[GSoC PATCH v2 1/4] path: introduce format_path() for centralized path formatting

[K Jayatheerth <jayatheerthkulkarni2005@gmail.com>]

The path-formatting logic inside `builtin/rev-parse.c` handles absolute,
canonical, and relative formatting rules based on user-supplied options.
However, this logic is tightly coupled to `rev-parse` and writes directly
to stdout.

To allow other builtins (such as the upcoming `git repo` path keys) to
re-use this logic, extract the core path-formatting algorithm into a centralized
helper function, `format_path()`, in `path.c`.

Expose a single, streamlined `path_format` enum in `path.h` to let callers
explicitly declare their formatting strategy (UNMODIFIED, RELATIVE,
RELATIVE_IF_SHARED, or CANONICAL). This decouples the core algorithm from
the localized fallback mechanics specific to `rev-parse`.

Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Mentored-by: Justin Tobler <jltobler@gmail.com>
Mentored-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
---
 path.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 path.h | 30 ++++++++++++++++++++++++++++++
 2 files changed, 88 insertions(+)

diff --git a/path.c b/path.c
index d7e17bf174..2fcd24c5eb 100644
--- a/path.c
+++ b/path.c
@@ -1579,6 +1579,64 @@ char *xdg_cache_home(const char *filename)
 	return NULL;
 }
 
+void format_path(struct strbuf *buf, const char *path,
+		 const char *prefix, enum path_format format)
+{
+	if (format == PATH_FORMAT_UNMODIFIED) {
+		strbuf_addstr(buf, path);
+		return;
+	}
+
+	if (format == 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(buf, relative_path(path, prefix, &relative_buf));
+
+		strbuf_release(&relative_buf);
+		strbuf_release(&real_path);
+		strbuf_release(&real_prefix);
+		free(cwd);
+	} else if (format == 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(buf, relative_path(path, prefix, &relative_buf));
+		strbuf_release(&relative_buf);
+	} else if (format == PATH_FORMAT_CANONICAL) {
+		struct strbuf canonical_buf = STRBUF_INIT;
+
+		strbuf_realpath_forgiving(&canonical_buf, path, 1);
+		strbuf_addbuf(buf, &canonical_buf);
+
+		strbuf_release(&canonical_buf);
+	}
+}
+
 REPO_GIT_PATH_FUNC(squash_msg, "SQUASH_MSG")
 REPO_GIT_PATH_FUNC(merge_msg, "MERGE_MSG")
 REPO_GIT_PATH_FUNC(merge_rr, "MERGE_RR")
diff --git a/path.h b/path.h
index 0434ba5e07..a78e0fc141 100644
--- a/path.h
+++ b/path.h
@@ -262,6 +262,36 @@ 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 {
+	/* 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.
+ *
+ * `buf`    : 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 format_path(struct strbuf *buf, const char *path,
+		 const char *prefix, enum path_format format);
+
 # ifdef USE_THE_REPOSITORY_VARIABLE
 #  include "strbuf.h"
 #  include "repository.h"
-- 
2.54.0