free_array.c

#define MAX_SIZE (64*1024)
static struct my_data data[MAX_SIZE];
static short free_idx[MAX_SIZE];
static short first_free_idx = -1;

// init; populate free items
free_idx[MAX_SIZE-1] = -1;
for (int i = 0; i < MAX_SIZE-1; ++i) {
  free_idx[i] = i+1;
}
first_free_idx = 0;

// insert; store index elsewhere
index = first_free_idx;
first_free_idx = free_idx[index];
free_idx[index] = -1;
data[index] = ...input;

// lookup using stored index
output = data[idx];

// update existing stored element
data[idx] = input;

// remove used stored element
free_idx[idx] = first_free_idx;
first_free_idx = idx;

// branchless iterate over all elements
for (int i = 0; i < MAX_SIZE; ++i) {
  var = free_idx[i] == -1 ? ... : ...; 
}