1 files changed, 85 insertions, 0 deletions
diff --git a/2020/eleven.nim b/2020/eleven.nim
new file mode 100644
index 0000000..7324b2d
--- /dev/null
+++ b/2020/eleven.nim
@@ -0,0 +1,85 @@
+# Day Eleven: Seating System
+import os, strutils, sequtils, sugar
+
+let input: string = paramStr(1)
+var terminal: seq[seq[char]] = map(split(strip(readFile(input)), '\n'), row => @row)
+let height: int = len(terminal)
+let width: int = len(terminal[0])
+
+while true:
+  var prev, future: seq[char] = @(repeat('.', width)) # dummy floor
+  var buffer: seq[seq[char]]  = terminal
+  for i, row in buffer:
+    if height - i == 1:
+      future = @(repeat('.', width))
+    else:
+      future = buffer[i+1]
+    for j, seat in row:
+      let threshold: int =
+        if j == 0:
+          count(prev[j..j+1], '#') + count($row[j+1], "#") + count(future[j..j+1], '#')
+        elif j == width - 1:
+          count(prev[j-1..j], '#') + count($row[j-1], "#") + count(future[j-1..j], '#')
+        else:
+          count(prev[j-1..j+1], '#') + count($row[j-1], "#") +
+          count($row[j+1], "#") + count(future[j-1..j+1], '#')
+      if seat == '#' and threshold >= 4:
+        buffer[i][j] = 'L'
+      elif seat == 'L' and threshold == 0:
+        buffer[i][j] = '#'
+    prev = row
+  if terminal == buffer: break
+  terminal = buffer
+echo count($terminal, '#')
+
+terminal = map(split(strip(readFile(input)), '\n'), row => @row)
+while true:
+  var prev, future: seq[char] = @(repeat('.', width)) # dummy floor
+  var buffer: seq[seq[char]] = terminal
+  for i, row in buffer:
+    if height - i == 1:
+      future = @(repeat('.', width))
+    else:
+      future = buffer[i+1]
+    for j, seat in row:
+      var threshold: int = 0
+      # left
+      for k in countdown(j-1, 0):
+        if terminal[i][k] == '#': inc(threshold)
+        if terminal[i][k] != '.': break
+      # right
+      for k in j+1 .. width - 1:
+        if terminal[i][k] == '#': inc(threshold)
+        if terminal[i][k] != '.': break
+      # up
+      for k in countdown(i-1, 0):
+        if terminal[k][j] == '#': inc(threshold)
+        if terminal[k][j] != '.': break
+      # down
+      for k in i+1 .. height - 1:
+        if terminal[k][j] == '#': inc(threshold)
+        if terminal[k][j] != '.': break
+      # down-left
+      for k in 1 .. min(i, j):
+        if terminal[i-k][j-k] == '#': inc(threshold)
+        if terminal[i-k][j-k] != '.': break
+      # up-left
+      for k in 1 .. min(height-1-i, j):
+        if terminal[i+k][j-k] == '#': inc(threshold)
+        if terminal[i+k][j-k] != '.': break
+      # down-right
+      for k in 1 .. min(i, width-1-j):
+        if terminal[i-k][j+k] == '#': inc(threshold)
+        if terminal[i-k][j+k] != '.': break
+      # up-right
+      for k in 1 .. min(height-1-i, width-1-j):
+        if terminal[i+k][j+k] == '#': inc(threshold)
+        if terminal[i+k][j+k] != '.': break
+      if seat == '#' and threshold >= 5:
+        buffer[i][j] = 'L'
+      elif seat == 'L' and threshold == 0:
+        buffer[i][j] = '#'
+    prev = row
+  if terminal == buffer: break
+  terminal = buffer
+echo count($terminal, '#')