go: Reduce memory usage and improve speed of Decode by 26%

Use handwritten version of strings.Map. This avoids having to dynamically
allocate and resize the byte slice, and allows the compiler to fully
remove memory allocations from the operation. As a result, the time
spent in StripCode is almost completely removed.

Author: willbeason

go/decode.go

@@ -31,8 +31,8 @@ func Decode(code string) (CodeArea, error) {
 	}
 	// Strip out separator character, padding characters and convert to upper
 	// case.
-	code = StripCode(code)
-	codeLen := len(code)
+	codeBytes := StripCode(code)
+	codeLen := len(codeBytes)
 	if codeLen < 2 {
 		return area, errors.New("code too short")
 	}
@@ -50,8 +50,8 @@ func Decode(code string) (CodeArea, error) {
 		lng *= encBase
 		height *= encBase
 		if i < codeLen {
-			lat += int64(strings.IndexByte(Alphabet, code[i]))
-			lng += int64(strings.IndexByte(Alphabet, code[i+1]))
+			lat += int64(strings.IndexByte(Alphabet, codeBytes[i]))
+			lng += int64(strings.IndexByte(Alphabet, codeBytes[i+1]))
 			height = 1
 		}
 	}
@@ -64,7 +64,7 @@ func Decode(code string) (CodeArea, error) {
 		lng *= gridCols
 		width *= gridCols
 		if i < codeLen {
-			dval := int64(strings.IndexByte(Alphabet, code[i]))
+			dval := int64(strings.IndexByte(Alphabet, codeBytes[i]))
 			lat += dval / gridCols
 			lng += dval % gridCols
 			height = 1

go/olc.go

@@ -187,19 +187,20 @@ func upper(b byte) byte {
 //
 // The code is truncated to the first 15 digits, as Decode won't use more,
 // to avoid underflow errors.
-func StripCode(code string) string {
-	code = strings.Map(
-		func(r rune) rune {
-			if r == Separator || r == Padding {
-				return -1
-			}
-			return rune(upper(byte(r)))
-		},
-		code)
-	if len(code) > maxCodeLen {
-		return code[:maxCodeLen]
+func StripCode(code string) []byte {
+	result := make([]byte, maxCodeLen)
+	pos := 0
+	for _, r := range code {
+		if r == Separator || r == Padding {
+			continue
+		}
+		result[pos] = upper(byte(r))
+		pos++
+		if pos >= maxCodeLen {
+			break
+		}
 	}
-	return code
+	return result[:pos]
 }
 
 // Because the OLC codes are an area, they can't start at 180 degrees, because they would then have something > 180 as their upper bound.