array.c

#include "array.h"

// arenasize:
//   virtual address size of the backing arenai
void *
make_array(usize arenasize, usize elementsize, char *name)
{
    struct arena *arena = make_arena(arenasize, name);

    struct array_header *array = arena_alloc_align(arena, 16, sizeof *array);
    array->arena = arena;
    array->count = 0;
    array->elementsize = elementsize;

    return array->data;
}

void
free_array(void *data)
{
    struct array_header *array = array_header_ptr(data);
    free_arena(array->arena);
}

void *
_array_alloc(void *data, usize count)
{
    struct array_header *array = array_header_ptr(data);

    // alignment is 1 because elementsize will ensure elements are correctly
    // packed
    void *slot = arena_alloc_align(array->arena, 1, array->elementsize * count);

    array->count += count;
    return slot;
}

void
_array_appendn(void *data, void *values, usize elementsize, usize n)
{
    struct array_header *array = array_header_ptr(data);
    assert(elementsize == array->elementsize);

    void *slot = _array_alloc(data, n);
    memcpy(slot, values, elementsize * n);
}

void
_array_remove(void *data, usize index)
{
    struct array_header *array = array_header_ptr(data);
    assert(array->elementsize);
    assert(index < array->count);

    // swap slot with the last element
    void *end = array->data + array->elementsize * (array->count - 1);
    void *slot = array->data + array->elementsize * index;
    memcpy(slot, end, array->elementsize);

    array->arena->allocated -= array->elementsize;
    array->count -= 1;
}

array.h

struct array_header {
    // 16 bytes for alignment
    struct arena *arena;
    uint32 elementsize;
    uint32 count;
    char data[];
};

#define array_header_ptr(data) ((struct array_header *)((void *)data - sizeof(struct array_header)))
#define array_alloc(data, count) _array_alloc(data, count)
#define array_append(data, value) _array_appendn(data, &value, sizeof(value), 1)
#define array_appendn(data, values, n) _array_appendn(data, values, sizeof(*values), n)
#define array_remove(data, index) _array_remove(data, index)
#define array_count(data) (array_header_ptr(data)->count)


void *make_array(usize arenasize, usize elementsize, char *name);
void free_array(void *data);
void *_array_alloc(void *data, usize count);
void _array_append(void *data, void *value, usize elementsize);
void _array_appendn(void *data, void *value, usize elementsize, usize n);
void _array_remove(void *data, usize index);