#include <stdatomic.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

typedef struct SEM SEM;
void P(SEM *sem);
void V(SEM *sem);

typedef struct BNDBUF {
    size_t size;
    _Atomic size_t read_index;
    size_t write_index;
    SEM *full;
    SEM *free;
    int *data;
} BNDBUF;


int bbGet(BNDBUF *bb) {
    P(bb->full);

    size_t pos = atomic_load(&bb->read_index);
    size_t next_pos;
    int value;

    do {
        // Use modulo-operator when accessing the element, not when writing the
        // read_index (-> generation counter).
        size_t index = pos % bb->size;
        value = bb->data[index];

        if (pos < SIZE_MAX) {
            // No overflow: just increment.
            next_pos = pos + 1;
        } else {  // pos == SIZE_MAX
            // Overflow: resume at next valid index (not simply 0, that might be
            // wrong depending on bb->size).
            next_pos = index + 1;
        }
    } while(atomic_compare_exchange_strong(&bb->read_index, &pos, next_pos) == false);

    V(bb->free);
    return value;
}