Alignment calculation for arena

align.c

// Trying to understand the padding calculation from https://nullprogram.com/blog/2023/09/27/

byte *alloc(arena *a, size objsize, size align, size count) {
  size avail = a->end - a->beg;
  /*
   Padding is how far the next aligned address is beyond the beginning of the arena.
   (The "beginning" of the arena advances as data is added, and is really the beginning of the remaining avilable space.)

   Use wrapping unsigned integer negation of the beginning address to measure what's left rather than what's in use.
   Calculate how far this address is beyond the previous aligned address using modulo:
   
     addr % align == addr & (align - 1) // because align is a power of 2 (i.e. > 0)

   Example with u4 address and 4 byte alignment:
          0x  0   4   8   c   
         beg  ---------->..... 0xb 0b1011
        -beg  .....<---------- 0x5 0b0101
       align  |   |   |   |    4   0b0100
     align-1                       0b0011
     padding      x            1   0b0001 == -beg & (align-1)
      giving  ----------->.... 0xc = 0xb + 1
   */
  size padding = -(uptr)a->beg & (align - 1);
  if (count > (avail - padding)/objsize) oom();
  size total = count * objsize;
  byte *p = a->beg + padding;
  a->beg += padding + total;
  for (size i = 0; i < total; i++) p[i] = 0;
  return p;
}