tustvold commented on code in PR #3690:
URL: https://github.com/apache/arrow-rs/pull/3690#discussion_r1103859302
##########
arrow-arith/src/arithmetic.rs:
##########
@@ -1241,6 +1243,74 @@ pub fn multiply_dyn_checked(
}
}
+/// Perform `left * right` operation on two decimal arrays. If either left or
right value is
+/// null then the result is also null.
+///
+/// This performs decimal multiplication which allows precision loss if an
exact representation
+/// is not possible for the result. In the case, the result will be rounded.
+pub fn multiply_decimal(
+ left: &PrimitiveArray<Decimal128Type>,
+ right: &PrimitiveArray<Decimal128Type>,
+) -> Result<ArrayRef, ArrowError> {
+ let precision = left.precision();
+ let mut product_scale = left.scale() + right.scale();
+
+ math_checked_op(left, right, |a, b| {
+ let a = i256::from_i128(a);
+ let b = i256::from_i128(b);
+
+ a.checked_mul(b)
+ .map(|mut a| {
+ // Round the value if an exact representation is not possible.
+ // ref: java.math.BigDecimal#doRound
+ let mut digits = a.to_string().len() as i8;
+ let mut diff = digits - (Decimal128Type::MAX_PRECISION as i8);
+
+ while diff > 0 {
+ let divisor = i256::from_i128(10).pow_wrapping(diff as
u32);
+ a = divide_and_round::<Decimal256Type>(a, divisor);
+ product_scale -= diff;
Review Comment:
If say the 5th value of the array triggers a reduction in precision, we will
apply the truncation to all values that follow, but the first four values will
have been computed with the original product_scale?
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