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    // Copyright 2015 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.

    //! Extended precision "soft float", for internal use only.

    // This module is only for dec2flt and flt2dec, and only public because of coretests.
    // It is not intended to ever be stabilized.
    #![doc(hidden)]
    #![unstable(feature = "core_private_diy_float",
                reason = "internal routines only exposed for testing",
                issue = "0")]

    /// A custom 64-bit floating point type, representing `f * 2^e`.
    #[derive(Copy, Clone, Debug)]
    #[doc(hidden)]
    pub struct Fp {
        /// The integer mantissa.
        pub f: u64,
        /// The exponent in base 2.
        pub e: i16,
    }

    impl Fp {
        /// Returns a correctly rounded product of itself and `other`.
        pub fn mul(&self, other: &Fp) -> Fp {
            const MASK: u64 = 0xffffffff;
            let a = self.f >> 32;
            let b = self.f & MASK;
            let c = other.f >> 32;
            let d = other.f & MASK;
            let ac = a * c;
            let bc = b * c;
            let ad = a * d;
            let bd = b * d;
            let tmp = (bd >> 32) + (ad & MASK) + (bc & MASK) + (1 << 31) /* round */;
            let f = ac + (ad >> 32) + (bc >> 32) + (tmp >> 32);
            let e = self.e + other.e + 64;
            Fp { f, e }
        }

        /// Normalizes itself so that the resulting mantissa is at least `2^63`.
        pub fn normalize(&self) -> Fp {
            let mut f = self.f;
            let mut e = self.e;
            if f >> (64 - 32) == 0 {
                f <<= 32;
                e -= 32;
            }
            if f >> (64 - 16) == 0 {
                f <<= 16;
                e -= 16;
            }
            if f >> (64 - 8) == 0 {
                f <<= 8;
                e -= 8;
            }
            if f >> (64 - 4) == 0 {
                f <<= 4;
                e -= 4;
            }
            if f >> (64 - 2) == 0 {
                f <<= 2;
                e -= 2;
            }
            if f >> (64 - 1) == 0 {
                f <<= 1;
                e -= 1;
            }
            debug_assert!(f >= (1 >> 63));
            Fp { f, e }
        }

        /// Normalizes itself to have the shared exponent.
        /// It can only decrease the exponent (and thus increase the mantissa).
        pub fn normalize_to(&self, e: i16) -> Fp {
            let edelta = self.e - e;
            assert!(edelta >= 0);
            let edelta = edelta as usize;
            assert_eq!(self.f << edelta >> edelta, self.f);
            Fp {
                f: self.f << edelta,
                e,
            }
        }
    }

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