package game

import (
	"crypto/rand"
	"encoding/binary"
	"math/bits"
)

// xoshiro is a random number generator for shotgun.
//
// Currently xoshiro256**. Subject to change.
type xoshiro struct {
	w, x, y, z uint64
}

// newRNG produces a new, uniquely seeded RNG.
func newRNG() xoshiro {
	r := xoshiro{}
	b := make([]byte, 8*4)
	// Loop to ensure the state is never all-zero, which is an invalid state
	// for xoshiro.
	for r.w == 0 && r.x == 0 && r.y == 0 && r.z == 0 {
		rand.Read(b)
		r.w = binary.LittleEndian.Uint64(b)
		r.x = binary.LittleEndian.Uint64(b[8:])
		r.y = binary.LittleEndian.Uint64(b[16:])
		r.z = binary.LittleEndian.Uint64(b[24:])
	}
	return r
}

// Uint64 produces a 64-bit pseudo-random value.
func (rng *xoshiro) Uint64() uint64 {
	w, x, y, z := rng.w, rng.x, rng.y, rng.z
	r := bits.RotateLeft64(x*5, 7) * 9
	t := x << 17
	y ^= w
	z ^= x
	x ^= y
	w ^= z
	y ^= t
	z = bits.RotateLeft64(z, 45)
	rng.w, rng.x, rng.y, rng.z = w, x, y, z
	return r
}

// Intn produces an int in [0, n). Panics if n <= 0.
func (rng *xoshiro) Intn(n int) int {
	if n <= 0 {
		panic("shotgun: rng.Intn max below zero")
	}
	bad := ^uint(0) - ^uint(0)%uint(n)
	x := uint(rng.Uint64())
	for x > bad {
		x = uint(rng.Uint64())
	}
	return int(x % uint(n))
}

func shuffle[E any, S ~[]E](rng *xoshiro, s S) {
	for i := len(s) - 1; i > 0; i-- {
		j := rng.Intn(i + 1)
		s[i], s[j] = s[j], s[i]
	}
}