From 79aa7d7a0318db9913d7cec5473ef51ef2e04593 Mon Sep 17 00:00:00 2001
From: Rasmus Dahlberg <rasmus@mullvad.net>
Date: Mon, 20 Dec 2021 14:37:25 +0100
Subject: hex: Add lower-case hex parser and tests

---
 pkg/hex/hex.go      | 54 +++++++++++++++++++++++++++++++++++++++++++
 pkg/hex/hex_test.go | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 120 insertions(+)
 create mode 100644 pkg/hex/hex.go
 create mode 100644 pkg/hex/hex_test.go

diff --git a/pkg/hex/hex.go b/pkg/hex/hex.go
new file mode 100644
index 0000000..1b5e324
--- /dev/null
+++ b/pkg/hex/hex.go
@@ -0,0 +1,54 @@
+// package hex implements a lower-case hex parser.
+package hex
+
+import (
+	"fmt"
+)
+
+const (
+	language = "0123456789abcdef"
+)
+
+// Serialize serializes a buffer as lower-case hex
+func Serialize(buf []byte) string {
+	out := make([]byte, len(buf)*2)
+	for i, b := range buf {
+		offset := i * 2
+		out[offset] = language[b>>4]
+		out[offset+1] = language[b&0x0f]
+	}
+	return string(out)
+}
+
+// Deserialize tries to deserialize a lower-case hex string
+func Deserialize(str string) ([]byte, error) {
+	if len(str)%2 != 0 {
+		return nil, fmt.Errorf("hex: string must have even length")
+	}
+
+	buf := make([]byte, len(str)/2)
+	for i := 0; i < len(buf); i++ {
+		offset := i * 2
+		first, ok := deserializeOne(str[offset])
+		if !ok {
+			return nil, fmt.Errorf("hex: invalid character at index %d: %d", i, first)
+		}
+		second, ok := deserializeOne(str[offset+1])
+		if !ok {
+			return nil, fmt.Errorf("hex: invalid character at index %d: %d", i, second)
+		}
+		buf[i] = first << 4
+		buf[i] = buf[i] | second
+	}
+	return buf, nil
+}
+
+func deserializeOne(b byte) (byte, bool) {
+	if b >= '0' && b <= '9' {
+		return b - '0', true
+	}
+	if b >= 'a' && b <= 'f' {
+		return b - 'a' + 10, true
+	}
+	return 0, false
+}
diff --git a/pkg/hex/hex_test.go b/pkg/hex/hex_test.go
new file mode 100644
index 0000000..222bd5e
--- /dev/null
+++ b/pkg/hex/hex_test.go
@@ -0,0 +1,66 @@
+package hex
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestSerialize(t *testing.T) {
+	for _, table := range []struct {
+		desc  string
+		input []byte
+		want  string
+	}{
+		{
+			desc:  "valid",
+			input: []byte{0, 9, 10, 15, 16, 17, 254, 255},
+			want:  "00090a0f1011feff",
+		},
+	} {
+		str := Serialize(table.input)
+		if got, want := str, table.want; got != want {
+			t.Errorf("got %q but wanted %q in test %q", got, want, table.desc)
+		}
+	}
+}
+
+func TestDeserialize(t *testing.T) {
+	for _, table := range []struct {
+		desc  string
+		input string
+		want  []byte
+		err   bool
+	}{
+		{
+			desc:  "invalid: length is odd",
+			input: "0",
+			err:   true,
+		},
+		{
+			desc:  "invalid: even index has invalid character",
+			input: "A0",
+			err:   true,
+		},
+		{
+			desc:  "invalid: odd index has invalid character",
+			input: "0A",
+			err:   true,
+		},
+		{
+			desc:  "valid",
+			input: "00090a0f1011feff",
+			want:  []byte{0, 9, 10, 15, 16, 17, 254, 255},
+		},
+	} {
+		buf, err := Deserialize(table.input)
+		if got, want := err != nil, table.err; got != want {
+			t.Errorf("got error %v but wanted %v in test %q: %v", got, want, table.desc, err)
+		}
+		if err != nil {
+			continue
+		}
+		if got, want := buf, table.want; !bytes.Equal(got, want) {
+			t.Errorf("got %v but wanted %v in test %q", got, want, table.desc)
+		}
+	}
+}
-- 
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