Encode white pawns and black pawns into bitmasks

pawn_encoder.rb

# Program to encode pawns as bitmasks from a FEN
# Usage: ruby pawn_encoder.rb "rn3rk1/4bpp1/pq2bn1p/2pp4/B6B/2N1PN2/PP3PPP/2RQ1RK1"
def encode(fen)
  board = fen.gsub("8"," 7").gsub("7"," 6").gsub("6"," 5").gsub("5"," 4").gsub("4"," 3").gsub("3"," 2").gsub("2"," 1").gsub("1"," ")
  wpawnsA = board.gsub(/[^P\/]/,"0").gsub(/P/,"1").split("/")
  bpawnsA = board.gsub(/[^p\/]/,"0").gsub(/p/,"1").split("/")

  wpawnsB = wpawnsA.rotate
  bpawnsB = bpawnsA.rotate(-1)

  wpawns1 = wpawnsA.join.split(//).map {|c| c.ord - '0'.ord}
  bpawns1 = bpawnsA.join.split(//).map {|c| c.ord - '0'.ord}

  wpawns2 = wpawnsB.join.split(//).map {|c| c.ord - '0'.ord}
  bpawns2 = bpawnsB.join.split(//).map {|c| c.ord - '0'.ord}

  wpawns = wpawns1.zip(wpawns2).map {|i, j| (i+j+1)/2}
  bpawns = bpawns1.zip(bpawns2).map {|i, j| (i+j+1)/2}

  [wpawns.map {|i| i.to_s}.join.slice(24, 32), bpawns.map {|i| i.to_s}.join.slice(8, 32)].join
end

puts encode(ARGV[0])