src/d2s_intrinsics.rs

// Translated from C to Rust. The original C code can be found at
// https://github.com/ulfjack/ryu and carries the following license:
//
// Copyright 2018 Ulf Adams
//
// The contents of this file may be used under the terms of the Apache License,
// Version 2.0.
//
//    (See accompanying file LICENSE-Apache or copy at
//     http://www.apache.org/licenses/LICENSE-2.0)
//
// Alternatively, the contents of this file may be used under the terms of
// the Boost Software License, Version 1.0.
//    (See accompanying file LICENSE-Boost or copy at
//     https://www.boost.org/LICENSE_1_0.txt)
//
// Unless required by applicable law or agreed to in writing, this software
// is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.

use core::ptr;

// Returns (lo, hi).
#[cfg(not(integer128))]
#[cfg_attr(feature = "no-panic", inline)]
pub fn umul128(a: u64, b: u64) -> (u64, u64) {
    let a_lo = a as u32;
    let a_hi = (a >> 32) as u32;
    let b_lo = b as u32;
    let b_hi = (b >> 32) as u32;

    let b00 = a_lo as u64 * b_lo as u64;
    let b01 = a_lo as u64 * b_hi as u64;
    let b10 = a_hi as u64 * b_lo as u64;
    let b11 = a_hi as u64 * b_hi as u64;

    let b00_lo = b00 as u32;
    let b00_hi = (b00 >> 32) as u32;

    let mid1 = b10 + b00_hi as u64;
    let mid1_lo = mid1 as u32;
    let mid1_hi = (mid1 >> 32) as u32;

    let mid2 = b01 + mid1_lo as u64;
    let mid2_lo = mid2 as u32;
    let mid2_hi = (mid2 >> 32) as u32;

    let p_hi = b11 + mid1_hi as u64 + mid2_hi as u64;
    let p_lo = ((mid2_lo as u64) << 32) | b00_lo as u64;

    (p_lo, p_hi)
}

#[cfg(not(integer128))]
#[cfg_attr(feature = "no-panic", inline)]
pub fn shiftright128(lo: u64, hi: u64, dist: u32) -> u64 {
    // We don't need to handle the case dist >= 64 here (see above).
    debug_assert!(dist > 0);
    debug_assert!(dist < 64);
    (hi << (64 - dist)) | (lo >> dist)
}

#[cfg_attr(feature = "no-panic", inline)]
pub fn div5(x: u64) -> u64 {
    x / 5
}

#[cfg_attr(feature = "no-panic", inline)]
pub fn div10(x: u64) -> u64 {
    x / 10
}

#[cfg_attr(feature = "no-panic", inline)]
pub fn div100(x: u64) -> u64 {
    x / 100
}

#[cfg_attr(feature = "no-panic", inline)]
fn pow5_factor(mut value: u64) -> u32 {
    let mut count = 0u32;
    loop {
        debug_assert!(value != 0);
        let q = div5(value);
        let r = (value as u32).wrapping_sub(5u32.wrapping_mul(q as u32));
        if r != 0 {
            break;
        }
        value = q;
        count += 1;
    }
    count
}

// Returns true if value is divisible by 5^p.
#[cfg_attr(feature = "no-panic", inline)]
pub fn multiple_of_power_of_5(value: u64, p: u32) -> bool {
    // I tried a case distinction on p, but there was no performance difference.
    pow5_factor(value) >= p
}

// Returns true if value is divisible by 2^p.
#[cfg_attr(feature = "no-panic", inline)]
pub fn multiple_of_power_of_2(value: u64, p: u32) -> bool {
    debug_assert!(value != 0);
    debug_assert!(p < 64);
    // __builtin_ctzll doesn't appear to be faster here.
    (value & ((1u64 << p) - 1)) == 0
}

#[cfg(integer128)]
#[cfg_attr(feature = "no-panic", inline)]
pub fn mul_shift_64(m: u64, mul: &(u64, u64), j: u32) -> u64 {
    let b0 = m as u128 * mul.0 as u128;
    let b2 = m as u128 * mul.1 as u128;
    (((b0 >> 64) + b2) >> (j - 64)) as u64
}

#[cfg(integer128)]
#[cfg_attr(feature = "no-panic", inline)]
pub unsafe fn mul_shift_all_64(
    m: u64,
    mul: &(u64, u64),
    j: u32,
    vp: *mut u64,
    vm: *mut u64,
    mm_shift: u32,
) -> u64 {
    ptr::write(vp, mul_shift_64(4 * m + 2, mul, j));
    ptr::write(vm, mul_shift_64(4 * m - 1 - mm_shift as u64, mul, j));
    mul_shift_64(4 * m, mul, j)
}

#[cfg(not(integer128))]
#[cfg_attr(feature = "no-panic", inline)]
pub unsafe fn mul_shift_all_64(
    mut m: u64,
    mul: &(u64, u64),
    j: u32,
    vp: *mut u64,
    vm: *mut u64,
    mm_shift: u32,
) -> u64 {
    m <<= 1;
    // m is maximum 55 bits
    let (lo, tmp) = umul128(m, mul.0);
    let (mut mid, mut hi) = umul128(m, mul.1);
    mid = mid.wrapping_add(tmp);
    hi = hi.wrapping_add((mid < tmp) as u64); // overflow into hi

    let lo2 = lo.wrapping_add(mul.0);
    let mid2 = mid.wrapping_add(mul.1).wrapping_add((lo2 < lo) as u64);
    let hi2 = hi.wrapping_add((mid2 < mid) as u64);
    ptr::write(vp, shiftright128(mid2, hi2, j - 64 - 1));

    if mm_shift == 1 {
        let lo3 = lo.wrapping_sub(mul.0);
        let mid3 = mid.wrapping_sub(mul.1).wrapping_sub((lo3 > lo) as u64);
        let hi3 = hi.wrapping_sub((mid3 > mid) as u64);
        ptr::write(vm, shiftright128(mid3, hi3, j - 64 - 1));
    } else {
        let lo3 = lo + lo;
        let mid3 = mid.wrapping_add(mid).wrapping_add((lo3 < lo) as u64);
        let hi3 = hi.wrapping_add(hi).wrapping_add((mid3 < mid) as u64);
        let lo4 = lo3.wrapping_sub(mul.0);
        let mid4 = mid3.wrapping_sub(mul.1).wrapping_sub((lo4 > lo3) as u64);
        let hi4 = hi3.wrapping_sub((mid4 > mid3) as u64);
        ptr::write(vm, shiftright128(mid4, hi4, j - 64));
    }

    shiftright128(mid, hi, j - 64 - 1)
}