2023 Day 10 Part 2

2023/day10.nim

@@ -2,12 +2,14 @@ import std/algorithm
 import std/deques
 import std/enumerate
 import std/math
+import std/options
 import std/sequtils
 import std/sets
 import std/strscans
 import std/strutils
 import std/sugar
 import std/tables
+import std/terminal
 
 proc dbg[T](v: T): T = (echo(v); v)
 
@@ -33,8 +35,11 @@ func parseSketch(text: string): Sketch =
       if sym == 'S':
         result.start = (r, c)
 
+func inBounds(s: Sketch, p: Point): bool =
+  p.r in 0 ..< s.rows and p.c in 0 ..< s.cols
+
 func pipeAt(s: Sketch, p: Point): char =
-  if p.r in 0 ..< s.rows and p.c in 0 ..< s.cols:
+  if s.inBounds(p):
     s.grid[p]
   else:
     '.'
@@ -65,6 +70,8 @@ func neighbors(s: Sketch, p: Point): seq[Point] =
     (0, +1, E, W), # Right  E-W
   ]:
     let n = (p.r + dr, p.c + dc)
+    if not s.inBounds(n):
+      continue
     if s.hasConn(p, pDir) and s.hasConn(n, nDir):
       result.add(n)
 
@@ -93,7 +100,190 @@ proc part1(): int =
   let text = readFile("input/day10.txt")
   let sketch = parseSketch(text)
 
-  for d in values(sketch.reachable):
+  let reachable = sketch.reachable
+  for d in values(reachable):
     result = max(result, d)
 
+proc path(s: Sketch): seq[Point] =
+  var queue: Deque[seq[Point]]
+
+  queue.addLast @[s.start]
+
+  while queue.len > 0:
+    let path = queue.popFirst
+    let point = path[^1]
+
+    if path.len > 1 and s.pipeAt(point) == 'S':
+      return path
+
+    for n in s.neighbors(point):
+      if path.len > 1 and path[^2] == n:
+        continue
+      queue.addLast(path & @[n])
+
+func inBoundsPadding(s: Sketch, p: Point): bool =
+  p.r in -1 .. s.rows and p.c in -1 .. s.cols
+
+func neighborsFill(s: Sketch, p: Point): seq[Point] =
+  for (dr, dc) in [
+    (-1, 0), # Up
+    (+1, 0), # Down
+    (0, -1), # Left
+    (0, +1), # Right
+  ]:
+    let n = (p.r + dr, p.c + dc)
+    if s.inBoundsPadding(n):
+      result.add n
+
+proc floodFill(s: Sketch, start: Point, path: HashSet[Point]): HashSet[Point] =
+  var queue: Deque[Point]
+
+  queue.addLast start
+
+  while queue.len > 0:
+    let state = queue.popFirst
+
+    if state in result:
+      continue
+    result.incl state
+
+    for n in s.neighborsFill(state):
+      if n in path:
+        continue
+      queue.addLast n
+
+func cells(s: Sketch): HashSet[Point] =
+  for r in 0 ..< s.rows:
+    for c in 0 ..< s.cols:
+      result.incl (r, c)
+
+proc heading(a: Point, b: Point): Direction =
+  let dr = b.r - a.r
+  let dc = b.c - a.c
+  assert(abs(dr) + abs(dc) == 1)
+
+  if dr == -1: return N
+  if dr == +1: return S
+  if dc == -1: return W
+  if dc == +1: return E
+  assert(false)
+
+func north(p: Point): Point = (p.r - 1, p.c)
+func south(p: Point): Point = (p.r + 1, p.c)
+func east(p: Point): Point =  (p.r   , p.c + 1)
+func west(p: Point): Point =  (p.r   , p.c - 1)
+
+func northwest(p: Point): Point = (p.r - 1, p.c - 1)
+func northeast(p: Point): Point = (p.r - 1, p.c + 1)
+func southwest(p: Point): Point = (p.r + 1, p.c - 1)
+func southeast(p: Point): Point = (p.r + 1, p.c + 1)
+
+func sides3(p: Point, h: Direction): (seq[Point], seq[Point]) =
+  case h:
+    of N: (
+      @[p.northwest, p.west, p.southwest],
+      @[p.northeast, p.east, p.southeast],
+    )
+    of S: (
+      @[p.southeast, p.east, p.northeast],
+      @[p.southwest, p.west, p.northwest],
+    )
+    of E: (
+      @[p.northeast, p.north, p.northwest],
+      @[p.southeast, p.south, p.southwest],
+    )
+    of W: (
+      @[p.southwest, p.south, p.southeast],
+      @[p.northwest, p.north, p.northeast],
+    )
+
+func sides2(p: Point, h: Direction): (seq[Point], seq[Point]) =
+  let (l, r) = sides3(p, h)
+  (l[0..^2], r[0..^2])
+
+proc walkPath(s: Sketch, path: seq[Point]): (HashSet[Point], HashSet[Point]) =
+  for (a, b) in zip(path[0..^2], path[1..^1]):
+    let h = a.heading(b)
+
+    let (left, right) = sides2(a, h)
+    for p in left:
+      result[0].incl p
+    for p in right:
+      result[1].incl p
+
+type Side = enum
+  Left
+  Right
+
+func whichSide(p: Point, left: HashSet[Point], right: HashSet[Point]): Option[Side] =
+  if p in left and p in right:
+    return none(Side)
+  if p in left:
+    return some(Left)
+  if p in right:
+    return some(Right)
+  return none(Side)
+
+proc part2(): int =
+  let text = readFile("input/day10.txt")
+  let sketch = parseSketch(text)
+
+  let path = sketch.path
+  assert sketch.pipeAt(path[0]) == 'S'
+  assert sketch.pipeAt(path[^1]) == 'S'
+
+  let (left, right) = sketch.walkPath(path)
+
+  var candidates = sketch.cells
+
+  let pathSet = path.toHashSet
+  for p in pathSet:
+    candidates.excl p
+
+  var foundOutsideSide = false
+  var outside: Side
+  for p in sketch.floodFill((-1, -1), pathSet):
+    candidates.excl p
+
+    if p in pathSet:
+      continue
+
+    # Marked both ways, inconclusve?
+    if p in left and p in right:
+      continue
+
+    if p in left:
+      if foundOutsideSide:
+        assert(outside == Left)
+      foundOutsideSide = true
+      outside = Left
+    elif p in right:
+      if foundOutsideSide:
+        assert(outside == Right)
+      foundOutsideSide = true
+      outside = Right
+
+  var inside: HashSet[Point]
+  for r in 0 ..< sketch.rows:
+    for c in 0 ..< sketch.cols:
+      let p = (r, c)
+
+      # Already known to be outside or on path
+      if p notin candidates:
+        continue
+
+      let s = whichSide(p, left, right)
+      if s.isSome and s.get == outside:
+        for pOut in sketch.floodFill(p, pathSet):
+          candidates.excl pOut
+      elif s.isSome and s.get != outside:
+        for pIn in sketch.floodFill(p, pathSet):
+          inside.incl pIn
+      else:
+        discard "Inconclusive! I hope this doesn't matter!"
+
+  assert(candidates.len == inside.len)
+  inside.len
+
 echo part1()
+echo part2()