/// Wrapper around Vec which supports fast indexing and rotation
///
/// The rotation implemented by grid::Storage is a simple integer addition.
/// Compare with standard library rotation which requires rearranging items in
/// memory.
///
/// As a consequence, the indexing operators need to be reimplemented for this
/// type to account for the 0th element not always being at the start of the
/// allocation.
///
/// Because certain Vec operations are no longer valid on this type, no Deref
/// implementation is provided. Anything from Vec that should be exposed must be
/// done so manually.
use std::ops::{Index, IndexMut};

#[derive(Clone, Debug, Deserialize, Serialize, Eq, PartialEq)]
pub struct Storage<T> {
    inner: Vec<T>,
    zero: usize,
}

impl<T> Storage<T> {
    #[inline]
    pub fn with_capacity(cap: usize) -> Storage<T> {
        Storage {
            inner: Vec::with_capacity(cap),
            zero: 0
        }
    }

    #[inline]
    pub fn push(&mut self, item: T) {
        self.inner.push(item)
    }

    #[inline]
    pub fn pop(&mut self) -> Option<T> {
        self.inner.pop()
    }

    #[inline]
    pub fn len(&self) -> usize {
        self.inner.len()
    }

    /// Compute actual index in underlying storage given the requested index.
    #[inline]
    fn compute_index(&self, requested: usize) -> usize {
        (requested + self.zero) % self.len()
    }

    pub fn swap(&mut self, a: usize, b: usize) {
        let a = self.compute_index(a);
        let b = self.compute_index(b);
        self.inner.swap(a, b);
    }

    pub fn iter(&self) -> Iter<T> {
        Iter { storage: self, index: 0 }
    }

    pub fn iter_mut(&mut self) -> IterMut<T> {
        IterMut { storage: self, index: 0 }
    }

    pub fn rotate(&mut self, count: isize) {
        let len = self.len();
        assert!(count.abs() as usize <= len);
        self.zero += (count + len as isize) as usize % len;
    }
}

impl<T> Index<usize> for Storage<T> {
    type Output = T;
    #[inline]
    fn index(&self, index: usize) -> &T {
        let index = self.compute_index(index); // borrowck
        &self.inner[index]
    }
}

impl<T> IndexMut<usize> for Storage<T> {
    #[inline]
    fn index_mut(&mut self, index: usize) -> &mut T {
        let index = self.compute_index(index); // borrowck
        &mut self.inner[index]
    }
}

pub struct Iter<'a, T: 'a> {
    storage: &'a Storage<T>,
    index: usize,
}

pub struct IterMut<'a, T: 'a> {
    storage: &'a mut Storage<T>,
    index: usize,
}

impl<'a, T: 'a> Iterator for Iter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        if self.index == self.storage.len() {
            None
        } else {
            let res = Some(&self.storage[self.index]);
            self.index += 1;
            res
        }
    }
}

impl<'a, T: 'a> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;

    fn next(&mut self) -> Option<Self::Item> {
        if self.index == self.storage.len() {
            None
        } else {
            let index = self.index;
            self.index += 1;

            unsafe {
                Some(&mut *(&mut self.storage[index] as *mut _))
            }
        }
    }
}