solution.py

# Project Euler problem 102

# Which side of the line AB is the point P?
# (The dot product of AP and the perpendicular to AB)
def side((ax,ay), (bx,by), (px,py)):
    return (bx - ax) * (py - ay) - (by - ay) * (px - ax)

def same_side(a, b, p, q):
    sp = side(a, b, p)
    sq = side(a, b, q)
    return (sp >= 0 and sq >= 0) or (sp <= 0 and sq <= 0)

def contains_origin(a, b, c):
    sa = same_side(b, c, a, (0,0))
    sb = same_side(c, a, b, (0,0))
    sc = same_side(a, b, c, (0,0))
    return (sa and sb and sc)

if __name__ == '__main__':
    count = 0
    with open('triangles.txt') as f:
        for line in f:
            ax,ay, bx,by, cx,cy = map(int, line.split(','))
            if contains_origin((ax,ay), (bx,by), (cx,cy)):
                count += 1
    print count