core: added core::num::flt2dec for floating-point formatting.

This is a fork of the flt2dec portion of rust-strconv [1] with
a necessary relicensing (the original code was licensed CC0-1.0).
Each module is accompanied with large unit tests, integrated
in this commit as coretest::num::flt2dec. This module is added
in order to replace the existing core::fmt::float method.

The forked revision of rust-strconv is from 2015-04-20, with a commit ID
9adf6d3571c6764a6f240a740c823024f70dc1c7.

[1] https://github.com/lifthrasiir/rust-strconv/

src/libcore/num/flt2dec/bignum.rs

@@ -0,0 +1,355 @@
+// 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.
+
+//! Custom arbitrary-precision number (bignum) implementation.
+//!
+//! This is designed to avoid the heap allocation at expense of stack memory.
+//! The most used bignum type, `Big32x36`, is limited by 32 × 36 = 1,152 bits
+//! and will take at most 152 bytes of stack memory. This is (barely) enough
+//! for handling all possible finite `f64` values.
+//!
+//! In principle it is possible to have multiple bignum types for different
+//! inputs, but we don't do so to avoid the code bloat. Each bignum is still
+//! tracked for the actual usages, so it normally doesn't matter.
+
+#![macro_use]
+
+use prelude::*;
+use mem;
+use intrinsics;
+
+/// Arithmetic operations required by bignums.
+pub trait FullOps {
+    /// Returns `(carry', v')` such that `carry' * 2^W + v' = self + other + carry`,
+    /// where `W` is the number of bits in `Self`.
+    fn full_add(self, other: Self, carry: bool) -> (bool /*carry*/, Self);
+
+    /// Returns `(carry', v')` such that `carry' * 2^W + v' = self * other + carry`,
+    /// where `W` is the number of bits in `Self`.
+    fn full_mul(self, other: Self, carry: Self) -> (Self /*carry*/, Self);
+
+    /// Returns `(carry', v')` such that `carry' * 2^W + v' = self * other + other2 + carry`,
+    /// where `W` is the number of bits in `Self`.
+    fn full_mul_add(self, other: Self, other2: Self, carry: Self) -> (Self /*carry*/, Self);
+
+    /// Returns `(quo, rem)` such that `borrow * 2^W + self = quo * other + rem`
+    /// and `0 <= rem < other`, where `W` is the number of bits in `Self`.
+    fn full_div_rem(self, other: Self, borrow: Self) -> (Self /*quotient*/, Self /*remainder*/);
+}
+
+macro_rules! impl_full_ops {
+    ($($ty:ty: add($addfn:path), mul/div($bigty:ident);)*) => (
+        $(
+            impl FullOps for $ty {
+                fn full_add(self, other: $ty, carry: bool) -> (bool, $ty) {
+                    // this cannot overflow, the output is between 0 and 2*2^nbits - 1
+                    // FIXME will LLVM optimize this into ADC or similar???
+                    let (v, carry1) = unsafe { $addfn(self, other) };
+                    let (v, carry2) = unsafe { $addfn(v, if carry {1} else {0}) };
+                    (carry1 || carry2, v)
+                }
+
+                fn full_mul(self, other: $ty, carry: $ty) -> ($ty, $ty) {
+                    // this cannot overflow, the output is between 0 and 2^nbits * (2^nbits - 1)
+                    let nbits = mem::size_of::<$ty>() * 8;
+                    let v = (self as $bigty) * (other as $bigty) + (carry as $bigty);
+                    ((v >> nbits) as $ty, v as $ty)
+                }
+
+                fn full_mul_add(self, other: $ty, other2: $ty, carry: $ty) -> ($ty, $ty) {
+                    // this cannot overflow, the output is between 0 and 2^(2*nbits) - 1
+                    let nbits = mem::size_of::<$ty>() * 8;
+                    let v = (self as $bigty) * (other as $bigty) + (other2 as $bigty) +
+                            (carry as $bigty);
+                    ((v >> nbits) as $ty, v as $ty)
+                }
+
+                fn full_div_rem(self, other: $ty, borrow: $ty) -> ($ty, $ty) {
+                    debug_assert!(borrow < other);
+                    // this cannot overflow, the dividend is between 0 and other * 2^nbits - 1
+                    let nbits = mem::size_of::<$ty>() * 8;
+                    let lhs = ((borrow as $bigty) << nbits) | (self as $bigty);
+                    let rhs = other as $bigty;
+                    ((lhs / rhs) as $ty, (lhs % rhs) as $ty)
+                }
+            }
+        )*
+    )
+}
+
+impl_full_ops! {
+    u8:  add(intrinsics::u8_add_with_overflow),  mul/div(u16);
+    u16: add(intrinsics::u16_add_with_overflow), mul/div(u32);
+    u32: add(intrinsics::u32_add_with_overflow), mul/div(u64);
+//  u64: add(intrinsics::u64_add_with_overflow), mul/div(u128); // damn!
+}
+
+macro_rules! define_bignum {
+    ($name:ident: type=$ty:ty, n=$n:expr) => (
+        /// Stack-allocated arbitrary-precision (up to certain limit) integer.
+        ///
+        /// This is backed by an fixed-size array of given type ("digit").
+        /// While the array is not very large (normally some hundred bytes),
+        /// copying it recklessly may result in the performance hit.
+        /// Thus this is intentionally not `Copy`.
+        ///
+        /// All operations available to bignums panic in the case of over/underflows.
+        /// The caller is responsible to use large enough bignum types.
+        pub struct $name {
+            /// One plus the offset to the maximum "digit" in the use.
+            /// This does not decrease, so be aware of the computation order.
+            /// `base[size..]` should be zero.
+            size: usize,
+            /// Digits. `[a, b, c, ...]` represents `a + b*n + c*n^2 + ...`.
+            base: [$ty; $n]
+        }
+
+        impl $name {
+            /// Makes a bignum from one digit.
+            pub fn from_small(v: $ty) -> $name {
+                let mut base = [0; $n];
+                base[0] = v;
+                $name { size: 1, base: base }
+            }
+
+            /// Makes a bignum from `u64` value.
+            pub fn from_u64(mut v: u64) -> $name {
+                use mem;
+
+                let mut base = [0; $n];
+                let mut sz = 0;
+                while v > 0 {
+                    base[sz] = v as $ty;
+                    v >>= mem::size_of::<$ty>() * 8;
+                    sz += 1;
+                }
+                $name { size: sz, base: base }
+            }
+
+            /// Returns true if the bignum is zero.
+            pub fn is_zero(&self) -> bool {
+                self.base[..self.size].iter().all(|&v| v == 0)
+            }
+
+            /// Adds `other` to itself and returns its own mutable reference.
+            pub fn add<'a>(&'a mut self, other: &$name) -> &'a mut $name {
+                use cmp;
+                use num::flt2dec::bignum::FullOps;
+
+                let mut sz = cmp::max(self.size, other.size);
+                let mut carry = false;
+                for (a, b) in self.base[..sz].iter_mut().zip(other.base[..sz].iter()) {
+                    let (c, v) = (*a).full_add(*b, carry);
+                    *a = v;
+                    carry = c;
+                }
+                if carry {
+                    self.base[sz] = 1;
+                    sz += 1;
+                }
+                self.size = sz;
+                self
+            }
+
+            /// Subtracts `other` from itself and returns its own mutable reference.
+            pub fn sub<'a>(&'a mut self, other: &$name) -> &'a mut $name {
+                use cmp;
+                use num::flt2dec::bignum::FullOps;
+
+                let sz = cmp::max(self.size, other.size);
+                let mut noborrow = true;
+                for (a, b) in self.base[..sz].iter_mut().zip(other.base[..sz].iter()) {
+                    let (c, v) = (*a).full_add(!*b, noborrow);
+                    *a = v;
+                    noborrow = c;
+                }
+                assert!(noborrow);
+                self.size = sz;
+                self
+            }
+
+            /// Multiplies itself by a digit-sized `other` and returns its own
+            /// mutable reference.
+            pub fn mul_small<'a>(&'a mut self, other: $ty) -> &'a mut $name {
+                use num::flt2dec::bignum::FullOps;
+
+                let mut sz = self.size;
+                let mut carry = 0;
+                for a in self.base[..sz].iter_mut() {
+                    let (c, v) = (*a).full_mul(other, carry);
+                    *a = v;
+                    carry = c;
+                }
+                if carry > 0 {
+                    self.base[sz] = carry;
+                    sz += 1;
+                }
+                self.size = sz;
+                self
+            }
+
+            /// Multiplies itself by `2^bits` and returns its own mutable reference.
+            pub fn mul_pow2<'a>(&'a mut self, bits: usize) -> &'a mut $name {
+                use mem;
+
+                let digitbits = mem::size_of::<$ty>() * 8;
+                let digits = bits / digitbits;
+                let bits = bits % digitbits;
+
+                assert!(digits < $n);
+                debug_assert!(self.base[$n-digits..].iter().all(|&v| v == 0));
+                debug_assert!(bits == 0 || (self.base[$n-digits-1] >> (digitbits - bits)) == 0);
+
+                // shift by `digits * digitbits` bits
+                for i in (0..self.size).rev() {
+                    self.base[i+digits] = self.base[i];
+                }
+                for i in 0..digits {
+                    self.base[i] = 0;
+                }
+
+                // shift by `nbits` bits
+                let mut sz = self.size + digits;
+                if bits > 0 {
+                    let last = sz;
+                    let overflow = self.base[last-1] >> (digitbits - bits);
+                    if overflow > 0 {
+                        self.base[last] = overflow;
+                        sz += 1;
+                    }
+                    for i in (digits+1..last).rev() {
+                        self.base[i] = (self.base[i] << bits) |
+                                       (self.base[i-1] >> (digitbits - bits));
+                    }
+                    self.base[digits] <<= bits;
+                    // self.base[..digits] is zero, no need to shift
+                }
+
+                self.size = sz;
+                self
+            }
+
+            /// Multiplies itself by a number described by `other[0] + other[1] * n +
+            /// other[2] * n^2 + ...` and returns its own mutable reference.
+            pub fn mul_digits<'a>(&'a mut self, other: &[$ty]) -> &'a mut $name {
+                // the internal routine. works best when aa.len() <= bb.len().
+                fn mul_inner(ret: &mut [$ty; $n], aa: &[$ty], bb: &[$ty]) -> usize {
+                    use num::flt2dec::bignum::FullOps;
+
+                    let mut retsz = 0;
+                    for (i, &a) in aa.iter().enumerate() {
+                        if a == 0 { continue; }
+                        let mut sz = bb.len();
+                        let mut carry = 0;
+                        for (j, &b) in bb.iter().enumerate() {
+                            let (c, v) = a.full_mul_add(b, ret[i + j], carry);
+                            ret[i + j] = v;
+                            carry = c;
+                        }
+                        if carry > 0 {
+                            ret[i + sz] = carry;
+                            sz += 1;
+                        }
+                        if retsz < i + sz {
+                            retsz = i + sz;
+                        }
+                    }
+                    retsz
+                }
+
+                let mut ret = [0; $n];
+                let retsz = if self.size < other.len() {
+                    mul_inner(&mut ret, &self.base[..self.size], other)
+                } else {
+                    mul_inner(&mut ret, other, &self.base[..self.size])
+                };
+                self.base = ret;
+                self.size = retsz;
+                self
+            }
+
+            /// Divides itself by a digit-sized `other` and returns its own
+            /// mutable reference *and* the remainder.
+            pub fn div_rem_small<'a>(&'a mut self, other: $ty) -> (&'a mut $name, $ty) {
+                use num::flt2dec::bignum::FullOps;
+
+                assert!(other > 0);
+
+                let sz = self.size;
+                let mut borrow = 0;
+                for a in self.base[..sz].iter_mut().rev() {
+                    let (q, r) = (*a).full_div_rem(other, borrow);
+                    *a = q;
+                    borrow = r;
+                }
+                (self, borrow)
+            }
+        }
+
+        impl ::cmp::PartialEq for $name {
+            fn eq(&self, other: &$name) -> bool { self.base[..] == other.base[..] }
+        }
+
+        impl ::cmp::Eq for $name {
+        }
+
+        impl ::cmp::PartialOrd for $name {
+            fn partial_cmp(&self, other: &$name) -> ::option::Option<::cmp::Ordering> {
+                ::option::Option::Some(self.cmp(other))
+            }
+        }
+
+        impl ::cmp::Ord for $name {
+            fn cmp(&self, other: &$name) -> ::cmp::Ordering {
+                use cmp::max;
+                use iter::order;
+
+                let sz = max(self.size, other.size);
+                let lhs = self.base[..sz].iter().cloned().rev();
+                let rhs = other.base[..sz].iter().cloned().rev();
+                order::cmp(lhs, rhs)
+            }
+        }
+
+        impl ::clone::Clone for $name {
+            fn clone(&self) -> $name {
+                $name { size: self.size, base: self.base }
+            }
+        }
+
+        impl ::fmt::Debug for $name {
+            fn fmt(&self, f: &mut ::fmt::Formatter) -> ::fmt::Result {
+                use mem;
+
+                let sz = if self.size < 1 {1} else {self.size};
+                let digitlen = mem::size_of::<$ty>() * 2;
+
+                try!(write!(f, "{:#x}", self.base[sz-1]));
+                for &v in self.base[..sz-1].iter().rev() {
+                    try!(write!(f, "_{:01$x}", v, digitlen));
+                }
+                ::result::Result::Ok(())
+            }
+        }
+    )
+}
+
+/// The digit type for `Big32x36`.
+pub type Digit32 = u32;
+
+define_bignum!(Big32x36: type=Digit32, n=36);
+
+// this one is used for testing only.
+#[doc(hidden)]
+pub mod tests {
+    use prelude::*;
+    define_bignum!(Big8x3: type=u8, n=3);
+}
+