File dp_hash64.h¶

FileList > doppler > dp_hash64.h

FNV-1a 64 over bytes, incrementally: the one content hash in doppler. More...

#include <stddef.h>
#include <stdint.h>

Public Static Functions¶

Type	Name
uint64_t	dp_hash64 (uint64_t h, const void * data, size_t n) Continue an FNV-1a 64 hash over `n` more bytes.

Macros¶

Type	Name
define	DP_HASH64_INIT `UINT64\_C (0xcbf29ce484222325)` The FNV-1a 64 offset basis: the hash of no bytes, and where a hash starts.
define	DP_HASH64_PRIME `UINT64\_C (0x100000001b3)` The FNV-1a 64 prime.

Detailed Description¶

It identifies content and does not protect it. A record names a --data-from-file source by its path, its length in bits and this hash, and a replay refuses a file whose hash differs (docs/design/payload-data-source.md §4.8). A path alone replays whatever the file holds on the day, and a length alone misses an edit of the same size.

FNV-1a rather than xxHash64 because identity needs no speed: the hash runs at the rate a file is read, and a file is read at the rate frames are sent. FNV-1a is a few lines, works a byte at a time and so is incremental by construction, and has published test vectors. Not for anything an adversary chooses: a collision can be constructed.

Header-only, for the reason dp_crc16.h gives: no component grows a link-line dependency for a few lines of arithmetic.

Public Static Functions Documentation¶

function dp_hash64¶

Continue an FNV-1a 64 hash over n more bytes.

static inline uint64_t dp_hash64 (
    uint64_t h,
    const void * data,
    size_t n
)

Pass DP_HASH64_INIT to start, then feed each block as it is read, passing back what the last call returned. The result does not depend on where the input was split, so a file hashed read by read gives the same value as the whole file hashed at once, and is never read twice. The state is the returned uint64_t itself, so a source that serializes its state carries the hash by value.

Parameters:

h the hash so far; DP_HASH64_INIT for none.
data the next bytes; may be NULL when n is 0.
n how many.

Returns:

the hash over everything fed so far.

uint64_t h = DP_HASH64_INIT;
h = dp_hash64 (h, "foo", 3);
h = dp_hash64 (h, "bar", 3);   // 0x85944171f73967e8, as for "foobar"

Macro Definition Documentation¶

define DP_HASH64_INIT¶

The FNV-1a 64 offset basis: the hash of no bytes, and where a hash starts.

#define DP_HASH64_INIT `UINT64_C (0xcbf29ce484222325)`

define DP_HASH64_PRIME¶

The FNV-1a 64 prime.

#define DP_HASH64_PRIME `UINT64_C (0x100000001b3)`

The documentation for this class was generated from the following file native/inc/doppler/dp_hash64.h