// 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 crate::common::{log10_pow2, log10_pow5, pow5bits};

#[cfg(not(feature = "small"))]

pub use crate::d2s_full_table::{DOUBLE_POW5_INV_SPLIT, DOUBLE_POW5_SPLIT};

use crate::d2s_intrinsics::{

    div10, div100, div5, mul_shift_all_64, multiple_of_power_of_2, multiple_of_power_of_5,

};

#[cfg(feature = "small")]

pub use crate::d2s_small_table::{compute_inv_pow5, compute_pow5};

use core::mem::MaybeUninit;

pub const DOUBLE_MANTISSA_BITS: u32 = 52;

pub const DOUBLE_EXPONENT_BITS: u32 = 11;

pub const DOUBLE_BIAS: i32 = 1023;

pub const DOUBLE_POW5_INV_BITCOUNT: i32 = 125;

pub const DOUBLE_POW5_BITCOUNT: i32 = 125;

#[cfg_attr(feature = "no-panic", inline)]

pub fn decimal_length17(v: u64) -> u32 {

    // This is slightly faster than a loop.

    // The average output length is 16.38 digits, so we check high-to-low.

    // Function precondition: v is not an 18, 19, or 20-digit number.

    // (17 digits are sufficient for round-tripping.)

    debug_assert!(v < 100000000000000000);

    if v >= 10000000000000000 {

        17

    } else if v >= 1000000000000000 {

        16

    } else if v >= 100000000000000 {

        15

    } else if v >= 10000000000000 {

        14

    } else if v >= 1000000000000 {

        13

    } else if v >= 100000000000 {

        12

    } else if v >= 10000000000 {

        11

    } else if v >= 1000000000 {

        10

    } else 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

    }

}

// A floating decimal representing m * 10^e.

pub struct FloatingDecimal64 {

    pub mantissa: u64,

    // Decimal exponent's range is -324 to 308

    // inclusive, and can fit in i16 if needed.

    pub exponent: i32,

}

#[cfg_attr(feature = "no-panic", inline)]

pub fn d2d(ieee_mantissa: u64, ieee_exponent: u32) -> FloatingDecimal64 {

    let (e2, m2) = if ieee_exponent == 0 {

        (

            // We subtract 2 so that the bounds computation has 2 additional bits.

            1 - DOUBLE_BIAS - DOUBLE_MANTISSA_BITS as i32 - 2,

            ieee_mantissa,

        )

    } else {

        (

            ieee_exponent as i32 - DOUBLE_BIAS - DOUBLE_MANTISSA_BITS as i32 - 2,

            (1u64 << DOUBLE_MANTISSA_BITS) | ieee_mantissa,

        )

    };

    let even = (m2 & 1) == 0;

    let accept_bounds = even;

    // Step 2: Determine the interval of valid decimal representations.

    let mv = 4 * m2;

    // Implicit bool -> int conversion. True is 1, false is 0.

    let mm_shift = (ieee_mantissa != 0 || ieee_exponent <= 1) as u32;

    // We would compute mp and mm like this:

    // uint64_t mp = 4 * m2 + 2;

    // uint64_t mm = mv - 1 - mm_shift;

    // Step 3: Convert to a decimal power base using 128-bit arithmetic.

    let mut vr: u64;

    let mut vp: u64;

    let mut vm: u64;

    let mut vp_uninit: MaybeUninit<u64> = MaybeUninit::uninit();

    let mut vm_uninit: MaybeUninit<u64> = MaybeUninit::uninit();

    let e10: i32;

    let mut vm_is_trailing_zeros = false;

    let mut vr_is_trailing_zeros = false;

    if e2 >= 0 {

        // I tried special-casing q == 0, but there was no effect on performance.

        // This expression is slightly faster than max(0, log10_pow2(e2) - 1).

        let q = log10_pow2(e2) - (e2 > 3) as u32;

        e10 = q as i32;

        let k = DOUBLE_POW5_INV_BITCOUNT + pow5bits(q as i32) - 1;

        let i = -e2 + q as i32 + k;

        vr = unsafe {

            mul_shift_all_64(

                m2,

                #[cfg(feature = "small")]

                &compute_inv_pow5(q),

                #[cfg(not(feature = "small"))]

                {

                    debug_assert!(q < DOUBLE_POW5_INV_SPLIT.len() as u32);

                    DOUBLE_POW5_INV_SPLIT.get_unchecked(q as usize)

                },

                i as u32,

                vp_uninit.as_mut_ptr(),

                vm_uninit.as_mut_ptr(),

                mm_shift,

            )

        };

        vp = unsafe { vp_uninit.assume_init() };

        vm = unsafe { vm_uninit.assume_init() };

        if q <= 21 {

            // This should use q <= 22, but I think 21 is also safe. Smaller values

            // may still be safe, but it's more difficult to reason about them.

            // Only one of mp, mv, and mm can be a multiple of 5, if any.

            let mv_mod5 = (mv as u32).wrapping_sub(5u32.wrapping_mul(div5(mv) as u32));

            if mv_mod5 == 0 {

                vr_is_trailing_zeros = multiple_of_power_of_5(mv, q);

            } else if accept_bounds {

                // Same as min(e2 + (~mm & 1), pow5_factor(mm)) >= q

                // <=> e2 + (~mm & 1) >= q && pow5_factor(mm) >= q

                // <=> true && pow5_factor(mm) >= q, since e2 >= q.

                vm_is_trailing_zeros = multiple_of_power_of_5(mv - 1 - mm_shift as u64, q);

            } else {

                // Same as min(e2 + 1, pow5_factor(mp)) >= q.

                vp -= multiple_of_power_of_5(mv + 2, q) as u64;

            }

        }

    } else {

        // This expression is slightly faster than max(0, log10_pow5(-e2) - 1).

        let q = log10_pow5(-e2) - (-e2 > 1) as u32;

        e10 = q as i32 + e2;

        let i = -e2 - q as i32;

        let k = pow5bits(i) - DOUBLE_POW5_BITCOUNT;

        let j = q as i32 - k;

        vr = unsafe {

            mul_shift_all_64(

                m2,

                #[cfg(feature = "small")]

                &compute_pow5(i as u32),

                #[cfg(not(feature = "small"))]

                {

                    debug_assert!(i < DOUBLE_POW5_SPLIT.len() as i32);

                    DOUBLE_POW5_SPLIT.get_unchecked(i as usize)

                },

                j as u32,

                vp_uninit.as_mut_ptr(),

                vm_uninit.as_mut_ptr(),

                mm_shift,

            )

        };

        vp = unsafe { vp_uninit.assume_init() };

        vm = unsafe { vm_uninit.assume_init() };

        if q <= 1 {

            // {vr,vp,vm} is trailing zeros if {mv,mp,mm} has at least q trailing 0 bits.

            // mv = 4 * m2, so it always has at least two trailing 0 bits.

            vr_is_trailing_zeros = true;

            if accept_bounds {

                // mm = mv - 1 - mm_shift, so it has 1 trailing 0 bit iff mm_shift == 1.

                vm_is_trailing_zeros = mm_shift == 1;

            } else {

                // mp = mv + 2, so it always has at least one trailing 0 bit.

                vp -= 1;

            }

        } else if q < 63 {

            // TODO(ulfjack): Use a tighter bound here.

            // We want to know if the full product has at least q trailing zeros.

            // We need to compute min(p2(mv), p5(mv) - e2) >= q

            // <=> p2(mv) >= q && p5(mv) - e2 >= q

            // <=> p2(mv) >= q (because -e2 >= q)

            vr_is_trailing_zeros = multiple_of_power_of_2(mv, q);

        }

    }

    // Step 4: Find the shortest decimal representation in the interval of valid representations.

    let mut removed = 0i32;

    let mut last_removed_digit = 0u8;

    // On average, we remove ~2 digits.

    let output = if vm_is_trailing_zeros || vr_is_trailing_zeros {

        // General case, which happens rarely (~0.7%).

        loop {

            let vp_div10 = div10(vp);

            let vm_div10 = div10(vm);

            if vp_div10 <= vm_div10 {

                break;

            }

            let vm_mod10 = (vm as u32).wrapping_sub(10u32.wrapping_mul(vm_div10 as u32));

            let vr_div10 = div10(vr);

            let vr_mod10 = (vr as u32).wrapping_sub(10u32.wrapping_mul(vr_div10 as u32));

            vm_is_trailing_zeros &= vm_mod10 == 0;

            vr_is_trailing_zeros &= last_removed_digit == 0;

            last_removed_digit = vr_mod10 as u8;

            vr = vr_div10;

            vp = vp_div10;

            vm = vm_div10;

            removed += 1;

        }

        if vm_is_trailing_zeros {

            loop {

                let vm_div10 = div10(vm);

                let vm_mod10 = (vm as u32).wrapping_sub(10u32.wrapping_mul(vm_div10 as u32));

                if vm_mod10 != 0 {

                    break;

                }

                let vp_div10 = div10(vp);

                let vr_div10 = div10(vr);

                let vr_mod10 = (vr as u32).wrapping_sub(10u32.wrapping_mul(vr_div10 as u32));

                vr_is_trailing_zeros &= last_removed_digit == 0;

                last_removed_digit = vr_mod10 as u8;

                vr = vr_div10;

                vp = vp_div10;

                vm = vm_div10;

                removed += 1;

            }

        }

        if vr_is_trailing_zeros && last_removed_digit == 5 && vr % 2 == 0 {

            // Round even if the exact number is .....50..0.

            last_removed_digit = 4;

        }

        // We need to take vr + 1 if vr is outside bounds or we need to round up.

        vr + ((vr == vm && (!accept_bounds || !vm_is_trailing_zeros)) || last_removed_digit >= 5)

            as u64

    } else {

        // Specialized for the common case (~99.3%). Percentages below are relative to this.

        let mut round_up = false;

        let vp_div100 = div100(vp);

        let vm_div100 = div100(vm);

        // Optimization: remove two digits at a time (~86.2%).

        if vp_div100 > vm_div100 {

            let vr_div100 = div100(vr);

            let vr_mod100 = (vr as u32).wrapping_sub(100u32.wrapping_mul(vr_div100 as u32));

            round_up = vr_mod100 >= 50;

            vr = vr_div100;

            vp = vp_div100;

            vm = vm_div100;

            removed += 2;

        }

        // Loop iterations below (approximately), without optimization above:

        // 0: 0.03%, 1: 13.8%, 2: 70.6%, 3: 14.0%, 4: 1.40%, 5: 0.14%, 6+: 0.02%

        // Loop iterations below (approximately), with optimization above:

        // 0: 70.6%, 1: 27.8%, 2: 1.40%, 3: 0.14%, 4+: 0.02%

        loop {

            let vp_div10 = div10(vp);

            let vm_div10 = div10(vm);

            if vp_div10 <= vm_div10 {

                break;

            }

            let vr_div10 = div10(vr);

            let vr_mod10 = (vr as u32).wrapping_sub(10u32.wrapping_mul(vr_div10 as u32));

            round_up = vr_mod10 >= 5;

            vr = vr_div10;

            vp = vp_div10;

            vm = vm_div10;

            removed += 1;

        }

        // We need to take vr + 1 if vr is outside bounds or we need to round up.

        vr + (vr == vm || round_up) as u64

    };

    let exp = e10 + removed;

    FloatingDecimal64 {

        exponent: exp,

        mantissa: output,

    }

}