git-2.22.0/pack-revindex.c
/*
 * This is a least-significant-digit radix sort.
 *
 * It sorts each of the "n" items in "entries" by its offset field. The "max"
 * parameter must be at least as large as the largest offset in the array,
 * and lets us quit the sort early.
 */
This is a least-significant-digit radix sort.
It sorts each of the "n" items in "entries" by its offset field. The "max"
parameter must be at least as large as the largest offset in the array,
and lets us quit the sort early.
This is a least - significant - digit radix sort . It sorts each of the " n " items in " entries " by its offset field . The " max " parameter must be at least as large as the largest offset in the array and lets us quit the sort early .
static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
{
#define DIGIT_SIZE (16)
#define BUCKETS (1 << DIGIT_SIZE)
#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
	struct revindex_entry *tmp, *from, *to;
	int bits;
	unsigned *pos;
	ALLOC_ARRAY(pos, BUCKETS);
	ALLOC_ARRAY(tmp, n);
	from = entries;
	to = tmp;
	for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
		unsigned i;
		memset(pos, 0, BUCKETS * sizeof(*pos));
		for (i = 0; i < n; i++)
			pos[BUCKET_FOR(from, i, bits)]++;
		for (i = 1; i < BUCKETS; i++)
			pos[i] += pos[i-1];
		for (i = n - 1; i != UINT_MAX; i--)
			to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
		SWAP(from, to);
	}
	if (from != entries)
		COPY_ARRAY(entries, tmp, n);
	free(tmp);
	free(pos);
#undef BUCKET_FOR
#undef BUCKETS
#undef DIGIT_SIZE
}
static void sort_revindex ( struct revindex_entry * entries , unsigned n , off_t max ) { #define DIGIT_SIZE  (16) #define BUCKETS  (1 << DIGIT_SIZE) #define BUCKET_FOR ( a , i , bits )  (((a)[(i)].offset >> (bits)) & (BUCKETS-1)) struct revindex_entry * tmp , * from , * to ; int bits ; unsigned * pos ; ALLOC_ARRAY ( pos , BUCKETS ) ; ALLOC_ARRAY ( tmp , n ) ; from = entries ; to = tmp ; for ( bits = 0 ; max >> bits ; bits += DIGIT_SIZE ) { unsigned i ; memset ( pos , 0 , BUCKETS * sizeof ( * pos ) ) ; for ( i = 0 ; i < n ; i ++ ) pos [ BUCKET_FOR ( from , i , bits ) ] ++ ; for ( i = 1 ; i < BUCKETS ; i ++ ) pos [ i ] += pos [ i - 1 ] ; for ( i = n - 1 ; i != UINT_MAX ; i -- ) to [ -- pos [ BUCKET_FOR ( from , i , bits ) ] ] = from [ i ] ; SWAP ( from , to ) ; } if ( from != entries ) COPY_ARRAY ( entries , tmp , n ) ; free ( tmp ) ; free ( pos ) ; #undef  BUCKET_FOR #undef  BUCKETS #undef  DIGIT_SIZE }
This is a least-significant-digit radix sort.
This is a least - significant - digit radix sort .
/*
	 * We use a "digit" size of 16 bits. That keeps our memory
	 * usage reasonable, and we can generally (for a 4G or smaller
	 * packfile) quit after two rounds of radix-sorting.
	 */ /*
	 * We want to know the bucket that a[i] will go into when we are using
	 * the digit that is N bits from the (least significant) end.
	 */ /*
	 * We need O(n) temporary storage. Rather than do an extra copy of the
	 * partial results into "entries", we sort back and forth between the
	 * real array and temporary storage. In each iteration of the loop, we
	 * keep track of them with alias pointers, always sorting from "from"
	 * to "to".
	 */ /*
	 * If (max >> bits) is zero, then we know that the radix digit we are
	 * on (and any higher) will be zero for all entries, and our loop will
	 * be a no-op, as everybody lands in the same zero-th bucket.
	 */ /*
		 * We want pos[i] to store the index of the last element that
		 * will go in bucket "i" (actually one past the last element).
		 * To do this, we first count the items that will go in each
		 * bucket, which gives us a relative offset from the last
		 * bucket. We can then cumulatively add the index from the
		 * previous bucket to get the true index.
		 */ /*
		 * Now we can drop the elements into their correct buckets (in
		 * our temporary array).  We iterate the pos counter backwards
		 * to avoid using an extra index to count up. And since we are
		 * going backwards there, we must also go backwards through the
		 * array itself, to keep the sort stable.
		 *
		 * Note that we use an unsigned iterator to make sure we can
		 * handle 2^32-1 objects, even on a 32-bit system. But this
		 * means we cannot use the more obvious "i >= 0" loop condition
		 * for counting backwards, and must instead check for
		 * wrap-around with UINT_MAX.
		 */ /*
		 * Now "to" contains the most sorted list, so we swap "from" and
		 * "to" for the next iteration.
		 */ /*
	 * If we ended with our data in the original array, great. If not,
	 * we have to move it back from the temporary storage.
	 */