Documentation
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Overview ¶
Package hash implements the hash function used throughout Noms.
Noms serialization from version 4-onward uses the first 20 bytes of sha-512 for hashes.
sha-512 was chosen because:
- sha-1 is no longer recommended. - sha-3 is brand new, not a lot of platform support. - blake is not commonly used, not a lot of platform support. - within sha-2, sha-512 is faster than sha-256 on 64 bit.
Our specific truncation scheme (first 20 bytes) was chosen because:
- The "standard" truncation schemes are not widely supported. For example, at time of writing, there is no fast native implementation of sha512/256 on Node. - The smallest standard truncation of sha512 is 28 bytes, but we don't need this many. And because we are a database, the size of the hashes matters. Bigger hashes mean less data in each chunk, which means less tree fan-out, which means slower iteration and searching. 20 bytes is a good balance between collision resistance and wide trees. - 20 bytes leads to a nice round number of base32 digits: 32.
The textual serialization of hashes uses big-endian base32 with the alphabet {0-9,a-v}. This scheme was chosen because:
- It's easy to convert to and from base32 without bignum arithemetic. - No special chars: you can double-click to select in GUIs. - Sorted hashes will be sorted textually, making it easy to scan for humans.
In Noms, the hash function is a component of the serialization version, which is constant over the entire lifetime of a single database. So clients do not need to worry about encountering multiple hash functions in the same database.
Index ¶
Constants ¶
const ( // ByteLen is the number of bytes used to represent the Hash. ByteLen = 20 // StringLen is the number of characters need to represent the Hash using Base32. StringLen = 32 // 20 * 8 / log2(32) )
Variables ¶
This section is empty.
Functions ¶
This section is empty.
Types ¶
type Hash ¶
Hash is used to represent the hash of a Noms Value.
func MaybeParse ¶
MaybeParse parses a string representing a hash as a Base32 encoded byte array. If the string is not well formed then this returns (emptyHash, false).
func Parse ¶
Parse parses a string representing a hash as a Base32 encoded byte array. If the string is not well formed then this panics.
func (Hash) Greater ¶
Greater compares two hashes returning whether this Hash is greater than other.
type HashSet ¶
type HashSet map[Hash]struct{}
HashSet is a set of Hashes.
Source Files