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    // Copyright 2015 The Ramp Developers
    //
    //    Licensed under the Apache License, Version 2.0 (the "License");
    //    you may not use this file except in compliance with the License.
    //    You may obtain a copy of the License at
    //
    //        http://www.apache.org/licenses/LICENSE-2.0
    //
    //    Unless required by applicable law or agreed to in writing, software
    //    distributed under the License is distributed on an "AS IS" BASIS,
    //    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    //    See the License for the specific language governing permissions and
    //    limitations under the License.

    use ll::limb::Limb;

    use std::{fmt, ops};
    #[cfg(debug_assertions)]
    use std::mem;
    use std::cmp::Ordering;

    /// A version of `*const Limb` that is bounds-checked when debug assertions are on
    #[derive(Copy, Clone, Debug)]
    pub struct Limbs {
        ptr: *const Limb,
        bounds: Bounds,
    }

    /// A version of `*mut Limb` that is bounds-checked when debug assertions are on
    #[derive(Copy, Clone)]
    pub struct LimbsMut {
        ptr: *mut Limb,
        bounds: Bounds,
    }

    macro_rules! api {
        ($ty: ident, $ptr: ty) => {
            impl $ty {
                /// Create a new instance, pointing at `base` and valid
                /// from `base.offset(start)` to `base.offset(end)`.
                pub unsafe fn new(base: $ptr, start: i32, end: i32) -> $ty {
                    $ty {
                        ptr: base,
                        bounds: Bounds::new(base as usize, start, end)
                    }
                }

                /// Move `self` to point to the `x`th Limbs from the
                /// current location.
                #[inline]
                pub unsafe fn offset(self, x: isize) -> $ty {
                    debug_assert!(self.bounds.offset_valid(self.ptr as usize, x),
                                  "invalid offset of {:?} by {}, which should be in {:?}", self.ptr, x, self.bounds);
                    $ty {
                        ptr: self.ptr.offset(x),
                        bounds: self.bounds,
                    }
                }
            }

            impl PartialEq for $ty {
                fn eq(&self, other: &$ty) -> bool {
                    self.ptr == other.ptr
                }
            }
            impl PartialOrd for $ty {
                fn partial_cmp(&self, other: &$ty) -> Option<Ordering> {
                    self.ptr.partial_cmp(&other.ptr)
                }
            }
            impl Eq for $ty {}
            impl Ord for $ty {
                fn cmp(&self, other: &$ty) -> Ordering {
                    self.ptr.cmp(&other.ptr)
                }
            }

            impl ops::Deref for $ty {
                type Target = Limb;
                fn deref(&self) -> &Limb {
                    debug_assert!(self.bounds.can_deref(self.ptr as usize),
                                  "invalid deref of {:?}, which should be in {:?}", self.ptr, self.bounds);
                    unsafe { &*self.ptr }
                }
            }
        }
    }

    api!(Limbs, *const Limb);
    api!(LimbsMut, *mut Limb);
    impl LimbsMut {
        /// View the `LimbsMut` as a `Limbs` (an explicit `*const
        /// Limb` -> `*mut Limb` conversion)
        pub fn as_const(self) -> Limbs {
            Limbs {
                ptr: self.ptr,
                bounds: self.bounds,
            }
        }
    }
    impl ops::DerefMut for LimbsMut {
        fn deref_mut(&mut self) -> &mut Limb {
            debug_assert!(self.bounds.can_deref(self.ptr as usize),
                          "invalid mut deref of {:?}, which should be in {:?}", self.ptr, self.bounds);
            unsafe { &mut *self.ptr }
        }
    }

    // This is where the magic is at: the bounds are only stored/checked
    // in debug mode; release mode just powers ahead without any overhead.

    #[derive(Copy, Clone)]
    #[cfg(debug_assertions)]
    struct Bounds {
        lo: usize,
        hi: usize,
    }
    #[derive(Copy, Clone)]
    #[cfg(not(debug_assertions))]
    struct Bounds;

    #[cfg(debug_assertions)]
    impl Bounds {
        fn new(ptr: usize, start: i32, end: i32) -> Bounds {
            assert!(start <= end);
            Bounds {
                lo: ptr + start as usize * mem::size_of::<Limb>(),
                hi: ptr + end as usize * mem::size_of::<Limb>(),
            }
        }
        fn can_deref(self, ptr: usize) -> bool {
            // a deref can't deref when we're at the limit
            self.lo <= ptr && ptr < self.hi
        }
        fn offset_valid(self, ptr: usize, offset: isize) -> bool {
            let bytes = offset * mem::size_of::<Limb>() as isize;
            let new = ptr.wrapping_add(bytes as usize);
            // an offset can point to the limit (i.e. one byte past the end)
            self.lo <= new && new <= self.hi
        }
    }
    #[cfg(not(debug_assertions))]
    impl Bounds {
        fn new(_ptr: usize, _start: i32, _end: i32) -> Bounds { Bounds }
        #[inline]
        fn can_deref(self, _ptr: usize) -> bool { true }
        #[inline]
        fn offset_valid(self, _ptr: usize, _offset: isize) -> bool { true }
    }
    impl fmt::Debug for Bounds {
        #[cfg(debug_assertions)]
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Bounds {{ lo: 0x{:x}, hi: 0x{:x} }}", self.lo, self.hi)
        }
        #[cfg(not(debug_assertions))]
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Bounds {{ <optimised out> }}")
        }
    }

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