Refactor check digit calculation

cont/check_digit.go

@@ -1,28 +1,38 @@
 package cont
 
 // CalcCheckDigit calculates check digit for owner, equipment category ID and serial number.
+// This function was optimized for fun and has a suboptimal reading experience.
 func CalcCheckDigit(ownerCode string, equipCatID rune, serialNum int) int {
-	n := 0
-	d := 1
+	var n uint16 = 0
+	var d uint16 = 1
+
 	for _, c := range ownerCode {
-		n += d * charValue(c)
+		n += d * charValue(uint16(c))
 		d *= 2
 	}
-	n += d * charValue(equipCatID)
-	d *= 2
-	divider := 100000
-	for divider > 0 {
-		n += d * ((serialNum / divider) % 10)
-		d *= 2
-		divider /= 10
+
+	n += d * charValue(uint16(equipCatID))
+
+	// Handle the case for the serial number when it is
+	// out of range of uint16.
+	n += 512 * uint16(serialNum%10)
+	serialNum /= 10
+	n += 256 * uint16(serialNum%10)
+
+	// uint16 can be used because we are now <10000
+	s := uint16(serialNum / 10)
+	d = 128
+	for d >= 16 {
+		n += d * (s % 10)
+		d /= 2
+		s /= 10
 	}
-	return n % 11
+	return int(n % 11)
 }
 
 // charValue returns the index of character plus 10.
 // A?BCDEFGHIJK?LMNOPQRSTU?VWXYZ
-// A=10, B=12, C=13, ... , K=21, L=23, ...
-func charValue(char rune) int {
-	n := int(char)
-	return n - 55 + (n-56)/10
+// A=10, (no 11) B=12, C=13, ... , K=21, (no 22) L=23, ...
+func charValue(char uint16) uint16 {
+	return char - 55 + (char-56)/10
 }

cont/check_digit_test.go

@@ -45,6 +45,11 @@ func TestCalcCheckDigit(t *testing.T) {
 			args{"CSQ", 'U', 305438},
 			3,
 		},
+		{
+			"Test CSQ U 999998 3",
+			args{"CSQ", 'U', 999998},
+			3,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {