src/common.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.

// Returns the number of decimal digits in v, which must not contain more than 9
// digits.
#[cfg_attr(feature = "no-panic", inline)]
pub fn decimal_length9(v: u32) -> u32 {
    // Function precondition: v is not a 10-digit number.
    // (f2s: 9 digits are sufficient for round-tripping.)
    debug_assert!(v < 1000000000);

    if v >= 100000000 {
        9
    } else if v >= 10000000 {
        8
    } else if v >= 1000000 {
        7
    } else if v >= 100000 {
        6
    } else if v >= 10000 {
        5
    } else if v >= 1000 {
        4
    } else if v >= 100 {
        3
    } else if v >= 10 {
        2
    } else {
        1
    }
}

// Returns e == 0 ? 1 : [log_2(5^e)]; requires 0 <= e <= 3528.
#[cfg_attr(feature = "no-panic", inline)]
#[allow(dead_code)]
pub fn log2_pow5(e: i32) -> i32 /* or u32 -> u32 */ {
    // This approximation works up to the point that the multiplication
    // overflows at e = 3529. If the multiplication were done in 64 bits, it
    // would fail at 5^4004 which is just greater than 2^9297.
    debug_assert!(e >= 0);
    debug_assert!(e <= 3528);
    ((e as u32 * 1217359) >> 19) as i32
}

// Returns e == 0 ? 1 : ceil(log_2(5^e)); requires 0 <= e <= 3528.
#[cfg_attr(feature = "no-panic", inline)]
pub fn pow5bits(e: i32) -> i32 /* or u32 -> u32 */ {
    // This approximation works up to the point that the multiplication
    // overflows at e = 3529. If the multiplication were done in 64 bits, it
    // would fail at 5^4004 which is just greater than 2^9297.
    debug_assert!(e >= 0);
    debug_assert!(e <= 3528);
    (((e as u32 * 1217359) >> 19) + 1) as i32
}

#[cfg_attr(feature = "no-panic", inline)]
#[allow(dead_code)]
pub fn ceil_log2_pow5(e: i32) -> i32 /* or u32 -> u32 */ {
    log2_pow5(e) + 1
}

// Returns floor(log_10(2^e)); requires 0 <= e <= 1650.
#[cfg_attr(feature = "no-panic", inline)]
pub fn log10_pow2(e: i32) -> u32 /* or u32 -> u32 */ {
    // The first value this approximation fails for is 2^1651 which is just greater than 10^297.
    debug_assert!(e >= 0);
    debug_assert!(e <= 1650);
    (e as u32 * 78913) >> 18
}

// Returns floor(log_10(5^e)); requires 0 <= e <= 2620.
#[cfg_attr(feature = "no-panic", inline)]
pub fn log10_pow5(e: i32) -> u32 /* or u32 -> u32 */ {
    // The first value this approximation fails for is 5^2621 which is just greater than 10^1832.
    debug_assert!(e >= 0);
    debug_assert!(e <= 2620);
    (e as u32 * 732923) >> 20
}