https://twitter.com/sup39x1207/status/1484772160581562368

sms-rng-index.c

#include <stdint.h>
uint32_t rng(uint32_t x);
uint32_t rnginv(uint32_t x);

#define mul4k1(a, b) (4*a*b+a+b)
#define d 275878811
#define bdinv 15530683
#define dbinv 317143667
#define qmask 0x7FFFFFFF

static uint32_t mxmodq(uint32_t x) {
  uint32_t u = d;
  uint32_t ans = 0;
  while (x) {
    if (x & 1)
      ans = mul4k1(ans, u);
    u = mul4k1(u, u);
    x >>= 1;
  }
  return ans & qmask;
}

uint32_t rng(uint32_t x) {
  return mxmodq(x)*bdinv & qmask;
}

uint32_t rnginv(uint32_t r) {
  uint32_t s = dbinv*r & qmask;
  uint32_t x = 0;
  while (1) {
    uint32_t g = mxmodq(x);
    if (g == s) return x;
    x |= 1 << __builtin_ctz(g-s);
  }
  return x;
}