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chirizxc wants to merge 1 commit into from
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a bit optimize four-digit chunks in integer formatting #71

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88 changes: 62 additions & 26 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -347,15 +347,19 @@ impl Unsigned for u128 {
(mod_1e16, u128::MAX_STR_LEN)
} else {
// Write digits at buf[23..39].
enc_16lsd::<{ u128::MAX_STR_LEN - 16 }>(buf, mod_1e16);
//
// SAFETY: `mod_1e16 < 1e16` (remainder), and `U128_MAX_DEC_N - 16 + 16 == buf.len()`.
unsafe { enc_16lsd::<{ u128::MAX_STR_LEN - 16 }>(buf, mod_1e16) };

// Take another 16 decimals.
let (quot2, mod2) = div_rem_1e16(quot_1e16);
if quot2 == 0 {
(mod2, u128::MAX_STR_LEN - 16)
} else {
// Write digits at buf[7..23].
enc_16lsd::<{ u128::MAX_STR_LEN - 32 }>(buf, mod2);
//
// SAFETY: `mod2 < 1e16` (remainder), and `U128_MAX_DEC_N - 32 + 16 <= buf.len()`.
unsafe { enc_16lsd::<{ u128::MAX_STR_LEN - 32 }>(buf, mod2) };
// Quot2 has at most 7 decimals remaining after two 1e16 divisions.
(quot2 as u64, u128::MAX_STR_LEN - 32)
}
Expand All @@ -369,11 +373,10 @@ impl Unsigned for u128 {
let quad = remain % 1_00_00;
remain /= 1_00_00;
let (pair1, pair2) = divmod100(quad as u32);
// SAFETY: quad is a remainder modulo 10_000. The offset checks
// above reserve exactly four bytes in buf.
unsafe {
buf[offset + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 0));
buf[offset + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 1));
buf[offset + 2].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 0));
buf[offset + 3].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 1));
write_quad(buf.get_unchecked_mut(offset..offset + 4), quad);
}
}

Expand Down Expand Up @@ -403,37 +406,70 @@ impl Unsigned for u128 {
}
}

// Encodes the 16 least-significant decimals of n into `buf[OFFSET..OFFSET + 16]`.
// Writes `quad` as exactly four digits (for example: `42` becomes `"0042"`).
//
// # Safety
//
// `quad` must be below 10_000 and `buf` must contain exactly four bytes.
#[cfg_attr(feature = "no-panic", no_panic)]
fn enc_16lsd<const OFFSET: usize>(buf: &mut [MaybeUninit<u8>], n: u64) {
// Consume the least-significant decimals from a working copy.
#[inline(always)]
unsafe fn write_quad(buf: &mut [MaybeUninit<u8>], quad: u64) {
// SAFETY: These are this function's caller-provided invariants.
unsafe {
core::hint::assert_unchecked(quad < 10_000);
core::hint::assert_unchecked(buf.len() == 4);
}

let quad = quad as u32;

// Note: this is equivalent to `quad / 100`, but contains no division instructions.
let high = (quad * const { (1 << 19) / 100 + 1 }) >> 19;
let low = quad - high * 100;
let high = high as usize;
let low = low as usize;

// SAFETY: `high` and `low` are below 100 because `quad` is below 10_000.
unsafe { core::hint::assert_unchecked(high < 100 && low < 100) }

buf[0..2].write_copy_of_slice(&DECIMAL_PAIRS[high * 2..high * 2 + 2]);
buf[2..4].write_copy_of_slice(&DECIMAL_PAIRS[low * 2..low * 2 + 2]);
}

// Encodes the 16 least-significant decimals of n into `buf[OFFSET .. OFFSET + 16 ]`.
//
// # Safety
//
// `n` must be below 1e16, and `buf` must be at least `OFFSET + 16` bytes long.
#[cfg_attr(feature = "no-panic", no_panic)]
unsafe fn enc_16lsd<const OFFSET: usize>(buf: &mut [MaybeUninit<u8>], n: u64) {
// SAFETY: Every caller passes a remainder produced by division by 10^16,
// and every used `OFFSET` specialization reserves sixteen bytes in `buf`.
unsafe {
core::hint::assert_unchecked(n < 10_000_000_000_000_000);
core::hint::assert_unchecked(OFFSET + 16 <= buf.len());
}

// Peel four digits at a time from right to left (12345678 -> 1234 | 5678).
// Since 10_000 is constant, LLVM replaces each division with multiply or shift.
let mut remain = n;

// Format per four digits from the lookup table.
for quad_index in (1..4).rev() {
// pull two pairs
let quad = remain % 1_00_00;
remain /= 1_00_00;
let (pair1, pair2) = divmod100(quad as u32);

// SAFETY: `OFFSET + quad_index * 4` starts one of the four
// non-overlapping four-byte regions proven in bounds above.
unsafe {
buf[quad_index * 4 + OFFSET + 0]
.write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 0));
buf[quad_index * 4 + OFFSET + 1]
.write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 1));
buf[quad_index * 4 + OFFSET + 2]
.write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 0));
buf[quad_index * 4 + OFFSET + 3]
.write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 1));
write_quad(
buf.get_unchecked_mut(OFFSET + quad_index * 4..OFFSET + (quad_index + 1) * 4),
quad,
);
}
}

// final two pairs
let (pair1, pair2) = divmod100(remain as u32);
// SAFETY: OFFSET starts the first four-byte region proven in bounds above.
unsafe {
buf[OFFSET + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 0));
buf[OFFSET + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 1));
buf[OFFSET + 2].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 0));
buf[OFFSET + 3].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 1));
write_quad(buf.get_unchecked_mut(OFFSET..OFFSET + 4), remain);
}
}

Expand Down
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