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#![feature(alloc)]
#![feature(heap_api)]
#![feature(thread_local)]
#![feature(const_fn)]
#![no_std]

/**
# Jenga: A stack based allocator.

*/

extern crate alloc;

use core::cell::Cell;
use core::{mem, slice, ptr};

#[thread_local]
static STASH: Stash = Stash {
    top:    Cell::new(0),
    cap:    Cell::new(0)
};
const  STASH_SIZE:   usize = 1 << 20;

struct Stash {
    top:    Cell<usize>,
    cap:    Cell<usize>
}
unsafe impl Sync for Stash {} // it isn't sync! don't ever expose this type
impl Stash {
    #[inline(always)]
    fn top(&self) -> usize {
        let top = self.top.get();
        if top != 0 {
            top
        } else {
            self.init();
            self.top.get()
        }
    }
    #[inline(always)]
    fn set_top(&self, top: usize) {
        self.top.set(top);
    }
    
    fn init_size(&self, size: usize) {
        use alloc;
        
        assert_eq!(STASH.top.get(), 0);
        let base = unsafe { alloc::heap::allocate(size, 64) } as usize;
        self.top.set(base);
        self.cap.set(base + size);
    }
    #[cold]
    fn init(&self) {
        self.init_size(STASH_SIZE);
    }
}

#[derive(Debug)]
pub enum InitError {
    AlreadyInitialized
}

/// reserve `size` bytes of memory for the allocator
/// This will fail if it was already initialized
pub fn init_with_capacity(size: usize) -> Result<(), InitError> {
    if STASH.top.get() != 0 {
        Ok(STASH.init_size(size))
    } else {
        Err(InitError::AlreadyInitialized)
    }
}

#[derive(Debug)]
pub enum PlaceError {
    OutOfMemory
}


#[inline(always)]
fn padding_for<T>(ptr: usize) -> usize {
    let align = mem::align_of::<T>();
    align - (ptr - 1) % align - 1
}

/// this function will run the provided closure with pointer to uninitialized memory.
/// drop is not called afterwards.
#[inline(always)]
pub unsafe fn place_raw<F, O, T>(count: usize, f: F) -> Result<O, PlaceError>
    where F: FnOnce(*mut T) -> O
{
    let old_top = STASH.top();
    
    let size = count * mem::size_of::<T>();
    let pad = padding_for::<T>(old_top);
    let ptr = old_top + pad;
    let new_top = ptr + size;
    
    if new_top < STASH.cap.get() {
        STASH.set_top(new_top);
        
        let r = f(ptr as *mut T);
        
        STASH.set_top(old_top);
        Ok(r)
    } else {
        Err(PlaceError::OutOfMemory)
    }
}

/// allocate `count` elements of T
#[inline]
pub fn place<F, O, T>(count: usize, f: F) -> Result<O, PlaceError>
    where F: FnOnce(&mut [T]) -> O, T: Default
{   
    unsafe {
        place_raw(count, |ptr| {
            // initialize memory
            for i in 0 .. count {
                ptr::write(ptr.offset(i as isize), T::default());
            }
            let slice = slice::from_raw_parts_mut(ptr, count);
            
            let r = f(slice);
        
            // drop elements
            for i in 0 .. count {
                ptr::drop_in_place(ptr.offset(i as isize));
            }
            r
        })
    }
}

/// allocate `count` elements of T
#[inline]
pub fn place_iter<I, F, T, O>(mut iter: I, f: F) -> Result<O, PlaceError>
    where I: Iterator<Item=T>, F: FnOnce(&mut [I::Item]) -> O
{
    
    let old_top = STASH.top();
    
    let cap = STASH.cap.get();
    let pad = padding_for::<T>(old_top);
    let ptr_base = old_top + pad;
    let mut p = ptr_base;
    let mut len = 0;
    
    // drain the iterator
    while let Some(e) = iter.next() {
        if p + mem::size_of::<T>() > cap {
            // abort
            while p > ptr_base {
                p -= mem::size_of::<T>();
                unsafe { ptr::drop_in_place(p as *mut T) };
            }
            #[cold]
            return Err(PlaceError::OutOfMemory);
        }
        unsafe {
            ptr::write(p as *mut T, e);
        }
        p += mem::size_of::<T>();
        len += 1;
        
        // update top: next() might use the allocator too!
        STASH.set_top(p as usize);
    }
    
    let slice = unsafe {
        slice::from_raw_parts_mut(p as *mut T, len)
    };
    let r = f(slice);
    
    // drop elements
    while p > ptr_base {
        p -= mem::size_of::<T>();
        unsafe {
            ptr::drop_in_place(p as *mut T);
        }
    }
    
    STASH.set_top(old_top);
    
    Ok(r)
}