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/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Copyright by the Board of Trustees of the University of Illinois. *
* All rights reserved. *
* *
* This file is part of HDF5. The full HDF5 copyright notice, including *
* terms governing use, modification, and redistribution, is contained in *
* the files COPYING and Copyright.html. COPYING can be found at the root *
* of the source code distribution tree; Copyright.html can be found at the *
* root level of an installed copy of the electronic HDF5 document set and *
* is linked from the top-level documents page. It can also be found at *
* http://hdf.ncsa.uiuc.edu/HDF5/doc/Copyright.html. If you do not have *
* access to either file, you may request a copy from hdfhelp@ncsa.uiuc.edu. *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
/*-------------------------------------------------------------------------
*
* Created: H5checksum.c
* Aug 21 2006
* Quincey Koziol <koziol@hdfgroup.org>
*
* Purpose: Internal code for computing fletcher32 checksums
*
*-------------------------------------------------------------------------
*/
/****************/
/* Module Setup */
/****************/
/***********/
/* Headers */
/***********/
#include "H5private.h" /* Generic Functions */
/****************/
/* Local Macros */
/****************/
/******************/
/* Local Typedefs */
/******************/
/********************/
/* Package Typedefs */
/********************/
/********************/
/* Local Prototypes */
/********************/
/*********************/
/* Package Variables */
/*********************/
/*****************************/
/* Library Private Variables */
/*****************************/
/*******************/
/* Local Variables */
/*******************/
/*-------------------------------------------------------------------------
* Function: H5_fletcher32
*
* Purpose: This routine provides a generic, fast checksum algorithm for
* use in the library.
*
* Note: See the Wikipedia page for Fletcher's checksum:
* http://en.wikipedia.org/wiki/Fletcher%27s_checksum
* for more details, etc.
*
* Note #2: The algorithm below differs from that given in the Wikipedia
* page by copying the data into 'sum1' in a more portable way
* and also by initializing 'sum1' and 'sum2' to 0 instead of
* 0xffff (for backward compatibility reasons, mostly).
*
* Return: 32-bit fletcher checksum of input buffer (can't fail)
*
* Programmer: Quincey Koziol
* Monday, August 21, 2006
*
*-------------------------------------------------------------------------
*/
uint32_t
H5_fletcher32(const void *_data, size_t _len)
{
const uint8_t *data = (const uint8_t *)_data; /* Pointer to the data to be summed */
size_t len = _len / 2; /* Length in 16-bit words */
uint32_t sum1 = 0, sum2 = 0;
FUNC_ENTER_NOAPI_NOINIT_NOFUNC(H5_fletcher32)
/* Sanity check */
HDassert(_data);
HDassert(_len > 0);
/* Compute checksum for pairs of bytes */
/* (the magic "360" value is is the largest number of sums that can be
* performed without numeric overflow)
*/
while (len) {
unsigned tlen = len > 360 ? 360 : len;
len -= tlen;
do {
sum1 += (((uint16_t)data[0]) << 8) | ((uint16_t)data[1]);
data += 2;
sum2 += sum1;
} while (--tlen);
sum1 = (sum1 & 0xffff) + (sum1 >> 16);
sum2 = (sum2 & 0xffff) + (sum2 >> 16);
}
/* Check for odd # of bytes */
if(_len % 2) {
sum1 += ((uint16_t)*data) << 8;
sum2 += sum1;
sum1 = (sum1 & 0xffff) + (sum1 >> 16);
sum2 = (sum2 & 0xffff) + (sum2 >> 16);
} /* end if */
/* Second reduction step to reduce sums to 16 bits */
sum1 = (sum1 & 0xffff) + (sum1 >> 16);
sum2 = (sum2 & 0xffff) + (sum2 >> 16);
FUNC_LEAVE_NOAPI((sum2 << 16) | sum1)
} /* end H5_fletcher32() */
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