checkout: add fuzzy branch name suggestions

Makefile

@@ -1091,6 +1091,7 @@ LIB_OBJS += blame.o
 LIB_OBJS += blob.o
 LIB_OBJS += bloom.o
 LIB_OBJS += branch.o
+LIB_OBJS += branch-suggestions.o
 LIB_OBJS += bundle-uri.o
 LIB_OBJS += bundle.o
 LIB_OBJS += cache-tree.o

branch-suggestions.c

@@ -0,0 +1,99 @@
+#define USE_THE_REPOSITORY_VARIABLE
+
+#include "git-compat-util.h"
+#include "branch-suggestions.h"
+#include "refs.h"
+#include "string-list.h"
+#include "levenshtein.h"
+#include "gettext.h"
+#include "repository.h"
+#include "strbuf.h"
+
+#define SIMILARITY_FLOOR 7
+#define SIMILAR_ENOUGH(x) ((x) < SIMILARITY_FLOOR)
+#define MAX_SUGGESTIONS 5
+
+struct branch_suggestion_cb {
+	const char *attempted_name;
+	struct string_list *suggestions;
+};
+
+static int collect_branch_cb(const char *refname, const char *referent UNUSED,
+			     const struct object_id *oid UNUSED,
+			     int flags UNUSED, void *cb_data)
+{
+	struct branch_suggestion_cb *cb = cb_data;
+	const char *branch_name;
+
+	/* Since we're using refs_for_each_ref_in with "refs/heads/",
+	 * the refname might already be stripped or might still have the prefix */
+	if (starts_with(refname, "refs/heads/")) {
+		branch_name = refname + strlen("refs/heads/");
+	} else {
+		branch_name = refname;
+	}
+
+	/* Skip the attempted name itself */
+	if (!strcmp(branch_name, cb->attempted_name))
+		return 0;
+
+	string_list_append(cb->suggestions, branch_name);
+	return 0;
+}
+
+void suggest_similar_branch_names(const char *attempted_name)
+{
+	struct string_list branches = STRING_LIST_INIT_DUP;
+	struct branch_suggestion_cb cb_data;
+	size_t i;
+	int best_similarity = INT_MAX;
+	int suggestion_count = 0;
+
+	cb_data.attempted_name = attempted_name;
+	cb_data.suggestions = &branches;
+
+	/* Collect all local branch names */
+	refs_for_each_ref_in(get_main_ref_store(the_repository), "refs/heads/",
+			     collect_branch_cb, &cb_data);
+
+	if (!branches.nr)
+		goto cleanup;
+
+	/* Calculate Levenshtein distances */
+	for (i = 0; i < branches.nr; i++) {
+		const char *branch_name = branches.items[i].string;
+		int distance;
+
+		/* Give prefix matches a very good score */
+		if (starts_with(branch_name, attempted_name)) {
+			distance = 0;
+		} else {
+			distance = levenshtein(attempted_name, branch_name, 0, 2, 1, 3);
+		}
+
+		branches.items[i].util = (void *)(intptr_t)distance;
+
+		if (distance < best_similarity)
+			best_similarity = distance;
+	}
+
+	/* Only show suggestions if they're similar enough */
+	if (!SIMILAR_ENOUGH(best_similarity))
+		goto cleanup;
+
+	/* Count and display similar branches */
+	for (i = 0; i < branches.nr && suggestion_count < MAX_SUGGESTIONS; i++) {
+		int distance = (int)(intptr_t)branches.items[i].util;
+
+		if (distance <= best_similarity && SIMILAR_ENOUGH(distance)) {
+			if (suggestion_count == 0) {
+				fprintf(stderr, "%s\n", _("hint: Did you mean one of these?"));
+			}
+			fprintf(stderr, "hint:     %s\n", branches.items[i].string);
+			suggestion_count++;
+		}
+	}
+
+cleanup:
+	string_list_clear(&branches, 0);
+}

branch-suggestions.h

@@ -0,0 +1,11 @@
+#ifndef BRANCH_SUGGESTIONS_H
+#define BRANCH_SUGGESTIONS_H
+
+/**
+ * Suggest similar branch names when a branch checkout fails.
+ * This function analyzes local branches and suggests ones that are
+ * similar to the attempted branch name using fuzzy matching.
+ */
+void suggest_similar_branch_names(const char *attempted_name);
+
+#endif /* BRANCH_SUGGESTIONS_H */

builtin/checkout.c

@@ -4,6 +4,7 @@
 #include "builtin.h"
 #include "advice.h"
 #include "branch.h"
+#include "branch-suggestions.h"
 #include "cache-tree.h"
 #include "checkout.h"
 #include "commit.h"
@@ -605,6 +606,10 @@ static int checkout_paths(const struct checkout_opts *opts,
 							opts);
 
 	if (report_path_error(ps_matched, &opts->pathspec)) {
+		/* If there's only one pathspec and it looks like a branch name, suggest similar branches */
+		if (opts->pathspec.nr == 1 && !strchr(opts->pathspec.items[0].original, '/')) {
+			suggest_similar_branch_names(opts->pathspec.items[0].original);
+		}
 		free(ps_matched);
 		return 1;
 	}
@@ -1447,8 +1452,10 @@ static int parse_branchname_arg(int argc, const char **argv,
 		}
 
 		if (!recover_with_dwim) {
-			if (has_dash_dash)
+			if (has_dash_dash) {
+				suggest_similar_branch_names(arg);
 				die(_("invalid reference: %s"), arg);
+			}
 			return argcount;
 		}
 	}
@@ -1635,9 +1642,11 @@ static int checkout_branch(struct checkout_opts *opts,
 	} else if (opts->track == BRANCH_TRACK_UNSPECIFIED)
 		opts->track = git_branch_track;
 
-	if (new_branch_info->name && !new_branch_info->commit)
+	if (new_branch_info->name && !new_branch_info->commit) {
+		suggest_similar_branch_names(new_branch_info->name);
 		die(_("Cannot switch branch to a non-commit '%s'"),
 		    new_branch_info->name);
+	}
 
 	if (noop_switch &&
 	    !opts->switch_branch_doing_nothing_is_ok)

meson.build

@@ -287,6 +287,7 @@ libgit_sources = [
   'blob.c',
   'bloom.c',
   'branch.c',
+  'branch-suggestions.c',
   'bundle-uri.c',
   'bundle.c',
   'cache-tree.c',

t/t2028-checkout-branch-suggestions.sh

@@ -0,0 +1,37 @@
+#!/bin/sh
+
+test_description='checkout branch suggestions'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit initial &&
+	git branch feature-authentication &&
+	git branch feature-authorization &&
+	git branch bugfix-auth-issue
+'
+
+test_expect_success 'suggest similar branch names on checkout failure' '
+	test_must_fail git checkout feature-auth 2>err &&
+	grep "hint: Did you mean one of these?" err &&
+	grep "feature-authentication" err &&
+	grep "feature-authorization" err
+'
+
+test_expect_success 'suggest single branch name on close match' '
+	test_must_fail git checkout feature-authent 2>err &&
+	grep "hint: Did you mean one of these?" err &&
+	grep "feature-authentication" err
+'
+
+test_expect_success 'no suggestions for very different names' '
+	test_must_fail git checkout completely-different-name 2>err &&
+	! grep "hint: Did you mean" err
+'
+
+test_expect_success 'no suggestions for paths with slashes' '
+	test_must_fail git checkout nonexistent/file.txt 2>err &&
+	! grep "hint: Did you mean" err
+'
+
+test_done