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Task/N-queens-problem/Sparkling/n-queens-problem.sparkling

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+let print_table = function (pos) {
+	pos.foreach(function (_, i) {
+		stdout.printf("  %c", 'a' + i);
+	});
+
+	stdout.write("\n");
+
+	pos.foreach(function (col, row) {
+		stdout.printf("%d", row + 1);
+		stdout.printf("%s #\n", range(col).reduce("", function (s, t) {
+			return s .. "   ";
+		}));
+	});
+
+	stdout.write("\n\n");
+};
+
+let threatens = function (row_a, col_a, row_b, col_b) {
+	return row_a == row_b
+	    or col_a == col_b
+	    or abs(row_a - row_b) == abs(col_a - col_b);
+};
+
+let good = function(pos, end_idx) {
+	return pos.all(function (col_a, row_a) {
+		return range(row_a + 1, end_idx).all(function (row_b) {
+			let col_b = pos[row_b];
+			return not threatens(row_a, col_a, row_b, col_b);
+		});
+	});
+};
+
+// Returns number of solutions
+let n_queens = function (pos, index) {
+	if index >= pos.length {
+		if good(pos, index) {
+			print_table(pos);
+			return 1;
+		}
+
+		return 0;
+	}
+
+	if not good(pos, index) {
+		return 0;
+	}
+
+	return pos.map(function (_, col) {
+		pos[index] = col;
+		return n_queens(pos, index + 1);
+	}).reduce(0, function (a, b) { return a + b; });
+};
+
+stdout.printf("%d solutions\n", n_queens(range(8), 0));