svg_path.py

"""
svg_path.py
-----------

Provides utility functions for manipulating svgs.

"""
import logging
from parser_combinators import char, white, integer
from parser_combinators import seq, any_, many, one_or_more, change_result

logger = logging.getLogger(__name__)

class ParseError(Exception): 
    """Raised when parsing fails."""
    pass

def draw_path(svg_path):
    """Converts a path specification into a list of points.
    
    Args:
        svg_path: A path specification. A comma seperated non-empty 
            sequence of:
            
            - ``z``, representing the initial point of the path.

            - ``m X Y``, where ``X`` and ``Y`` are sequences of digits.
            Represents a move to the coordinate point (X, Y) relative
            to the origin.

            - ``l X Y``, where ``X`` and ``Y`` are sequences of digits.
            Represents a move to the coordinate point (X, Y) relative
            to the current position.

            The first element of the sequence must be of the form
            ``m X Y``.
    
    Returns: A list of points, which are pairs of floats.

    Raises:
        ParseError: If ``svg_path`` does not conform to the 
            specification.
        
    """
    def new_Z(result):
        """Creates a Z command from a parse result."""
        return 'Z', None

    Z = change_result(char('z'), new_Z)
    
    def new_M(result):
        """Creates an M command from a parse result."""
        [_, ws_and_coords] = result
        _, coords = zip(*ws_and_coords)
        return 'M', coords
    
    M = change_result(seq(char('m'), many(seq(many(white), integer))), new_M)

    def new_L(result):
        """Creates an L command from a parse result."""
        [_, ws_and_coords] = result
        _, coords = zip(*ws_and_coords)
        return 'M', coords

    L = change_result(seq(char('l'), many(seq(many(white), integer))), new_L)

    def new_path(result):
        """Flattens the list of commands, discarding commas."""
        [first, rest] = result
        # rest is a sequence of pairs of (commas + whitespace, command)
        _, rest_ = zip(*rest)
        return rest_.insert(0, first)

    svg_path_parser = change_result(
            seq(M, many(seq(char(,), many(white)), any_(Z, L, M))),
            new_path
    )
    
    result, remaining = svg_path_parser(svg_path.lower())

    if result is None or remaining != '':
        raise ParseError('svg path "' + svg_path + '" is not valid')
    else:
        points = []
        position = (0, 0)

        for command in result:
            kind, coords = command

            if kind == 'Z':
                position = points[0]
                points.append(position)

            if kind == 'M':
                [X, Y] = coords
                position = (X, Y)
                points.append(position)

            if kind == 'L':
                [X, Y] = coords
                X_, Y_ = position
                position = (X_ + X, Y_ + Y)
                points.append(position)
        
        logger.info('drew path "{}", got points {}'.format(svg_path, points))
        return points