# Constants for line-segment tests
DONT_INTERSECT = 0
COLINEAR = -1

def have_same_signs(a, b):
    return ((a ^ b) >= 0)



def line_seg_intersect(line1point1, line1point2, line2point1, line2point2):
    x1 = line1point1[0]
    y1 = line1point1[1]
    x2 = line1point2[0]
    y2 = line1point2[1]
    x3 = line2point1[0]
    y3 = line2point1[1]
    x4 = line2point2[0]
    y4 = line2point2[1]

    a1 = y2 - y1  
    b1 = x1 - x2  
    c1 = (x2 * y1) - (x1 * y2)

    r3 = (a1 * x3) + (b1 * y3) + c1  
    r4 = (a1 * x4) + (b1 * y4) + c1

    if ((r3 != 0) and (r4 != 0) and have_same_signs(r3, r4)):
        return(DONT_INTERSECT)

    a2 = y4 - y3  
    b2 = x3 - x4  
    c2 = x4 * y3 - x3 * y4

    r1 = a2 * x1 + b2 * y1 + c2  
    r2 = a2 * x2 + b2 * y2 + c2

    if ((r1 != 0) and (r2 != 0) and have_same_signs(r1, r2)):  
         return(DONT_INTERSECT)

    denom = (a1 * b2) - (a2 * b1)  
    if denom == 0:  
        return(COLINEAR)
    elif denom < 0:
        offset = (-1 * denom / 2)
    else:
        offset = denom / 2
    
    num = (b1 * c2) - (b2 * c1)
    if num < 0:
        x = (num - offset) / denom
    else:
        x = (num + offset) / denom

    num = (a2 * c1) - (a1 * c2)  
    if num <0:
        y = (num - offset) / denom
    else:
        y = (num - offset) / denom

    return (x, y)


def can_see(source, target, blocking_rects_list):
    
    """
    Performs a los check from the center of the source to the center of the target.
    Makes the following assumtion:
        1 - Both the source and target are objects that include a pygame.Rect() member object
            called object.rect.

    Returns 1 of line of sight is clear. Returns 0 if it is blocked.
    """
    
    los_line_p1 = source.rect.center
    los_line_p2 = target.rect.center
 

    # check each candidate rect against this los line. If any of them
    # intersect, the los is blocked.

    for rect in blocking_rects_list:
        block_p1 = rect.topleft
        block_p2 = rect.bottomright
        if line_seg_intersect(los_line_p1, los_line_p2, block_p1, block_p2):
            return 0
        block_p1 = rect.topright
        block_p2 = rect.bottomleft
        if line_seg_intersect(los_line_p1, los_line_p2, block_p1, block_p2):
            return 0
    return 1