Merge pull request JuliaLang#37486 from sostock/zeroperiods

Make zero-periods compare equal

NEWS.md

@@ -157,6 +157,8 @@ Standard library changes
 
 #### Dates
 * `Quarter` period is defined ([#35519]).
+* Zero-valued `FixedPeriod`s and `OtherPeriod`s now compare equal, e.g.,
+ `Year(0) == Day(0)`. The behavior of non-zero `Period`s is not changed. ([#37486])
 
 #### Statistics
 

stdlib/Dates/src/periods.jl

@@ -456,19 +456,20 @@ Base.convert(::Type{Quarter}, x::Month) = Quarter(divexact(value(x), 3))
 Base.promote_rule(::Type{Quarter}, ::Type{Month}) = Month
 
 
-# fixed is not comparable to other periods, as per discussion in issue #21378
-(==)(x::FixedPeriod, y::OtherPeriod) = false
-(==)(x::OtherPeriod, y::FixedPeriod) = false
+# fixed is not comparable to other periods, except when both are zero (#37459)
+(==)(x::FixedPeriod, y::OtherPeriod) = iszero(x) & iszero(y)
+(==)(x::OtherPeriod, y::FixedPeriod) = y == x
 
-const fixedperiod_seed = UInt === UInt64 ? 0x5b7fc751bba97516 : 0xeae0fdcb
-const otherperiod_seed = UInt === UInt64 ? 0xe1837356ff2d2ac9 : 0x170d1b00
+const zero_or_fixedperiod_seed = UInt === UInt64 ? 0x5b7fc751bba97516 : 0xeae0fdcb
+const nonzero_otherperiod_seed = UInt === UInt64 ? 0xe1837356ff2d2ac9 : 0x170d1b00
+otherperiod_seed(x::OtherPeriod) = iszero(value(x)) ? zero_or_fixedperiod_seed : nonzero_otherperiod_seed
 # tons() will overflow for periods longer than ~300,000 years, implying a hash collision
 # which is relatively harmless given how infrequent such periods should appear
-Base.hash(x::FixedPeriod, h::UInt) = hash(tons(x), h + fixedperiod_seed)
+Base.hash(x::FixedPeriod, h::UInt) = hash(tons(x), h + zero_or_fixedperiod_seed)
 # Overflow can also happen here for really long periods (~8e17 years)
-Base.hash(x::Year, h::UInt) = hash(12 * value(x), h + otherperiod_seed)
-Base.hash(x::Quarter, h::UInt) = hash(3 * value(x), h + otherperiod_seed)
-Base.hash(x::Month, h::UInt) = hash(value(x), h + otherperiod_seed)
+Base.hash(x::Year, h::UInt) = hash(12 * value(x), h + otherperiod_seed(x))
+Base.hash(x::Quarter, h::UInt) = hash(3 * value(x), h + otherperiod_seed(x))
+Base.hash(x::Month, h::UInt) = hash(value(x), h + otherperiod_seed(x))
 
 Base.isless(x::FixedPeriod, y::OtherPeriod) = throw(MethodError(isless, (x, y)))
 Base.isless(x::OtherPeriod, y::FixedPeriod) = throw(MethodError(isless, (x, y)))

stdlib/Dates/test/periods.jl

@@ -460,6 +460,21 @@ end
  @test hash(y) == hash(z)
  end
 end
+@testset "Equality and hashing between FixedPeriod/OtherPeriod/CompoundPeriod (#37459)" begin
+ function test_hash_equality(x, y)
+ @test x == y
+ @test y == x
+ @test isequal(x, y)
+ @test isequal(y, x)
+ @test hash(x) == hash(y)
+ end
+ for FP = (Dates.Week, Dates.Day, Dates.Hour, Dates.Minute,
+ Dates.Second, Dates.Millisecond, Dates.Microsecond, Dates.Nanosecond)
+ for OP = (Dates.Year, Dates.Quarter, Dates.Month)
+ test_hash_equality(FP(0), OP(0))
+ end
+ end
+end
 
 @testset "#30832" begin
  @test Dates.toms(Dates.Second(1) + Dates.Nanosecond(1)) == 1e3