#include <stdio.h>
#include <stdlib.h>

void* AllocateLargestFreeBlock(size_t* Size)
{
  size_t s0, s1;
  void* p;

  s0 = ~(size_t)0 ^ (~(size_t)0 >> 1);

  while (s0 && (p = malloc(s0)) == NULL)
    s0 >>= 1;

  if (p)
    free(p);

  s1 = s0 >> 1;

  while (s1)
  {
    if ((p = malloc(s0 + s1)) != NULL)
    {
      s0 += s1;
      free(p);
    }
    s1 >>= 1;
  }

  while (s0 && (p = malloc(s0)) == NULL)
    s0 ^= s0 & -s0;

  *Size = s0;
  return p;
}

size_t GetFreeSize(void)
{
  size_t total = 0;
  void* pFirst = NULL;
  void* pLast = NULL;

  for (;;)
  {
    size_t largest;
    void* p = AllocateLargestFreeBlock(&largest);

    if (largest < sizeof(void*))
    {
      if (p != NULL)
        free(p);
      break;
    }

    *(void**)p = NULL;

    total += largest;

    if (pFirst == NULL)
      pFirst = p;

    if (pLast != NULL)
      *(void**)pLast = p;

    pLast = p;
  }

  while (pFirst != NULL)
  {
    void* p = *(void**)pFirst;
    free(pFirst);
    pFirst = p;
  }

  return total;
}

int main(void)
{
  printf("Total free: %zu\n", GetFreeSize());
  printf("Total free: %zu\n", GetFreeSize());
  printf("Total free: %zu\n", GetFreeSize());
  printf("Total free: %zu\n", GetFreeSize());
  printf("Total free: %zu\n", GetFreeSize());
  return 0;
}