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diff --git a/src/libcore/num/flt2dec/decoder.rs b/src/libcore/num/flt2dec/decoder.rs
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+++ b/src/libcore/num/flt2dec/decoder.rs
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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.
+
+//! Decodes a floating-point value into individual parts and error ranges.
+
+use prelude::*;
+
+use {f32, f64};
+use num::{Float, FpCategory};
+
+/// Decoded unsigned finite value, such that:
+///
+/// - The original value equals to `mant * 2^exp`.
+///
+/// - Any number from `(mant - minus) * 2^exp` to `(mant + plus) * 2^exp` will
+///   round to the original value. The range is inclusive only when
+///   `inclusive` is true.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Decoded {
+    /// The scaled mantissa.
+    pub mant: u64,
+    /// The lower error range.
+    pub minus: u64,
+    /// The upper error range.
+    pub plus: u64,
+    /// The shared exponent in base 2.
+    pub exp: i16,
+    /// True when the error range is inclusive.
+    ///
+    /// In IEEE 754, this is true when the original mantissa was even.
+    pub inclusive: bool,
+}
+
+/// Decoded unsigned value.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum FullDecoded {
+    /// Not-a-number.
+    Nan,
+    /// Infinities, either positive or negative.
+    Infinite,
+    /// Zero, either positive or negative.
+    Zero,
+    /// Finite numbers with further decoded fields.
+    Finite(Decoded),
+}
+
+/// A floating point type which can be `decode`d.
+pub trait DecodableFloat: Float + Copy {
+    /// Returns `x * 2^exp`. Almost same to `std::{f32,f64}::ldexp`.
+    /// This is used for testing.
+    fn ldexpi(f: i64, exp: isize) -> Self;
+    /// The minimum positive normalized value.
+    fn min_pos_norm_value() -> Self;
+}
+
+impl DecodableFloat for f32 {
+    fn ldexpi(f: i64, exp: isize) -> Self { f as Self * (exp as Self).exp2() }
+    fn min_pos_norm_value() -> Self { f32::MIN_POSITIVE }
+}
+
+impl DecodableFloat for f64 {
+    fn ldexpi(f: i64, exp: isize) -> Self { f as Self * (exp as Self).exp2() }
+    fn min_pos_norm_value() -> Self { f64::MIN_POSITIVE }
+}
+
+/// Returns a sign (true when negative) and `FullDecoded` value
+/// from given floating point number.
+pub fn decode<T: DecodableFloat>(v: T) -> (/*negative?*/ bool, FullDecoded) {
+    let (mant, exp, sign) = v.integer_decode();
+    let even = (mant & 1) == 0;
+    let decoded = match v.classify() {
+        FpCategory::Nan => FullDecoded::Nan,
+        FpCategory::Infinite => FullDecoded::Infinite,
+        FpCategory::Zero => FullDecoded::Zero,
+        FpCategory::Subnormal => {
+            // neighbors: (mant - 2, exp) -- (mant, exp) -- (mant + 2, exp)
+            // Float::integer_decode always preserves the exponent,
+            // so the mantissa is scaled for subnormals.
+            FullDecoded::Finite(Decoded { mant: mant, minus: 1, plus: 1,
+                                          exp: exp, inclusive: even })
+        }
+        FpCategory::Normal => {
+            let minnorm = <T as DecodableFloat>::min_pos_norm_value().integer_decode();
+            if mant == minnorm.0 {
+                // neighbors: (maxmant, exp - 1) -- (minnormmant, exp) -- (minnormmant + 1, exp)
+                // where maxmant = minnormmant * 2 - 1
+                FullDecoded::Finite(Decoded { mant: mant << 2, minus: 1, plus: 2,
+                                              exp: exp - 2, inclusive: even })
+            } else {
+                // neighbors: (mant - 1, exp) -- (mant, exp) -- (mant + 1, exp)
+                FullDecoded::Finite(Decoded { mant: mant << 1, minus: 1, plus: 1,
+                                              exp: exp - 1, inclusive: even })
+            }
+        }
+    };
+    (sign < 0, decoded)
+}
+