2 * adler32.c : routines for handling Adler-32 checksums
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28 #include "private/svn_adler32.h"
31 * An Adler-32 implementation per RFC1950.
33 * "The Adler-32 algorithm is much faster than the CRC32 algorithm yet
34 * still provides an extremely low probability of undetected errors"
38 * 65521 is the largest prime less than 65536.
39 * "That 65521 is prime is important to avoid a possible large class of
40 * two-byte errors that leave the check unchanged."
42 #define ADLER_MOD_BASE 65521
45 * Start with CHECKSUM and update the checksum by processing a chunk
49 svn__adler32(apr_uint32_t checksum
, const char *data
, apr_off_t len
)
51 /* The actual limit can be set somewhat higher but should
52 * not be lower because the SIMD code would not be used
55 * However, it must be lower than 5552 to make sure our local
56 * implementation does not suffer from overflows.
60 /* Larger buffers can be effiently handled by Marc Adler's
61 * optimized code. Also, new zlib versions will come with
62 * SIMD code for x86 and x64.
64 return (apr_uint32_t
)adler32(checksum
,
70 const unsigned char *input
= (const unsigned char *)data
;
71 apr_uint32_t s1
= checksum
& 0xFFFF;
72 apr_uint32_t s2
= checksum
>> 16;
75 /* Some loop unrolling
76 * (approx. one clock tick per byte + 2 ticks loop overhead)
78 for (; len
>= 8; len
-= 8, input
+= 8)
80 s1
+= input
[0]; s2
+= s1
;
81 s1
+= input
[1]; s2
+= s1
;
82 s1
+= input
[2]; s2
+= s1
;
83 s1
+= input
[3]; s2
+= s1
;
84 s1
+= input
[4]; s2
+= s1
;
85 s1
+= input
[5]; s2
+= s1
;
86 s1
+= input
[6]; s2
+= s1
;
87 s1
+= input
[7]; s2
+= s1
;
90 /* Adler-32 calculation as a simple two ticks per iteration loop.
99 return ((s2
% ADLER_MOD_BASE
) << 16) | (s1
% ADLER_MOD_BASE
);