use crate::block_device::{BlockDevice, BLOCK_SIZE};
use crate::disk::block::Block;
use crate::AyaFS;
use lru::LruCache;
use std::num::NonZeroUsize;
use std::sync::Arc;

#[derive(Clone)]
pub struct CachedBlock<T: Block> {
    pub block: T,
    pub index: usize,
    pub dirty: bool,
}

pub fn convert_mut<U: Block, T: Block>(input_block: &mut CachedBlock<U>) -> &mut CachedBlock<T> {
    let ptr = input_block as *const CachedBlock<U> as *mut u8;
    let block = ptr.cast::<CachedBlock<T>>();
    unsafe { &mut *block }
}

pub fn convert<U: Block, T: Block>(input_block: &CachedBlock<U>) -> &CachedBlock<T> {
    let ptr = input_block as *const CachedBlock<U> as *mut u8;
    let block = ptr.cast::<CachedBlock<T>>();
    unsafe { &*block }
}

pub(crate) struct BlockCache<T: Block> {
    device: Arc<dyn BlockDevice>,
    cache: LruCache<usize, CachedBlock<T>>,
}

impl<T: Block> BlockCache<T> {
    pub(crate) fn new(device: Arc<dyn BlockDevice>, cache_size: usize) -> Self {
        Self {
            device,
            cache: LruCache::new(NonZeroUsize::new(cache_size).unwrap()),
        }
    }

    pub(crate) fn load_block(&mut self, index: usize) -> bool {
        if self.cache.contains(&index) == false {
            let mut buffer = [0u8; BLOCK_SIZE];
            self.device.read(index, &mut buffer);
            let block: T = unsafe { std::mem::transmute_copy(&buffer) };
            let cached_block = CachedBlock {
                block,
                index,
                dirty: false,
            };
            if let Some((old_index, old_block)) = self.cache.push(index, cached_block) {
                assert_ne!(old_index, index); // 只有 block 不在 cache 里的时候才会插入
                if old_block.dirty {
                    let old_block_ptr = &old_block.block as *const T as *mut u8;
                    let old_block_buffer =
                        unsafe { std::slice::from_raw_parts(old_block_ptr, BLOCK_SIZE) };
                    self.device.write(old_index, old_block_buffer);
                }
            }
        }
        true
    }

    /// 从 LRU cache 里获取一个 block 的引用, 如果没有在 cache 中会加载.
    /// 这个函数不应该返回 None
    pub(crate) fn get_block<U: Block>(&mut self, index: usize) -> Option<&CachedBlock<U>> {
        if !self.cache.contains(&index) {
            self.load_block(index);
        }
        self.cache.get(&index).map(convert::<T, U>)
    }

    /// 从 LRU cache 里获取一个 block 的可变引用, 如果没有在 cache 中会加载.
    /// 这个函数不应该返回 None
    pub(crate) fn get_block_mut<U: Block>(&mut self, index: usize) -> Option<&mut CachedBlock<U>> {
        if !self.cache.contains(&index) {
            self.load_block(index);
        }
        self.cache.get_mut(&index).map(|block| {
            block.dirty = true;
            convert_mut::<T, U>(block)
        })
    }

    /// 向 LRU cache 中插入一个全新初始化的 block
    /// 这个 block 全 0, 而且是 dirty 的, 即使被挤出去也会触发一次落盘
    pub(crate) fn init_block(&mut self, index: usize) {
        let allocated_block = CachedBlock {
            block: T::default(),
            index,
            dirty: true,
        };
        if let Some((old_index, old_block)) = self.cache.push(index, allocated_block) {
            if old_block.dirty {
                let old_block_ptr = &old_block.block as *const T as *mut u8;
                let old_block_buffer =
                    unsafe { std::slice::from_raw_parts(old_block_ptr, BLOCK_SIZE) };
                self.device.write(old_index, old_block_buffer);
            }
        }
    }

    #[allow(unused)]
    /// 从 LRU cache 中读取一个 block 的引用, *不会* 影响 LRU cache 的结构, 如果没有在 cache 中不会加载.
    pub(crate) fn peek_block<U: Block>(&self, index: usize) -> Option<&CachedBlock<U>> {
        self.cache.peek(&index).map(convert::<T, U>)
    }

    #[allow(unused)]
    /// 从 LRU cache 中读取一个 block 的可变引用, *不会* 影响 LRU cache 的结构, 如果没有在 cache 中不会加载.
    pub(crate) fn peek_block_mut<U: Block>(&mut self, index: usize) -> Option<&mut CachedBlock<U>> {
        self.cache.peek_mut(&index).map(convert_mut::<T, U>)
    }

    pub(crate) fn update_block<U: Block>(&mut self, block: CachedBlock<U>) -> bool {
        if self.cache.contains(&block.index) {
            let mut data_block = convert::<U, T>(&block).clone();
            data_block.dirty = true; // TODO 需要把显式写回的都标记为 dirty 吗
            self.cache.push(block.index, data_block);
            true
        } else {
            false
        }
    }

    fn pop(&mut self, entry: &usize) -> Option<CachedBlock<T>> {
        self.cache.pop(entry)
    }
}

impl AyaFS {
    pub(crate) fn init_block(&mut self, index: usize) {
        self.cached_blocks.init_block(index);
    }

    pub(crate) fn get_block<T: Block>(&mut self, index: usize) -> Option<&CachedBlock<T>> {
        if self.data_bitmap.query(index) {
            Some(self.cached_blocks.get_block::<T>(index).unwrap())
        } else {
            self.cached_blocks.pop(&index);
            None
        }
        // self.data_bitmap
        //     .query(index)
        //     .then(|| self.cached_blocks.get_block::<T>(index).unwrap())
        // 返回 None 当且仅当 data_bitmap 中这个 block 为 invalid
    }

    pub(crate) fn get_block_mut<T: Block>(&mut self, index: usize) -> Option<&mut CachedBlock<T>> {
        if self.data_bitmap.query(index) {
            Some(self.cached_blocks.get_block_mut::<T>(index).unwrap())
        } else {
            self.cached_blocks.pop(&index);
            None
        }
        // self.data_bitmap
        //     .query(index)
        //     .then(|| self.cached_blocks.get_block_mut::<T>(index).unwrap())
        // 返回 None 当且仅当 data_bitmap 中这个 block 为 invalid
    }

    #[allow(unused)]
    pub(crate) fn peek_block<T: Block>(&self, index: usize) -> Option<&CachedBlock<T>> {
        self.data_bitmap
            .query(index)
            .then(|| self.cached_blocks.peek_block::<T>(index).unwrap())
        // 返回 None 当且仅当 data_bitmap 中这个 block 为 invalid
    }

    #[allow(unused)]
    pub(crate) fn peek_block_mut<T: Block>(&mut self, index: usize) -> Option<&mut CachedBlock<T>> {
        self.data_bitmap
            .query(index)
            .then(|| self.cached_blocks.peek_block_mut::<T>(index).unwrap())
        // 返回 None 当且仅当 data_bitmap 中这个 block 为 invalid
    }

    pub(crate) fn update_block<T: Block>(&mut self, block: CachedBlock<T>) -> bool {
        self.cached_blocks.update_block(block)
    }
}