93 lines
3.2 KiB
Rust
93 lines
3.2 KiB
Rust
// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Utility functions for bignums that don't make too much sense to turn into methods.
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// FIXME This module's name is a bit unfortunate, since other modules also import `core::num`.
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use cmp::Ordering::{self, Less, Equal, Greater};
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pub use num::bignum::Big32x40 as Big;
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/// Test whether truncating all bits less significant than `ones_place` introduces
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/// a relative error less, equal, or greater than 0.5 ULP.
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pub fn compare_with_half_ulp(f: &Big, ones_place: usize) -> Ordering {
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if ones_place == 0 {
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return Less;
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}
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let half_bit = ones_place - 1;
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if f.get_bit(half_bit) == 0 {
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// < 0.5 ULP
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return Less;
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}
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// If all remaining bits are zero, it's = 0.5 ULP, otherwise > 0.5
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// If there are no more bits (half_bit == 0), the below also correctly returns Equal.
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for i in 0..half_bit {
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if f.get_bit(i) == 1 {
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return Greater;
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}
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}
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Equal
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}
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/// Convert an ASCII string containing only decimal digits to a `u64`.
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///
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/// Does not perform checks for overflow or invalid characters, so if the caller is not careful,
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/// the result is bogus and can panic (though it won't be `unsafe`). Additionally, empty strings
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/// are treated as zero. This function exists because
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///
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/// 1. using `FromStr` on `&[u8]` requires `from_utf8_unchecked`, which is bad, and
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/// 2. piecing together the results of `integral.parse()` and `fractional.parse()` is
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/// more complicated than this entire function.
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pub fn from_str_unchecked<'a, T>(bytes: T) -> u64 where T : IntoIterator<Item=&'a u8> {
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let mut result = 0;
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for &c in bytes {
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result = result * 10 + (c - b'0') as u64;
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}
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result
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}
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/// Convert a string of ASCII digits into a bignum.
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///
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/// Like `from_str_unchecked`, this function relies on the parser to weed out non-digits.
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pub fn digits_to_big(integral: &[u8], fractional: &[u8]) -> Big {
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let mut f = Big::from_small(0);
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for &c in integral.iter().chain(fractional) {
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let n = (c - b'0') as u32;
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f.mul_small(10);
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f.add_small(n);
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}
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f
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}
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/// Unwraps a bignum into a 64 bit integer. Panics if the number is too large.
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pub fn to_u64(x: &Big) -> u64 {
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assert!(x.bit_length() < 64);
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let d = x.digits();
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if d.len() < 2 {
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d[0] as u64
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} else {
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(d[1] as u64) << 32 | d[0] as u64
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}
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}
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/// Extract a range of bits.
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/// Index 0 is the least significant bit and the range is half-open as usual.
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/// Panics if asked to extract more bits than fit into the return type.
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pub fn get_bits(x: &Big, start: usize, end: usize) -> u64 {
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assert!(end - start <= 64);
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let mut result: u64 = 0;
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for i in (start..end).rev() {
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result = result << 1 | x.get_bit(i) as u64;
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}
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result
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}
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