andygrove commented on code in PR #5136:
URL: https://github.com/apache/datafusion-comet/pull/5136#discussion_r3692057216
##########
native/spark-expr/src/conversion_funcs/numeric.rs:
##########
@@ -939,6 +961,79 @@ where
Ok(Arc::new(result.with_precision_and_scale(precision, scale)?))
}
+/// Convert a double to a decimal unscaled value with Spark semantics.
+///
+/// Spark converts through `BigDecimal(Double.toString(d)).setScale(scale,
HALF_UP)`: it
+/// rounds the shortest decimal string form of the value, not its exact binary
expansion.
+/// The two disagree for values like 0.5153125 whose binary value
(0.51531249999...) sits
+/// just below the rounding tie that the string form lands on, so a plain
+/// `(f * 10^scale).round()` produces results that differ from Spark.
+///
+/// Returns `None` for NaN / infinity and for results that do not fit
`precision`.
+fn float_to_decimal128(f: f64, precision: u8, scale: i8) -> Option<i128> {
+ if !f.is_finite() {
+ return None;
+ }
+
+ // Shortest round-trip decimal form, same digits as Java's Double.toString
+ let mut buf = ryu::Buffer::new();
+ let (mantissa, exp10) = parse_decimal_notation(buf.format_finite(f));
+
+ // value = mantissa * 10^exp10, so unscaled = round(mantissa * 10^(exp10 +
scale))
+ let shift = exp10 + scale as i32;
+ let unscaled = if shift >= 0 {
+ // Overflowing i128 here means the result cannot fit any decimal
precision
+ mantissa.checked_mul(pow10_i128(shift.try_into().ok()?)?)?
Review Comment:
Fixed in 069b1e0c1 — now `pow10_i128(shift as u32)?`, matching the sibling
`else` branch and the other callers.
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