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#pragma once
#ifndef ALIGNMENT
#define ALIGNMENT (2*sizeof(void*))
#endif
struct Arena {
unsigned char* buffer;
size_t prev_offset;
size_t curr_offset;
size_t capacity;
};
// definition
b8 is_power_of_two(uintptr_t x);
uintptr_t fast_modulo(uintptr_t p, uintptr_t a);
uintptr_t align_forward(uintptr_t ptr, uintptr_t alignment);
void arena_init(Arena* a, unsigned char *memory, size_t capacity);
void* arena_alloc(Arena* a, size_t size);
void arena_clear(Arena* a);
// implementation
b8 is_power_of_two(uintptr_t x) {
return (x & (x-1)) == 0;
}
uintptr_t fast_modulo(uintptr_t p, uintptr_t a) {
return (p & (a - 1));
}
uintptr_t align_forward(uintptr_t ptr, size_t alignment) {
uintptr_t p, a, modulo;
assert(is_power_of_two(alignment));
p = ptr;
a = (uintptr_t)alignment;
modulo = fast_modulo(p, a);
if (modulo != 0) {
p += (a - modulo);
}
return p;
}
void arena_init(Arena* a, unsigned char *memory, size_t capacity) {
a->buffer = memory;
a->prev_offset = 0;
a->curr_offset = 0;
a->capacity = capacity;
}
void* arena_alloc(Arena* a, size_t size) {
void *ptr = NULL;
assert(is_power_of_two(ALIGNMENT));
uintptr_t curr_ptr = (uintptr_t)a->buffer + a->curr_offset;
uintptr_t offset = align_forward(curr_ptr, ALIGNMENT);
offset = offset - (uintptr_t)a->buffer;
size_t next_offset = offset + size;
assert(next_offset <= a->capacity);
ptr = &a->buffer[offset];
a->prev_offset = a->curr_offset;
a->curr_offset = next_offset;
memset(ptr, 0, size);
return ptr;
}
void arena_clear(Arena* a) {
a->curr_offset = 0;
a->prev_offset = 0;
}
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