Merge pull request #9957 from xtqqczze/bigdecimal-powi

uucore: switch to `BigDecimal::powi` implementation

Author: cakebaker

src/uucore/src/lib/features/parser/num_parser.rs

@@ -7,10 +7,8 @@
 
 // spell-checker:ignore powf copysign prec ilog inity infinit infs bigdecimal extendedbigdecimal biguint underflowed muls
 
-use std::num::NonZeroU64;
-
 use bigdecimal::{
-    BigDecimal, Context,
+    BigDecimal,
     num_bigint::{BigInt, BigUint, Sign},
 };
 use num_traits::Signed;
@@ -398,71 +396,6 @@ fn make_error(overflow: bool, negative: bool) -> ExtendedParserError<ExtendedBig
     }
 }
 
-/// Compute bd**exp using exponentiation by squaring algorithm, while maintaining the
-/// precision specified in ctx (the number of digits would otherwise explode).
-///
-/// Algorithm comes from <https://en.wikipedia.org/wiki/Exponentiation_by_squaring>
-///
-/// TODO: Still pending discussion in <https://github.com/akubera/bigdecimal-rs/issues/147>,
-/// we do lose a little bit of precision, and the last digits may not be correct.
-/// Note: This has been copied from the latest revision in <https://github.com/akubera/bigdecimal-rs/pull/148>,
-/// so it's using minimum Rust version of `bigdecimal-rs`.
-fn pow_with_context(bd: &BigDecimal, exp: i64, ctx: &Context) -> BigDecimal {
-    if exp == 0 {
-        return 1.into();
-    }
-
-    // When performing a multiplication between 2 numbers, we may lose up to 2 digits
-    // of precision.
-    // "Proof": https://github.com/akubera/bigdecimal-rs/issues/147#issuecomment-2793431202
-    const MARGIN_PER_MUL: u64 = 2;
-    // When doing many multiplication, we still introduce additional errors, add 1 more digit
-    // per 10 multiplications.
-    const MUL_PER_MARGIN_EXTRA: u64 = 10;
-
-    fn trim_precision(bd: BigDecimal, ctx: &Context, margin: u64) -> BigDecimal {
-        let prec = ctx.precision().get() + margin;
-        if bd.digits() > prec {
-            bd.with_precision_round(NonZeroU64::new(prec).unwrap(), ctx.rounding_mode())
-        } else {
-            bd
-        }
-    }
-
-    // Count the number of multiplications we're going to perform, one per "1" binary digit
-    // in exp, and the number of times we can divide exp by 2.
-    let mut n = exp.unsigned_abs();
-    // Note: 63 - n.leading_zeros() == n.ilog2, but that's only available in recent Rust versions.
-    let muls = (n.count_ones() + (63 - n.leading_zeros()) - 1) as u64;
-    // Note: div_ceil would be nice to use here, but only available in recent Rust versions.
-    // (see note above about minimum Rust version in use)
-    let margin_extra = (muls + MUL_PER_MARGIN_EXTRA / 2) / MUL_PER_MARGIN_EXTRA;
-    let mut margin = margin_extra + MARGIN_PER_MUL * muls;
-
-    let mut bd_y: BigDecimal = 1.into();
-    let mut bd_x = if exp >= 0 {
-        bd.clone()
-    } else {
-        bd.inverse_with_context(&ctx.with_precision(
-            NonZeroU64::new(ctx.precision().get() + margin + MARGIN_PER_MUL).unwrap(),
-        ))
-    };
-
-    while n > 1 {
-        if n % 2 == 1 {
-            bd_y = trim_precision(&bd_x * bd_y, ctx, margin);
-            margin -= MARGIN_PER_MUL;
-            n -= 1;
-        }
-        bd_x = trim_precision(bd_x.square(), ctx, margin);
-        margin -= MARGIN_PER_MUL;
-        n /= 2;
-    }
-    debug_assert_eq!(margin, margin_extra);
-
-    trim_precision(bd_x * bd_y, ctx, 0)
-}
-
 /// Construct an [`ExtendedBigDecimal`] based on parsed data
 fn construct_extended_big_decimal(
     digits: BigUint,
@@ -510,7 +443,7 @@ fn construct_extended_big_decimal(
         let bd = BigDecimal::from_bigint(signed_digits, 0)
             / BigDecimal::from_bigint(BigInt::from(16).pow(scale as u32), 0);
 
-        // pow_with_context "only" supports i64 values. Just overflow/underflow if the value provided
+        // powi "only" supports i64 values. Just overflow/underflow if the value provided
         // is > 2**64 or < 2**-64.
         let Some(exponent) = exponent.to_i64() else {
             return Err(make_error(exponent.is_positive(), negative));
@@ -520,7 +453,7 @@ fn construct_extended_big_decimal(
         let base: BigDecimal = 2.into();
         // Note: We cannot overflow/underflow BigDecimal here, as we will not be able to reach the
         // maximum/minimum scale (i64 range).
-        let pow2 = pow_with_context(&base, exponent, &Context::default());
+        let pow2 = base.powi(exponent);
 
         bd * pow2
     } else {