sidef-scripts/Game solvers/chess960_initial_positions.sf

#!/usr/bin/ruby

# Generate the initial positions in Fischer Random Chess (also known as chess960).

# See also:
#   https://en.wikipedia.org/wiki/Chess960

var pieces = %w(♛ ♚ ♜ ♜ ♝ ♝ ♞ ♞)

func is_valid_960 (backrank) {
    var king = backrank.index('♚')
    var (rook1, rook2) = backrank.indices_of('♜')...
    king.is_between(rook1, rook2) || return false
    var (bishop1, bishop2) = backrank.indices_of('♝')...
    bishop1+bishop2 -> is_odd
}

func random_960_position {
    loop {
        var a = pieces.random_permutation
        return a if is_valid_960(a)
    }
}

say "Here's a random 960 position: #{random_960_position().join(' ')}"

var chess960 = gather {

    var arr = pieces.clone

    loop {
        take(arr.clone) if is_valid_960(arr)
        arr.next_permutation || break
    }
}

say "There are #{chess960.len} initial positions."

__END__
Here's a random 960 position: ♞ ♜ ♝ ♛ ♞ ♚ ♜ ♝
There are 960 initial positions.