StableHashTraits

The aim of StableHashTraits is to make it easy to compute a stable hash of any Julia value with minimal boilerplate using trait-based dispatch; here, "stable" means the value will not change across Julia versions (or between Julia sessions).

For example:

using StableHashTraits
using StableHashTraits: Transformer
using Dates

struct MyType
   data::Vector{UInt8}
   metadata::Dict{Symbol, Any}
end
# ignore `metadata`, `data` will be hashed using fallbacks for `AbstractArray` type
StableHashTraits.transformer(::Type{<:MyType}) = Transformer(pick_fields(:data))
# NOTE: `pick_fields` is a helper function implemented by `StableHashTraits`
# it creates a named tuple with the given object fields; in the above code it is used
# in its curried form e.g. `pick_fields(:data)` is the same as `x -> pick_fields(x, :data)`
a = MyType(read("myfile.txt"), Dict{Symbol, Any}(:read => Dates.now()))
b = MyType(read("myfile.txt"), Dict{Symbol, Any}(:read => Dates.now()))
stable_hash(a; version=4) == stable_hash(b; version=4) # true

In many cases, users can simply call stable_hash(x; version=4) on the type they want to hash. However, a method of transformer can be used to customize how an object is hashed. It should dispatch on the type to be transformed, and return a function wrapped in Transformer. During hashing, this function is called and its result is the value that is actually hashed.

StableHashTraits aims to guarantee a stable hash so long as you only upgrade to non-breaking versions (e.g. StableHashTraits = "1" in [compat] of Project.toml); any changes in an object's hash in this case would be considered a bug.

Note

In versions prior to 2.0, StableHashTraits included hash versions 1-3. These have been removed in version 2.0 of StableHashTraits, and the existing version 4 has been left unchanged, to avoid confusion: calling stable_hash(x; version=4) will yield the same result, regardless of whether you are using StableHashTraits 1.3 or 2.0. Calls to the earlier hash versions will error in 2.0.

Use Case and Design Rationale

StableHashTraits is designed to be used in cases where there is an object you wish to serialize in a content-addressed cache. How and when objects pass the same input to a hashing algorithm is meant to be predictable and well defined, so that the user can reliably define methods of transformer to modify this behavior.

What gets hashed?

When you call stable_hash(x; version=4), StableHashTraits hashes both the value x and its type T. Rather than hashing the type T itself directly, in most cases instead StructTypes.StructType(T) is hashed, using StructTypes.jl. For example, since the "StructType" of Float64 and Float32 are both NumberType, when hashing Float64 and Float32 values, value and NumberType are hashed. This provides a simple trait-based system that doesn't need to rely on internal details. See below for more details.

You can customize how the value is hashed using StableHashTraits.transformer, and how its type is hashed using StableHashTraits.transform_type. If you need to customize either of these functions for a type that you don't own, you can use a @context to avoid type piracy.

StructType.DataType

StructType.DataType denotes a type that is some kind of "record"; i.e. its content is defined by the fields (getfield(f) for f in fieldnames(T)) of the type. Since it is the default, it is used to hash most types.

To hash the value, each field value (getfield(f) for f in fieldnames(T)) is hashed.

If StructType(T) <: StructTypes.UnorderedStruct (the default), the field values are first sorted by the lexographic order of the field names.

The type of a data type is hashed using string(nameof(T)), the fieldnames(T), (sorting them for UnorderedStruct), along with a hash of the type of each element of fieldtypes(T) according to their StructType.

No type parameters are hashed by default. To hash these you need to specialize on StableHashTraits.transform_type for your struct. Note that because fieldtypes(T) is hashed, you don't need to do this unless your type parameters are not used in the specification of your field types.

StructType.ArrayType

ArrayType is used when hashing a sequence of values.

To hash the value, each element of an array type is hashed using iterate. If the object isa AbstractArray, the size(x) of the object is also hashed.

If StableHashTraits.is_ordered returns false the elements are first sorted according to StableHashTraits.hash_sort_by.

To hash the type, the string "StructTypes.ArrayType" is hashed (meaning that the kind of array won't matter to the hash value), and the type of the elype is hashed, according to its StructType. If the type <: AbstractArray, the ndims(T) is hashed.

StructTypes.DictType

To hash the value, each key-value pair of a dict type is hashed, as returned by StructType.keyvaluepairs(x).

If StableHashTraits.is_ordered returns false (which is the default return value) the pairs are first sorted according their keys using StableHashTraits.hash_sort_by.

To hash the type, the string "StructTypes.DictType" is hashed (meaning that the kind of dictionary won't matter), and the type of the keytype and valtype is hashed, according to its StructType.

AbstractRange

AbstractRange constitutes an exception to the rule that we use StructType: for efficient hashing, ranges are treated as another first-class container object, separate from array types.

The value is hashed as (first(x), step(x), last(x)).

The type is hashed as "Base.AbstractRange" along with the type of the eltype, according to its StructType. Thus, the type of range doesn't matter (just that it is a range).

StructTypes{Number/String/Bool}Type

To hash the value, the result of Base.writeing the object is hashed.

To hash the type, the value of string("StructType.", nameof_string(StructType(T)))) is used (c.f. StableHashTraits.nameof_string for details). Note that this means the type of the value itself is not being hashed, rather a string related to its struct type.

StructType.CustomStruct

For any StructType.CustomStruct, the object is first StructType.lowered and the result is hashed according to the lowered StructType.

missing and nothing

There is no value hashed for missing or nothing; the type is hashed as the string "Base.Missing" and "Base.Nothing" respectively. Note in particular the string "Base.Missing" does not have the same hash as missing, since the former would have its struct type hashed.

StructType.{Null/Singleton}Type

Null and Singleton types are hashed solely according to their type (no value is hashed)

Their types is hashed by StableHashTraits.nameof_string This means the module of the type does not matter: the module of a type is often considered an implementation detail, so it is left out to avoid unexpected hash changes from non-breaking releases that change the module of a type.

Note

If you wish to disambiguate functions or types that have the same name but that come from different modules you can overload StableHashTraits.transform_type for those functions. If you want to include the module name for a broad set of types, rather than explicitly specifying a module name for each type, you may want to consider calling StableHashTraits.module_nameof_string in the body of your transform_type method. This can avoid a number of footguns when including the module names: for example, module_nameof_string renames Core to Base to elide Base julia changes to the location of a functions between these two modules and it renames pluto workspace modules to prevent structs from having a different hash each time the notebook is run.

Function

Function values are hashed according to their their fields (getfield(f) for f in fieldnames(f)) as per StructType.UnorderedStruct; functions can have fields when they are curried (e.g. ==(2)), and so, for this reason, the fields are included in the hash by default.

The type of the function is hashed according to its StableHashTraits.nameof_string, therefore excluding its module. The exact module of a function is often considered an implementation detail, so it is left out to avoid unexpected hash changes from non-breaking releases that change the module of a function.

Type

When hashing a type as a value (e.g. stable_hash(Int; version=4)) the value of `StableHashTraits.nameof_string is hashed. The exact module of a type is often considered an implementation detail, so it is left out to avoid unexpected hash changes from non-breaking releases that change the module of a type.

Examples

All of the following hash examples follow directly from the definitions above, but may not be so obvious to the reader.

Most of the behaviors described below can be customized/changed by using your own hash StableHashTraits.@context, which can be passed as the second argument to stable_hash. StableHashTraits tries to defer to StructTypes for most defaults instead of making more opinionated choices.

The order of NamedTuple pairs does not matter, because NamedTuple has a struct type of UnorderedStruct:

stable_hash((;a=1,b=2); version=4) == stable_hash((;b=2,a=1); version=4)

Two structs with the same fields and name hash equivalently, because the default struct type is UnorderedStruct:

module A
    struct X
        bar::Int
        foo::Float64
    end
end

module B
    struct X
        foo::Float64
        bar::Int
    end
end

stable_hash(B.X(2, 1); version=4) == stable_hash(A.X(1, 2); version=4)

Different array types with the same content hash to the same value:

stable_hash(view([1,2,3], 1:2); version=4) == stable_hash([1,2]; version=4)

Byte-equivalent arrays of all NumberType values will hash to the same value:

stable_hash([0.0, 0.0]; version=4) == stable_hash([0, 0]; version=4)
stable_hash([0.0f0, 0.0f0]; version=4) != stable_hash([0, 0]; version=4) # not byte equivalent

Also, even though the bytes are the same, since the size is hashed, we have:

stable_hash([0.0f0, 0.0f0]; version=4) != stable_hash([0]; version=4)

If the eltype has a different StructType, no collision will occur:

stable_hash(Any[0.0, 0.0]; version=4) != stable_hash([0, 0]; version=4)

Even if the mathematical values are the same, if the bytes are not the same no collision will occur:

stable_hash([1.0, 2.0]; version=4) != stable_hash([1, 2]; version=4)

Two types with the same name but different type parameters will hash the same (unless you define a transform_type_value method for your type to include those type parameters in its return value):

struct MyType{T} end
stable_hash(MyType{:a}) == stable_hash(MyType{:b}) # true

Numerical changes will, of course, change the hash. One way this can catch you off guard are some differences in StaticArray outputs between julia versions:

julia> using StaticArrays, StableHashTraits;

julia> begin
        rotmatrix2d(a) = @SMatrix [cos(a) sin(a); -sin(a) cos(a)]
        rotate(a, p) = rotmatrix2d(a) * p
        rotate((pi / 4), SVector{2}(0.42095778959006, -0.42095778959006))
    end;

In julia 1.9.4:

julia> bytes2hex(stable_hash(rotate((pi / 4), SVector{2}(0.42095778959006, -0.42095778959006)); version=4))
"4ccdc172688dd2b5cd50ba81071a19217c3efe2e3b625e571542004c8f96c797"

julia> rotate((pi / 4), SVector{2}(0.42095778959006, -0.42095778959006))
2-element SVector{2, Float64} with indices SOneTo(2):
  7.419375817039376e-17
 -0.5953242152248626

In julia 1.6.7

julia> bytes2hex(stable_hash(rotate((pi / 4), SVector{2}(0.42095778959006, -0.42095778959006)); version=4))
"3b8d998f3106c05f8b74ee710267775d0d0ce0e6780c1256f4926d3b7dcddf9e"

julia> rotate((pi / 4), SVector{2}(0.42095778959006, -0.42095778959006))
2-element SVector{2, Float64} with indices SOneTo(2):
  5.551115123125783e-17
 -0.5953242152248626

Breaking changes

In 2.0

All deprecated behavior has been removed; the only hash version available is 4. Users must now explicitly specify the hash version to use (e.g. stable_hash(x; version=4))

In 1.3

This release includes a new hash version 4 that has breaking API changes relative to earlier versions, documented above. The prior API is deprecated, however remains the default to avoid breaking users's code. In version 2 of StableHashTraits, which will be released in relatively short order, only hash version 4 will be available.

In 1.2

This release includes a bugfix to stable_type_id and the underlying hashes that depend on it (true for most types). This bug caused stable_type_id to yield a different value depending on the scope in which stable_type_id was first called for a given type.

Now that 1.3 is available, 1.2 should not be used, as it addresses the same bug with a better API, rather than the hotfix applied here.

1.2 defines hash version 3, which uses a fixed version of stable_type_id that can be used by leveraging hash version 3. E.g. if you call stable_hash(x, version=3) or use HashVersion{3}() where you would have used HashVersion{2}() you will not be susceptible to the bug. If you make use of stable_type_id directly and want to avoid this bug, you should use StableHashTraits.stable_type_id_fixed.

Because existing uses of StableHashTraits might depend on the extant, broken behavior, versions 1 and 2 of hashing remain unchanged.

In 1.1

This release includes speed improvements of about 100 fold.

• Feature: HashVersion{2} is a new hash context that can be up to ~100x faster than HashVersion{1}.
• Feature: The requirements for HashVersion{2} on the passed hash function have been relaxed, such that alg=crc32 should again work (no need to call alg=(x,s=UInt32(0)) -> crc32c(copy(x),s)).
• Feature: @ConstantHash allows for precomputed hash values of constant strings and numbers.
• Feature: stable_typename_id and stable_type_id provide compile-time 64 bit hashes of the types of objects
• Feature: root_version: Most users can safely ignore this new function. If you are implementing a root context (one that returns parent_context(::MyContext) = nothing) you will need to define this function. It indicates what version of the hashing implementations to use (1 or 2). It defaults to 1 to avoid changing the hash values of existing root contexts, but should be defined to return 2 to make use of the more optimized implementations used by HashVersion{2}.
• Deprecation: HashVersion{1} has been deprecated, favor version 2 over 1 in all cases where backwards compatibility is not required.
• Deprecation: qualified_name and qualified_type have been deprecated, in favor of stable_typename_id and stable_type_id.
• Deprecation: ConstantHash has been deprecated in favor of the more efficient @ConstantHash. To remove deprecated API: any call to ConstantHash(x) where x is a constant literal should be changed to @ConstantHash(x). If x is an expression you can use FnHash(_ -> x) to achieve the same result. Note however that the use of a non-literal is probably a code smell, as hash_method should normally only depend on the type of its arguments.

In 1.0:

This is a very breaking release, almost all values hash differently and the API has changed. However, far fewer manual definitions of hash_method become necessary. The fallback for Any should handle many more cases.

• Breaking: transform has been removed, its features are covered by FnHash and HashAndContext.
• Breaking: stable_hash no longer accepts multiple objects to hash (wrap them in a tuple instead); it now accepts a single object to hash, and the second positional argument is the context (see below for details on contexts).
• Breaking: The default alg for stable_hash is sha256; to use the old default (crc32c) you can pass alg=(x,s=UInt32(0)) -> crc32c(copy(x),s).
• Deprecation: The traits to return from hash_method have changed quite a bit. You will need to replace the old names as follows to avoid deprecation warnings during your tests:
  • Favor StructHash() (which uses fieldnames instead of propertynames) to UseProperties().
  • BUT to reproduce UseProperties(), call StructHash(propertynames => getproperty)
  • Replace UseQualifiedName() with FnHash(qualified_name, HashWrite())
  • Replace UseSize(method) with (FnHash(size), method)
  • Replace UseTable with FnHash(Tables.columns, StructHash(Tables.columnnames => Tables.getcolumn))
• Deprecation: The fallback methods for hashing are defined within a specific context (HashVersion{1}). Any contexts you make should define a parent_context method that returns e.g. HashVersion{1} so that the fallback implementation for any methods of hash_method you don't implement work properly. (A default version of parent_context raises a deprecation warning and returns HashVersion{1}). Refer to the discussion below about contexts.

In 0.3:

To prevent reshaped arrays from having the same hash (stable_hash([1 2; 3 4]) == stable_hash(vec([1 2; 3 4]))) the hashes for all arrays with more than 1 dimension have changed.

In 0.2:

To support hasing of all tables (Tables.istable(x) == true), hashes have changed for such objects when:

1. calling stable_hash(x) did not previously error
2. x is not a DataFrame (these previously errored)
3. x is not a NamedTuple of tables columns (these have the same hash as before)
4. x is not an AbstractArray of NamedTuple rows (these have the same hash as before)
5. x can be successfully written to an IO buffer via Base.write or StableHashTraits.write (otherwise it previously errored)
6. x has no specialized stable_hash method defined for it (otherwise the hash will be the same)

Any such table now uses the method UseTable, rather than UseWrite, and so would have the same hash as a DataFrame or NamedTuple with the same column contents instead of its previous hash value. For example if you had a custom table type MyCustomTable for which you only defined a StableHashTraits.write method and no hash_method, its hash will be changed unless you now define hash_method(::MyCustomTable) = UseWrite().