DD::Image::Hash Class Reference

Public Types
typedef U64	HashType

Public Member Functions
	Hash (const Hash &h)
	Hash (U64 v)
const Hash &	operator= (const Hash &h)
void	reset ()
void	reset (U64 v)
bool	operator== (const Hash &h) const
bool	operator!= (const Hash &h) const
bool	operator< (const Hash &h) const
U64	value () const
U64	getHash () const
void	append (const void *data, size_t length)
void	append (const char *)
void	append (const std::string &s)
void	append (bool)
void	append (int)
void	append (unsigned)
void	append (float)
void	append (double)
void	append (const Hash &)
void	append (const std::set< int > &intset)
void	append (const std::map< int, int > &iimap)
void	append (U64)
template<class T >
void	append (const std::vector< T > &vec)
template<class T >
Hash &	operator<< (const T &v)
void	newvalue ()

Detailed Description

Generates and maintaines a hash sum of data. If the hashes of two pieces of data are equal it is extremely likely the two pieces of data are equal.

To generate a hash, make one of these objects, then reset() it, then call append() with each piece of data you want to include. You can then compare it with other hashes, or extract the numerical value with value().

The algorithim used is a 64-bit CRC checksum, using the CRC-64-ECMA-182 polynominal (lsb representation 0x42F0E1EBA9EA3693). Some people worry that this is not a cryptographcally sound checksum, but that is not needed when the data is not being produced by something that is actively trying to subvert the checksum. For random data it is equally strong.

Member Function Documentation

void DD::Image::Hash::reset ( )

inline

Reset the hash to it's initial value. This is not zero.

Referenced by DD::Image::ReadGeo::_validate(), DD::Image::GeometryList::add_object(), DD::Image::GeoOp::get_geometry(), DD::Image::GeoOp::get_geometry_hash(), and DD::Image::LUT::validate().

void Hash::append	(	const void *	data,
		size_t	length
	)

Add the n bytes starting at pointer to the hash.

Warning: this call is for appending blocks of data together. A zero-length block will not change the hash, and the hash is the same no matter how the same block is split into calls to this. This will cause the hash to fail if you actually care about this, for instance if you have several variable-sized arrays of data. Swapping a zero-length and non-zero-length one, or moving data from the end of one to the start of the next, will not change the hash. The solution is to append the length as well.

void Hash::append

(

const char *

ptr

)

Add a null-terminated string or a null pointer to the hash.

The zero-length string and the null pointer hash to different values, and those are different than not calling this.

void Hash::append

(

bool

v

)

Add a bool value to the hash. This is different than any other possible append (as it only adds 1 bit to the hash rather than a multiple of 8) and thus it is also useful for marking where zero-length arrays are.

void Hash::append

(

int

value

)

Add an integer value to the hash. Same as using append(&value, sizeof(int)) on a LSB-first machine. This is intended for hashing enumeration values.

void Hash::append

(

unsigned

value

)

Add an unsigned value to the hash. Same as using append(&value, sizeof(unsigned)) on a LSB-first machine. This is intended for hashing enumeration or size values.

void Hash::append

(

float

value

)

Add a float value to the hash. Same as using append(&value, sizeof(float)) (this may change on MSB machines).

void Hash::append

(

double

value

)

Add a double value to the hash. Same as using append(&value, sizeof(float)) (this may change on MSB machines).

void Hash::append

(

const Hash &

h

)

Add another hash to this hash.

void Hash::append

(

const std::set< int > &

intset

)

Add an intset to the hash.

void Hash::append

(

const std::map< int, int > &

iimap

)

Add a std::map<int, int> to the hash.

void Hash::append

(

U64

value

)

Add a 64 bit value to this hash.