Merge pull request #81 from mick4711/chess-moh

Faster implementation using math.Abs and switch

chess/chess.go

@@ -1,38 +1,55 @@
 package chess
 
-import "errors"
+import (
+	"errors"
+	"math"
+)
 
-// CanKnightAttack checks if two knights on specified positions can attack each other
+var invalidSquareErr = errors.New("invalid square")
+
+// Knights can attack when seperated by 2 squares in one direction and 1 square in the other direction.
+// Horizontal axis is called 'file' labelled from 'a' to 'f'
+// Forward axis is called 'rank' labelled from 1 to 8
 func CanKnightAttack(white, black string) (bool, error) {
-	// Do initial validation
-	if len(white) < 2 || len(black) < 2 {
-		return false, errors.New("args too short")
+	// square must have 2 characters
+	if len(white) != 2 || len(black) != 2 {
+		return true, invalidSquareErr
 	}
 
-	// Check if pieces are on board
-	if white[0]-'a' > 7 || white[1]-'1' > 7 {
-		return false, errors.New("invalid white position")
+	// cannot be on the same square
+	if white == black {
+		return false, invalidSquareErr
 	}
-	if black[0]-'a' > 7 || black[1]-'1' > 7 {
-		return false, errors.New("invalid black position")
+
+	// assign integer values (1 - 8) to file letters and rank digits
+	wFile := int(white[0] - 'a' + 1)
+	if wFile < 1 || wFile > 8 {
+		return false, invalidSquareErr
 	}
 
-	// Calculate distance between knights in each axis
-	d0 := abs(int(white[0]) - int(black[0]))
-	d1 := abs(int(white[1]) - int(black[1]))
+	bFile := int(black[0] - 'a' + 1)
+	if bFile < 1 || bFile > 8 {
+		return false, invalidSquareErr
+	}
 
-	// If the difference is 1 on one axis and 2 on the other we're happy
-	if (d0 == 1 && d1 == 2) || (d0 == 2 && d1 == 1) {
-		return true, nil
-	} else if d0 == 0 && d1 == 0 {
-		return false, errors.New("you cannot stack them you silly")
+	wRank := int(white[1] - '1' + 1)
+	if wRank < 1 || wRank > 8 {
+		return false, invalidSquareErr
 	}
-	return false, nil
-}
 
-func abs(x int) int {
-	if x < 0 {
-		return -x
+	bRank := int(black[1] - '1' + 1)
+	if bRank < 1 || bRank > 8 {
+		return false, invalidSquareErr
 	}
-	return x
+
+	// check file and rank relative positions
+	switch int(math.Abs(float64(wFile - bFile))) {
+	case 1:
+		return int(math.Abs(float64(wRank-bRank))) == 2, nil
+	case 2:
+		return int(math.Abs(float64(wRank-bRank))) == 1, nil
+	}
+
+	// white and black valid but not in knight range of each other
+	return false, nil
 }