From 4e2e05c1b9db6c9d59c865520d2add4afa6f0fa6 Mon Sep 17 00:00:00 2001
From: Claude Code <aneurin+claude@aneur.in>
Date: Mon, 7 Sep 2026 16:27:22 +0100
Subject: [PATCH] length: count runes, not bytes

MinLength/MaxLength measured len(value), so a string of 8 accented
characters (9+ bytes) failed MaxLength(8) despite the error message
promising "characters". Count with utf8.RuneCountInString so the check
matches the wording and the common-sense intent.

Still not grapheme-cluster aware (combining marks, emoji ZWJ sequences
count as several runes), which is out of scope for a stdlib-only helper.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 length.go      | 10 ++++++----
 length_test.go | 18 ++++++++++++++++--
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/length.go b/length.go
index 02d77f6..22987cd 100644
--- a/length.go
+++ b/length.go
@@ -1,15 +1,17 @@
 package validate
 
+import "unicode/utf8"
+
 var (
 	ErrMustBeLonger  = NewError("must contain at least %d characters")
 	ErrMustBeShorter = NewError("must contain no more than %d characters")
 )
 
 // MaxLength validates that a string is no longer than a given maximum.
-// Length is measured in bytes (len), not runes.
+// Length is counted in runes, so multi-byte characters count as one.
 func MaxLength(l int) func(string) error {
 	return func(value string) error {
-		if len(value) > l {
+		if utf8.RuneCountInString(value) > l {
 			return ErrMustBeShorter.With(l)
 		}
 		return nil
@@ -17,10 +19,10 @@ func MaxLength(l int) func(string) error {
 }
 
 // MinLength validates that a string is at least a given minimum length.
-// Length is measured in bytes (len), not runes.
+// Length is counted in runes, so multi-byte characters count as one.
 func MinLength(l int) func(string) error {
 	return func(value string) error {
-		if len(value) < l {
+		if utf8.RuneCountInString(value) < l {
 			return ErrMustBeLonger.With(l)
 		}
 		return nil
diff --git a/length_test.go b/length_test.go
index defd24e..b471c38 100644
--- a/length_test.go
+++ b/length_test.go
@@ -20,7 +20,14 @@ func ExampleMinLength() {
 
 func TestMaxLength(t *testing.T) {
 	testCases := map[int]map[string]error{
-		8: {"abcd": nil, "abcdefgh": nil, "abcd efg": nil, "abcdefghi": ErrMustBeShorter.With(8)},
+		8: {
+			"abcd":      nil,
+			"abcdefgh":  nil,
+			"abcd efg":  nil,
+			"abcdéfgh":  nil, // 8 runes, 9 bytes
+			"abcdefghi": ErrMustBeShorter.With(8),
+			"abcdéfghi": ErrMustBeShorter.With(8), // 9 runes, 10 bytes
+		},
 	}
 
 	for setup, values := range testCases {
@@ -41,7 +48,14 @@ func TestMaxLength(t *testing.T) {
 
 func TestMinLength(t *testing.T) {
 	testCases := map[int]map[string]error{
-		8: {"abcd": ErrMustBeLonger.With(8), "abcdefgh": nil, "abcd efg": nil, "abcdefghi": nil},
+		8: {
+			"abcd":      ErrMustBeLonger.With(8),
+			"abcdéfg":   ErrMustBeLonger.With(8), // 7 runes, 8 bytes
+			"abcdefgh":  nil,
+			"abcdéfgh":  nil, // 8 runes, 9 bytes
+			"abcd efg":  nil,
+			"abcdefghi": nil,
+		},
 	}
 
 	for setup, values := range testCases {