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Improved comments on the sqlar.c and compress.c extensions that describe

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the differences between the "zlib format" used by SQLAR, the raw deflate
format used by ZIP, and the custom format used by compress.c.  No changes
to code.

FossilOrigin-Name: c13415c5caf06eaa73086c500907451dfcdfd1354ac4a9ab63edb7255edbfa1b
This commit is contained in:
drh
2018-01-07 19:52:28 +00:00
parent 1fa6d9f96f
commit acd6fdee98
4 changed files with 31 additions and 8 deletions
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15
ext/misc/compress.c
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@ -27,6 +27,21 @@ SQLITE_EXTENSION_INIT1
** seven bits per integer stored in the lower seven bits of each byte.
** More significant bits occur first. The most significant bit (0x80)
** is a flag to indicate the end of the integer.
**
** This function, SQLAR, and ZIP all use the same "deflate" compression
** algorithm, but each is subtly different:
**
** * ZIP uses raw deflate.
**
** * SQLAR uses the "zlib format" which is raw deflate with a two-byte
** algorithm-identification header and a four-byte checksum at the end.
**
** * This utility uses the "zlib format" like SQLAR, but adds the variable-
** length integer uncompressed size value at the beginning.
**
** This function might be extended in the future to support compression
** formats other than deflate, by providing a different algorithm-id
** mark following the variable-length integer size parameter.
*/
static void compressFunc(
sqlite3_context *context,

8
ext/misc/sqlar.c
View File

@ -24,6 +24,14 @@ SQLITE_EXTENSION_INIT1
** If the type of X is SQLITE_BLOB, and compressing that blob using
** zlib utility function compress() yields a smaller blob, return the
** compressed blob. Otherwise, return a copy of X.
**
** SQLar uses the "zlib format" for compressed content. The zlib format
** contains a two-byte identification header and a four-byte checksum at
** the end. This is different from ZIP which uses the raw deflate format.
**
** Future enhancements to SQLar might add support for new compression formats.
** If so, those new formats will be identified by alternative headers in the
** compressed data.
*/
static void sqlarCompressFunc(
sqlite3_context *context,