"""Generate a simple test STL (binary) — a unit cube centered at origin."""
import struct

vertices = [
    (-1,-1,-1), (1,-1,-1), (1,1,-1), (-1,1,-1),
    (-1,-1,1),  (1,-1,1),  (1,1,1),  (-1,1,1),
]

faces = [
    (0,2,1), (0,3,2),  # bottom
    (4,5,6), (4,6,7),  # top
    (0,1,5), (0,5,4),  # front
    (2,3,7), (2,7,6),  # back
    (0,4,7), (0,7,3),  # left
    (1,2,6), (1,6,5),  # right
]

def normal(v0, v1, v2):
    e1 = [v1[i]-v0[i] for i in range(3)]
    e2 = [v2[i]-v0[i] for i in range(3)]
    n = [e1[1]*e2[2]-e1[2]*e2[1], e1[2]*e2[0]-e1[0]*e2[2], e1[0]*e2[1]-e1[1]*e2[0]]
    l = sum(x*x for x in n)**0.5
    return [x/l for x in n] if l > 0 else [0,0,0]

with open("examples/cube.stl", "wb") as f:
    f.write(b'\0' * 80)
    f.write(struct.pack('<I', len(faces)))
    for face in faces:
        v = [vertices[i] for i in face]
        n = normal(*v)
        f.write(struct.pack('<3f', *n))
        for vert in v:
            f.write(struct.pack('<3f', *[float(x) for x in vert]))
        f.write(struct.pack('<H', 0))