mod1.c

#include <stdint.h>
#include <stdio.h>

typedef float f32;
typedef int32_t i32;

f32 mod1(f32 f) {
  i32 i = *(i32*)&f;
  i32 e = (i >> 23) & 0xff;
  i32 m = i & 0x7fffff;
  if (e == 255) { /* inf or nan */
    i = 1 | (e << 23); /* nan */
  } else if (e >= 127) { /* >= 1 */
    /* keep the fractional bits */
    if (e - 127 >= 32) return 0.0f;
    m &= 0x7fffff >> (e - 127);
    if (m == 0) return 0.0f;
    /* renormalize */
    while (!(m & 0x800000)) e--, m <<= 1;
    m &= 0x7fffff;
    i = m | (e << 23) | (i & 0x80000000);
  }
  return *(f32*)&i;
}

int main() {
  printf("%f\n", mod1(123.45));
  printf("%f\n", mod1(1.0/0.0));
  printf("%f\n", mod1(0.000123));
  printf("%f\n", mod1(-1234.000123));
  return 0;
}