faster mergewith! (#40893)

Co-authored-by: matthias314 <matthias314@posteo.net>
Co-authored-by: Jameson Nash <vtjnash@gmail.com>

Author: matthias314

base/abstractdict.jl

@@ -234,12 +234,14 @@ Dict{Int64, Int64} with 3 entries:
 ```
 """
 function mergewith!(combine, d::AbstractDict, others::AbstractDict...)
-    for other in others
-        for (k,v) in other
-            d[k] = haskey(d, k) ? combine(d[k], v) : v
-        end
+    foldl(mergewith!(combine), others; init = d)
+end
+
+function mergewith!(combine, d1::AbstractDict, d2::AbstractDict)
+    for (k, v) in d2
+        d1[k] = haskey(d1, k) ? combine(d1[k], v) : v
     end
-    return d
+    return d1
 end
 
 mergewith!(combine) = (args...) -> mergewith!(combine, args...)

base/dict.jl

@@ -729,6 +729,21 @@ function map!(f, iter::ValueIterator{<:Dict})
     return iter
 end
 
+function mergewith!(combine, d1::Dict{K, V}, d2::AbstractDict) where {K, V}
+    for (k, v) in d2
+        i = ht_keyindex2!(d1, k)
+        if i > 0
+            d1.vals[i] = combine(d1.vals[i], v)
+        else
+            if !isequal(k, convert(K, k))
+                throw(ArgumentError("$(limitrepr(k)) is not a valid key for type $K"))
+            end
+            @inbounds _setindex!(d1, convert(V, v), k, -i)
+        end
+    end
+    return d1
+end
+
 struct ImmutableDict{K,V} <: AbstractDict{K,V}
     parent::ImmutableDict{K,V}
     key::K

test/dict.jl

@@ -1070,6 +1070,26 @@ end
     check_merge([Dict(3=>4), Dict(:a=>5)], Dict(:a => 5, 3 => 4))
 end
 
+@testset "AbstractDict mergewith!" begin
+# we use IdDict to test the mergewith! implementation for AbstractDict
+    d1 = IdDict(1 => 1, 2 => 2)
+    d2 = IdDict(2 => 3, 3 => 4)
+    d3 = IdDict{Int, Float64}(1 => 5, 3 => 6)
+    d = copy(d1)
+    @inferred mergewith!(-, d, d2)
+    @test d == IdDict(1 => 1, 2 => -1, 3 => 4)
+    d = copy(d1)
+    @inferred mergewith!(-, d, d3)
+    @test d == IdDict(1 => -4, 2 => 2, 3 => 6)
+    d = copy(d1)
+    @inferred mergewith!(+, d, d2, d3)
+    @test d == IdDict(1 => 6, 2 => 5, 3 => 10)
+    @inferred mergewith(+, d1, d2, d3)
+    d = mergewith(+, d1, d2, d3)
+    @test d isa Dict{Int, Float64}
+    @test d == Dict(1 => 6, 2 => 5, 3 => 10)
+end
+
 @testset "misc error/io" begin
     d = Dict('a'=>1, 'b'=>1, 'c'=> 3)
     @test_throws ErrorException 'a' in d