[Variant] Allow lossless casting from integer to floating point (#8357)

# Which issue does this PR close?

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enhancements and this helps us generate change logs for our releases.
You can link an issue to this PR using the GitHub syntax.

- Closes #NNN.

# Rationale for this change

Historically, `Variant::as_fXX` methods don't even try to cast int
values as floating point, which is counter-intuitive.

# What changes are included in this PR?

Allow lossless casting of variant integer values to variant floating
point values, by a naive determination of precision:
* Every floating point number has some number of bits of precision
   * 53 (double)
   * 24 (single)
   * 11 (half)
* Any integer that fits entirely inside the target floating point type's
precision can be converted losslessly
* This produces an intuitive result: "too big" numbers fail to convert,
while "small enough" numbers do convert.
   * This is a sufficient but _not_ a necessary condition.
* Technically, wider integer can be represented losslessly as well, as
long as they have enough trailing zeros
* It's unclear whether allowing those wider values to cast is actually
helpful in practice, because only 1 in 2**k values can cast (where k is
the number of bits of excess precision); it would certainly make input
testing more expensive.

# Are these changes tested?

New unit tests and doc tests.

# Are there any user-facing changes?

Yes. Values that failed to cast before now succeed.

Author: scovich

parquet-variant/src/utils.rs

@@ -144,3 +144,20 @@ pub(crate) const fn expect_size_of<T>(expected: usize) {
         let _ = [""; 0][size];
     }
 }
+
+pub(crate) fn fits_precision<const N: u32>(n: impl Into<i64>) -> bool {
+    n.into().unsigned_abs().leading_zeros() >= (i64::BITS - N)
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_fits_precision() {
+        assert!(fits_precision::<10>(1023));
+        assert!(!fits_precision::<10>(1024));
+        assert!(fits_precision::<10>(-1023));
+        assert!(!fits_precision::<10>(-1024));
+    }
+}

parquet-variant/src/variant.rs

@@ -28,7 +28,7 @@ use crate::decoder::{
     self, get_basic_type, get_primitive_type, VariantBasicType, VariantPrimitiveType,
 };
 use crate::path::{VariantPath, VariantPathElement};
-use crate::utils::{first_byte_from_slice, slice_from_slice};
+use crate::utils::{first_byte_from_slice, fits_precision, slice_from_slice};
 use std::ops::Deref;
 
 use arrow_schema::ArrowError;
@@ -1082,8 +1082,8 @@ impl<'m, 'v> Variant<'m, 'v> {
 
     /// Converts this variant to an `f16` if possible.
     ///
-    /// Returns `Some(f16)` for float and double variants,
-    /// `None` for non-floating-point variants.
+    /// Returns `Some(f16)` for floating point values, and integers with up to 11 bits of
+    /// precision. `None` otherwise.
     ///
     /// # Example
     ///
@@ -1099,21 +1099,29 @@ impl<'m, 'v> Variant<'m, 'v> {
     /// let v2 = Variant::from(std::f64::consts::PI);
     /// assert_eq!(v2.as_f16(), Some(f16::from_f64(std::f64::consts::PI)));
     ///
+    /// // and from integers with no more than 11 bits of precision
+    /// let v3 = Variant::from(2047);
+    /// assert_eq!(v3.as_f16(), Some(f16::from_f32(2047.0)));
+    ///
     /// // but not from other variants
-    /// let v3 = Variant::from("hello!");
-    /// assert_eq!(v3.as_f16(), None);
+    /// let v4 = Variant::from("hello!");
+    /// assert_eq!(v4.as_f16(), None);
     pub fn as_f16(&self) -> Option<f16> {
         match *self {
             Variant::Float(i) => Some(f16::from_f32(i)),
             Variant::Double(i) => Some(f16::from_f64(i)),
+            Variant::Int8(i) => Some(i.into()),
+            Variant::Int16(i) if fits_precision::<11>(i) => Some(f16::from_f32(i as _)),
+            Variant::Int32(i) if fits_precision::<11>(i) => Some(f16::from_f32(i as _)),
+            Variant::Int64(i) if fits_precision::<11>(i) => Some(f16::from_f32(i as _)),
             _ => None,
         }
     }
 
     /// Converts this variant to an `f32` if possible.
     ///
-    /// Returns `Some(f32)` for float and double variants,
-    /// `None` for non-floating-point variants.
+    /// Returns `Some(f32)` for floating point values, and integer values with up to 24 bits of
+    /// precision.  `None` otherwise.
     ///
     /// # Examples
     ///
@@ -1128,23 +1136,31 @@ impl<'m, 'v> Variant<'m, 'v> {
     /// let v2 = Variant::from(std::f64::consts::PI);
     /// assert_eq!(v2.as_f32(), Some(std::f32::consts::PI));
     ///
+    /// // and from integers with no more than 24 bits of precision
+    /// let v3 = Variant::from(16777215i64);
+    /// assert_eq!(v3.as_f32(), Some(16777215.0));
+    ///
     /// // but not from other variants
-    /// let v3 = Variant::from("hello!");
-    /// assert_eq!(v3.as_f32(), None);
+    /// let v4 = Variant::from("hello!");
+    /// assert_eq!(v4.as_f32(), None);
     /// ```
     #[allow(clippy::cast_possible_truncation)]
     pub fn as_f32(&self) -> Option<f32> {
         match *self {
             Variant::Float(i) => Some(i),
             Variant::Double(i) => Some(i as f32),
+            Variant::Int8(i) => Some(i.into()),
+            Variant::Int16(i) => Some(i.into()),
+            Variant::Int32(i) if fits_precision::<24>(i) => Some(i as _),
+            Variant::Int64(i) if fits_precision::<24>(i) => Some(i as _),
             _ => None,
         }
     }
 
     /// Converts this variant to an `f64` if possible.
     ///
-    /// Returns `Some(f64)` for float and double variants,
-    /// `None` for non-floating-point variants.
+    /// Returns `Some(f64)` for floating point values, and integer values with up to 53 bits of
+    /// precision.  `None` otherwise.
     ///
     /// # Examples
     ///
@@ -1159,14 +1175,22 @@ impl<'m, 'v> Variant<'m, 'v> {
     /// let v2 = Variant::from(std::f64::consts::PI);
     /// assert_eq!(v2.as_f64(), Some(std::f64::consts::PI));
     ///
+    /// // and from integers with no more than 53 bits of precision
+    /// let v3 = Variant::from(9007199254740991i64);
+    /// assert_eq!(v3.as_f64(), Some(9007199254740991.0));
+    ///
     /// // but not from other variants
-    /// let v3 = Variant::from("hello!");
-    /// assert_eq!(v3.as_f64(), None);
+    /// let v4 = Variant::from("hello!");
+    /// assert_eq!(v4.as_f64(), None);
     /// ```
     pub fn as_f64(&self) -> Option<f64> {
         match *self {
             Variant::Float(i) => Some(i.into()),
             Variant::Double(i) => Some(i),
+            Variant::Int8(i) => Some(i.into()),
+            Variant::Int16(i) => Some(i.into()),
+            Variant::Int32(i) => Some(i.into()),
+            Variant::Int64(i) if fits_precision::<53>(i) => Some(i as _),
             _ => None,
         }
     }