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// Copyright © 2016–2017 University of Malta

// This program is free software: you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public License
// as published by the Free Software Foundation, either version 3 of
// the License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License and a copy of the GNU General Public License along with
// this program. If not, see <http://www.gnu.org/licenses/>.

use {Assign, Integer};
use gmp_mpfr_sys::gmp::{self, mpz_t};
use std::mem;
use std::ops::Deref;
use std::os::raw::c_int;
use std::sync::atomic::{AtomicPtr, Ordering};

#[repr(C)]
/// A small integer that does not require any memory allocation.
///
/// This can be useful when you have a `u64`, `i64`, `u32` or `i32`
/// but need a reference to an `Integer`.
///
/// If there are functions that take a `u32` or `i32` directly instead
/// of an `Integer` reference, using them can still be faster than
/// using a `SmallInteger`; the functions would still need to check
/// for the size of an `Integer` obtained using `SmallInteger`.
///
/// The `SmallInteger` type can be coerced to an `Integer`, as it
/// implements `Deref` with an `Integer` target.
///
/// # Examples
///
/// ```rust
/// use rug::Integer;
/// use rug::integer::SmallInteger;
/// // `a` requires a heap allocation
/// let mut a = Integer::from(250);
/// // `b` can reside on the stack
/// let b = SmallInteger::from(-100);
/// a.lcm(&b);
/// assert_eq!(a, 500);
/// // another computation:
/// a.lcm(&SmallInteger::from(30));
/// assert_eq!(a, 1500);
/// ```
pub struct SmallInteger {
    inner: Mpz,
    limbs: [gmp::limb_t; LIMBS_IN_SMALL_INTEGER],
}

const LIMBS_IN_SMALL_INTEGER: usize = 64 / gmp::LIMB_BITS as usize;

#[repr(C)]
pub struct Mpz {
    alloc: c_int,
    size: c_int,
    d: AtomicPtr<gmp::limb_t>,
}

impl Default for SmallInteger {
    fn default() -> SmallInteger {
        SmallInteger::new()
    }
}

impl SmallInteger {
    /// Creates a `SmallInteger` with value 0.
    pub fn new() -> SmallInteger {
        SmallInteger {
            inner: Mpz {
                size: 0,
                alloc: LIMBS_IN_SMALL_INTEGER as c_int,
                d: Default::default(),
            },
            limbs: [0; LIMBS_IN_SMALL_INTEGER],
        }
    }

    fn update_d(&self) {
        // sanity check
        assert_eq!(mem::size_of::<Mpz>(), mem::size_of::<mpz_t>());
        // Since this is borrowed, the limbs won't move around, and we
        // can set the d field.
        let d = &self.limbs[0] as *const _ as *mut _;
        self.inner.d.store(d, Ordering::Relaxed);
    }
}

impl Deref for SmallInteger {
    type Target = Integer;
    fn deref(&self) -> &Integer {
        self.update_d();
        let ptr = (&self.inner) as *const _ as *const _;
        unsafe { &*ptr }
    }
}

impl<T> From<T> for SmallInteger
where
    SmallInteger: Assign<T>,
{
    fn from(val: T) -> SmallInteger {
        let mut ret = SmallInteger::new();
        ret.assign(val);
        ret
    }
}

impl Assign<i32> for SmallInteger {
    fn assign(&mut self, val: i32) {
        self.assign(val.wrapping_abs() as u32);
        if val < 0 {
            self.inner.size = -self.inner.size;
        }
    }
}

impl Assign<i64> for SmallInteger {
    fn assign(&mut self, val: i64) {
        self.assign(val.wrapping_abs() as u64);
        if val < 0 {
            self.inner.size = -self.inner.size;
        }
    }
}

impl Assign<u32> for SmallInteger {
    fn assign(&mut self, val: u32) {
        if val == 0 {
            self.inner.size = 0;
        } else {
            self.inner.size = 1;
            self.limbs[0] = val as gmp::limb_t;
        }
    }
}

impl Assign<u64> for SmallInteger {
    fn assign(&mut self, val: u64) {
        match gmp::LIMB_BITS {
            64 => {
                if val == 0 {
                    self.inner.size = 0;
                } else {
                    self.inner.size = 1;
                    self.limbs[0] = val as gmp::limb_t;
                }
            }
            32 => {
                if val == 0 {
                    self.inner.size = 0;
                } else if val <= 0xffff_ffff {
                    self.inner.size = 1;
                    self.limbs[0] = val as u32 as gmp::limb_t;
                } else {
                    self.inner.size = 2;
                    self.limbs[0] = val as u32 as gmp::limb_t;
                    self.limbs[1 + 0] = (val >> 32) as u32 as gmp::limb_t;
                }
            }
            _ => {
                unimplemented!();
            }
        }
    }
}