Add comments and slight variable name change

modhex.go

@@ -86,31 +86,40 @@ func (e InvalidByteError) Error() string {
 // even length. If the input is malformed, Decode returns the number of bytes
 // decoded before the error.
 func Decode(dst, src []byte) (int, error) {
-	i, j := 0, 1
-	for ; j < len(src); j += 2 {
-		a, ok := modhexMap[src[j-1]]
+	// decode the src bytes two at a time
+	n, i := 0, 1
+	for ; i < len(src); i += 2 {
+
+		// try to decode the upper nibble
+		a, ok := modhexMap[src[i-1]]
 		if !ok {
-			return i, InvalidByteError(src[j-1])
+			return n, InvalidByteError(src[i-1])
 		}
 
-		b, ok := modhexMap[src[j]]
+		// try to decode the lower nibble
+		b, ok := modhexMap[src[i]]
 		if !ok {
-			return i, InvalidByteError(src[j])
+			return n, InvalidByteError(src[i])
 		}
 
-		dst[i] = (a << 4) | b
-		i++
+		// store the decoded value into dst
+		dst[n] = (a << 4) | b
+		n++
 	}
 
+	// check for an odd length input
 	if len(src)%2 == 1 {
-		if _, ok := modhexMap[src[j-1]]; !ok {
-			return i, InvalidByteError(src[j-1])
+		// first check for an invalid byte
+		if _, ok := modhexMap[src[i-1]]; !ok {
+			return n, InvalidByteError(src[i-1])
 		}
 
-		return i, ErrLength
+		// report the number of bytes written and the length error
+		return n, ErrLength
 	}
 
-	return i, nil
+	// return the number of bytes written to dst
+	return n, nil
 }
 
 // DecodeString returns the bytes represented by the modhex string s.