create Chess960 starting positions

chess960sp.rb

#!/usr/bin/env ruby

# create Chess960 starting position
# algorithm from https://en.wikipedia.org/wiki/Fischer_random_chess_numbering_scheme

knight_positions = [
  ['N', 'N', '.', '.', '.'],
  ['N', '.', 'N', '.', '.'],
  ['N', '.', '.', 'N', '.'],
  ['N', '.', '.', '.', 'N'],
  ['.', 'N', 'N', '.', '.'],
  ['.', 'N', '.', 'N', '.'],
  ['.', 'N', '.', '.', 'N'],
  ['.', '.', 'N', 'N', '.'],
  ['.', '.', 'N', '.', 'N'],
  ['.', '.', '.', 'N', 'N']
]

# place knight on free square according to
# position table
def place_knights(row, knight_positions, n)
  i = 0
  knights = knight_positions[n]
  row.each_with_index do |square, index|
    next unless square == '.'

    row[index] = 'N' if knights[i] == 'N'
    i += 1
  end
  row
end

# place queen on Nth free column
def place_queen(row, n)
  i = 0
  row.each_with_index do |square, index|
    next unless square == '.'

    if i == n
      row[index] = 'Q'
      break
    end
    i += 1
  end
  row
end

def place_king_and_rooks(row)
  i = 0
  row.each_with_index do |square, index|
    next unless square == '.'
    case i
      when 0
      row[index] = 'R'
      when 1
      row[index] = 'K'
      when 2
      row[index] = 'R'
    end
    i = i + 1
  end
  row
end

# starting position in FEN
def fen(row)
  "#{row.join.downcase}/pppppppp/8/8/8/8/PPPPPPPP/#{row.join} w KQkq - 0 1"
end

row = Array.new(8) { '.' }

N1 = rand(960)
# N1 = 518 # standard chess
N2, B1 = N1.divmod(4)
# light squared bishop
row[B1 * 2 + 1] = 'B'
N3, B2 = N2.divmod(4)
# dark squared bishop
row[B2 * 2] = 'B'
N4, Q = N3.divmod(6)
place_queen(row, Q)
place_knights(row, knight_positions, N4)
place_king_and_rooks(row)
puts "Chess960 starting position #{N1}"
puts fen(row)