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Remove unnecessary constants from flt2dec dragon #128609

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Merged
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Aug 5, 2024
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Remove unnecessary constants from flt2dec dragon #128609

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49 changes: 26 additions & 23 deletions library/core/src/num/flt2dec/strategy/dragon.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,48 +12,51 @@ use crate::num::flt2dec::{round_up, Decoded, MAX_SIG_DIGITS};

static POW10: [Digit; 10] =
[1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000];
static TWOPOW10: [Digit; 10] =
[2, 20, 200, 2000, 20000, 200000, 2000000, 20000000, 200000000, 2000000000];

// precalculated arrays of `Digit`s for 10^(2^n)
static POW10TO16: [Digit; 2] = [0x6fc10000, 0x2386f2];
static POW10TO32: [Digit; 4] = [0, 0x85acef81, 0x2d6d415b, 0x4ee];
static POW10TO64: [Digit; 7] = [0, 0, 0xbf6a1f01, 0x6e38ed64, 0xdaa797ed, 0xe93ff9f4, 0x184f03];
static POW10TO128: [Digit; 14] = [
0, 0, 0, 0, 0x2e953e01, 0x3df9909, 0xf1538fd, 0x2374e42f, 0xd3cff5ec, 0xc404dc08, 0xbccdb0da,
0xa6337f19, 0xe91f2603, 0x24e,
// precalculated arrays of `Digit`s for 5^(2^n).
static POW5TO16: [Digit; 2] = [0x86f26fc1, 0x23];
static POW5TO32: [Digit; 3] = [0x85acef81, 0x2d6d415b, 0x4ee];
static POW5TO64: [Digit; 5] = [0xbf6a1f01, 0x6e38ed64, 0xdaa797ed, 0xe93ff9f4, 0x184f03];
static POW5TO128: [Digit; 10] = [
0x2e953e01, 0x3df9909, 0xf1538fd, 0x2374e42f, 0xd3cff5ec, 0xc404dc08, 0xbccdb0da, 0xa6337f19,
0xe91f2603, 0x24e,
];
static POW10TO256: [Digit; 27] = [
0, 0, 0, 0, 0, 0, 0, 0, 0x982e7c01, 0xbed3875b, 0xd8d99f72, 0x12152f87, 0x6bde50c6, 0xcf4a6e70,
0xd595d80f, 0x26b2716e, 0xadc666b0, 0x1d153624, 0x3c42d35a, 0x63ff540e, 0xcc5573c0, 0x65f9ef17,
0x55bc28f2, 0x80dcc7f7, 0xf46eeddc, 0x5fdcefce, 0x553f7,
static POW5TO256: [Digit; 19] = [
0x982e7c01, 0xbed3875b, 0xd8d99f72, 0x12152f87, 0x6bde50c6, 0xcf4a6e70, 0xd595d80f, 0x26b2716e,
0xadc666b0, 0x1d153624, 0x3c42d35a, 0x63ff540e, 0xcc5573c0, 0x65f9ef17, 0x55bc28f2, 0x80dcc7f7,
0xf46eeddc, 0x5fdcefce, 0x553f7,
];

#[doc(hidden)]
pub fn mul_pow10(x: &mut Big, n: usize) -> &mut Big {
debug_assert!(n < 512);
// Save ourself the left shift for the smallest cases.
if n < 8 {
return x.mul_small(POW10[n & 7]);
}
// Multiply by the powers of 5 and shift the 2s in at the end.
// This keeps the intermediate products smaller and faster.
if n & 7 != 0 {
x.mul_small(POW10[n & 7]);
x.mul_small(POW10[n & 7] >> (n & 7));
}
if n & 8 != 0 {
x.mul_small(POW10[8]);
x.mul_small(POW10[8] >> 8);
}
if n & 16 != 0 {
x.mul_digits(&POW10TO16);
x.mul_digits(&POW5TO16);
}
if n & 32 != 0 {
x.mul_digits(&POW10TO32);
x.mul_digits(&POW5TO32);
}
if n & 64 != 0 {
x.mul_digits(&POW10TO64);
x.mul_digits(&POW5TO64);
}
if n & 128 != 0 {
x.mul_digits(&POW10TO128);
x.mul_digits(&POW5TO128);
}
if n & 256 != 0 {
x.mul_digits(&POW10TO256);
x.mul_digits(&POW5TO256);
}
x
x.mul_pow2(n)
}

fn div_2pow10(x: &mut Big, mut n: usize) -> &mut Big {
Expand All @@ -62,7 +65,7 @@ fn div_2pow10(x: &mut Big, mut n: usize) -> &mut Big {
x.div_rem_small(POW10[largest]);
n -= largest;
}
x.div_rem_small(TWOPOW10[n]);
x.div_rem_small(POW10[n] << 1);
x
}

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