/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

//! Utility functions for doubly-linked lists.

use std::collections::DList;
use std::mem;
use std::ptr;

struct RawDList<T> {
    length: uint,
    head: Option<Box<RawNode<T>>>,
    tail: *mut RawNode<T>,
}

#[allow(dead_code)]
struct RawNode<T> {
    next: Option<Box<RawNode<T>>>,
    prev: *mut RawNode<T>,
    value: T,
}

#[unsafe_destructor]
impl<T> Drop for RawDList<T> {
    fn drop(&mut self) {
        fail!("shouldn't happen")
    }
}

/// Workaround for a missing method on Rust's `DList` type. Splits the head off a list in O(1)
/// time.
pub fn split<T>(list: &mut DList<T>) -> DList<T> {
    let list = unsafe {
        mem::transmute::<&mut DList<T>,&mut RawDList<T>>(list)
    };

    if list.length == 0 {
        fail!("split_dlist(): empty list")
    }
    let mut head_node = mem::replace(&mut list.head, None);
    let head_node_ptr: *mut RawNode<T> = &mut **head_node.as_mut().unwrap();
    let mut head_list = RawDList {
        length: 1,
        head: head_node,
        tail: head_node_ptr,
    };
    debug_assert!(list.head.is_none());
    mem::swap(&mut head_list.head.as_mut().unwrap().next, &mut list.head);
    debug_assert!(head_list.head.as_mut().unwrap().next.is_none());
    debug_assert!(head_list.head.as_mut().unwrap().prev.is_null());
    head_list.head.as_mut().unwrap().prev = ptr::null_mut();

    list.length -= 1;
    if list.length == 0 {
        list.tail = ptr::null_mut()
    } else {
        if list.length == 1 {
            list.tail = &mut **list.head.as_mut().unwrap() as *mut RawNode<T>
        }
        list.head.as_mut().unwrap().prev = ptr::null_mut()
    }

    unsafe {
        mem::transmute::<RawDList<T>,DList<T>>(head_list)
    }
}