Question

I would like to convert a few Python lines on Ruby, from this excellent article signed by Thomas Guest at: http://wordaligned.org/articles/drawing-chess-positions

(note: I’m a really bigger Python noob)

Here is a copy of the original Python version:

def expand_blanks(fen):
    '''Expand the digits in an FEN string into spaces

    >>> expand_blanks("rk4q3")
    'rk    q   '
    '''
    def expand(match):
        return ' ' * int(match.group(0))
    return re.compile(r'\d').sub(expand, fen)

def outer_join(sep, ss):
    '''Like string.join, but encloses the result with outer separators.

    Example:
    >>> outer_join('|', ['1', '2', '3'])
    '|1|2|3|'
    '''
    return '%s%s%s' % (sep, sep.join(ss), sep)

def ascii_draw_chess_position(fen):
    '''Returns an ASCII picture of pieces on a chessboard.'''
    pieces = expand_blanks(fen).replace('/', '')
    divider = '+-+-+-+-+-+-+-+-+\n'
    rows = ((outer_join('|', pieces[r: r + 8]) + '\n')
            for r in range(0, 8 * 8, 8))
    return outer_join(divider, rows)

Usage example:

>>> fen = "r2q1rk1/pp2ppbp/1np2np1/2Q3B1/3PP1b1/2N2N2/PP3PPP/3RKB1R"
>>> print ascii_draw_chess_position(fen)
+-+-+-+-+-+-+-+-+
|r| | |q| |r|k| |
+-+-+-+-+-+-+-+-+
|p|p| | |p|p|b|p|
+-+-+-+-+-+-+-+-+
| |n|p| | |n|p| |
+-+-+-+-+-+-+-+-+
| | |Q| | | |B| |
+-+-+-+-+-+-+-+-+
| | | |P|P| |b| |
+-+-+-+-+-+-+-+-+
| | |N| | |N| | |
+-+-+-+-+-+-+-+-+
|P|P| | | |P|P|P|
+-+-+-+-+-+-+-+-+
| | | |R|K|B| |R|
+-+-+-+-+-+-+-+-+

I have trying to do some modifications to convert each line in Ruby… And I wonder if it’s bad start :s But I publish it anyway:

def expand_blanks(fen)
  def expand(match)
    return ' ' * int(match.group(0))
  end

  # bugged:
  re.compile(r'\d').sub(expand, fen)
end

def outer_join(sep, ss)
  sep + sep.join(ss) + sep
end

def ascii_draw_chess_position(fen)
  pieces = expand_blanks(fen).replace('/', '')
  puts pieces.class
  divider = "+-+-+-+-+-+-+-+-+\n"

  # bugged lines:
  rows = ((outer_join('|', pieces[r, r + 8]) + '\n')
  for r in range(0, 8 * 8, 8))
    return outer_join(divider, rows)
  end
end

fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"
puts ascii_draw_chess_position(fen)

If you see some lines that I can fix, I would be cool for me. Thank you all.

Answer 1

Looking at each method in turn, starting with outer_join.

In Python, join is a method on strings that takes a sequence and joins the elements delimited by the string. e.g. '|'.join(['p, 'p', 'p'])

In Ruby, join is a method on arrays that takes the delimiter as an argument e.g. ['p', 'p', 'p'].join('|')

so the Ruby version of outer_join would be:

def outer_join(sep, ss):
  sep + ss.join(sep) + sep
end

Next look at expand_blanks, the intention here is to replace a digit with the equivalent number of spaces. In Ruby this can be achieved using gsub to replace all occurrences of a pattern in a string. gsub can be called with a block that will be passed each match substring and returns the string that the match should be replaced with. So the Ruby code would be:

def expand_blanks(fen)
  fen.gsub(/\d/) { |match| ' ' * match.to_i }
end

Finally in ascii_draw_chess_position we can use gsub again as an equivalent to Python’s replace method and use Ruby’s map function in place of what was achieved using a list comprehension in Python as follows:

def ascii_draw_chess_position(fen)
  pieces = expand_blanks(fen).gsub('/', '')
  divider = "+-+-+-+-+-+-+-+-+\n"
  rows = (0...8).map do |i|
    row = pieces[i * 8...(i + 1) * 8].split('')
    outer_join("|",row) + "\n"
  end
  puts outer_join(divider, rows)
end

Let me know if you need any more details on the above.