1 files changed, 66 insertions, 0 deletions
diff --git a/2022/nim/day09/src/main.nim b/2022/nim/day09/src/main.nim
new file mode 100644
index 0000000..7432fd2
--- /dev/null
+++ b/2022/nim/day09/src/main.nim
@@ -0,0 +1,66 @@
+# Day 9: Rope Bridge
+import std/[os, strutils, sequtils, sets]
+
+let input = paramStr(1).readFile().strip().split("\n")
+let directions = input.mapIt(it.split(" ")).mapIt((it[0][0], it[1].parseInt()))
+
+type Pos = ref object # must be ref for mutation
+  x: int
+  y: int
+
+type Knot = ref object
+  pos: Pos
+  prev: Knot
+
+func init(_: typedesc[Knot], prev: Knot): Knot =
+  return Knot(pos: Pos(x: 0, y: 0), prev: prev)
+
+proc update(loc, knot: Knot) =
+  let dx = loc.pos.x - knot.pos.x
+  let dy = loc.pos.y - knot.pos.y
+
+  if abs(dx) == 2:
+    knot.pos.x += dx div 2
+    if dy != 0:
+      knot.pos.y += dy div abs(dy)
+  elif abs(dy) == 2:
+    knot.pos.y += dy div 2
+    if dx != 0:
+      knot.pos.x += dx div abs(dx)
+
+proc update(knots: varargs[Knot]) =
+  for i, knot in knots:
+    update(knot.prev, knot)
+
+var visited = initHashSet[(int, int)]()
+var visitedAgain = initHashSet[(int, int)]()
+
+var
+  head = Knot.init(nil)
+  tail = Knot.init(head)
+
+var
+  one = Knot.init(head)
+  two = Knot.init(one)
+  three = Knot.init(two)
+  four = Knot.init(three)
+  five = Knot.init(four)
+  six = Knot.init(five)
+  seven = Knot.init(six)
+  eight = Knot.init(seven)
+  nine = Knot.init(eight)
+
+for dir in directions:
+  for i in 0 ..< dir[1]:
+    if dir[0]   == 'R': head.pos.x += 1
+    elif dir[0] == 'L': head.pos.x -= 1
+    elif dir[0] == 'U': head.pos.y += 1
+    elif dir[0] == 'D': head.pos.y -= 1
+
+    update(head, tail)
+    visited.incl((tail.pos.x, tail.pos.y))
+    update(one, two, three, four, five, six, seven, eight, nine)
+    visitedAgain.incl((nine.pos.x, nine.pos.y))
+
+echo visited.len
+echo visitedAgain.len