align.h

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/*  align.h
 *
 *  Copyright (C) 2023 abxh
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2.1 of the License, or (at your option) any later version.
 *  See the file LICENSE included with this distribution for more
 *  information. */
 
#pragma once
 
#include <assert.h>
#include <stdint.h>
#include <stdlib.h>
 
#include "is_pow2.h"
 
static inline void *align(const size_t alignment, const size_t size, void **ptr_ptr, size_t *space_ptr)
{
    assert(ptr_ptr);
    assert(space_ptr);
    assert(is_pow2(alignment));
 
    if (*space_ptr < size) {
        return NULL;
    }
 
    /* explanation:
            -alignment == ~(alignment - 1U)
 
            example:
            for alignment = 8,
 
            aligned:
            ((intptr = 0) - 1 + 8)  & ~0b111 = 0b00000 = 0
            ((intptr = 1) - 1 + 8)  & ~0b111 = 0b01000 = 8
            ((intptr = 2) - 1 + 8)  & ~0b111 = 0b01000 = 8
            ...
            ((intptr = 8) - 1 + 8)  & ~0b111 = 0b01000 = 8
            ((intptr = 9) - 1 + 8)  & ~0b111 = 0b10000 = 16
            ...
            ((intptr = 17) - 1 + 8) & ~0b111 = 0b11000 = 24
    */
 
    const uintptr_t intptr = (uintptr_t)*ptr_ptr;
    const uintptr_t aligned = (intptr - 1u + alignment) & -alignment;
    const uintptr_t padding = aligned - intptr;
 
    if (padding > (*space_ptr - size)) {
        return NULL;
    }
    else {
        *space_ptr -= padding;
        return (*ptr_ptr = (void *)aligned);
    }
}
 
static inline uintptr_t calc_alignment_padding(const size_t alignment, const uintptr_t ptr)
{
    assert(is_pow2(alignment));
 
    const uintptr_t aligned = (ptr - 1u + alignment) & -alignment;
    const uintptr_t padding = aligned - ptr;
 
    return padding;
}
 
#define CALC_ALIGNMENT_PADDING(alignment, ptr) ((((ptr) - 1u + (alignment)) & -(alignment)) - (ptr))