package kjv

// MT19937 seeded with init_by_array from one integer, and a choice made by
// rejection sampling over its bits, so the verse of the day for any given date
// never changes.

type mtRandom struct {
	mt  [624]uint32
	idx int
}

func newMTRandom(seed uint32) *mtRandom {
	r := &mtRandom{}
	r.initGenrand(19650218)
	key := []uint32{seed}
	i, j := 1, 0
	k := max(624, len(key))
	for ; k > 0; k-- {
		r.mt[i] = (r.mt[i] ^ ((r.mt[i-1] ^ (r.mt[i-1] >> 30)) * 1664525)) + key[j] + uint32(j)
		i++
		j++
		if i >= 624 {
			r.mt[0] = r.mt[623]
			i = 1
		}
		if j >= len(key) {
			j = 0
		}
	}
	for k = 623; k > 0; k-- {
		r.mt[i] = (r.mt[i] ^ ((r.mt[i-1] ^ (r.mt[i-1] >> 30)) * 1566083941)) - uint32(i)
		i++
		if i >= 624 {
			r.mt[0] = r.mt[623]
			i = 1
		}
	}
	r.mt[0] = 0x80000000
	r.idx = 624
	return r
}

func (r *mtRandom) initGenrand(s uint32) {
	r.mt[0] = s
	for i := 1; i < 624; i++ {
		r.mt[i] = 1812433253*(r.mt[i-1]^(r.mt[i-1]>>30)) + uint32(i)
	}
	r.idx = 624
}

func (r *mtRandom) next() uint32 {
	if r.idx >= 624 {
		for k := 0; k < 624; k++ {
			y := (r.mt[k] & 0x80000000) | (r.mt[(k+1)%624] & 0x7fffffff)
			v := r.mt[(k+397)%624] ^ (y >> 1)
			if y&1 != 0 {
				v ^= 0x9908b0df
			}
			r.mt[k] = v
		}
		r.idx = 0
	}
	y := r.mt[r.idx]
	r.idx++
	y ^= y >> 11
	y ^= (y << 7) & 0x9d2c5680
	y ^= (y << 15) & 0xefc60000
	y ^= y >> 18
	return y
}

// choiceIndex is random.choice's _randbelow(n) for n < 2**32.
func (r *mtRandom) choiceIndex(n int) int {
	if n <= 0 {
		return 0
	}
	k := 0
	for v := n; v > 0; v >>= 1 {
		k++
	}
	for {
		if x := int(r.next() >> (32 - k)); x < n {
			return x
		}
	}
}