specialize argmin/argmax on ranges (#34315)

Also: document and add test_broken on argmin of degenerate ranges

base/range.jl

@@ -606,6 +606,40 @@ maximum(r::AbstractUnitRange) = isempty(r) ? throw(ArgumentError("range must be
 minimum(r::AbstractRange)  = isempty(r) ? throw(ArgumentError("range must be non-empty")) : min(first(r), last(r))
 maximum(r::AbstractRange)  = isempty(r) ? throw(ArgumentError("range must be non-empty")) : max(first(r), last(r))
 
+"""
+    argmin(r::AbstractRange)
+
+Ranges can have multiple minimal elements. In that case
+`argmin` will return a minimal index, but not necessarily the
+first one.
+"""
+function argmin(r::AbstractRange)
+    if isempty(r)
+        throw(ArgumentError("range must be non-empty"))
+    elseif step(r) > 0
+        firstindex(r)
+    else
+        first(searchsorted(r, last(r)))
+    end
+end
+
+"""
+    argmax(r::AbstractRange)
+
+Ranges can have multiple maximal elements. In that case
+`argmax` will return a maximal index, but not necessarily the
+first one.
+"""
+function argmax(r::AbstractRange)
+    if isempty(r)
+        throw(ArgumentError("range must be non-empty"))
+    elseif step(r) > 0
+        first(searchsorted(r, last(r)))
+    else
+        firstindex(r)
+    end
+end
+
 extrema(r::AbstractRange) = (minimum(r), maximum(r))
 
 # Ranges are immutable

test/ranges.jl

@@ -1212,6 +1212,31 @@ end
     @test convert(Array{Float64,1}, r) == a
 end
 
+@testset "extrema" begin
+    @test_throws ArgumentError minimum(1:2:-1)
+    @test_throws ArgumentError argmin(Base.OneTo(-1))
+    @test_throws ArgumentError maximum(Base.OneTo(-1))
+    @test_throws ArgumentError argmax(1:-1)
+
+    for (r, imin, imax) in [
+            (Base.OneTo(5), 1, 5),
+            (1:10, 1, 10),
+            (10:-1:0, 11, 1),
+            (range(10, stop=20, length=5), 1, 5),
+            (range(10.3, step=-2, length=7), 7, 1),
+           ]
+        @test minimum(r) === r[imin]
+        @test maximum(r) === r[imax]
+        @test imin === argmin(r)
+        @test imax === argmax(r)
+        @test extrema(r) === (r[imin], r[imax])
+    end
+
+    r = 1f8-10:1f8
+    @test_broken argmin(f) == argmin(collect(r))
+    @test_broken argmax(f) == argmax(collect(r))
+end
+
 @testset "OneTo" begin
     let r = Base.OneTo(-5)
         @test isempty(r)
@@ -1227,6 +1252,8 @@ end
         @test last(r) == 3
         @test minimum(r) == 1
         @test maximum(r) == 3
+        @test argmin(r) == 1
+        @test argmax(r) == 3
         @test r[2] == 2
         @test r[2:3] === 2:3
         @test_throws BoundsError r[4]

test/sorting.jl

@@ -189,6 +189,11 @@ end
                 @test searchsorted(reverse(coll), -huge, rev=true) === lastindex(coll)+1:lastindex(coll)
             end
         end
+        @testset "issue ##34408" begin
+            r = 1f8-10:1f8
+            # collect(r) = Float32[9.999999e7, 9.999999e7, 9.999999e7, 9.999999e7, 1.0e8, 1.0e8, 1.0e8, 1.0e8, 1.0e8]
+            @test_broken searchsorted(collect(r)) == searchsorted(r)
+        end
     end
     @testset "issue #35272" begin
         for v0 = (3:-1:1, 3.0:-1.0:1.0), v = (v0, collect(v0))