From 825a3c18fbc8e0497eafea9254baadb2951f23c6 Mon Sep 17 00:00:00 2001
From: Jim Myhrberg <contact@jimeh.me>
Date: Tue, 16 Mar 2021 02:14:56 +0000
Subject: [PATCH] feat(strings): add UUID function to generate random RFC 4122
 UUID v4 strings

---
 strings.go              | 27 +++++++++++++++++++++++++++
 strings_example_test.go |  5 +++++
 strings_test.go         | 29 +++++++++++++++++++++++++++++
 3 files changed, 61 insertions(+)

diff --git a/strings.go b/strings.go
index ddea864..f3152d3 100644
--- a/strings.go
+++ b/strings.go
@@ -18,6 +18,7 @@ const (
 	alphabeticChars   = upperChars + lowerChars
 	alphanumericChars = alphabeticChars + numericChars
 	dnsLabelChars     = lowerNumericChars + "-"
+	uuidHyphen        = byte('-')
 )
 
 var (
@@ -233,6 +234,32 @@ func DNSLabel(n int) (string, error) {
 	}
 }
 
+// UUID returns a random UUID v4 in string format as defined by RFC 4122,
+// section 4.4.
+func UUID() (string, error) {
+	b, err := Bytes(16)
+	if err != nil {
+		return "", err
+	}
+
+	b[6] = (b[6] & 0x0f) | 0x40 // Version: 4 (random)
+	b[8] = (b[8] & 0x3f) | 0x80 // Variant: RFC 4122
+
+	// Construct a UUID v4 string according to RFC 4122 specifications.
+	dst := make([]byte, 36)
+	hex.Encode(dst[0:8], b[0:4]) // time-low
+	dst[8] = uuidHyphen
+	hex.Encode(dst[9:13], b[4:6]) // time-mid
+	dst[13] = uuidHyphen
+	hex.Encode(dst[14:18], b[6:8]) // time-high-and-version
+	dst[18] = uuidHyphen
+	hex.Encode(dst[19:23], b[8:10]) // clock-seq-and-reserved, clock-seq-low
+	dst[23] = uuidHyphen
+	hex.Encode(dst[24:], b[10:]) // node
+
+	return string(dst), nil
+}
+
 func isASCII(s string) bool {
 	for _, c := range s {
 		if c > unicode.MaxASCII {
diff --git a/strings_example_test.go b/strings_example_test.go
index 9ca4380..e59a97a 100644
--- a/strings_example_test.go
+++ b/strings_example_test.go
@@ -70,3 +70,8 @@ func ExampleDNSLabel() {
 	s, _ := rands.DNSLabel(16)
 	fmt.Println(s) // => z0ij9o8qkbs0ru-h
 }
+
+func ExampleUUID() {
+	s, _ := rands.UUID()
+	fmt.Println(s) // => a62b8712-f238-43ba-a47e-333f5fffe785
+}
diff --git a/strings_test.go b/strings_test.go
index 03b5c31..1df898e 100644
--- a/strings_test.go
+++ b/strings_test.go
@@ -2,6 +2,7 @@ package rands
 
 import (
 	"encoding/base64"
+	"encoding/hex"
 	"errors"
 	"regexp"
 	"strings"
@@ -579,6 +580,34 @@ func BenchmarkDNSLabel(b *testing.B) {
 	}
 }
 
+func TestUUID(t *testing.T) {
+	m := regexp.MustCompile(
+		`^[a-f0-9]{8}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{12}$`,
+	)
+
+	for i := 0; i < 10000; i++ {
+		got, err := UUID()
+		require.NoError(t, err)
+		require.Regexp(t, m, got)
+
+		raw := strings.ReplaceAll(got, "-", "")
+		b := make([]byte, 16)
+		_, err = hex.Decode(b, []byte(raw))
+		require.NoError(t, err)
+
+		require.Equal(t, 4, int(b[6]>>4), "version is not 4")
+		require.Equal(t, byte(0x80), b[8]&0xc0,
+			"variant is not RFC 4122",
+		)
+	}
+}
+
+func BenchmarkUUID(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		_, _ = UUID()
+	}
+}
+
 //
 // Helpers
 //