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    // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! A Big integer (signed version: `BigInt`, unsigned version: `BigUint`).
    //!
    //! A `BigUint` is represented as a vector of `BigDigit`s.
    //! A `BigInt` is a combination of `BigUint` and `Sign`.
    //!
    //! Common numerical operations are overloaded, so we can treat them
    //! the same way we treat other numbers.
    //!
    //! ## Example
    //!
    //! ```rust
    //! extern crate num_bigint;
    //! extern crate num_traits;
    //!
    //! # fn main() {
    //! use num_bigint::BigUint;
    //! use num_traits::{Zero, One};
    //! use std::mem::replace;
    //!
    //! // Calculate large fibonacci numbers.
    //! fn fib(n: usize) -> BigUint {
    //!     let mut f0: BigUint = Zero::zero();
    //!     let mut f1: BigUint = One::one();
    //!     for _ in 0..n {
    //!         let f2 = f0 + &f1;
    //!         // This is a low cost way of swapping f0 with f1 and f1 with f2.
    //!         f0 = replace(&mut f1, f2);
    //!     }
    //!     f0
    //! }
    //!
    //! // This is a very large number.
    //! println!("fib(1000) = {}", fib(1000));
    //! # }
    //! ```
    //!
    //! It's easy to generate large random numbers:
    //!
    //! ```rust
    //! # #[cfg(feature = "rand")]
    //! extern crate rand;
    //! extern crate num_bigint as bigint;
    //!
    //! # #[cfg(feature = "rand")]
    //! # fn main() {
    //! use bigint::{ToBigInt, RandBigInt};
    //!
    //! let mut rng = rand::thread_rng();
    //! let a = rng.gen_bigint(1000);
    //!
    //! let low = -10000.to_bigint().unwrap();
    //! let high = 10000.to_bigint().unwrap();
    //! let b = rng.gen_bigint_range(&low, &high);
    //!
    //! // Probably an even larger number.
    //! println!("{}", a * b);
    //! # }
    //!
    //! # #[cfg(not(feature = "rand"))]
    //! # fn main() {
    //! # }
    //! ```
    //!
    //! ## Compatibility
    //!
    //! The `num-bigint` crate is tested for rustc 1.15 and greater.

    #![doc(html_root_url = "https://docs.rs/num-bigint/0.2")]
    // We don't actually support `no_std` yet, and probably won't until `alloc` is stable.  We're just
    // reserving this ability with the "std" feature now, and compilation will fail without.
    #![cfg_attr(not(feature = "std"), no_std)]

    #[cfg(feature = "rand")]
    extern crate rand;
    #[cfg(feature = "serde")]
    extern crate serde;

    extern crate num_integer as integer;
    extern crate num_traits as traits;
    #[cfg(feature = "quickcheck")]
    extern crate quickcheck;

    use std::error::Error;
    use std::fmt;

    #[macro_use]
    mod macros;

    mod bigint;
    mod biguint;

    #[cfg(feature = "rand")]
    mod bigrand;

    #[cfg(target_pointer_width = "32")]
    type UsizePromotion = u32;
    #[cfg(target_pointer_width = "64")]
    type UsizePromotion = u64;

    #[cfg(target_pointer_width = "32")]
    type IsizePromotion = i32;
    #[cfg(target_pointer_width = "64")]
    type IsizePromotion = i64;

    #[derive(Debug, Clone, PartialEq, Eq)]
    pub struct ParseBigIntError {
        kind: BigIntErrorKind,
    }

    #[derive(Debug, Clone, PartialEq, Eq)]
    enum BigIntErrorKind {
        Empty,
        InvalidDigit,
    }

    impl ParseBigIntError {
        fn __description(&self) -> &str {
            use BigIntErrorKind::*;
            match self.kind {
                Empty => "cannot parse integer from empty string",
                InvalidDigit => "invalid digit found in string",
            }
        }

        fn empty() -> Self {
            ParseBigIntError {
                kind: BigIntErrorKind::Empty,
            }
        }

        fn invalid() -> Self {
            ParseBigIntError {
                kind: BigIntErrorKind::InvalidDigit,
            }
        }
    }

    impl fmt::Display for ParseBigIntError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            self.__description().fmt(f)
        }
    }

    impl Error for ParseBigIntError {
        fn description(&self) -> &str {
            self.__description()
        }
    }

    pub use biguint::BigUint;
    pub use biguint::ToBigUint;

    pub use bigint::BigInt;
    pub use bigint::Sign;
    pub use bigint::ToBigInt;

    #[cfg(feature = "rand")]
    pub use bigrand::{RandBigInt, RandomBits, UniformBigInt, UniformBigUint};

    mod big_digit {
        /// A `BigDigit` is a `BigUint`'s composing element.
        pub type BigDigit = u32;

        /// A `DoubleBigDigit` is the internal type used to do the computations.  Its
        /// size is the double of the size of `BigDigit`.
        pub type DoubleBigDigit = u64;

        /// A `SignedDoubleBigDigit` is the signed version of `DoubleBigDigit`.
        pub type SignedDoubleBigDigit = i64;

        // `DoubleBigDigit` size dependent
        pub const BITS: usize = 32;

        const LO_MASK: DoubleBigDigit = (-1i32 as DoubleBigDigit) >> BITS;

        #[inline]
        fn get_hi(n: DoubleBigDigit) -> BigDigit {
            (n >> BITS) as BigDigit
        }
        #[inline]
        fn get_lo(n: DoubleBigDigit) -> BigDigit {
            (n & LO_MASK) as BigDigit
        }

        /// Split one `DoubleBigDigit` into two `BigDigit`s.
        #[inline]
        pub fn from_doublebigdigit(n: DoubleBigDigit) -> (BigDigit, BigDigit) {
            (get_hi(n), get_lo(n))
        }

        /// Join two `BigDigit`s into one `DoubleBigDigit`
        #[inline]
        pub fn to_doublebigdigit(hi: BigDigit, lo: BigDigit) -> DoubleBigDigit {
            DoubleBigDigit::from(lo) | (DoubleBigDigit::from(hi) << BITS)
        }
    }

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