Add comment explaining checksum mechanism.

FossilOrigin-Name: 3e9ef5153ebf0543ad0f75a7561f73d22171da53

diff --git a/manifest b/manifest
index d67960a052e90666a399d4b938afbd7d8a38937a..ed63519a6fad73f0af26b40b3776813ff5ab92b1 100644 (file)
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Fix\sbugs\sin\sWAL\smode\srollback.
-D 2010-04-24T18:44:05
+C Add\scomment\sexplaining\schecksum\smechanism.
+D 2010-04-24T19:07:29
 F Makefile.arm-wince-mingw32ce-gcc fcd5e9cd67fe88836360bb4f9ef4cb7f8e2fb5a0
 F Makefile.in 4f2f967b7e58a35bb74fb7ec8ae90e0f4ca7868b
 F Makefile.linux-gcc d53183f4aa6a9192d249731c90dbdffbd2c68654
@@ -131,7 +131,7 @@ F src/journal.c b0ea6b70b532961118ab70301c00a33089f9315c
 F src/legacy.c a199d7683d60cef73089e892409113e69c23a99f
 F src/lempar.c 7f026423f4d71d989e719a743f98a1cbd4e6d99e
 F src/loadext.c 1c7a61ce1281041f437333f366a96aa0d29bb581
-F src/log.c 6097555f746e9992b3bb7a257d2e60ced69e3a84
+F src/log.c 538da9c42f6da426e82b114f37c27cf730fcba34
 F src/log.h 06eacedac8b5a720cb88aa3c9e5d886ba7b394fe
 F src/main.c 867de6aa444abd97771b2b70472f448d65c1c77e
 F src/malloc.c a08f16d134f0bfab6b20c3cd142ebf3e58235a6a
@@ -808,7 +808,7 @@ F tool/speedtest2.tcl ee2149167303ba8e95af97873c575c3e0fab58ff
 F tool/speedtest8.c 2902c46588c40b55661e471d7a86e4dd71a18224
 F tool/speedtest8inst1.c 293327bc76823f473684d589a8160bde1f52c14e
 F tool/vdbe-compress.tcl d70ea6d8a19e3571d7ab8c9b75cba86d1173ff0f
-P a352f6285e33a806fbe4475e720e763fdc5bb47d
-R a372a014eac56d1df8e79bcff4ecc6e7
+P 31215969f59be536fe87431bb9fbfa7d13027e35
+R 1d48ccc5eb3586f62961518732858aa1
 U dan
-Z 3d8f3676c5fc9fd427fe6aa0aa15e428
+Z d82402bdae4dea648cb0a1aad66c722d

diff --git a/manifest.uuid b/manifest.uuid
index b8874609f243b76e49000dd94ca9c63277f8ed5d..aa217ce025a366f67188cfa39459be895887ce17 100644 (file)
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-31215969f59be536fe87431bb9fbfa7d13027e35
\ No newline at end of file
+3e9ef5153ebf0543ad0f75a7561f73d22171da53
\ No newline at end of file

diff --git a/src/log.c b/src/log.c
index d9637de024a1e925a2f72491ad4bdb560a9755d0..c0357e87de00e3aa251815ff946a5c5c66a5753e 100644 (file)
--- a/src/log.c
+++ b/src/log.c
@@ -316,15 +316,26 @@ static LogSummary *pLogSummary = 0;
 ** Generate an 8 byte checksum based on the data in array aByte[] and the
 ** initial values of aCksum[0] and aCksum[1]. The checksum is written into
 ** aCksum[] before returning.
+**
+** The range of bytes to checksum is treated as an array of 32-bit 
+** little-endian unsigned integers. For each integer X in the array, from
+** start to finish, do the following:
+**
+**   aCksum[0] += X;
+**   aCksum[1] += aCksum[0];
+**
+** For the calculation above, use 64-bit unsigned accumulators. Before
+** returning, truncate the values to 32-bits as follows: 
+**
+**   aCksum[0] = (u32)(aCksum[0] + (aCksum[0]>>24));
+**   aCksum[1] = (u32)(aCksum[1] + (aCksum[1]>>24));
 */
-#define LOG_CKSM_BYTES 8
 static void logChecksumBytes(u8 *aByte, int nByte, u32 *aCksum){
   u64 sum1 = aCksum[0];
   u64 sum2 = aCksum[1];
   u32 *a32 = (u32 *)aByte;
   u32 *aEnd = (u32 *)&aByte[nByte];
 
-  assert( LOG_CKSM_BYTES==2*sizeof(u32) );
   assert( (nByte&0x00000003)==0 );
 
   if( SQLITE_LITTLEENDIAN ){
@@ -1666,7 +1677,7 @@ int sqlite3LogFrames(
   PgHdr *pLast;                   /* Last frame in list */
   int nLast = 0;                  /* Number of extra copies of last page */
 
-  assert( LOG_FRAME_HDRSIZE==(4 * 2 + LOG_CKSM_BYTES) );
+  assert( LOG_FRAME_HDRSIZE==(4 * 2 + 2*sizeof(u32)) );
   assert( pList );
 
   /* If this is the first frame written into the log, write the log