Ok, I think I figured out day 20s cleverness after many angry wrong guesses

2021/nim/day20.nim

@@ -0,0 +1,84 @@
+import ./common, std/[sequtils, algorithm, sugar, sets, strformat, strutils, tables, options, json, hashes]
+
+type
+  Point = (int, int)
+  Image = object
+    pixels: TableRef[Point, bool]
+    topLeft: Point
+    bottomRight: Point
+    infinite: bool
+
+proc parse(s: openArray[string]): Image =
+  var pixels = newTable[Point, bool]()
+  for y,l in s.pairs:
+    for x,c in l:
+      pixels[(x, y)] = c == '#'
+  return Image(topLeft: (0, 0), bottomRight: (s[0].len-1, s.len-1), pixels: pixels, infinite: false)
+
+proc enhance(image: var Image, algo: string) =
+  var (tlx, tly) = image.topLeft
+  var (brx, bry) = image.bottomRight
+  var newPixels = newTable[Point, bool]()
+  var xr = tlx-1..brx+1
+  var yr = tly-1..bry+1
+  for x in xr:
+    for y in yr:
+      var sample = 0
+      for sy in y-1..y+1:
+        for sx in x-1..x+1:
+          if image.pixels.getOrDefault((sx, sy), image.infinite):
+            inc sample
+          sample = sample shl 1
+      sample = sample shr 1
+      # echo &"{x}, {y} => {sample}"
+      newPixels[(x, y)] = algo[sample] == '#'
+      if x < tlx: tlx = x
+      if y < tly: tly = y
+      if x > brx: brx = x
+      if y > bry: bry = y
+  image.topLeft = (tlx, tly)
+  image.bottomRight = (brx, bry)
+  image.pixels = newPixels
+  if image.infinite:
+    image.infinite = algo[^1] == '#'
+  else:
+    image.infinite = algo[0] == '#'
+
+proc `$`(image: Image): string =
+  var s: seq[string] = @[]
+  var (tlx, tly) = image.topLeft
+  var (brx, bry) = image.bottomRight
+  for _ in tly..bry:
+    s.add ".".repeat((brx-tlx)+1)
+  for p in image.pixels.keys:
+    let (x, y) = p
+    if image.pixels[p]: s[y-tly][x-tlx] = '#'
+  s.join("\n")
+
+
+proc p1(input: Lines): uint64 =
+  var image = input[2..^1].parse()
+  echo &"{image.pixels.len} dim: {image.topLeft} {image.bottomRight}:"
+  echo image
+  image.enhance(input[0])
+  echo &"{image.pixels.len} dim: {image.topLeft} {image.bottomRight}:"
+  echo image
+  image.enhance(input[0])
+  echo &"{image.pixels.len} dim: {image.topLeft} {image.bottomRight}:"
+  echo image
+  for v in image.pixels.values:
+    if v: inc result
+
+proc p2(input: Lines): uint64 =
+  0
+
+const input = """
+..#.#..#####.#.#.#.###.##.....###.##.#..###.####..#####..#....#..#..##..###..######.###...####..#..#####..##..#.#####...##.#.#..#.##..#.#......#.###.######.###.####...#.##.##..#..#..#####.....#.#....###..#.##......#.....#..#..#..##..#...##.######.####.####.#.#...#.......#..#.#.#...####.##.#......#..#...##.#.##..#...##.#.##..###.#......#.#.......#.#.#.####.###.##...#.....####.#..#..#.##.#....##..#.####....##...##..#...#......#.#.......#.......##..####..#...#.#.#...##..#.#..###..#####........#..####......#..#
+
+#..#.
+#....
+##..#
+..#..
+..###
+""".strip().split('\n')
+doDayX 20, (n: int) => n.loadInput, p1, p2, (input, 35'u64, 0'u64)