//! Buddy alloc,
//! This code heavily references from https://pdos.csail.mit.edu/6.828/2019/lec/malloc.c
//! Check wiki to learn the algorithm: https://en.wikipedia.org/wiki/Buddy_memory_allocation
//!
//! The idea to to initialize base..end memory to leaf size, then merge them up.

#![allow(clippy::needless_range_loop)]

/// the smallest allocation bytes
pub const LEAF_SIZE: usize = 16;

const OOM_MSG: &str = "requires more memory space to initialize BuddyAlloc";

pub const fn block_size(k: usize) -> usize {
    (1 << k) * LEAF_SIZE
}

const fn nblock(k: usize, entries_size: usize) -> usize {
    1 << (entries_size - k - 1)
}

const fn roundup(n: usize, sz: usize) -> usize {
    ((n - 1) / sz + 1) * sz
}

fn log2(mut n: usize) -> usize {
    let mut k = 0;
    while n > 1 {
        k += 1;
        n >>= 1;
    }
    k
}

fn bit_isset(bit_array: *const u8, i: usize) -> bool {
    unsafe {
        let b = bit_array.add(i / 8);
        let m = 1 << (i % 8);
        *b & m == m
    }
}

fn bit_set(bit_array: *mut u8, i: usize) {
    debug_assert!(!bit_isset(bit_array, i));
    unsafe {
        let b = bit_array.add(i / 8);
        let m = 1 << (i % 8);
        *b |= m;
    }
}

fn bit_clear(bit_array: *mut u8, i: usize) {
    debug_assert!(bit_isset(bit_array, i));
    unsafe {
        let b = bit_array.add(i / 8);
        let m = 1 << (i % 8);
        *b &= !m;
    }
}

// find a min k that great than n bytes
pub fn first_up_k(n: usize) -> usize {
    let mut k = 0;
    let mut size = LEAF_SIZE;
    while size < n {
        k += 1;
        size *= 2;
    }
    k
}

struct BuddyList {
    next: *mut BuddyList,
    prev: *mut BuddyList,
}

impl core::fmt::Debug for BuddyList {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        write!(f, "BuddyList {{ ")?;
        if Self::is_empty(self as *const BuddyList) {
            write!(f, "empty")?;
        } else {
            let mut count = 0;
            let mut p = self as *const BuddyList;
            while count == 0 || p != self as *const BuddyList {
                unsafe {
                    write!(
                        f,
                        "item({}, self: {:?}, next: {:?}, prev: {:?}) ",
                        count,
                        p,
                        (*p).next,
                        (*p).prev
                    )?;
                }
                count += 1;
                p = unsafe { (*p).next };

                if count > 10 {
                    write!(f, "items...")?;
                    break;
                }
            }
        }
        write!(f, " }}")
    }
}

impl BuddyList {
    fn init(list: *mut BuddyList) {
        unsafe {
            (*list).next = list;
            (*list).prev = list;
        }
    }

    fn remove(list: *mut BuddyList) {
        unsafe {
            (*(*list).prev).next = (*list).next;
            (*(*list).next).prev = (*list).prev;
        }
    }

    fn pop(list: *mut BuddyList) -> *mut BuddyList {
        assert!(!Self::is_empty(list));
        let n_list: *mut BuddyList = unsafe { (*list).next };
        Self::remove(n_list);
        n_list
    }

    fn push(list: *mut BuddyList, p: *mut u8) {
        let p = p.cast::<BuddyList>();
        unsafe {
            let n_list: BuddyList = BuddyList {
                prev: list,
                next: (*list).next,
            };
            p.write_unaligned(n_list);
            (*(*list).next).prev = p;
            (*list).next = p;
        }
    }

    fn is_empty(list: *const BuddyList) -> bool {
        unsafe { (*list).next as *const BuddyList == list }
    }
}

struct Entry {
    free: *mut BuddyList,
    /// Bit array to keep tracking alloc
    alloc: *mut u8,
    /// Bit array to keep tracking split
    split: *mut u8,
}

impl Default for Entry {
    fn default() -> Self {
        Entry {
            free: core::ptr::null_mut(),
            alloc: core::ptr::null_mut(),
            split: core::ptr::null_mut(),
        }
    }
}

pub struct BuddyAlloc {
    /// memory start addr
    base_addr: usize,
    /// memory end addr
    end_addr: usize,
    /// unavailable memories at end_addr
    unavailable: usize,
    entries: *mut Entry,
    entries_size: usize,
}

impl BuddyAlloc {
    /// # Safety
    ///
    /// The `base_addr..(base_addr + len)` must be allocated before using,
    /// and must guarantee no others write to the memory range, to avoid undefined behaviors.
    /// The new function panic if memory space not enough for initialize BuddyAlloc.
    pub unsafe fn new(base_addr: *const u8, len: usize) -> Self {
        let mut base_addr = base_addr as usize;
        let end_addr = base_addr + len;
        base_addr = roundup(base_addr, LEAF_SIZE);
        // we use (k + 1)-th entry's split flag to test existence of k-th entry's blocks;
        // to accoding this convention, we make a dummy (entries_size - 1)-th entry.
        // so we plus 2 on entries_size.
        let entries_size = log2((end_addr - base_addr) / LEAF_SIZE) + 2;

        // alloc buddy allocator memory
        let used_bytes = core::mem::size_of::<Entry>() * entries_size;
        assert!(end_addr >= base_addr + used_bytes, OOM_MSG);
        let entries = base_addr as *mut Entry;
        base_addr += used_bytes;

        // init entries free
        for k in 0..entries_size {
            // use one bit for per memory block
            let used_bytes = core::mem::size_of::<BuddyList>();
            assert!(end_addr >= base_addr + used_bytes, OOM_MSG);
            let entry = entries.add(k).as_mut().expect("entry");
            entry.free = base_addr as *mut BuddyList;
            core::ptr::write_bytes(entry.free, 0, used_bytes);
            BuddyList::init(entry.free);
            base_addr += used_bytes;
        }

        // init alloc
        for k in 0..entries_size {
            // use one bit for per memory block
            let used_bytes = roundup(nblock(k, entries_size), 8) / 8;
            assert!(end_addr >= base_addr + used_bytes, OOM_MSG);
            let entry = entries.add(k).as_mut().expect("entry");
            entry.alloc = base_addr as *mut u8;
            // mark all blocks as allocated
            core::ptr::write_bytes(entry.alloc, 0, used_bytes);
            base_addr += used_bytes;
        }

        // init split
        for k in 1..entries_size {
            // use one bit for per memory block
            let used_bytes = roundup(nblock(k, entries_size), 8) / 8;
            assert!(end_addr >= base_addr + used_bytes, OOM_MSG);
            let entry = entries.add(k).as_mut().expect("entry");
            entry.split = base_addr as *mut u8;
            core::ptr::write_bytes(entry.split, 0, used_bytes);
            base_addr += used_bytes;
        }

        assert!(end_addr >= base_addr, OOM_MSG);

        let mut allocator = BuddyAlloc {
            base_addr,
            end_addr,
            entries,
            entries_size,
            unavailable: 0,
        };
        allocator.init_free_list();
        allocator
    }

    fn init_free_list(&mut self) {
        let mut base_addr = self.base_addr;
        let end_addr = self.end_addr;
        let entries_size = self.entries_size;

        // try alloc blocks
        for k in (0..(entries_size - 1)).rev() {
            let block_size = block_size(k);
            let entry = self.entry(k);
            let parent_entry = self.entry(k + 1);

            // alloc free blocks
            while base_addr + block_size <= end_addr {
                debug_assert!(!bit_isset(
                    entry.alloc,
                    self.block_index(k, base_addr as *const u8)
                ));
                BuddyList::push(entry.free, base_addr as *mut u8);
                // mark parent's split and alloc
                let block_index = self.block_index(k, base_addr as *const u8);
                if block_index % 2 == 0 {
                    let parent_index = self.block_index(k + 1, base_addr as *const u8);
                    if !bit_isset(parent_entry.alloc, parent_index) {
                        bit_set(parent_entry.alloc, parent_index);
                    }
                    bit_set(parent_entry.split, parent_index);
                }
                base_addr += block_size;
            }

            // mark unavailable blocks as allocated
            let n = nblock(k, entries_size);
            let unavailable_block_index = self.block_index(k, base_addr as *const u8);
            assert!(unavailable_block_index < n);
            bit_set(entry.alloc, unavailable_block_index);
        }

        self.unavailable = end_addr - base_addr;
    }

    pub fn malloc(&mut self, nbytes: usize) -> *mut u8 {
        let fk = first_up_k(nbytes);
        let mut k =
            match (fk..self.entries_size).find(|&k| !BuddyList::is_empty(self.entry(k).free)) {
                Some(k) => k,
                None => return core::ptr::null_mut(),
            };
        let p: *mut u8 = BuddyList::pop(self.entry(k).free) as *mut u8;
        bit_set(self.entry(k).alloc, self.block_index(k, p));
        while k > fk {
            let q: *mut u8 = (p as usize + block_size(k - 1)) as *mut u8;
            bit_set(self.entry(k).split, self.block_index(k, p));
            bit_set(self.entry(k - 1).alloc, self.block_index(k - 1, p));
            debug_assert!(!bit_isset(
                self.entry(k - 1).alloc,
                self.block_index(k - 1, q)
            ));
            BuddyList::push(self.entry(k - 1).free, q);
            k -= 1;
        }
        p
    }

    pub fn free(&mut self, mut p: *mut u8) {
        let mut k = self.block_k(p);
        while k < (self.entries_size - 1) {
            let block_index = self.block_index(k, p);
            bit_clear(self.entry(k).alloc, block_index);
            let buddy = if block_index % 2 == 0 {
                block_index + 1
            } else {
                block_index - 1
            };
            if bit_isset(self.entry(k).alloc, buddy) {
                break;
            }
            // merge buddy since its free
            // 1. clear split of k + 1
            // 2. set p to the address of merged block
            // 3. repeat for k = k + 1 until reach MAX_K
            // 4. push p back to k entry free list
            let q = self.block_addr(k, buddy);
            BuddyList::remove(q as *mut BuddyList);
            if buddy % 2 == 0 {
                p = q as *mut u8;
            }
            bit_clear(self.entry(k + 1).split, self.block_index(k + 1, p));
            k += 1;
        }
        debug_assert!(!bit_isset(self.entry(k).alloc, self.block_index(k, p)));
        BuddyList::push(self.entry(k).free, p);
    }

    /// available bytes
    pub fn available_bytes(&self) -> usize {
        self.end_addr - self.unavailable - self.base_addr
    }

    fn entry(&self, i: usize) -> &Entry {
        if i >= self.entries_size {
            panic!(
                "index out of range, len: {} index: {}",
                self.entries_size, i
            );
        }
        unsafe { self.entries.add(i).as_ref().expect("entry") }
    }

    /// find k of p
    fn block_k(&self, p: *const u8) -> usize {
        for k in 0..(self.entries_size - 1) {
            if bit_isset(self.entry(k + 1).split, self.block_index(k + 1, p)) {
                debug_assert!(bit_isset(self.entry(k).alloc, self.block_index(k, p)));
                return k;
            }
        }
        0
    }

    /// block index of p under k
    fn block_index(&self, k: usize, p: *const u8) -> usize {
        let n = p as usize - self.base_addr;
        let index = n / block_size(k);
        debug_assert!(index < nblock(k, self.entries_size));
        index
    }

    /// block addr of index under k
    fn block_addr(&self, k: usize, i: usize) -> usize {
        let n = i * block_size(k);
        self.base_addr + n
    }
}