//! Structures for interpreting slices of variable length as arrays.
//!
//! `nslice` provides `MinSlice` and `ExactSlice` for representing slices known to have
//! either exactly or at least some compile-time-known number of values.
//! ```
//! use nslice::MinSlice;
//! let slice = &[1, 2, 3, 4, 5, 6];
//! let minslice: &MinSlice<_, 3> = MinSlice::from_slice(slice).unwrap();
//! assert_eq!(minslice.tail.len(), 3);
//! assert_eq!(minslice.head[0], 1);
//! assert_eq!(minslice.tail[2], 6);
//! ```
use std::slice;

/// A reference to a region of memory which is known to contain `N` or more elements
/// of type `T`.
///
/// Much like `[T]` itself, it is not possible to construct an owned `MinSlice`.
/// `MinSlice` is merely a way of reinterpreting an existing slice
/// (`&[T]` or `&mut [T]`), and it is exactly the same size as a slice:
/// one pointer and one `usize`.
pub struct MinSlice<T, const N: usize> {
    /// The bounded region of memory. Exactly `N` `T`s.
    pub head: [T; N],
    /// Zero or more remaining `T`s after the `N` in the bounded region.
    pub tail: [T],
}

/// A reference to a region of memory which contains exactly `N` elements of type `T`.
///
/// `ExactSlice` is merely a way of reinterpreting an existing slice
/// (`&[T]` or `&mut [T]`), but because there is no need to store a length
/// for retrieval at runtime, its representation consists of just one pointer.

pub struct ExactSlice<T, const N: usize> {
    /// The bounded region of memory. Exactly `N` `T`s.
    head: [T; N]
}

impl<T, const N: usize> MinSlice<T, N> {
    /// Produce a `&MinSlice` from a slice of `T`s.
    /// Returns `None` if there are not enough elements in `slice`.
    pub fn from_slice(slice: &[T]) -> Option<&MinSlice<T, N>> {
        if slice.len() >= N {
            Some(unsafe { Self::from_slice_unchecked(slice) })
        } else {
            None
        }
    }

    /// Produce a `&mut MinSlice` from a mutable slice of `T`s.
    /// Returns `None` if there are not enough elements in `slice`.
    pub fn from_mut(slice: &mut [T]) -> Option<&mut MinSlice<T, N>> {
        if slice.len() >= N {
            Some(unsafe { Self::from_mut_unchecked(slice) })
        } else {
            None
        }
    }

    /// Produce a `&MinSlice` from a slice of `T`s without checking its length.
    ///
    /// # Safety
    ///
    /// The caller is responsible for upholding the length invariant
    /// `slice.len() >= N`, in addition to all normal slice invariants.
    pub unsafe fn from_slice_unchecked(slice: &[T]) -> &MinSlice<T, N> {
        let resized = slice::from_raw_parts(slice.as_ptr(), slice.len() - N);
        &*(resized as *const [T] as *const MinSlice<T, N>)
    }

    /// Produce a `&mut MinSlice` from a slice of `T`s without checking its length.
    ///
    /// # Safety
    ///
    /// The caller is responsible for upholding the length invariant
    /// `slice.len() >= N`, in addition to all normal slice invariants.
    pub unsafe fn from_mut_unchecked(slice: &mut [T]) -> &mut MinSlice<T, N> {
        let resized = slice::from_raw_parts_mut(slice.as_mut_ptr(), slice.len() - N);
        &mut *(resized as *mut [T] as *mut MinSlice<T, N>)
    }
}

impl<T, const N: usize> ExactSlice<T, N> {
    /// Produce an `&ExactSlice` from a slice of `T`s.
    /// Returns `None` if there are not the correct number of elements in `slice`.
    pub fn from_slice(slice: &[T]) -> Option<&ExactSlice<T, N>> {
        if slice.len() == N {
            Some(unsafe { Self::from_slice_unchecked(slice) })
        } else {
            None
        }
    }

    /// Produce an `&mut ExactSlice` from a mutable slice of `T`s.
    /// Returns `None` if there are not the correct number of elements in `slice`.
    pub fn from_mut(slice: &mut [T]) -> Option<&mut ExactSlice<T, N>> {
        if slice.len() == N {
            Some(unsafe { Self::from_mut_unchecked(slice) })
        } else {
            None
        }
    }

    /// Produce a `&MinSlice` from this `&ExactSlice`. Its `tail` will be empty.
    pub fn as_min_slice(&self) -> Option<&MinSlice<T, N>> {
        MinSlice::from_slice(&self.head[..])
    }

    /// Produce a `&mut MinSlice` from this `&min ExactSlice`. Its `tail` will be empty.
    pub fn as_mut_min_slice(&mut self) -> Option<&mut MinSlice<T, N>> {
        MinSlice::from_mut(&mut self.head[..])
    }

    /// Produce an `&ExactSlice` from a slice of `T`s without checking its length.
    ///
    /// # Safety
    ///
    /// The caller is responsible for upholding the length invariant
    /// `slice.len() == N`, in addition to all normal slice invariants.
    pub unsafe fn from_slice_unchecked(slice: &[T]) -> &ExactSlice<T, N> {
        &*(slice.as_ptr() as *const ExactSlice<T, N>)
    }

    /// Produce an `&mut ExactSlice` from a slice of `T`s without checking its length.
    ///
    /// # Safety
    ///
    /// The caller is responsible for upholding the length invariant
    /// `slice.len() == N`, in addition to all normal slice invariants.
    pub unsafe fn from_mut_unchecked(slice: &mut [T]) -> &mut ExactSlice<T, N> {
        &mut *(slice.as_mut_ptr() as *mut ExactSlice<T, N>)
    }
}

#[test]
fn basic_min_success() {
    let slice = &[1, 2, 3, 4, 5, 6];
    let minslice: &MinSlice<_, 3> = MinSlice::from_slice(slice).unwrap();
    assert_eq!(minslice.tail.len(), 3);
    assert_eq!(minslice.head[0], 1);
    assert_eq!(minslice.tail[2], 6);
}


#[test]
fn basic_min_failure() {
    let slice = &[1, 2, 3, 4, 5, 6];
    let minslice: Option<&MinSlice<_, 7>> = MinSlice::from_slice(slice);
    assert!(minslice.is_none());
}