Added ldexp to list of diffrules (#73)

* Update rules.jl

Added a rule for the function ldexp https://docs.julialang.org/en/v1/base/math/#Base.Math.ldexp

* Update src/rules.jl

use exp2 instead of 2^x to avoid overflows, indicate the derivative w.r.t. the second argument of ldexp as not defined since that argument is of integer type.

Co-authored-by: David Widmann <[email protected]>

* added special test for ldexp since its second argument is required to be an integer and thus non-differentiable. Changed variable name from "non_numeric_arg_functions" to "non_diffable_arg_functions" to be more general and account for the numeric but non-differentiable nature of the second argument of ldexp.

* Delete .vscode directory

* Update tests

* Bump version

Co-authored-by: David Widmann <[email protected]>
Co-authored-by: David Widmann <[email protected]>

Author: david-hofmann

Project.toml

@@ -1,6 +1,6 @@
 name = "DiffRules"
 uuid = "b552c78f-8df3-52c6-915a-8e097449b14b"
-version = "1.8.0"
+version = "1.9.0"
 
 [deps]
 LogExpFunctions = "2ab3a3ac-af41-5b50-aa03-7779005ae688"

src/rules.jl

@@ -86,6 +86,7 @@ _abs_deriv(x) = signbit(x) ? -one(x) : one(x)
 @define_diffrule Base.atan(x, y)    = :( $y / ($x^2 + $y^2)                                 ), :( -$x / ($x^2 + $y^2)                                                     )
 @define_diffrule Base.hypot(x, y)  = :( $x / hypot($x, $y)                                      ), :(  $y / hypot($x, $y)                                                     )
 @define_diffrule Base.log(b, x)    = :( log($x) * inv(-log($b)^2 * $b)                          ), :( inv($x) / log($b)                                                       )
+@define_diffrule Base.ldexp(x, y)  = :( exp2($y)                                                ), :NaN
 
 @define_diffrule Base.mod(x, y)    = :( first(promote(ifelse(isinteger($x / $y), NaN, 1), NaN)) ), :(  z = $x / $y; first(promote(ifelse(isinteger(z), NaN, -floor(z)), NaN)) )
 @define_diffrule Base.rem(x, y)    = :( first(promote(ifelse(isinteger($x / $y), NaN, 1), NaN)) ), :(  z = $x / $y; first(promote(ifelse(isinteger(z), NaN, -trunc(z)), NaN)) )

test/runtests.jl

@@ -13,10 +13,10 @@ end
 @testset "DiffRules" begin
 @testset "check rules" begin
 
-non_numeric_arg_functions = [(:Base, :rem2pi, 2), (:Base, :ifelse, 3)]
+non_diffeable_arg_functions = [(:Base, :rem2pi, 2), (:Base, :ldexp, 2), (:Base, :ifelse, 3)]
 
 for (M, f, arity) in DiffRules.diffrules(; filter_modules=nothing)
-    (M, f, arity) ∈ non_numeric_arg_functions && continue
+    (M, f, arity) ∈ non_diffeable_arg_functions && continue
     if arity == 1
         @test DiffRules.hasdiffrule(M, f, 1)
         deriv = DiffRules.diffrule(M, f, :goo)
@@ -95,6 +95,23 @@ for xtype in [:Float64, :BigFloat, :Int64]
         end
     end
 end
+
+# Treat ldexp separately because of its integer second argument:
+derivs = DiffRules.diffrule(:Base, :ldexp, :x, :y)
+for xtype in [:Float64, :BigFloat]
+    for ytype in [:Integer, :UInt64, :Int64]
+        @eval begin
+            let
+                x = rand($xtype)
+                y = $ytype(rand(1 : 10))
+                dx, dy = $(derivs[1]), $(derivs[2])
+                @test isapprox(dx, finitediff(z -> ldexp(z, y), x), rtol=0.05)
+                @test isnan(dy)
+            end
+        end
+    end
+end
+
 end
 
     @testset "diffrules" begin