impl arbitrary::Arbitrary for NotNan and OrderedFloat.

Author: kpreid

Cargo.toml

@@ -19,6 +19,7 @@ serde      = { version = "1.0", optional = true, default-features = false }
 rkyv       = { version = "0.7", optional = true, default-features = false, features = ["size_32"] }
 schemars   = { version = "0.6.5", optional = true }
 rand       = { version = "0.8.3", optional = true, default-features = false }
+arbitrary  = { version = "1.0.0", optional = true }
 proptest   = { version = "1.0.0", optional = true }
 
 [dev-dependencies]

src/lib.rs

@@ -1977,3 +1977,65 @@ mod impl_proptest {
     }
     impl_arbitrary! { f32, f64 }
 }
+
+#[cfg(feature = "arbitrary")]
+mod impl_arbitrary {
+    use super::{FloatIsNan, NotNan, OrderedFloat};
+    use arbitrary::{Arbitrary, Unstructured};
+    use num_traits::FromPrimitive;
+
+    macro_rules! impl_arbitrary {
+        ($($f:ident),+) => {
+            $(
+                impl<'a> Arbitrary<'a> for NotNan<$f> {
+                    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {
+                        let float: $f = u.arbitrary()?;
+                        match NotNan::new(float) {
+                            Ok(notnan_value) => Ok(notnan_value),
+                            Err(FloatIsNan) => {
+                                // If our arbitrary float input was a NaN (encoded by exponent = max
+                                // value), then replace it with a finite float, reusing the mantissa
+                                // bits.
+                                //
+                                // This means the output is not uniformly distributed among all
+                                // possible float values, but Arbitrary makes no promise that that
+                                // is true.
+                                //
+                                // An alternative implementation would be to return an
+                                // `arbitrary::Error`, but that is not as useful since it forces the
+                                // caller to retry with new random/fuzzed data; and the precendent of
+                                // `arbitrary`'s built-in implementations is to prefer the approach of
+                                // mangling the input bits to fit.
+
+                                let (mantissa, _exponent, sign) =
+                                    num_traits::Float::integer_decode(float);
+                                let revised_float = <$f>::from_i64(
+                                    i64::from(sign) * mantissa as i64
+                                ).unwrap();
+
+                                // If this unwrap() fails, then there is a bug in the above code.
+                                Ok(NotNan::new(revised_float).unwrap())
+                            }
+                        }
+                    }
+
+                    fn size_hint(depth: usize) -> (usize, Option<usize>) {
+                        <$f as Arbitrary>::size_hint(depth)
+                    }
+                }
+
+                impl<'a> Arbitrary<'a> for OrderedFloat<$f> {
+                    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {
+                        let float: $f = u.arbitrary()?;
+                        Ok(OrderedFloat::from(float))
+                    }
+
+                    fn size_hint(depth: usize) -> (usize, Option<usize>) {
+                        <$f as Arbitrary>::size_hint(depth)
+                    }
+                }
+            )*
+        }
+    }
+    impl_arbitrary! { f32, f64 }
+}

tests/test.rs

@@ -692,3 +692,46 @@ fn from_ref() {
     assert_eq!(*o, 2.0f64);
     assert_eq!(f, 2.0f64);
 }
+
+#[cfg(feature = "arbitrary")]
+mod arbitrary_test {
+    use super::{NotNan, OrderedFloat};
+    use arbitrary::{Arbitrary, Unstructured};
+
+    #[test]
+    fn exhaustive() {
+        // Exhaustively search all patterns of sign and exponent bits plus a few mantissa bits.
+        for high_bytes in 0..=u16::MAX {
+            let [h1, h2] = high_bytes.to_be_bytes();
+
+            // Each of these should not
+            //   * panic,
+            //   * return an error, or
+            //   * need more bytes than given.
+            let n32: NotNan<f32> = Unstructured::new(&[h1, h2, h1, h2])
+                .arbitrary()
+                .expect("NotNan<f32> failure");
+            let n64: NotNan<f64> = Unstructured::new(&[h1, h2, h1, h2, h1, h2, h1, h2])
+                .arbitrary()
+                .expect("NotNan<f64> failure");
+            let _: OrderedFloat<f32> = Unstructured::new(&[h1, h2, h1, h2])
+                .arbitrary()
+                .expect("OrderedFloat<f32> failure");
+            let _: OrderedFloat<f64> = Unstructured::new(&[h1, h2, h1, h2, h1, h2, h1, h2])
+                .arbitrary()
+                .expect("OrderedFloat<f64> failure");
+
+            // Check for violation of NotNan's property of never containing a NaN.
+            assert!(!n32.into_inner().is_nan());
+            assert!(!n64.into_inner().is_nan());
+        }
+    }
+
+    #[test]
+    fn size_hints() {
+        assert_eq!(NotNan::<f32>::size_hint(0), (4, Some(4)));
+        assert_eq!(NotNan::<f64>::size_hint(0), (8, Some(8)));
+        assert_eq!(OrderedFloat::<f32>::size_hint(0), (4, Some(4)));
+        assert_eq!(OrderedFloat::<f64>::size_hint(0), (8, Some(8)));
+    }
+}