fix overflow bug in prevpow (JuliaLang#43410)

LilithHafner · Feb 22, 2022 · 8207530 · 8207530
1 parent 9eb8c16
commit 8207530
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Showing 2 changed files with 11 additions and 3 deletions.
diff --git a/base/intfuncs.jl b/base/intfuncs.jl
@@ -490,14 +490,19 @@ julia> prevpow(4, 16)
 16
 ```
 """
-function prevpow(a::Real, x::Real)
+function prevpow(a::T, x::Real) where T <: Real
     x < 1 && throw(DomainError(x, "`x` must be ≥ 1."))
     # See comment in nextpos() for a == special case.
     a == 2 && isa(x, Integer) && return _prevpow2(x)
     a <= 1 && throw(DomainError(a, "`a` must be greater than 1."))
     n = floor(Integer,log(a, x))
-    p = a^(n+1)
-    p <= x ? p : a^n
+    p = a^n
+    if a isa Integer
+        wp, overflow = mul_with_overflow(a, p)
+        return (wp <= x && !overflow) ? wp : p
+    end
+    wp = p*a
+    return wp <= x ? wp : p
 end
 
 ## ndigits (number of digits) in base 10 ##

diff --git a/test/intfuncs.jl b/test/intfuncs.jl
@@ -264,6 +264,9 @@ end
     @test prevpow(2, 3) == 2
     @test prevpow(2, 4) == 4
     @test prevpow(2, 5) == 4
+    @test prevpow(Int64(10), Int64(1234567890123456789)) === Int64(1000000000000000000)
+    @test prevpow(10, 101.0) === 100
+    @test prevpow(10.0, 101) === 100.0
     @test_throws DomainError prevpow(0, 3)
     @test_throws DomainError prevpow(0, 3)
 end