Merge pull request #658 from JuliaSymbolics/hash-consing

Implement hash consing for `Sym`

Author: ChrisRackauckas

Project.toml

@@ -26,6 +26,7 @@ SymbolicIndexingInterface = "2efcf032-c050-4f8e-a9bb-153293bab1f5"
 TermInterface = "8ea1fca8-c5ef-4a55-8b96-4e9afe9c9a3c"
 TimerOutputs = "a759f4b9-e2f1-59dc-863e-4aeb61b1ea8f"
 Unityper = "a7c27f48-0311-42f6-a7f8-2c11e75eb415"
+WeakValueDicts = "897b6980-f191-5a31-bcb0-bf3c4585e0c1"
 
 [weakdeps]
 LabelledArrays = "2ee39098-c373-598a-b85f-a56591580800"
@@ -57,6 +58,7 @@ SymbolicIndexingInterface = "0.3"
 TermInterface = "2.0"
 TimerOutputs = "0.5"
 Unityper = "0.1.2"
+WeakValueDicts = "0.1.0"
 julia = "1.3"
 
 [extras]

src/SymbolicUtils.jl

@@ -20,6 +20,7 @@ import TermInterface: iscall, isexpr, head, children,
                       operation, arguments, metadata, maketerm, sorted_arguments
 # For ReverseDiffExt
 import ArrayInterface
+using WeakValueDicts: WeakValueDict
 
 Base.@deprecate istree iscall
 export istree, operation, arguments, sorted_arguments, iscall

src/types.jl

@@ -23,38 +23,38 @@ const EMPTY_DICT = sdict()
 const EMPTY_DICT_T = typeof(EMPTY_DICT)
 
 @compactify show_methods=false begin
-    @abstract struct BasicSymbolic{T} <: Symbolic{T}
+    @abstract mutable struct BasicSymbolic{T} <: Symbolic{T}
         metadata::Metadata     = NO_METADATA
     end
-    struct Sym{T} <: BasicSymbolic{T}
+    mutable struct Sym{T} <: BasicSymbolic{T}
         name::Symbol           = :OOF
     end
-    struct Term{T} <: BasicSymbolic{T}
+    mutable struct Term{T} <: BasicSymbolic{T}
         f::Any                 = identity  # base/num if Pow; issorted if Add/Dict
         arguments::Vector{Any} = EMPTY_ARGS
         hash::RefValue{UInt}   = EMPTY_HASH
     end
-    struct Mul{T} <: BasicSymbolic{T}
+    mutable struct Mul{T} <: BasicSymbolic{T}
         coeff::Any             = 0         # exp/den if Pow
         dict::EMPTY_DICT_T     = EMPTY_DICT
         hash::RefValue{UInt}   = EMPTY_HASH
         arguments::Vector{Any} = EMPTY_ARGS
         issorted::RefValue{Bool} = NOT_SORTED
     end
-    struct Add{T} <: BasicSymbolic{T}
+    mutable struct Add{T} <: BasicSymbolic{T}
         coeff::Any             = 0         # exp/den if Pow
         dict::EMPTY_DICT_T     = EMPTY_DICT
         hash::RefValue{UInt}   = EMPTY_HASH
         arguments::Vector{Any} = EMPTY_ARGS
         issorted::RefValue{Bool} = NOT_SORTED
     end
-    struct Div{T} <: BasicSymbolic{T}
+    mutable struct Div{T} <: BasicSymbolic{T}
         num::Any               = 1
         den::Any               = 1
         simplified::Bool       = false
         arguments::Vector{Any} = EMPTY_ARGS
     end
-    struct Pow{T} <: BasicSymbolic{T}
+    mutable struct Pow{T} <: BasicSymbolic{T}
         base::Any              = 1
         exp::Any               = 1
         arguments::Vector{Any} = EMPTY_ARGS
@@ -77,6 +77,8 @@ function exprtype(x::BasicSymbolic)
     end
 end
 
+const wvd = WeakValueDict{UInt, BasicSymbolic}()
+
 # Same but different error messages
 @noinline error_on_type() = error("Internal error: unreachable reached!")
 @noinline error_sym() = error("Sym doesn't have a operation or arguments!")
@@ -92,7 +94,11 @@ const SIMPLIFIED = 0x01 << 0
 function ConstructionBase.setproperties(obj::BasicSymbolic{T}, patch::NamedTuple)::BasicSymbolic{T} where T
     nt = getproperties(obj)
     nt_new = merge(nt, patch)
-    Unityper.rt_constructor(obj){T}(;nt_new...)
+    # Call outer constructor because hash consing cannot be applied in inner constructor
+    @compactified obj::BasicSymbolic begin
+        Sym => Sym{T}(nt_new.name; nt_new...)
+        _ => Unityper.rt_constructor(obj){T}(;nt_new...)
+    end
 end
 
 ###
@@ -265,6 +271,26 @@ function _isequal(a, b, E)
     end
 end
 
+"""
+$(TYPEDSIGNATURES)
+
+Checks for equality between two `BasicSymbolic` objects, considering both their
+values and metadata.
+
+The default `Base.isequal` function for `BasicSymbolic` only compares their expressions
+and ignores metadata. This does not help deal with hash collisions when metadata is 
+relevant for distinguishing expressions, particularly in hashing contexts. This function
+provides a stricter equality check that includes metadata comparison, preventing
+such collisions.
+
+Modifying `Base.isequal` directly breaks numerous tests in `SymbolicUtils.jl` and
+downstream packages like `ModelingToolkit.jl`, hence the need for this separate
+function.
+"""
+function isequal_with_metadata(a::BasicSymbolic, b::BasicSymbolic)::Bool
+    isequal(a, b) && isequal(metadata(a), metadata(b))
+end
+
 Base.one( s::Symbolic) = one( symtype(s))
 Base.zero(s::Symbolic) = zero(symtype(s))
 
@@ -307,12 +333,61 @@ function Base.hash(s::BasicSymbolic, salt::UInt)::UInt
     end
 end
 
+"""
+$(TYPEDSIGNATURES)
+
+Calculates a hash value for a `BasicSymbolic` object, incorporating both its metadata and 
+symtype.
+
+This function provides an alternative hashing strategy to `Base.hash` for `BasicSymbolic` 
+objects. Unlike `Base.hash`, which only considers the expression structure, `hash2` also 
+includes the metadata and symtype in the hash calculation. This can be beneficial for hash 
+consing, allowing for more effective deduplication of symbolically equivalent expressions 
+with different metadata or symtypes.
+"""
+hash2(s::BasicSymbolic) = hash2(s, zero(UInt))
+function hash2(s::BasicSymbolic{T}, salt::UInt)::UInt where {T}
+    hash(metadata(s), hash(T, hash(s, salt)))
+end
+
 ###
 ### Constructors
 ###
 
-function Sym{T}(name::Symbol; kw...) where T
-    Sym{T}(; name=name, kw...)
+"""
+$(TYPEDSIGNATURES)
+
+Implements hash consing (flyweight design pattern) for `BasicSymbolic` objects.
+
+This function checks if an equivalent `BasicSymbolic` object already exists. It uses a 
+custom hash function (`hash2`) incorporating metadata and symtypes to search for existing 
+objects in a `WeakValueDict` (`wvd`).  Due to the possibility of hash collisions (where 
+different objects produce the same hash), a custom equality check (`isequal_with_metadata`) 
+which includes metadata comparison, is used to confirm the equivalence of objects with 
+matching hashes. If an equivalent object is found, the existing object is returned; 
+otherwise, the input `s` is returned. This reduces memory usage, improves compilation time 
+for runtime code generation, and supports built-in common subexpression elimination, 
+particularly when working with symbolic objects with metadata.
+
+Using a `WeakValueDict` ensures that only weak references to `BasicSymbolic` objects are 
+stored, allowing objects that are no longer strongly referenced to be garbage collected. 
+Custom functions `hash2` and `isequal_with_metadata` are used instead of `Base.hash` and 
+`Base.isequal` to accommodate metadata without disrupting existing tests reliant on the 
+original behavior of those functions.
+"""
+function BasicSymbolic(s::BasicSymbolic)::BasicSymbolic
+    h = hash2(s)
+    t = get!(wvd, h, s)
+    if t === s || isequal_with_metadata(t, s)
+        return t
+    else
+        return s
+    end
+end
+
+function Sym{T}(name::Symbol; kw...) where {T}
+    s = Sym{T}(; name, kw...)
+    BasicSymbolic(s)
 end
 
 function Term{T}(f, args; kw...) where T

test/basics.jl

@@ -1,4 +1,4 @@
-using SymbolicUtils: Symbolic, Sym, FnType, Term, Add, Mul, Pow, symtype, operation, arguments, issym, isterm, BasicSymbolic, term
+using SymbolicUtils: Symbolic, Sym, FnType, Term, Add, Mul, Pow, symtype, operation, arguments, issym, isterm, BasicSymbolic, term, isequal_with_metadata
 using SymbolicUtils
 using IfElse: ifelse
 using Setfield
@@ -336,6 +336,13 @@ end
 
     @test !isequal(a, missing)
     @test !isequal(missing, b)
+
+    a1 = setmetadata(a, Ctx1, "meta_1")
+    a2 = setmetadata(a, Ctx1, "meta_1")
+    a3 = setmetadata(a, Ctx2, "meta_2")
+    @test !isequal_with_metadata(a, a1)
+    @test isequal_with_metadata(a1, a2)
+    @test !isequal_with_metadata(a1, a3)
 end
 
 @testset "subtyping" begin

test/hash_consing.jl

@@ -0,0 +1,26 @@
+using SymbolicUtils, Test
+
+struct Ctx1 end
+struct Ctx2 end
+
+@testset "Sym" begin
+    x1 = only(@syms x)
+    x2 = only(@syms x)
+    @test x1 === x2
+    x3 = only(@syms x::Float64)
+    @test x1 !== x3
+    x4 = only(@syms x::Float64)
+    @test x1 !== x4
+    @test x3 === x4
+    x5 = only(@syms x::Int)
+    x6 = only(@syms x::Int)
+    @test x1 !== x5
+    @test x3 !== x5
+    @test x5 === x6
+
+    xm1 = setmetadata(x1, Ctx1, "meta_1")
+    xm2 = setmetadata(x1, Ctx1, "meta_1")
+    @test xm1 === xm2
+    xm3 = setmetadata(x1, Ctx2, "meta_2")
+    @test xm1 !== xm3
+end

test/runtests.jl

@@ -16,5 +16,6 @@ using Pkg, Test, SafeTestsets
         # Disabled until https://github.com/JuliaMath/SpecialFunctions.jl/issues/446 is fixed
         @safetestset "Fuzz" begin include("fuzz.jl") end
         @safetestset "Adjoints" begin include("adjoints.jl") end
+        @safetestset "Hash Consing" begin include("hash_consing.jl") end
     end
 end