day 18: Compute an entire row at once

Bignums seem to perform just fine here, so all right, we'll just use
them exclusively.

Author: petertseng

18_its_a_trap.rb

@@ -1,18 +1,3 @@
-# Store rows in blocks of this size.
-# 1 = trap, 0 = safe.
-# Within each block, characters on the left are the most significant bits.
-BLOCK = 10
-
-# We'll pre-compute every single block of 12 -> 10,
-# since 4096 entries in a table is easy.
-RULE = (0...(1 << BLOCK + 2)).map { |i|
-  (0...BLOCK).select { |j|
-    (i >> j) & 1 != (i >> j + 2) & 1
-  }.map { |j| 1 << j }.reduce(0, :|)
-}.freeze
-
-SAFE_COUNT = (0...(1 << BLOCK)).map { |i| BLOCK - i.to_s(2).count(?1) }.freeze
-
 rows = begin
   arg = ARGV.find { |x| x.start_with?('-n') }
   arg && Integer(ARGV.delete(arg)[2..-1])
@@ -21,23 +6,17 @@
 input = (!ARGV.empty? && !File.exist?(ARGV.first) ? ARGV.first : ARGF.read).freeze
 
 bit = {?^ => 1, ?. => 0}.freeze
-prev_row = input.each_char.each_slice(BLOCK).map { |slice|
-  slice.reduce(0) { |i, c| i << 1 | bit.fetch(c) }
-}
-
-safe = prev_row.sum { |block| SAFE_COUNT[block] }
+row = input.each_char.reduce(0) { |i, c| i << 1 | bit.fetch(c) }
+mask = 2 ** input.size - 1
 
-current_row = Array.new(prev_row.size)
+total = input.size
+traps = row.to_s(2).count(?1)
 
 (rows ? [rows - 1] : [39, 399960]).each { |n|
   n.times {
-    window = 0
-    current_row.size.times { |i|
-      window = (window << BLOCK | prev_row[i] << 1 | (prev_row[i + 1] || 0) >> BLOCK - 1) & (1 << BLOCK + 2) - 1
-      current_row[i] = RULE[window]
-      safe += SAFE_COUNT[current_row[i]]
-    }
-    prev_row, current_row = [current_row, prev_row]
+    row = ((row << 1) ^ (row >> 1)) & mask
+    traps += row.to_s(2).count(?1)
   }
-  puts safe
+  total += n * input.size
+  puts total - traps
 }