This module implements efficient computations of hash values for diverse Nim types. All the procs are based on these two building blocks:
If you want to implement hash procs for your custom types, you will end up writing the following kind of skeleton of code:
Example:
import std/hashes
type
Something = object
foo: int
bar: string
iterator items(x: Something): Hash =
yield hash(x.foo)
yield hash(x.bar)
proc hash(x: Something): Hash =
## Computes a Hash from `x`.
var h: Hash = 0
# Iterate over parts of `x`.
for xAtom in x:
# Mix the atom with the partial hash.
h = h !& xAtom
# Finish the hash.
result = !$h
If your custom types contain fields for which there already is a hash proc, you can simply hash together the hash values of the individual fields:
Example:
import std/hashes
type
Something = object
foo: int
bar: string
proc hash(x: Something): Hash =
## Computes a Hash from `x`.
var h: Hash = 0
h = h !& hash(x.foo)
h = h !& hash(x.bar)
result = !$h
Important: Use -d:nimPreviewHashRef to enable hashing refs. It is expected that this behavior becomes the new default in upcoming versions.
Note: If the type has a \== operator, the following must hold: If two values compare equal, their hashes must also be equal.
See also
- md5 module for the MD5 checksum algorithm
- base64 module for a Base64 encoder and decoder
- sha1 module for the SHA-1 checksum algorithm
- tables module for hash tables
Imports
Types
Hash = int
A hash value. Hash tables using these values should always have a size of a power of two so they can use the and operator instead of mod for truncation of the hash value. Source Edit
Procs
proc `!$`(h: Hash): Hash {.inline, ...raises: [], tags: [], forbids: [].}
Finishes the computation of the hash value.
This is only needed if you need to implement a hash proc for a new datatype.
proc `!&`(h: Hash; val: int): Hash {.inline, ...raises: [], tags: [], forbids: [].}
Mixes a hash value h with val to produce a new hash value.
This is only needed if you need to implement a hash proc for a new datatype.
proc hash(sBuf: string; sPos, ePos: int): Hash {....raises: [], tags: [],
forbids: [].}
Efficient hashing of a string buffer, from starting position sPos to ending position ePos (included).
hash(myStr, 0, myStr.high) is equivalent to hash(myStr).
Example:
var a = "abracadabra"
doAssert hash(a, 0, 3) == hash(a, 7, 10)
proc hash(x: cstring): Hash {....raises: [], tags: [], forbids: [].}
Efficient hashing of null-terminated strings.
Example:
doAssert hash(cstring"abracadabra") == hash("abracadabra")
doAssert hash(cstring"AbracadabrA") == hash("AbracadabrA")
doAssert hash(cstring"abracadabra") != hash(cstring"AbracadabrA")
proc hash(x: float): Hash {.inline, ...raises: [], tags: [], forbids: [].}
Efficient hashing of floats. Source Edit
proc hash(x: pointer): Hash {.inline, ...raises: [], tags: [], forbids: [].}
Efficient hash overload. Source Edit
proc hash(x: string): Hash {....raises: [], tags: [], forbids: [].}
Efficient hashing of strings.
See also:
Example:
doAssert hash("abracadabra") != hash("AbracadabrA")
proc hash[A](aBuf: openArray[A]; sPos, ePos: int): Hash
Efficient hashing of portions of arrays and sequences, from starting position sPos to ending position ePos (included). There must be a hash proc defined for the element type A.
hash(myBuf, 0, myBuf.high) is equivalent to hash(myBuf).
Example:
let a = [1, 2, 5, 1, 2, 6]
doAssert hash(a, 0, 1) == hash(a, 3, 4)
proc hash[A](x: openArray[A]): Hash
proc hash[A](x: set[A]): Hash
proc hash[T: Ordinal | enum](x: T): Hash {.inline.}
Efficient hashing of integers. Source Edit
proc hash[T: tuple | object | proc | iterator {.closure.}](x: T): Hash
Efficient hash overload.
Example:
# for `tuple|object`, `hash` must be defined for each component of `x`.
type Obj = object
x: int
y: string
type Obj2[T] = object
x: int
y: string
assert hash(Obj(x: 520, y: "Nim")) != hash(Obj(x: 520, y: "Nim2"))
# you can define custom hashes for objects (even if they're generic):
proc hash(a: Obj2): Hash = hash((a.x))
assert hash(Obj2[float](x: 520, y: "Nim")) == hash(Obj2[float](x: 520, y: "Nim2"))
Example:
# proc
proc fn1() = discard
const fn1b = fn1
assert hash(fn1b) == hash(fn1)
# closure
proc outer =
var a = 0
proc fn2() = a.inc
assert fn2 is "closure"
let fn2b = fn2
assert hash(fn2b) == hash(fn2)
assert hash(fn2) != hash(fn1)
outer()
proc hash[T](x: ptr [T]): Hash {.inline.}
Efficient hash overload.
Example:
var a: array[10, uint8]
assert a[0].addr.hash != a[1].addr.hash
assert cast[pointer](a[0].addr).hash == a[0].addr.hash
proc hash[T](x: ref [T]): Hash {.inline.}
Efficient hash overload.
Important: Use -d:nimPreviewHashRef to enable hashing refs. It is expected that this behavior becomes the new default in upcoming versions.
Example: cmd: -d:nimPreviewHashRef
type A = ref object
x: int
let a = A(x: 3)
let ha = a.hash
assert ha != A(x: 3).hash # A(x: 3) is a different ref object from `a`.
a.x = 4
assert ha == a.hash # the hash only depends on the address
Example: cmd: -d:nimPreviewHashRef
# you can overload `hash` if you want to customize semantics
type A[T] = ref object
x, y: T
proc hash(a: A): Hash = hash(a.x)
assert A[int](x: 3, y: 4).hash == A[int](x: 3, y: 5).hash
proc hashData(data: pointer; size: int): Hash {....raises: [], tags: [],
forbids: [].}
Hashes an array of bytes of size size. Source Edit
proc hashIdentity[T: Ordinal | enum](x: T): Hash {.inline.}
The identity hash, i.e. hashIdentity(x) = x. Source Edit
proc hashIgnoreCase(sBuf: string; sPos, ePos: int): Hash {....raises: [], tags: [],
forbids: [].}
Efficient hashing of a string buffer, from starting position sPos to ending position ePos (included); case is ignored.
Note: This uses a different hashing algorithm than hash(string).
hashIgnoreCase(myBuf, 0, myBuf.high) is equivalent to hashIgnoreCase(myBuf).
Example:
var a = "ABracadabRA"
doAssert hashIgnoreCase(a, 0, 3) == hashIgnoreCase(a, 7, 10)
proc hashIgnoreCase(x: string): Hash {....raises: [], tags: [], forbids: [].}
Efficient hashing of strings; case is ignored.
Note: This uses a different hashing algorithm than hash(string).
See also:
Example:
doAssert hashIgnoreCase("ABRAcaDABRA") == hashIgnoreCase("abRACAdabra")
doAssert hashIgnoreCase("abcdefghi") != hash("abcdefghi")
proc hashIgnoreStyle(sBuf: string; sPos, ePos: int): Hash {....raises: [],
tags: [], forbids: [].}
Efficient hashing of a string buffer, from starting position sPos to ending position ePos (included); style is ignored.
Note: This uses a different hashing algorithm than hash(string).
hashIgnoreStyle(myBuf, 0, myBuf.high) is equivalent to hashIgnoreStyle(myBuf).
Example:
var a = "ABracada_b_r_a"
doAssert hashIgnoreStyle(a, 0, 3) == hashIgnoreStyle(a, 7, a.high)
proc hashIgnoreStyle(x: string): Hash {....raises: [], tags: [], forbids: [].}
Efficient hashing of strings; style is ignored.
Note: This uses a different hashing algorithm than hash(string).
See also:
Example:
doAssert hashIgnoreStyle("aBr_aCa_dAB_ra") == hashIgnoreStyle("abracadabra")
doAssert hashIgnoreStyle("abcdefghi") != hash("abcdefghi")
proc hashWangYi1(x: int64 | uint64 | Hash): Hash {.inline.}
Wang Yi’s hash_v1 for 64-bit ints (see https://github.com/rurban/smhasher for more details). This passed all scrambling tests in Spring 2019 and is simple.
Note: It’s ok to define proc(x: int16): Hash = hashWangYi1(Hash(x)).