Function array_seek

src/backend/utils/adt/arrayfuncs.c:4755–4796

* Advance ptr over nitems array elements * * ptr: starting location in array * offset: 0-based linear element number of first element (the one at *ptr) * nullbitmap: start of array's null bitmap, or NULL if none * nitems: number of array elements to advance over (>= 0) * typlen, typbyval, typalign: storage parameters of array element datatype * * It is caller's responsibility to ensure tha

 * It is caller's responsibility to ensure that nitems is within range
 */
static char *
array_seek(char *ptr, int offset, bits8 *nullbitmap, int nitems,
		   int typlen, bool typbyval, char typalign)
{
	int			bitmask;
	int			i;

	/* easy if fixed-size elements and no NULLs */
	if (typlen > 0 && !nullbitmap)
		return ptr + nitems * ((Size) att_align_nominal(typlen, typalign));

	/* seems worth having separate loops for NULL and no-NULLs cases */
	if (nullbitmap)
	{
		nullbitmap += offset / 8;
		bitmask = 1 << (offset % 8);

		for (i = 0; i < nitems; i++)
		{
			if (*nullbitmap & bitmask)
			{
				ptr = att_addlength_pointer(ptr, typlen, ptr);
				ptr = (char *) att_align_nominal(ptr, typalign);
			}
			bitmask <<= 1;
			if (bitmask == 0x100 /* (1<<8) */)
			{
				nullbitmap++;
				bitmask = 1;
			}
		}
	}
	else
	{
		for (i = 0; i < nitems; i++)
		{
			ptr = att_addlength_pointer(ptr, typlen, ptr);
			ptr = (char *) att_align_nominal(ptr, typalign);
		}
	}
	return ptr;
}

/*
 * Compute total size of the nitems array elements starting at *ptr