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authorJunio C Hamano <gitster@pobox.com>2011-08-23 22:40:33 (GMT)
committerJunio C Hamano <gitster@pobox.com>2011-08-23 22:40:33 (GMT)
commit6fcb384869720ed2e9ea7de1938e44c54e56a75a (patch)
treebac64a700b43df8700e7f3c4e7d93cc0fb29d3dc /sha1_file.c
parent52457205a2301d66848fd965c928bc4e0360e2aa (diff)
parent3f4ab627146fcb1e1887198f520f39fb84fde73a (diff)
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Merge branch 'rt/zlib-smaller-window'
* rt/zlib-smaller-window: test: consolidate definition of $LF Tolerate zlib deflation with window size < 32Kb
Diffstat (limited to 'sha1_file.c')
-rw-r--r--sha1_file.c32
1 files changed, 26 insertions, 6 deletions
diff --git a/sha1_file.c b/sha1_file.c
index d5616dc..44444ae 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1217,14 +1217,34 @@ static int experimental_loose_object(unsigned char *map)
unsigned int word;
/*
- * Is it a zlib-compressed buffer? If so, the first byte
- * must be 0x78 (15-bit window size, deflated), and the
- * first 16-bit word is evenly divisible by 31. If so,
- * we are looking at the official format, not the experimental
- * one.
+ * We must determine if the buffer contains the standard
+ * zlib-deflated stream or the experimental format based
+ * on the in-pack object format. Compare the header byte
+ * for each format:
+ *
+ * RFC1950 zlib w/ deflate : 0www1000 : 0 <= www <= 7
+ * Experimental pack-based : Stttssss : ttt = 1,2,3,4
+ *
+ * If bit 7 is clear and bits 0-3 equal 8, the buffer MUST be
+ * in standard loose-object format, UNLESS it is a Git-pack
+ * format object *exactly* 8 bytes in size when inflated.
+ *
+ * However, RFC1950 also specifies that the 1st 16-bit word
+ * must be divisible by 31 - this checksum tells us our buffer
+ * is in the standard format, giving a false positive only if
+ * the 1st word of the Git-pack format object happens to be
+ * divisible by 31, ie:
+ * ((byte0 * 256) + byte1) % 31 = 0
+ * => 0ttt10000www1000 % 31 = 0
+ *
+ * As it happens, this case can only arise for www=3 & ttt=1
+ * - ie, a Commit object, which would have to be 8 bytes in
+ * size. As no Commit can be that small, we find that the
+ * combination of these two criteria (bitmask & checksum)
+ * can always correctly determine the buffer format.
*/
word = (map[0] << 8) + map[1];
- if (map[0] == 0x78 && !(word % 31))
+ if ((map[0] & 0x8F) == 0x08 && !(word % 31))
return 0;
else
return 1;