Cut a linestring at a distance

gistfile1.py

from math import sqrt 

def cut(coords, distance):
    '''Splits a cartesian linestring at a given distance
    Returns None for either part if the length is 0    
    '''

    if distance <= 0:
        return None, list(tuple(j) for j in coords)

    i = iter(coords)
    px, py = i.next()
    result = [(px, py)]
    cur_distance = 0.0
    for cx, cy in i:
        cur = (cx, cy)
        dx = cx - px
        dy = cy - py

        l = sqrt(dx * dx + dy * dy)
        
        cur_distance += l

        if cur_distance == distance:
            result.append(cur)
            rest = list(tuple(j) for j in i)                        
            if len(rest) == 0:
                return result, None

            return result, [cur] + rest

        elif cur_distance > distance:
            j = l - (cur_distance - distance)

            ax = dx / l * j
            ay = dy / l  * j

            mid = (px + ax, py + ay)
            result.append(mid)
            return result, [mid, cur] + list(tuple(j) for j in i)
        
        result.append(cur)
        px, py = cx, cy

    result.extend(tuple(j) for j in i)

    return result, None
    

if __name__ == '__main__':
    
    coords = [(0, 0), (1, 1)]

    # cut at the beginning
    assert cut(coords, 0) == (None, coords)
    
    # cut in the middle
    a, b = cut(coords, sqrt(2) / 2.0)
    assert a == [(0, 0), (0.5, 0.5)]
    assert b == [(0.5, 0.5), (1, 1)]

    # cut at the end
    a, b = cut(coords, sqrt(2))
    assert a == coords
    assert b == None

    # cut at the end
    a, b = cut(coords, 1000)
    assert a == coords
    assert b == None