbuf *dest, const char *path,
+			   const char *prefix, enum path_format format)
+{
+	switch (format) {
+	case PATH_FORMAT_DEFAULT:
+	case PATH_FORMAT_UNMODIFIED:
+		strbuf_addstr(dest, path);
+		break;
+
+	case PATH_FORMAT_RELATIVE: {
+		struct strbuf relative_buf = STRBUF_INIT;
+		struct strbuf real_path = STRBUF_INIT;
+		struct strbuf real_prefix = STRBUF_INIT;
+		char *cwd = NULL;
+
+		/*
+		 * We don't ever produce a relative path if prefix is NULL,
+		 * so set the prefix to the current directory so that we can
+		 * produce a relative path whenever possible.
+		 */
+		if (!prefix)
+			prefix = cwd = xgetcwd();
+
+		if (!is_absolute_path(path)) {
+			strbuf_realpath_forgiving(&real_path, path, 1);
+			path = real_path.buf;
+		}
+		if (!is_absolute_path(prefix)) {
+			strbuf_realpath_forgiving(&real_prefix, prefix, 1);
+			prefix = real_prefix.buf;
+		}
+
+		strbuf_addstr(dest, relative_path(path, prefix, &relative_buf));
+
+		strbuf_release(&relative_buf);
+		strbuf_release(&real_path);
+		strbuf_release(&real_prefix);
+		free(cwd);
+		break;
+	}
+
+	case PATH_FORMAT_RELATIVE_IF_SHARED: {
+		struct strbuf relative_buf = STRBUF_INIT;
+
+		/*
+		 * If we're using RELATIVE_IF_SHARED mode, then we want an
+		 * absolute path unless the two share a common prefix, so don't
+		 * default the prefix to the current working directory. Doing so
+		 * would cause a relative path to always be produced if possible.
+		 */
+		strbuf_addstr(dest, relative_path(path, prefix, &relative_buf));
+		strbuf_release(&relative_buf);
+		break;
+	}
+
+	case PATH_FORMAT_CANONICAL: {
+		struct strbuf canonical_buf = STRBUF_INIT;
+
+		strbuf_realpath_forgiving(&canonical_buf, path, 1);
+		strbuf_addbuf(dest, &canonical_buf);
+
+		strbuf_release(&canonical_buf);
+		break;
+	}
+
+	default:
+		BUG("unknown path_format value %d", format);
+	}
+}
+
 REPO_GIT_PATH_FUNC(squash_msg, "SQUASH_MSG")
 REPO_GIT_PATH_FUNC(merge_msg, "MERGE_MSG")
 REPO_GIT_PATH_FUNC(merge_rr, "MERGE_RR")

path.h

@@ -262,6 +262,42 @@ enum scld_error safe_create_leading_directories_no_share(char *path);
 int safe_create_file_with_leading_directories(struct repository *repo,
 					      const char *path);
 
+/**
+ * The formatting strategy to apply when writing a path into a buffer.
+ */
+enum path_format {
+	/*
+	 * Represents the default formatting behavior. Treated as
+	 * PATH_FORMAT_UNMODIFIED by append_formatted_path().
+	 */
+	PATH_FORMAT_DEFAULT,
+
+	/* Output the path exactly as-is without any modifications. */
+	PATH_FORMAT_UNMODIFIED,
+
+	/* Output a path relative to the provided directory prefix. */
+	PATH_FORMAT_RELATIVE,
+
+	/* Output a relative path only if the path shares a root with the prefix. */
+	PATH_FORMAT_RELATIVE_IF_SHARED,
+
+	/* Output a fully resolved, absolute canonical path. */
+	PATH_FORMAT_CANONICAL
+};
+
+/**
+ * Format a path according to the specified formatting strategy and append
+ * the result to the given strbuf.
+ *
+ * `dest`   : The string buffer to append the formatted path to.
+ * `path`   : The path string that needs to be formatted.
+ * `prefix` : The directory prefix to calculate relative offsets against.
+ * Pass NULL to default to the current working directory where applicable.
+ * `format` : The formatting behavior rule to execute.
+ */
+void append_formatted_path(struct strbuf *dest, const char *path,
+			   const char *prefix, enum path_format format);
+
 # ifdef USE_THE_REPOSITORY_VARIABLE
 #  include "strbuf.h"
 #  include "repository.h"