_random.h

#pragma once
#include "_math.h"
 
namespace njin {

struct rng {
  u64 state = 0x853c49e6748fea9bULL; 
  u64 inc = 0xda3e39cb94b95bdbULL;   
 
  rng() = default;
  explicit rng(u64 seed) { reseed(seed); }

  void reseed(u64 seed) {
    state = 0;
    inc = (seed << 1u) | 1u;
    next_u32();
    state += seed;
    next_u32();
  }

  u32 next_u32() {
    const u64 old = state;
    state = old * 6364136223846793005ULL + inc;
    const u32 xorshifted = (u32)(((old >> 18u) ^ old) >> 27u);
    const u32 rot = (u32)(old >> 59u);
    return (xorshifted >> rot) | (xorshifted << ((-rot) & 31u));
  }

  f32 unit() { return (f32)(next_u32() >> 8) * (1.0f / 16777216.0f); }

  f32 range(f32 lo, f32 hi) { return lo + (hi - lo) * unit(); }

  i32 range(i32 lo, i32 hi) {
    if (hi <= lo)
      return lo;
    const u32 span = (u32)((i64)hi - (i64)lo + 1);
    // Loại bỏ phần dư để mọi số có xác suất như nhau.
    const u32 limit = (u32)(-span) % span;
    u32 r = next_u32();
    while (r < limit)
      r = next_u32();
    return (i32)((i64)lo + (i64)(r % span));
  }

  bool chance(f32 p) { return unit() < p; }

  vec2 direction() { return from_angle(range(0.0f, 360.0f)); }

  vec2 point_in(rect r) {
    return {r.pos.x + range(0.0f, r.size.x), r.pos.y + range(0.0f, r.size.y)};
  }
};

} // namespace njin