pg_lzcompress.c

来自「postgresql8.3.4源码,开源数据库」· C语言 代码 · 共 727 行 · 第 1/2 页

/* ----------
 * pglz_find_match -
 *
 *		Lookup the history table if the actual input stream matches
 *		another sequence of characters, starting somewhere earlier
 *		in the input buffer.
 * ----------
 */
static inline int
pglz_find_match(PGLZ_HistEntry **hstart, const char *input, const char *end,
				int *lenp, int *offp, int good_match, int good_drop)
{
	PGLZ_HistEntry *hent;
	int32		len = 0;
	int32		off = 0;

	/*
	 * Traverse the linked history list until a good enough match is found.
	 */
	hent = hstart[pglz_hist_idx(input, end)];
	while (hent)
	{
		const char *ip = input;
		const char *hp = hent->pos;
		int32		thisoff;
		int32		thislen;

		/*
		 * Stop if the offset does not fit into our tag anymore.
		 */
		thisoff = ip - hp;
		if (thisoff >= 0x0fff)
			break;

		/*
		 * Determine length of match. A better match must be larger than the
		 * best so far. And if we already have a match of 16 or more bytes,
		 * it's worth the call overhead to use memcmp() to check if this match
		 * is equal for the same size. After that we must fallback to
		 * character by character comparison to know the exact position where
		 * the diff occurred.
		 */
		thislen = 0;
		if (len >= 16)
		{
			if (memcmp(ip, hp, len) == 0)
			{
				thislen = len;
				ip += len;
				hp += len;
				while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
				{
					thislen++;
					ip++;
					hp++;
				}
			}
		}
		else
		{
			while (ip < end && *ip == *hp && thislen < PGLZ_MAX_MATCH)
			{
				thislen++;
				ip++;
				hp++;
			}
		}

		/*
		 * Remember this match as the best (if it is)
		 */
		if (thislen > len)
		{
			len = thislen;
			off = thisoff;
		}

		/*
		 * Advance to the next history entry
		 */
		hent = hent->next;

		/*
		 * Be happy with lesser good matches the more entries we visited. But
		 * no point in doing calculation if we're at end of list.
		 */
		if (hent)
		{
			if (len >= good_match)
				break;
			good_match -= (good_match * good_drop) / 100;
		}
	}

	/*
	 * Return match information only if it results at least in one byte
	 * reduction.
	 */
	if (len > 2)
	{
		*lenp = len;
		*offp = off;
		return 1;
	}

	return 0;
}


/* ----------
 * pglz_compress -
 *
 *		Compresses source into dest using strategy.
 * ----------
 */
bool
pglz_compress(const char *source, int32 slen, PGLZ_Header *dest,
			  const PGLZ_Strategy *strategy)
{
	unsigned char *bp = ((unsigned char *) dest) + sizeof(PGLZ_Header);
	unsigned char *bstart = bp;
	int			hist_next = 0;
	bool		hist_recycle = false;
	const char *dp = source;
	const char *dend = source + slen;
	unsigned char ctrl_dummy = 0;
	unsigned char *ctrlp = &ctrl_dummy;
	unsigned char ctrlb = 0;
	unsigned char ctrl = 0;
	int32		match_len;
	int32		match_off;
	int32		good_match;
	int32		good_drop;
	int32		result_size;
	int32		result_max;
	int32		need_rate;

	/*
	 * Our fallback strategy is the default.
	 */
	if (strategy == NULL)
		strategy = PGLZ_strategy_default;

	/*
	 * If the strategy forbids compression (at all or if source chunk too
	 * small), fail.
	 */
	if (strategy->match_size_good <= 0 ||
		slen < strategy->min_input_size)
		return false;

	/*
	 * Save the original source size in the header.
	 */
	dest->rawsize = slen;

	/*
	 * Limit the match size to the maximum implementation allowed value
	 */
	if ((good_match = strategy->match_size_good) > PGLZ_MAX_MATCH)
		good_match = PGLZ_MAX_MATCH;
	if (good_match < 17)
		good_match = 17;

	if ((good_drop = strategy->match_size_drop) < 0)
		good_drop = 0;
	if (good_drop > 100)
		good_drop = 100;

	/*
	 * Initialize the history lists to empty.  We do not need to zero the
	 * hist_entries[] array; its entries are initialized as they are used.
	 */
	memset((void *) hist_start, 0, sizeof(hist_start));

	/*
	 * Compute the maximum result size allowed by the strategy. If the input
	 * size exceeds force_input_size, the max result size is the input size
	 * itself. Otherwise, it is the input size minus the minimum wanted
	 * compression rate.
	 */
	if (slen >= strategy->force_input_size)
		result_max = slen;
	else
	{
		need_rate = strategy->min_comp_rate;
		if (need_rate < 0)
			need_rate = 0;
		else if (need_rate > 99)
			need_rate = 99;
		result_max = slen - ((slen * need_rate) / 100);
	}

	/*
	 * Compress the source directly into the output buffer.
	 */
	while (dp < dend)
	{
		/*
		 * If we already exceeded the maximum result size, fail.
		 *
		 * We check once per loop; since the loop body could emit as many as 4
		 * bytes (a control byte and 3-byte tag), PGLZ_MAX_OUTPUT() had better
		 * allow 4 slop bytes.
		 */
		if (bp - bstart >= result_max)
			return false;

		/*
		 * Try to find a match in the history
		 */
		if (pglz_find_match(hist_start, dp, dend, &match_len,
							&match_off, good_match, good_drop))
		{
			/*
			 * Create the tag and add history entries for all matched
			 * characters.
			 */
			pglz_out_tag(ctrlp, ctrlb, ctrl, bp, match_len, match_off);
			while (match_len--)
			{
				pglz_hist_add(hist_start, hist_entries,
							  hist_next, hist_recycle,
							  dp, dend);
				dp++;			/* Do not do this ++ in the line above!		*/
				/* The macro would do it four times - Jan.	*/
			}
		}
		else
		{
			/*
			 * No match found. Copy one literal byte.
			 */
			pglz_out_literal(ctrlp, ctrlb, ctrl, bp, *dp);
			pglz_hist_add(hist_start, hist_entries,
						  hist_next, hist_recycle,
						  dp, dend);
			dp++;				/* Do not do this ++ in the line above!		*/
			/* The macro would do it four times - Jan.	*/
		}
	}

	/*
	 * Write out the last control byte and check that we haven't overrun the
	 * output size allowed by the strategy.
	 */
	*ctrlp = ctrlb;
	result_size = bp - bstart;
	if (result_size >= result_max)
		return false;

	/*
	 * Success - need only fill in the actual length of the compressed datum.
	 */
	SET_VARSIZE_COMPRESSED(dest, result_size + sizeof(PGLZ_Header));

	return true;
}


/* ----------
 * pglz_decompress -
 *
 *		Decompresses source into dest.
 * ----------
 */
void
pglz_decompress(const PGLZ_Header *source, char *dest)
{
	const unsigned char *sp;
	const unsigned char *srcend;
	unsigned char *dp;
	unsigned char *destend;

	sp = ((const unsigned char *) source) + sizeof(PGLZ_Header);
	srcend = ((const unsigned char *) source) + VARSIZE(source);
	dp = (unsigned char *) dest;
	destend = dp + source->rawsize;

	while (sp < srcend && dp < destend)
	{
		/*
		 * Read one control byte and process the next 8 items (or as many
		 * as remain in the compressed input).
		 */
		unsigned char ctrl = *sp++;
		int		ctrlc;

		for (ctrlc = 0; ctrlc < 8 && sp < srcend; ctrlc++)
		{
			if (ctrl & 1)
			{
				/*
				 * Otherwise it contains the match length minus 3 and the
				 * upper 4 bits of the offset. The next following byte
				 * contains the lower 8 bits of the offset. If the length is
				 * coded as 18, another extension tag byte tells how much
				 * longer the match really was (0-255).
				 */
				int32		len;
				int32		off;

				len = (sp[0] & 0x0f) + 3;
				off = ((sp[0] & 0xf0) << 4) | sp[1];
				sp += 2;
				if (len == 18)
					len += *sp++;

				/*
				 * Check for output buffer overrun, to ensure we don't
				 * clobber memory in case of corrupt input.  Note: we must
				 * advance dp here to ensure the error is detected below
				 * the loop.  We don't simply put the elog inside the loop
				 * since that will probably interfere with optimization.
				 */
				if (dp + len > destend)
				{
					dp += len;
					break;
				}

				/*
				 * Now we copy the bytes specified by the tag from OUTPUT to
				 * OUTPUT. It is dangerous and platform dependent to use
				 * memcpy() here, because the copied areas could overlap
				 * extremely!
				 */
				while (len--)
				{
					*dp = dp[-off];
					dp++;
				}
			}
			else
			{
				/*
				 * An unset control bit means LITERAL BYTE. So we just copy
				 * one from INPUT to OUTPUT.
				 */
				if (dp >= destend)	/* check for buffer overrun */
					break;			/* do not clobber memory */

				*dp++ = *sp++;
			}

			/*
			 * Advance the control bit
			 */
			ctrl >>= 1;
		}
	}

	/*
	 * Check we decompressed the right amount.
	 */
	if (dp != destend || sp != srcend)
		elog(ERROR, "compressed data is corrupt");

	/*
	 * That's it.
	 */
}