add new solutions

solutions/8/q840/solution.go

@@ -0,0 +1,63 @@
+package q840
+
+func checkSquare(x, y int, grid [][]int) bool {
+	seen := uint16(0)
+	rowSum, colSum := [3]int{}, [3]int{}
+	diag1sum, diag2sum := 0, 0
+
+	for ty := range 3 {
+		for tx := range 3 {
+			num := grid[y+ty][x+tx]
+			if num < 1 || num > 9 {
+				return false
+			}
+			tSeen := seen | (1 << (num - 1))
+			if tSeen == seen { // duplicate number
+				return false
+			}
+			seen = tSeen
+
+			rowSum[ty] += num
+			colSum[tx] += num
+			if tx == ty {
+				diag1sum += num
+			}
+			if tx+ty == 2 {
+				diag2sum += num
+			}
+		}
+	}
+
+	if diag1sum != diag2sum {
+		return false
+	}
+	for i := range 3 {
+		if rowSum[i] != diag1sum || colSum[i] != diag1sum {
+			return false
+		}
+	}
+	return true
+}
+
+func numMagicSquaresInside(grid [][]int) int {
+	if len(grid) < 3 || len(grid[0]) < 3 {
+		return 0
+	}
+	w, h := len(grid[0]), len(grid)
+
+	count := 0
+	for y := range h - 2 {
+		for x := range w - 2 {
+			if grid[y+1][x+1] != 5 { // the center cell has to be 5
+				continue
+			}
+			if checkSquare(x, y, grid) {
+				count++
+			}
+		}
+	}
+
+	return count
+}
+
+var _ = numMagicSquaresInside