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Shrink floating binary point types similar to the Haskell version #103

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May 27, 2017
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Shrink floating binary point types similar to the Haskell version #103

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2 changes: 1 addition & 1 deletion src/Hedgehog/Gen.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -409,7 +409,7 @@ module Gen =

/// Generates a random 64-bit floating point number.
let double : Gen<double> =
create Shrink.double Random.sizedDouble
create (Shrink.towardsDouble 0.0) Random.sizedDouble
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I know that at some point we need to change Gen.double so that it takes a Range, similar to the other primitive combinators. I'd prefer to do this on a separate PR because Random.sizedDouble must be changed too and Seed.nextDouble must be added first. (Same also for Gen.float.)

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👍


/// Generates a random 64-bit floating point number.
let float : Gen<float> =
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34 changes: 23 additions & 11 deletions src/Hedgehog/Shrink.fs
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Expand Up @@ -139,16 +139,28 @@ module Shrink =
LazyList.consNub destination <|
LazyList.map (fun y -> x - y) (halves diff)

/// Shrink a floating point number.
let double (x : double) : LazyList<double> =
let positive =
if x < 0.0 then
LazyList.singleton (-x)
else
LazyList.empty
/// Shrink a floating-point number by edging towards a destination.
///
/// >>> List.take 7 << LazyList.toList <| Shrink.towardsDouble 0.0 100.0
/// [0.0; 50.0; 75.0; 87.5; 93.75; 96.875; 98.4375]
///
/// >>> List.take 7 << LazyList.toList <| Shrink.towardsDouble 1.0 0.5
/// [1.0; 0.75; 0.625; 0.5625; 0.53125; 0.515625; 0.5078125]
///
/// Note we always try the destination first, as that is the optimal shrink.
///
let towardsDouble (destination : double) (x : double) : LazyList<double> =
if destination = x then
LazyList.empty
else
let diff =
x - destination

let integrals =
towards 0I (bigint x)
|> LazyList.map double
let go n =
let x' = x - n
if x' <> x then
Some (x', n / 2.0)
else
None
LazyList.unfold go diff

LazyList.append positive integrals
35 changes: 24 additions & 11 deletions tests/Hedgehog.Tests/ShrinkTests.fs
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Expand Up @@ -109,14 +109,27 @@ let ``towards returns empty list when run out of shrinks`` x0 destination =
test <@ actual |> List.isEmpty @>

[<Theory>]
[<InlineData( 1.0)>]
[<InlineData( 2.1)>]
[<InlineData( 3.2)>]
[<InlineData( 30.3)>]
[<InlineData( 128.4)>]
[<InlineData( 256.5)>]
[<InlineData( 512.6)>]
[<InlineData(1024.7)>]
let ``double shrinks a floating point number`` x =
let actual = Shrink.double x |> LazyList.toList
test <@ actual |> List.forall (fun x' -> x' < x) @>
[<InlineData( 2.0, 1.0)>]
[<InlineData( 3.0, 1.0)>]
[<InlineData( 30.0, 1.0)>]
[<InlineData( 128.0, 64.0)>]
[<InlineData( 256.0, 128.0)>]
[<InlineData( 512.0, 256.0)>]
[<InlineData(1024.0, 512.0)>]
let ``towardsDouble shrinks by edging towards a destination number`` x0 destination =
let actual =
x0
|> Shrink.towardsDouble destination
|> LazyList.toList
test <@ actual |> List.forall (fun x1 -> x1 < x0 && x1 >= destination) @>

[<Theory>]
[<InlineData( 1.0, 1.0)>]
[<InlineData( 30.0, 30.0)>]
[<InlineData(1024.0, 1024.0)>]
let ``towardsDouble returns empty list when run out of shrinks`` x0 destination =
let actual =
x0
|> Shrink.towards destination
|> LazyList.toList
test <@ actual |> List.isEmpty @>