From 1c951f7fe8e5e923cc5f5b18c91e461b61641b14 Mon Sep 17 00:00:00 2001
From: Sebastian Stock <42280794+sostock@users.noreply.github.com>
Date: Wed, 28 Jul 2021 11:53:28 +0200
Subject: [PATCH] Fix `length(::AbstractUnitRange)` and speed up
 `length(::AbstractUnitRange{<:Rational})` (#41479)

* Fix length(::AbstractUnitRange), faster length(::AbstractUnitRange{<:Rational})
---
 base/range.jl    | 17 ++++++++---------
 base/rational.jl | 18 ++++++++++++++++++
 test/ranges.jl   | 15 ++++++++++++++-
 3 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/base/range.jl b/base/range.jl
index b6a8e333cf52e..4ddecd7bdd91c 100644
--- a/base/range.jl
+++ b/base/range.jl
@@ -663,8 +663,8 @@ function checked_length(r::OrdinalRange{T}) where T
     else
         diff = checked_sub(stop, start)
     end
-    a = Integer(div(diff, s))
-    return checked_add(a, oneunit(a))
+    a = div(diff, s)
+    return Integer(checked_add(a, oneunit(a)))
 end
 
 function checked_length(r::AbstractUnitRange{T}) where T
@@ -672,8 +672,8 @@ function checked_length(r::AbstractUnitRange{T}) where T
     if isempty(r)
         return Integer(first(r) - first(r))
     end
-    a = Integer(checked_add(checked_sub(last(r), first(r))))
-    return checked_add(a, oneunit(a))
+    a = checked_sub(last(r), first(r))
+    return Integer(checked_add(a, oneunit(a)))
 end
 
 function length(r::OrdinalRange{T}) where T
@@ -690,15 +690,14 @@ function length(r::OrdinalRange{T}) where T
     else
         diff = stop - start
     end
-    a = Integer(div(diff, s))
-    return a + oneunit(a)
+    a = div(diff, s)
+    return Integer(a + oneunit(a))
 end
 
-
 function length(r::AbstractUnitRange{T}) where T
     @_inline_meta
-    a = Integer(last(r) - first(r)) # even when isempty, by construction (with overflow)
-    return a + oneunit(a)
+    a = last(r) - first(r) # even when isempty, by construction (with overflow)
+    return Integer(a + oneunit(a))
 end
 
 length(r::OneTo) = Integer(r.stop - zero(r.stop))
diff --git a/base/rational.jl b/base/rational.jl
index 23c9298962f53..1c83443d10df3 100644
--- a/base/rational.jl
+++ b/base/rational.jl
@@ -533,3 +533,21 @@ function hash(x::Rational{<:BitInteger64}, h::UInt)
     h = hash_integer(num, h)
     return h
 end
+
+# These methods are only needed for performance. Since `first(r)` and `last(r)` have the
+# same denominator (because their difference is an integer), `length(r)` can be calulated
+# without calling `gcd`.
+function length(r::AbstractUnitRange{T}) where T<:Rational
+    @_inline_meta
+    f = first(r)
+    l = last(r)
+    return div(l.num - f.num + f.den, f.den)
+end
+function checked_length(r::AbstractUnitRange{T}) where T<:Rational
+    f = first(r)
+    l = last(r)
+    if isempty(r)
+        return f.num - f.num
+    end
+    return div(checked_add(checked_sub(l.num, f.num), f.den), f.den)
+end
diff --git a/test/ranges.jl b/test/ranges.jl
index d4ff83b81fe2d..1c31585ece45e 100644
--- a/test/ranges.jl
+++ b/test/ranges.jl
@@ -544,6 +544,19 @@ end
     end
 end
 
+# A number type with the overflow behavior of `UInt8`. Conversion to `Integer` returns an
+# `Int32`, i.e., a type with different `typemin`/`typemax`. See  #41479
+struct OverflowingReal <: Real
+    val::UInt8
+end
+OverflowingReal(x::OverflowingReal) = x
+Base.:<=(x::OverflowingReal, y::OverflowingReal) = x.val <= y.val
+Base.:+(x::OverflowingReal, y::OverflowingReal) = OverflowingReal(x.val + y.val)
+Base.:-(x::OverflowingReal, y::OverflowingReal) = OverflowingReal(x.val - y.val)
+Base.round(x::OverflowingReal, ::RoundingMode) = x
+Base.Integer(x::OverflowingReal) = Int32(x.val)
+@test length(OverflowingReal(1):OverflowingReal(0)) == 0
+
 @testset "loops involving typemin/typemax" begin
     n = 0
     s = 0
@@ -1095,7 +1108,7 @@ end
 @testset "issue 10950" begin
     r = 1//2:3
     @test length(r) == 3
-    @test_throws MethodError checked_length(r) == 3 # this would work if checked_sub is defined on Rational
+    @test checked_length(r) == 3
     i = 1
     for x in r
         @test x == i//2