// Package spiralmatrix creates spiral matrices.
package spiralmatrix
type direction int
const (
	right direction = iota
	down
	left
	up
)
type matrix struct {
	d      direction
	min    int
	max    int
	row    int
	column int
}
// SpiralMatrix returns slices of a spiral matrix.
func SpiralMatrix(size int) [][]int {
	ret := make([][]int, size)
	for i := range ret {
		ret[i] = make([]int, size)
	}
	m := matrix{max: size - 1}
	for i := 1; i <= size*size; i++ {
		ret[m.row][m.column] = i
		m.next()
		if m.row == m.min && m.column == m.min && i != 1 {
			// we reached top left corner, going to the inner circle
			m.min++
			m.max--
			m.row++
			m.column++
		}
	}
	return ret
}
func (m *matrix) direction() {
	switch {
	case m.d == right && m.column+1 <= m.max:
		// keep right
	case m.d == down && m.row+1 <= m.max:
		// keep down
	case m.d == left && m.column-1 >= m.min:
		// keep left
	case m.d == up && m.row-1 >= m.min:
		// keep up
	default:
		// go to next direction
		m.d = (m.d + 1) % 4
	}
}
func (m *matrix) next() {
	m.direction()
	switch m.d {
	case right:
		m.column++
	case down:
		m.row++
	case left:
		m.column--
	case up:
		m.row--
	}
}