線分の交差判定 (Test whether line segments are intersected) on Python

intersected.py

def intersected(a,b,c,d):
    x1=b[0]-a[0]
    x2=c[0]-d[0]
    x3=b[1]-a[1]
    x4=c[1]-d[1]
    d=float(x1*x4-x2*x3)
    if d==0: return False
    c1=c[0]-a[0]
    c2=c[1]-a[1]
    s=(c1*x4-c2*x2)/d
    t=(c2*x1-c1*x3)/d
    if 0<=s<=1 and 0<=t<=1: return True
    return False

if __name__ == "__main__":
    # should be intersected
    a,b=(-3,1),(3,1)
    c,d=(-2,2),(-2,-1)
    print intersected(a,b,c,d)

    # should not be intersected
    a,b=(-3,1),(3,1)
    c,d=(2,-1),(-2,-1)
    print intersected(a,b,c,d)