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Tweak Open01 and Closed01 #237

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Tweak Open01 and Closed01 #237

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cd830e7
Fix off-by-one error in mantissa_bits passed to float_impls
tspiteri Jan 15, 2018
cd18f48
remove bias in Open01
tspiteri Jan 15, 2018
a5da781
add tests for floating-point edge cases
tspiteri Feb 2, 2018
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35 changes: 32 additions & 3 deletions src/rand_impls.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -269,9 +269,38 @@ mod tests {

#[test]
fn floating_point_edge_cases() {
// the test for exact equality is correct here.
assert!(ConstantRng(0xffff_ffff).gen::<f32>() != 1.0);
assert!(ConstantRng(0xffff_ffff_ffff_ffff).gen::<f64>() != 1.0);
const EPSILON32: f32 = 1.0 / (1u32 << 23) as f32;
const EPSILON64: f64 = 1.0 / (1u64 << 52) as f64;
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Why not use core::f32::EPSILON?

ε is defined as the difference between 1.0 and the next largest representable number; since all numbers output (excepting Closed01) are strictly less than 1.0 the exponent will be 1 less than for 1.0, hence the largest representable number less than 1 should be 1 - ε / 2.

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I first used std::f32::EPSILON but that failed the continuous integration tests and I forgot about core, so I'll change this.


let mut zeros = ConstantRng(0);
let mut ones = ConstantRng(!0);

let zero32 = zeros.gen::<f32>();
let zero64 = zeros.gen::<f64>();
let one32 = ones.gen::<f32>();
let one64 = ones.gen::<f64>();
assert_eq!(zero32, 0.0);
assert_eq!(zero64, 0.0);
assert!(1.0 - EPSILON32 <= one32 && one32 < 1.0);
assert!(1.0 - EPSILON64 <= one64 && one64 < 1.0);
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As noted above, I believe this should be exactly 1 - ε / 2. Exact equality tests here would be preferable (excepting the Open01 case near 0, where there are many near representable values).

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Actually, one32 should be 1.0 - f32::EPSILON, not 1.0 - f32::EPSILON / 2, since the generator generates a multiple of ɛ in gen::<f32>. If we are to use the exact equality tests, they should be (zero32, one32) == (0.0, 1.0 - f32::EPSILON), (closed_zero32, closed_one32) == (0.0, 1.0) and (open_zero32, open_one32) == (f32::EPSILON / 2, 1.0 - f32::EPSILON / 2). Should I amend the commit to do that?


let Closed01(closed_zero32) = zeros.gen::<Closed01<f32>>();
let Closed01(closed_zero64) = zeros.gen::<Closed01<f64>>();
let Closed01(closed_one32) = ones.gen::<Closed01<f32>>();
let Closed01(closed_one64) = ones.gen::<Closed01<f64>>();
assert_eq!(closed_zero32, 0.0);
assert_eq!(closed_zero64, 0.0);
assert_eq!(closed_one32, 1.0);
assert_eq!(closed_one64, 1.0);

let Open01(open_zero32) = zeros.gen::<Open01<f32>>();
let Open01(open_zero64) = zeros.gen::<Open01<f64>>();
let Open01(open_one32) = ones.gen::<Open01<f32>>();
let Open01(open_one64) = ones.gen::<Open01<f64>>();
assert!(0.0 < open_zero32 && open_zero32 <= EPSILON32);
assert!(0.0 < open_zero64 && open_zero64 <= EPSILON64);
assert!(1.0 - EPSILON32 <= open_one32 && open_one32 < 1.0);
assert!(1.0 - EPSILON64 <= open_one64 && open_one64 < 1.0);
}

#[test]
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