feat: add discontinuity handling API

src/Symbolics.jl

@@ -238,4 +238,7 @@ end
 export inverse, left_inverse, right_inverse, @register_inverse, has_inverse, has_left_inverse, has_right_inverse
 include("inverse.jl")
 
+export rootfunction, left_continuous_function, right_continuous_function, @register_discontinuity
+include("discontinuities.jl")
+
 end # module

src/discontinuities.jl

@@ -0,0 +1,106 @@
+"""
+    rootfunction(f)
+
+Given a function `f` with a discontinuity or discontinuous derivative, return the rootfinding
+function of `f`. The rootfinding function `g` takes the same arguments as `f`, and is such
+that `f` can be described as a piecewise function based on the sign of `g`, where each piece
+is continuous and has a continuous derivative. The pieces are obtained using
+`left_continuous_function(f)` and `right_continuous_function(f)`.
+
+More formally,
+```julia
+f(args...) = if g(args...) < 0
+    left_continuous_function(f)(args...)
+else
+    right_continuous_function(f)(args...)
+end
+```
+
+For example, if `f` is `max(x, y)`, the root function is `(x, y) -> x - y` with
+`left_continuous_function` as `(x, y) -> y` and `right_continuous_function` as
+`(x, y) -> x`.
+
+See also: [`left_continuous_function`](@ref), [`right_continuous_function`](@ref).
+"""
+function rootfunction end
+
+"""
+    left_continuous_function(f)
+
+Given a function `f` with a discontinuity or discontinuous derivative, return a function
+taking the same arguments as `f` which is continuous and has a continuous derivative
+when `rootfinding_function(f)` is negative.
+
+See also: [`rootfunction`](@ref).
+"""
+function left_continuous_function end
+
+"""
+    right_continuous_function(f)
+
+Given a function `f` with a discontinuity or discontinuous derivative, return a function
+taking the same arguments as `f` which is continuous and has a continuous derivative
+when `rootfinding_function(f)` is positive.
+
+See also: [`rootfunction`](@ref).
+"""
+function right_continuous_function end
+
+"""
+    @register_discontinuity f(arg1, arg2, ...) root_expr left_expr right_expr
+
+Utility macro to register functions with discontinuities. The function `f` with
+arguments `arg1, arg2, ...` has a `rootfunction` of `root_expr`, a
+`left_continuous_function` of `left_expr` and `right_continuous_function` of
+`right_expr`. `root_expr`, `left_expr` and `right_expr` are all expressions in terms
+of `arg1, arg2, ...`.
+
+For example, `max(x, y)` can be registered as `@register_discontinuity max(x, y) x - y y x`.
+
+See also: [`rootfunction`](@ref)
+"""
+macro register_discontinuity(f, root, left, right)
+    Meta.isexpr(f, :call) || error("Expected function call as first argument")
+    args = f.args[2:end]
+    fn = esc(f.args[1])
+    rootname = gensym(:root)
+    rootfn = :(function $rootname($(args...))
+        $root
+    end)
+    leftname = gensym(:left)
+    leftfn = :(function $leftname($(args...))
+        $left
+    end)
+    rightname = gensym(:right)
+    rightfn = :(function $rightname($(args...))
+        $right
+    end)
+    return quote
+        $rootfn
+        (::$typeof($rootfunction))(::$typeof($fn)) = $rootname
+        $leftfn
+        (::$typeof($left_continuous_function))(::$typeof($fn)) = $leftname
+        $rightfn
+        (::$typeof($right_continuous_function))(::$typeof($fn)) = $rightname
+    end
+end
+
+# a triangle function which is zero when x is a multiple of period
+function _triangle(x, period)
+    x /= 2period
+    abs(x + 1 // 4 - floor(x + 3 // 4)) - 1 // 2
+end
+
+@register_discontinuity abs(x) x -x x
+# just needs a rootfind to hit the discontinuity
+@register_discontinuity mod(x, y) _triangle(x, y) mod(x, y) mod(x, y)
+@register_discontinuity rem(x, y) _triangle(x, y) rem(x, y) rem(x, y)
+@register_discontinuity div(x, y) _triangle(x, y) div(x, y) div(x, y)
+@register_discontinuity max(x, y) x - y y x
+@register_discontinuity min(x, y) x - y x y
+@register_discontinuity NaNMath.max(x, y) x - y y x
+@register_discontinuity NaNMath.min(x, y) x - y x y
+@register_discontinuity <(x, y) x - y true false
+@register_discontinuity <=(x, y) y - x false true
+@register_discontinuity >(x, y) y - x true false
+@register_discontinuity >=(x, y) x - y false true

test/discontinuities.jl

@@ -0,0 +1,30 @@
+using Symbolics, NaNMath, Test
+
+function discontinuity_eval(fn, args...)
+    if rootfunction(fn)(args...) < 0
+        left_continuous_function(fn)(args...)
+    else
+        right_continuous_function(fn)(args...)
+    end
+end
+
+@testset "abs" begin
+    for x in -1.0:0.001:1.0
+        @test abs(x) ≈ discontinuity_eval(abs, x)
+    end
+end
+
+@testset "$(nameof(f))" for f in (mod, rem, div)
+    y = 0.7
+    for x in -2y:0.001:2y
+        @test f(x, y) ≈ discontinuity_eval(f, x, y)
+    end
+end
+
+@testset "$(nameof(f))" for f in (min, max, NaNMath.min, NaNMath.max, <, <=, >, >=)
+    for x in 0.0:0.1:1.0
+        for y in 0.0:0.1:1.0
+            @test f(x, y) ≈ discontinuity_eval(f, x, y)
+        end
+    end
+end

test/runtests.jl

@@ -62,6 +62,7 @@ if GROUP == "All" || GROUP == "Core"
         @safetestset "RootFinding solver" begin include("solver.jl") end
         @safetestset "Function inverses test" begin include("inverse.jl") end
         @safetestset "Taylor Series Test" begin include("taylor.jl") end
+        @safetestset "Discontinuity registration test" begin include("discontinuities.jl") end
     end
 end