upstream/mercurial-mirror Files · mercurial/bdiff.h

revlog: make compressed size comparisons consistent...

revlog: make compressed size comparisons consistent revlog.compress() compares the compressed size to the input size and throws away the compressed data if it is larger than the input. This is the correct thing to do, as storing compressed data that is larger than the input takes up more storage space and makes reading slower. However, the comparison was implemented inconsistently. For the streaming compression mode, we threw away the result if it was greater than or equal to the input size. But for the one-shot compression, we threw away the compression only if it was greater than the input size! This patch changes the comparison for the simple case so it is consistent with the streaming case. As a few tests demonstrate, this adds 1 byte to some revlog entries. This is because of an added 'u' header on the chunk. It seems somewhat wrong to increase the revlog size here. However, IMO the cost of 1 byte in storage is insignificant compared to the performance gains of avoiding decompression. This patch should invite questions around the heuristic for throwing away compressed data. For example, I'd argue we should be more liberal about rejecting compressed data, additionally doing so where the number of bytes saved fails to reach a threshold. But we can have this discussion another time.

Maciej Fijalkowski - - Load All Authors

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      #ifndef _HG_BDIFF_H_

      #define _HG_BDIFF_H_

      struct bdiff_line {

      	int hash, n, e;

      	ssize_t len;

      	const char *l;

      };

      struct bdiff_hunk;

      struct bdiff_hunk {

      	int a1, a2, b1, b2;

      	struct bdiff_hunk *next;

      };

      int bdiff_splitlines(const char *a, ssize_t len, struct bdiff_line **lr);

      int bdiff_diff(struct bdiff_line *a, int an, struct bdiff_line *b, int bn,

      	struct bdiff_hunk *base);

      void bdiff_freehunks(struct bdiff_hunk *l);

      #endif

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				#ifndef _HG_BDIFF_H_
				#define _HG_BDIFF_H_

				struct bdiff_line {
				int hash, n, e;
				ssize_t len;
				const char *l;
				};

				struct bdiff_hunk;
				struct bdiff_hunk {
				int a1, a2, b1, b2;
				struct bdiff_hunk *next;
				};

				int bdiff_splitlines(const char a, ssize_t len, struct bdiff_line *lr);
				int bdiff_diff(struct bdiff_line a, int an, struct bdiff_line b, int bn,
				struct bdiff_hunk *base);
				void bdiff_freehunks(struct bdiff_hunk *l);

				#endif