Add day 9 solutions

day9.chpl

@@ -0,0 +1,39 @@
+use IO, Set;
+
+const moveList = [ "L" => (-1, 0), "R" => (1, 0), "U" => (0, 1), "D" => (0, -1) ];
+
+iter moves() {
+  for line in stdin.lines().strip() {
+    const (dir, _, n) = line.partition(" ");
+    yield (moveList[dir], n : int);
+  }
+}
+
+config const length = 2;
+var rope: [0..<length] (int, int) = (0, 0);
+
+proc move((hx, hy), (tx, ty)): (int, int) {
+  const (dx, dy) = (hx - tx, hy - ty);
+  if abs(dx) > 1 && dy == 0 {
+    tx += sgn(dx);
+  } else if abs(dy) > 1 && dx == 0 {
+    ty += sgn(dy);
+  } else if abs(dx) + abs(dy) > 2 {
+    tx += sgn(dx);
+    ty += sgn(dy);
+  }
+  return (tx, ty);
+}
+
+var visited = new set((int, int));
+
+for (delta, n) in moves() {
+  for 1..n {
+    rope[0] += delta;
+    for idx in 1..<length {
+      rope[idx] = move(rope[idx-1], rope[idx]);
+    }
+    visited.add(rope.last);
+  }
+}
+writeln(visited.size);