// Package rng provides a small deterministic PRNG (xoshiro256**) seeded via
// splitmix64. Streams are derived from a seed plus labels so that independent
// parts of the simulation never share (and thus never perturb) each other's
// random sequence.
package rng

import (
	"math/bits"

	"wh/fixed"
)

type R struct{ s [4]uint64 }

func splitmix(x *uint64) uint64 {
	*x += 0x9e3779b97f4a7c15
	z := *x
	z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9
	z = (z ^ (z >> 27)) * 0x94d049bb133111eb
	return z ^ (z >> 31)
}

// New returns a generator for the given seed.
func New(seed uint64) *R {
	r := &R{}
	for i := range r.s {
		r.s[i] = splitmix(&seed)
	}
	return r
}

// Derive returns a generator for an independent stream identified by seed and
// labels, e.g. Derive(worldSeed, day, shipID).
func Derive(seed uint64, labels ...uint64) *R {
	h := seed
	for _, l := range labels {
		h ^= l + 0x9e3779b97f4a7c15 + (h << 6) + (h >> 2)
		splitmix(&h)
	}
	return New(h)
}

func (r *R) Uint64() uint64 {
	s := &r.s
	res := bits.RotateLeft64(s[1]*5, 7) * 9
	t := s[1] << 17
	s[2] ^= s[0]
	s[3] ^= s[1]
	s[1] ^= s[2]
	s[0] ^= s[3]
	s[2] ^= t
	s[3] = bits.RotateLeft64(s[3], 45)
	return res
}

// Intn returns a uniform value in [0, n). n must be > 0.
func (r *R) Intn(n uint64) uint64 {
	hi, lo := bits.Mul64(r.Uint64(), n)
	if lo < n {
		thresh := -n % n
		for lo < thresh {
			hi, lo = bits.Mul64(r.Uint64(), n)
		}
	}
	return hi
}

// Fixed returns a uniform value in [0, 1).
func (r *R) Fixed() fixed.F { return fixed.F(r.Uint64() >> 32) }

// FixedRange returns a uniform value in [lo, hi).
func (r *R) FixedRange(lo, hi fixed.F) fixed.F {
	return lo + r.Fixed().Mul(hi-lo)
}