Encodes pawns as a bitwise-AND-comparable index (from a SFEN)

shogi_pawn_encoder.rb

# Encodes pawns as a bitwise-AND-comparable index (from a SFEN)
# Usage: ruby pawn_encoder.rb "lnsgkgsnl/1r5b1/ppppppppp/9/9/9/PPPPPPPPP/1B5R1/LNSGKGSNL"
def encode(fen)
  board = fen.gsub("9"," 8").gsub("8"," 7").gsub("7"," 6").gsub("6"," 5").gsub("5"," 4").gsub("4"," 3").gsub("3"," 2").gsub("2"," 1").gsub("1"," ")
  # replace 'P' with '1' and replace 'p' with '1'
  bpawnsA = board.gsub(/[^P\/]/,"0").gsub(/P/,"1").split("/")
  wpawnsA = board.gsub(/[^p\/]/,"0").gsub(/p/,"1").split("/")

  # shift black pawns upwards a rank (rotate left) etc.
  bpawnsB = bpawnsA.rotate(-1)
  wpawnsB = wpawnsA.rotate

  # replace '1' with 1
  bpawns1 = bpawnsA.join.split(//).map {|c| c.ord - '0'.ord}
  wpawns1 = wpawnsA.join.split(//).map {|c| c.ord - '0'.ord}
  bpawns2 = bpawnsB.join.split(//).map {|c| c.ord - '0'.ord}
  wpawns2 = wpawnsB.join.split(//).map {|c| c.ord - '0'.ord}

  # average the shifted and unshifted pawn masks
  bpawns = bpawns1.zip(bpawns2).map {|i, j| (i+j+1)/2}
  wpawns = wpawns1.zip(wpawns2).map {|i, j| (i+j+1)/2}

  # concatenate computed value into a single index
  # which has the property that bitwise-AND computes similarity
  # instead of relying upon an "ECO" code/transposition table
  [bpawns.map {|i| i.to_s}.join.slice(36, 36), wpawns.map {|i| i.to_s}.join.slice(9, 36)].join
end

puts encode(ARGV[0])