This is xnu-8019. See this file in:
#include "WKdm.h"

/***********************************************************************
 *                   THE PACKING ROUTINES
 */

/* WK_pack_2bits()
 * Pack some multiple of four words holding two-bit tags (in the low
 * two bits of each byte) into an integral number of words, i.e.,
 * one fourth as many.
 * NOTE: Pad the input out with zeroes to a multiple of four words!
 */
static WK_word*
WK_pack_2bits(WK_word* source_buf,
    WK_word* source_end,
    WK_word* dest_buf)
{
	WK_word* src_next = source_buf;
	WK_word* dest_next = dest_buf;

	while (src_next < source_end) {
		WK_word temp = src_next[0];
		temp |= (src_next[1] << 2);
		temp |= (src_next[2] << 4);
		temp |= (src_next[3] << 6);

		dest_next[0] = temp;
		dest_next++;
		src_next += 4;
	}

	return dest_next;
}

/* WK_pack_4bits()
 * Pack an even number of words holding 4-bit patterns in the low bits
 * of each byte into half as many words.
 * note: pad out the input with zeroes to an even number of words!
 */

static WK_word*
WK_pack_4bits(WK_word* source_buf,
    WK_word* source_end,
    WK_word* dest_buf)
{
	WK_word* src_next = source_buf;
	WK_word* dest_next = dest_buf;

	/* this loop should probably be unrolled */
	while (src_next < source_end) {
		WK_word temp = src_next[0];
		temp |= (src_next[1] << 4);

		dest_next[0] = temp;
		dest_next++;
		src_next += 2;
	}

	return dest_next;
}

/* pack_3_tenbits()
 * Pack a sequence of three ten bit items into one word.
 * note: pad out the input with zeroes to an even number of words!
 */
static WK_word*
WK_pack_3_tenbits(WK_word* source_buf,
    WK_word* source_end,
    WK_word* dest_buf)
{
	WK_word* src_next = source_buf;
	WK_word* dest_next = dest_buf;

	/* this loop should probably be unrolled */
	while (src_next < source_end) {
		WK_word temp = src_next[0];
		temp |= (src_next[1] << 10);
		temp |= (src_next[2] << 20);

		dest_next[0] = temp;
		dest_next++;
		src_next += 3;
	}

	return dest_next;
}

/***************************************************************************
 *  WKdm_compress()---THE COMPRESSOR
 */

unsigned int
WKdm_compress(WK_word* src_buf,
    WK_word* dest_buf,
    unsigned int num_input_words)
{
	DictionaryElement dictionary[DICTIONARY_SIZE];

	/* arrays that hold output data in intermediate form during modeling */
	/* and whose contents are packed into the actual output after modeling */

	/* sizes of these arrays should be increased if you want to compress
	 * pages larger than 4KB
	 */
	WK_word tempTagsArray[300];   /* tags for everything          */
	WK_word tempQPosArray[300];   /* queue positions for matches  */
	WK_word tempLowBitsArray[1200]; /* low bits for partial matches */

	/* boundary_tmp will be used for keeping track of what's where in
	 * the compressed page during packing
	 */
	WK_word* boundary_tmp;

	/* Fill pointers for filling intermediate arrays (of queue positions
	 * and low bits) during encoding.
	 * Full words go straight to the destination buffer area reserved
	 * for them.  (Right after where the tags go.)
	 */
	WK_word* next_full_patt;
	char* next_tag = (char *) tempTagsArray;
	char* next_qp = (char *) tempQPosArray;
	WK_word* next_low_bits = tempLowBitsArray;

	WK_word* next_input_word = src_buf;
	WK_word* end_of_input = src_buf + num_input_words;

	PRELOAD_DICTIONARY;

	next_full_patt = dest_buf + TAGS_AREA_OFFSET + (num_input_words / 16);

#ifdef WK_DEBUG
	printf("\nIn WKdm_compress\n");
	printf("About to actually compress, src_buf is %u\n", src_buf);
	printf("dictionary is at %u\n", dictionary);
	printf("dest_buf is %u next_full_patt is %u\n", dest_buf, next_full_patt);
	fflush(stdout);
#endif

	while (next_input_word < end_of_input) {
		WK_word *dict_location;
		WK_word dict_word;
		WK_word input_word = *next_input_word;

		/* compute hash value, which is a byte offset into the dictionary,
		 * and add it to the base address of the dictionary. Cast back and
		 * forth to/from char * so no shifts are needed
		 */
		dict_location =
		    (WK_word *)
		    ((void*) (((char*) dictionary) + HASH_TO_DICT_BYTE_OFFSET(input_word)));

		dict_word = *dict_location;

		if (input_word == dict_word) {
			RECORD_EXACT(dict_location - dictionary);
		} else if (input_word == 0) {
			RECORD_ZERO;
		} else {
			WK_word input_high_bits = HIGH_BITS(input_word);
			if (input_high_bits == HIGH_BITS(dict_word)) {
				RECORD_PARTIAL(dict_location - dictionary, LOW_BITS(input_word));
				*dict_location = input_word;
			} else {
				RECORD_MISS(input_word);
				*dict_location = input_word;
			}
		}
		next_input_word++;
	}

#ifdef WK_DEBUG
	printf("AFTER MODELING in WKdm_compress()\n"); fflush(stdout);
	printf("tempTagsArray holds %u bytes\n",
	    next_tag - (char *) tempTagsArray);
	printf("tempQPosArray holds %u bytes\n",
	    next_qp - (char *) tempQPosArray);
	printf("tempLowBitsArray holds %u bytes\n",
	    (char *) next_low_bits - (char *) tempLowBitsArray);

	printf("next_full_patt is %p\n",
	    next_full_patt);

	printf(" i.e., there are %u full patterns\n",
	    next_full_patt - (dest_buf + TAGS_AREA_OFFSET + (num_input_words / 16)));
	fflush(stdout);

	{ int i;
	  WK_word *arr = (dest_buf + TAGS_AREA_OFFSET + (num_input_words / 16));

	  printf("  first 20 full patterns are: \n");
	  for (i = 0; i < 20; i++) {
		  printf(" %d", arr[i]);
	  }
	  printf("\n");}
#endif

	/* Record (into the header) where we stopped writing full words,
	 * which is where we will pack the queue positions.  (Recall
	 * that we wrote the full words directly into the dest buffer
	 * during modeling.
	 */

	SET_QPOS_AREA_START(dest_buf, next_full_patt);

	/* Pack the tags into the tags area, between the page header
	 * and the full words area.  We don't pad for the packer
	 * because we assume that the page size is a multiple of 16.
	 */

#ifdef WK_DEBUG
	printf("about to pack %u bytes holding tags\n",
	    next_tag - (char *) tempTagsArray);

	{ int i;
	  char* arr = (char *) tempTagsArray;

	  printf("  first 200 tags are: \n");
	  for (i = 0; i < 200; i++) {
		  printf(" %d", arr[i]);
	  }
	  printf("\n");}
#endif

	boundary_tmp = WK_pack_2bits(tempTagsArray,
	    (WK_word *) ((void *) next_tag),
	    dest_buf + HEADER_SIZE_IN_WORDS);

#ifdef WK_DEBUG
	printf("packing tags stopped at %u\n", boundary_tmp);
#endif

	/* Pack the queue positions into the area just after
	 * the full words.  We have to round up the source
	 * region to a multiple of two words.
	 */

	{
		unsigned int num_bytes_to_pack = (unsigned int)(next_qp - (char *) tempQPosArray);
		unsigned int num_packed_words = (num_bytes_to_pack + 7) >> 3; // ceil((double) num_bytes_to_pack / 8);
		unsigned int num_source_words = num_packed_words * 2;
		WK_word* endQPosArray = tempQPosArray + num_source_words;

		/* Pad out the array with zeros to avoid corrupting real packed
		 *  values. */
		for (; /* next_qp is already set as desired */
		    next_qp < (char*)endQPosArray;
		    next_qp++) {
			*next_qp = 0;
		}

#ifdef WK_DEBUG
		printf("about to pack %u (bytes holding) queue posns.\n",
		    num_bytes_to_pack);
		printf("packing them from %u words into %u words\n",
		    num_source_words, num_packed_words);
		printf("dest is range %u to %u\n",
		    next_full_patt, next_full_patt + num_packed_words);
		{ int i;
		  char *arr = (char *) tempQPosArray;
		  printf("  first 200 queue positions are: \n");
		  for (i = 0; i < 200; i++) {
			  printf(" %d", arr[i]);
		  }
		  printf("\n");}
#endif

		boundary_tmp = WK_pack_4bits(tempQPosArray,
		    endQPosArray,
		    next_full_patt);
#ifdef WK_DEBUG
		printf("Packing of queue positions stopped at %u\n", boundary_tmp);
#endif // WK_DEBUG

		/* Record (into the header) where we stopped packing queue positions,
		 * which is where we will start packing low bits.
		 */
		SET_LOW_BITS_AREA_START(dest_buf, boundary_tmp);
	}

	/* Pack the low bit patterns into the area just after
	 * the queue positions.  We have to round up the source
	 * region to a multiple of three words.
	 */

	{
		unsigned int num_tenbits_to_pack =
		    (unsigned int)(next_low_bits - tempLowBitsArray);
		unsigned int num_packed_words = (num_tenbits_to_pack + 2) / 3; //ceil((double) num_tenbits_to_pack / 3);
		unsigned int num_source_words = num_packed_words * 3;
		WK_word* endLowBitsArray = tempLowBitsArray + num_source_words;

		/* Pad out the array with zeros to avoid corrupting real packed
		 *  values. */

		for (; /* next_low_bits is already set as desired */
		    next_low_bits < endLowBitsArray;
		    next_low_bits++) {
			*next_low_bits = 0;
		}

#ifdef WK_DEBUG
		printf("about to pack low bits\n");
		printf("num_tenbits_to_pack is %u\n", num_tenbits_to_pack);
		printf("endLowBitsArray is %u\n", endLowBitsArray);
#endif

		boundary_tmp = WK_pack_3_tenbits(tempLowBitsArray,
		    endLowBitsArray,
		    boundary_tmp);

		SET_LOW_BITS_AREA_END(dest_buf, boundary_tmp);
	}

	return (unsigned int)((char *) boundary_tmp - (char *) dest_buf);
}