std: Bring back f32::from_str_radix as an unstable API

This API was exercised in a few tests and mirrors the `from_str_radix`
functionality of the integer types.

Author: alexcrichton

Diff for: src/doc/trpl/traits.md

@@ -336,7 +336,7 @@ This shows off the additional feature of `where` clauses: they allow bounds
 where the left-hand side is an arbitrary type (`i32` in this case), not just a
 plain type parameter (like `T`).
 
-# Default methods
+## Default methods
 
 There’s one last feature of traits we should cover: default methods. It’s
 easiest just to show an example:

Diff for: src/libcollections/bit.rs

@@ -40,7 +40,6 @@
 //! ```
 //! # #![feature(collections, core, step_by)]
 //! use std::collections::{BitSet, BitVec};
-//! use std::num::Float;
 //! use std::iter;
 //!
 //! let max_prime = 10000;

Diff for: src/libcollections/fmt.rs

@@ -175,7 +175,6 @@
 //! # #![feature(core, std_misc)]
 //! use std::fmt;
 //! use std::f64;
-//! use std::num::Float;
 //!
 //! #[derive(Debug)]
 //! struct Vector2D {
@@ -200,10 +199,11 @@
 //!         let magnitude = magnitude.sqrt();
 //!
 //!         // Respect the formatting flags by using the helper method
-//!         // `pad_integral` on the Formatter object. See the method documentation
-//!         // for details, and the function `pad` can be used to pad strings.
+//!         // `pad_integral` on the Formatter object. See the method
+//!         // documentation for details, and the function `pad` can be used
+//!         // to pad strings.
 //!         let decimals = f.precision().unwrap_or(3);
-//!         let string = f64::to_str_exact(magnitude, decimals);
+//!         let string = format!("{:.*}", decimals, magnitude);
 //!         f.pad_integral(true, "", &string)
 //!     }
 //! }

Diff for: src/libcore/iter.rs

@@ -2327,9 +2327,8 @@ impl<I: RandomAccessIterator, F> RandomAccessIterator for Inspect<I, F>
 /// An iterator that yields sequential Fibonacci numbers, and stops on overflow.
 ///
 /// ```
-/// # #![feature(core)]
+/// #![feature(core)]
 /// use std::iter::Unfold;
-/// use std::num::Int; // For `.checked_add()`
 ///
 /// // This iterator will yield up to the last Fibonacci number before the max
 /// // value of `u32`. You can simply change `u32` to `u64` in this line if

Diff for: src/libcore/lib.rs

@@ -108,6 +108,7 @@ mod uint_macros;
 #[path = "num/f32.rs"]   pub mod f32;
 #[path = "num/f64.rs"]   pub mod f64;
 
+#[macro_use]
 pub mod num;
 
 /* The libcore prelude, not as all-encompassing as the libstd prelude */

Diff for: src/libcore/num/f32.rs

@@ -16,9 +16,11 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use prelude::*;
+
 use intrinsics;
 use mem;
-use num::Float;
+use num::{Float, ParseFloatError};
 use num::FpCategory as Fp;
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -153,6 +155,8 @@ impl Float for f32 {
     #[inline]
     fn one() -> f32 { 1.0 }
 
+    from_str_radix_float_impl! { f32 }
+
     /// Returns `true` if the number is NaN.
     #[inline]
     fn is_nan(self) -> bool { self != self }
@@ -234,9 +238,6 @@ impl Float for f32 {
     /// The fractional part of the number, satisfying:
     ///
     /// ```
-    /// # #![feature(core)]
-    /// use std::num::Float;
-    ///
     /// let x = 1.65f32;
     /// assert!(x == x.trunc() + x.fract())
     /// ```

Diff for: src/libcore/num/f64.rs

@@ -16,10 +16,12 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use prelude::*;
+
 use intrinsics;
 use mem;
-use num::Float;
 use num::FpCategory as Fp;
+use num::{Float, ParseFloatError};
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const RADIX: u32 = 2;
@@ -153,6 +155,8 @@ impl Float for f64 {
     #[inline]
     fn one() -> f64 { 1.0 }
 
+    from_str_radix_float_impl! { f64 }
+
     /// Returns `true` if the number is NaN.
     #[inline]
     fn is_nan(self) -> bool { self != self }
@@ -234,9 +238,6 @@ impl Float for f64 {
     /// The fractional part of the number, satisfying:
     ///
     /// ```
-    /// # #![feature(core)]
-    /// use std::num::Float;
-    ///
     /// let x = 1.65f64;
     /// assert!(x == x.trunc() + x.fract())
     /// ```

Diff for: src/libcore/num/float_macros.rs

@@ -18,3 +18,145 @@ macro_rules! assert_approx_eq {
                 "{} is not approximately equal to {}", *a, *b);
     })
 }
+
+macro_rules! from_str_radix_float_impl {
+    ($T:ty) => {
+        fn from_str_radix(src: &str, radix: u32)
+                          -> Result<$T, ParseFloatError> {
+            use num::FloatErrorKind::*;
+            use num::ParseFloatError as PFE;
+
+            // Special values
+            match src {
+                "inf"   => return Ok(Float::infinity()),
+                "-inf"  => return Ok(Float::neg_infinity()),
+                "NaN"   => return Ok(Float::nan()),
+                _       => {},
+            }
+
+            let (is_positive, src) =  match src.slice_shift_char() {
+                None             => return Err(PFE { kind: Empty }),
+                Some(('-', ""))  => return Err(PFE { kind: Empty }),
+                Some(('-', src)) => (false, src),
+                Some((_, _))     => (true,  src),
+            };
+
+            // The significand to accumulate
+            let mut sig = if is_positive { 0.0 } else { -0.0 };
+            // Necessary to detect overflow
+            let mut prev_sig = sig;
+            let mut cs = src.chars().enumerate();
+            // Exponent prefix and exponent index offset
+            let mut exp_info = None::<(char, usize)>;
+
+            // Parse the integer part of the significand
+            for (i, c) in cs.by_ref() {
+                match c.to_digit(radix) {
+                    Some(digit) => {
+                        // shift significand one digit left
+                        sig = sig * (radix as $T);
+
+                        // add/subtract current digit depending on sign
+                        if is_positive {
+                            sig = sig + ((digit as isize) as $T);
+                        } else {
+                            sig = sig - ((digit as isize) as $T);
+                        }
+
+                        // Detect overflow by comparing to last value, except
+                        // if we've not seen any non-zero digits.
+                        if prev_sig != 0.0 {
+                            if is_positive && sig <= prev_sig
+                                { return Ok(Float::infinity()); }
+                            if !is_positive && sig >= prev_sig
+                                { return Ok(Float::neg_infinity()); }
+
+                            // Detect overflow by reversing the shift-and-add process
+                            if is_positive && (prev_sig != (sig - digit as $T) / radix as $T)
+                                { return Ok(Float::infinity()); }
+                            if !is_positive && (prev_sig != (sig + digit as $T) / radix as $T)
+                                { return Ok(Float::neg_infinity()); }
+                        }
+                        prev_sig = sig;
+                    },
+                    None => match c {
+                        'e' | 'E' | 'p' | 'P' => {
+                            exp_info = Some((c, i + 1));
+                            break;  // start of exponent
+                        },
+                        '.' => {
+                            break;  // start of fractional part
+                        },
+                        _ => {
+                            return Err(PFE { kind: Invalid });
+                        },
+                    },
+                }
+            }
+
+            // If we are not yet at the exponent parse the fractional
+            // part of the significand
+            if exp_info.is_none() {
+                let mut power = 1.0;
+                for (i, c) in cs.by_ref() {
+                    match c.to_digit(radix) {
+                        Some(digit) => {
+                            // Decrease power one order of magnitude
+                            power = power / (radix as $T);
+                            // add/subtract current digit depending on sign
+                            sig = if is_positive {
+                                sig + (digit as $T) * power
+                            } else {
+                                sig - (digit as $T) * power
+                            };
+                            // Detect overflow by comparing to last value
+                            if is_positive && sig < prev_sig
+                                { return Ok(Float::infinity()); }
+                            if !is_positive && sig > prev_sig
+                                { return Ok(Float::neg_infinity()); }
+                            prev_sig = sig;
+                        },
+                        None => match c {
+                            'e' | 'E' | 'p' | 'P' => {
+                                exp_info = Some((c, i + 1));
+                                break; // start of exponent
+                            },
+                            _ => {
+                                return Err(PFE { kind: Invalid });
+                            },
+                        },
+                    }
+                }
+            }
+
+            // Parse and calculate the exponent
+            let exp = match exp_info {
+                Some((c, offset)) => {
+                    let base = match c {
+                        'E' | 'e' if radix == 10 => 10.0,
+                        'P' | 'p' if radix == 16 => 2.0,
+                        _ => return Err(PFE { kind: Invalid }),
+                    };
+
+                    // Parse the exponent as decimal integer
+                    let src = &src[offset..];
+                    let (is_positive, exp) = match src.slice_shift_char() {
+                        Some(('-', src)) => (false, src.parse::<usize>()),
+                        Some(('+', src)) => (true,  src.parse::<usize>()),
+                        Some((_, _))     => (true,  src.parse::<usize>()),
+                        None             => return Err(PFE { kind: Invalid }),
+                    };
+
+                    match (is_positive, exp) {
+                        (true,  Ok(exp)) => base.powi(exp as i32),
+                        (false, Ok(exp)) => 1.0 / base.powi(exp as i32),
+                        (_, Err(_))      => return Err(PFE { kind: Invalid }),
+                    }
+                },
+                None => 1.0, // no exponent
+            };
+
+            Ok(sig * exp)
+        }
+    }
+}