Initial commit

go.mod

@@ -0,0 +1,3 @@
+module git.trashheap.io/blinkthethings/modhex
+
+go 1.15

modhex.go

@@ -0,0 +1,66 @@
+package modhex
+
+import (
+	"errors"
+	"fmt"
+)
+
+const hexChars = "0123456789abcdef"
+const modhexChars = "cbdefghijklnrtuv"
+
+var modhex2HexMap map[byte]byte
+var hex2ModhexMap map[byte]byte
+
+func init() {
+	hex2ModhexMap = make(map[byte]byte)
+	for i, h := range []byte(hexChars) {
+		hex2ModhexMap[h] = modhexChars[i]
+	}
+
+	modhex2HexMap = make(map[byte]byte)
+	for i, m := range []byte(modhexChars) {
+		modhex2HexMap[m] = hexChars[i]
+	}
+}
+
+// EncodeHex encodes a hex string into a modhex string
+func EncodeHex(hex string) (string, error) {
+	size := len([]byte(hex))
+
+	if size%2 == 1 {
+		return "", errors.New("size of input hex input not even")
+	}
+
+	var modhex = make([]byte, size)
+
+	for i, h := range []byte(hex) {
+		if m, ok := hex2ModhexMap[h]; ok {
+			modhex[i] = m
+		} else {
+			return "", fmt.Errorf("input not hex encoded; position %d", i)
+		}
+	}
+
+	return string(modhex), nil
+}
+
+// DecodeHex decodes a modhex string into a hex string
+func DecodeHex(modhex string) (string, error) {
+	size := len([]byte(modhex))
+
+	if size%2 == 1 {
+		return "", errors.New("size of modhex input not even")
+	}
+
+	var hex = make([]byte, size)
+
+	for i, m := range []byte(modhex) {
+		if h, ok := modhex2HexMap[m]; ok {
+			hex[i] = h
+		} else {
+			return "", fmt.Errorf("input not modhex encoded; position %d", i)
+		}
+	}
+
+	return string(hex), nil
+}

modhex_test.go

@@ -0,0 +1,49 @@
+package modhex
+
+import "testing"
+
+type test struct {
+	hex    string
+	modhex string
+}
+
+func TestEncodeHexDecodeHex(t *testing.T) {
+	var tests = []test{
+		{
+			modhex: "dteffuje",
+			hex:    "2d344e83",
+		},
+
+		{
+			modhex: "hknhfjbrjnlnldnhcujvddbikngjrtgh",
+			hex:    "69b6481c8baba2b60e8f22179b58cd56",
+		},
+
+		{
+			modhex: "urtubjtnuihvntcreeeecvbregfjibtn",
+			hex:    "ecde18dbe76fbd0c33330f1c354871db",
+		},
+	}
+
+	for _, tc := range tests {
+		modhex, err := EncodeHex(tc.hex)
+
+		if err != nil {
+			t.Errorf("EncodeHex Error: %s\n", err)
+		}
+
+		if modhex != tc.modhex {
+			t.Errorf("EncodeHex Incorrect: Actual: %s; Expected: %s\n", modhex, tc.modhex)
+		}
+
+		hex, err := DecodeHex(tc.modhex)
+
+		if err != nil {
+			t.Errorf("DecodeHex Error: %s\n", err)
+		}
+
+		if hex != tc.hex {
+			t.Errorf("DecodeHex Incorrect: Actual: %s; Expected: %s\n", hex, tc.hex)
+		}
+	}
+}