https://leetcode.com/problems/robot-bounded-in-circle/

robotBoundedInCircle.go

/*
IDEA: Iff:
  - The robot is facing North after the instructions, AND
  - The robot was displaced by any non-zero amount, THEN
the robot will hare off into the sunset. 

Otherwise, the robot will loop endlessly. 

Proof: Lots of drawing on square-ruled paper. 
*/
func isRobotBounded(instructions string) bool {
	vectors := [4][2]int{{1, 0}, {0, 1}, {-1, 0}, {0, -1}}
	heading := uint8(0)
	pos := [2]int{0, 0}

	for _, c := range instructions {
		switch c {
		case 'G':
			pos[0] += vectors[heading][0]
			pos[1] += vectors[heading][1]
		case 'L':
			heading = (heading + 3) % 4
		case 'R':
			heading = (heading + 1) % 4
		}
	}

	return (heading != 0) || (pos[0] == 0 && pos[1] == 0)
}