Validate length of multibyte characters better

[JaTochNietDan]

If we length check on just the string when there's some foreign characters in there it can often come back as a length you wouldn't expect. This doesn't make much sense to me for validation purposes, you always want the length of a string to mean x characters, not bytes.

See my Go Playground: https://play.golang.org/p/GNDIG2Tzx4y

I would expect the behaviour of the runed string, not the latter when validating.

[codecov-io]

Codecov Report

Merging #24 into master will increase coverage by 0.02%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
validate.go	95.04% <100%> (+0.02%)	⬆️

[arp242]

Small thing: if you're creating an intermediate variable then you might as well use length := len([]rune(value))? That's only what it's used for.

There is also utf8.RuneCountInString(), which seems a bit faster:

Original:
	BenchmarkLenASCII-4     300000000                4.04 ns/op
	BenchmarkLenMB-4        300000000                4.04 ns/op

Original PR:
	BenchmarkLenASCII-4     30000000                48.1 ns/op
	BenchmarkLenMB-4        20000000                66.3 ns/op

RuneCountInString:
	BenchmarkLenASCII-4     50000000                26.1 ns/op
	BenchmarkLenMB-4        50000000                32.7 ns/op

Not that it really matters all that much in this context, but don't see a reason not to use it?

Full patch with benchmark:

diff --git i/validate.go w/validate.go
index 64bb805..ec9846d 100644
--- i/validate.go
+++ w/validate.go
@@ -52,6 +52,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode/utf8"
 
 	"github.com/teamwork/mailaddress"
 )
@@ -439,16 +440,16 @@ func (v *Validator) HexColor(key, value string, message ...string) {
 func (v *Validator) Len(key, value string, min, max int, message ...string) {
 	msg := getMessage(message, "")
 
-	runeVal := []rune(value)
+	l := utf8.RuneCountInString(value)
 
 	switch {
-	case len(runeVal) < min:
+	case l < min:
 		if msg != "" {
 			v.Append(key, msg)
 		} else {
 			v.Append(key, fmt.Sprintf(MessageLenLonger, min))
 		}
-	case max > 0 && len(runeVal) > max:
+	case max > 0 && l > max:
 		if msg != "" {
 			v.Append(key, msg)
 		} else {
diff --git i/validate_test.go w/validate_test.go
index 476c2be..1319d69 100644
--- i/validate_test.go
+++ w/validate_test.go
@@ -810,3 +810,17 @@ func TestErrorOrNil(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkLenASCII(b *testing.B) {
+	v := New()
+	for n := 0; n < b.N; n++ {
+		v.Len("x", "Hello, world, it's a sentence!", 0, 40)
+	}
+}
+
+func BenchmarkLenMB(b *testing.B) {
+	v := New()
+	for n := 0; n < b.N; n++ {
+		v.Len("x", "H€llo, world, it's a sentence¡", 0, 40)
+	}
+}

[arp242]

Could you clarify the comment that it counts characters, and not byte size? There are use cases for both, so seems to me that it's good to clarify in docs. e.g.

// Len sets the minimum and maximum length for a string.
//
// A maximum of 0 indicates there is no upper limit. The length is counted in
// characters (runes), not bytes.

[arp242]

👍