export function mat4_mul(a, b) {
	const out = new Float32Array(16);
	for (let c = 0; c < 4; c++) {
		for (let r = 0; r < 4; r++) {
			let s = 0;
			for (let k = 0; k < 4; k++) { s += a[k*4+r] * b[c*4+k]; }
			out[c*4+r] = s;
		}
	}
	return out;
}

export function mat4_perspective(fov, aspect, near, far) {
	const f = 1 / Math.tan(fov / 2), nf = 1 / (near - far);
	return new Float32Array([f/aspect,0,0,0, 0,f,0,0, 0,0,(far+near)*nf,-1, 0,0,2*far*near*nf,0]);
}

export function mat4_translate_z(z) {
	return new Float32Array([1,0,0,0, 0,1,0,0, 0,0,1,0, 0,0,z,1]);
}

export function quat_identity() { return [0, 0, 0, 1]; }

export function quat_from_axis_angle(ax, ay, az, a) {
	const s = Math.sin(a/2);
	return [ax*s, ay*s, az*s, Math.cos(a/2)];
}

export function quat_mul([ax,ay,az,aw], [bx,by,bz,bw]) {
	return [aw*bx+ax*bw+ay*bz-az*by, aw*by-ax*bz+ay*bw+az*bx,
	        aw*bz+ax*by-ay*bx+az*bw, aw*bw-ax*bx-ay*by-az*bz];
}

export function quat_conj([x,y,z,w]) { return [-x,-y,-z,w]; }

export function quat_rotate_vec([qx,qy,qz,qw], [vx,vy,vz]) {
	const tx = 2*(qy*vz - qz*vy);
	const ty = 2*(qz*vx - qx*vz);
	const tz = 2*(qx*vy - qy*vx);
	return [vx + qw*tx + qy*tz - qz*ty,
	        vy + qw*ty + qz*tx - qx*tz,
	        vz + qw*tz + qx*ty - qy*tx];
}

export function mat4_from_quat([x,y,z,w]) {
	return new Float32Array([
		1-2*(y*y+z*z), 2*(x*y+w*z),   2*(x*z-w*y),   0,
		2*(x*y-w*z),   1-2*(x*x+z*z), 2*(y*z+w*x),   0,
		2*(x*z+w*y),   2*(y*z-w*x),   1-2*(x*x+y*y), 0,
		0, 0, 0, 1,
	]);
}

export function mat3_from_mat4(m) {
	return new Float32Array([m[0],m[1],m[2], m[4],m[5],m[6], m[8],m[9],m[10]]);
}