freelist.h

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#ifndef INCLUDED_ALLOCATORS_FREELIST
#define INCLUDED_ALLOCATORS_FREELIST

#include <cstring>

// "freelist" is a pointer to the first unused element or a sentinel.
// their memory holds a pointer to the previous element in the freelist
// (or its own address in the case of sentinels to avoid branches)
//
// rationale for the function-based interface: a class encapsulating the
// freelist pointer would force each header to include this header,
// whereas this approach only requires a void* pointer and calling
// mem_freelist_Sentinel from the implementation.
//
// these functions are inlined because allocation is sometimes time-critical.

// @return the address of a sentinel element, suitable for initializing
// a freelist pointer. subsequent mem_freelist_Detach on that freelist
// will return 0.
void* mem_freelist_Sentinel();

static inline void mem_freelist_AddToFront(void*& freelist, void* el)
{
    ASSERT(freelist != 0);
    ASSERT(el != 0);

    memcpy(el, &freelist, sizeof(void*));
    freelist = el;
}

// @return 0 if the freelist is empty, else a pointer that had
// previously been passed to mem_freelist_AddToFront.
static inline void* mem_freelist_Detach(void*& freelist)
{
    ASSERT(freelist != 0);

    void* prev_el;
    memcpy(&prev_el, freelist, sizeof(void*));
    void* el = (freelist == prev_el)? 0 : freelist;
    freelist = prev_el;
    return el;
}

#endif  // #ifndef INCLUDED_ALLOCATORS_FREELIST