use tokio::io::{AsyncReadExt, AsyncWriteExt, AsyncSeekExt, SeekFrom};
use tokio::fs::{File, OpenOptions};
use std::path::Path;
use std::marker::PhantomData;

use async_trait::async_trait;

use bincode;
use bincode::{Decode, Encode};
use crate::binary_coding::encode;

use crate::entry::{Entry, EntryDetailed};
use crate::entry_header::EntryHeaderWithDataSize;
use crate::store_header::StoreHeader;
use crate::storage_engine::{Store, FilePosition, Result, ROWS_FILE_NAME};

#[async_trait]
trait PrimitiveCursor<T> {
    fn file(&mut self) -> &mut File;
    fn eof_file_position(&self) -> FilePosition;

    async fn read_bytes(&mut self, bytes: &mut [u8]) -> Result<()> {
        self.file().read_exact(bytes).await?;
        Ok(())
    }

    async fn get_bytes(&mut self, count: usize) -> Result<Vec<u8>> {
        let mut result: Vec<u8> = Vec::with_capacity(count);
        self.read_bytes(&mut result).await?;
        Ok(result)
    }

    async fn seek_to(&mut self, file_position: FilePosition) -> Result<()> {
        self.file().seek(SeekFrom::Start(file_position)).await?;
        Ok(())
    }

    async fn seek_to_start(&mut self) -> Result<()> {
        self.file().seek(SeekFrom::Start(0)).await?;
        Ok(())
    }

    async fn seek_to_end(&mut self) -> Result<()> {
        self.file().seek(SeekFrom::End(0)).await?;
        Ok(())
    }

    async fn seek_to_start_of_data(&mut self) -> Result<()> {
        self.seek_to(StoreHeader::SIZE as u64).await
    }

    async fn current_file_position(&mut self) -> Result<FilePosition> {
        let next_file_position: FilePosition = self.file().stream_position().await?;
        Ok(next_file_position)
    }

    async fn is_at_eof(&mut self) -> Result<bool> {
        Ok(self.current_file_position().await? == self.eof_file_position())
    }
}

#[async_trait]
pub trait CursorWithStoreHeader<T>: PrimitiveCursor<T> {
    fn header(&self) -> &StoreHeader;

    async fn read_entry_header(&mut self) -> Result<EntryHeaderWithDataSize> {
        let number_of_columns: usize = self.header().number_of_columns;
        let mut header_bytes: Vec<u8> = vec![0; EntryHeaderWithDataSize::size(number_of_columns)];
        self.read_bytes(&mut header_bytes).await?;
        let header = EntryHeaderWithDataSize::decode(&mut header_bytes[..], number_of_columns)?;

        Ok(header)
    }

    async fn read_entry_header_at(&mut self, file_position: FilePosition) -> Result<EntryHeaderWithDataSize>  {
        self.seek_to(file_position).await?;
        self.read_entry_header().await
    }

    // Returns None when file_position == eof_file_position
    async fn read_entry_at(&mut self, file_position: FilePosition) -> Result<Option<EntryDetailed<T>>> 
        where T: Decode
    {
        self.seek_to(file_position).await?;
        self.next().await
    }

    // ===Iteration===
    // Assumes that the current file position is at a valid entry or EOF.
    async fn next(&mut self) -> Result<Option<EntryDetailed<T>>> 
        where T: Decode
    {
        if self.is_at_eof().await? {
            return Ok(None)
        }

        let entry_header = self.read_entry_header().await?;

        let mut data_bytes: Vec<u8> = vec![0; entry_header.size_of_data()];
        self.read_bytes(&mut data_bytes).await?;
        let entry: EntryDetailed<T> =
            EntryDetailed::decode(entry_header, self.header().number_of_columns, &mut data_bytes)?;

        Ok(Some(entry))
    }

    // ===Debugging===
    async fn read_entries(&mut self) -> Result<()> 
        where T: Decode + std::fmt::Debug
    {
        self.seek_to_start_of_data().await?;
        while let Some(entry) = self.next().await? {
            println!("{:?}", entry);
        }
        println!("END of entries.");
        Ok(())
    }

    async fn read_all_bytes(&mut self) -> std::result::Result<Vec<u8>, std::io::Error> {
        let mut bytes: Vec<u8> = vec![];
        self.file().read_to_end(&mut bytes).await?;
        Ok(bytes)
    }
}


// ===Concrete Cursors===
pub struct ReadCursor<T> {
    header: StoreHeader,
    file: File,
    data_type: PhantomData<T>,

    eof_file_position: FilePosition,
}

pub struct WriteCursor<'a, T> {
    header: &'a mut StoreHeader,
    file: File,
    data_type: PhantomData<T>,

    eof_file_position: FilePosition,
}

// ===PrimitiveCursor===
impl <T>PrimitiveCursor<T> for ReadCursor<T> {
    fn file(&mut self) -> &mut File {
        &mut self.file
    }

    fn eof_file_position(&self) -> FilePosition {
        self.eof_file_position
    }
}

impl <T>PrimitiveCursor<T> for WriteCursor<'_, T> {
    fn file(&mut self) -> &mut File {
        &mut self.file
    }

    fn eof_file_position(&self) -> FilePosition {
        self.eof_file_position
    }
}

// ===CursorWithStoreHeader===
impl <T>CursorWithStoreHeader<T> for ReadCursor<T> {
    fn header(&self) -> &StoreHeader {
        &self.header
    }
}

impl <T>CursorWithStoreHeader<T> for WriteCursor<'_, T> {
    fn header(&self) -> &StoreHeader {
        &self.header
    }
}

impl <T> ReadCursor<T> {
    pub async fn new(store: &Store<T>) -> Result<Self> 
        where T: Send
    {
            let path_to_rows = Path::new(&store.table_folder).join(ROWS_FILE_NAME);
            let file: File =
                OpenOptions::new()
                    .read(true)
                    .open(path_to_rows)
                    .await?;

            let mut cursor = Self {
                header: store.header.clone(),
                file,
                data_type: store.data_type,

                eof_file_position: 0, // This will be overwriten by the seek_to_start_of_data
            };
            cursor.seek_to_start_of_data().await?;

            Ok(cursor)
    }

    pub async fn less_than_eq(&mut self, file_position0: FilePosition, file_position1: FilePosition) -> Result<bool> {
        todo!()
    }
}

impl <'cursor, T> WriteCursor<'cursor, T> {
    // 'store lives atleast as long as 'cursor
    pub async fn new<'store: 'cursor>(store: &'store mut Store<T>) -> Result<Self> 
        where T: Send
    {
            let path_to_rows = Path::new(&store.table_folder).join(ROWS_FILE_NAME);
            let file: File =
                OpenOptions::new()
                    .read(true)
                    .write(true)
                    .open(path_to_rows)
                    .await?;

            let mut cursor = Self {
                header: &mut store.header,
                file,
                data_type: store.data_type,

                eof_file_position: 0, // This will be overwriten by the seek_to_start_of_data
            };
            cursor.seek_to_start_of_data().await?;

            Ok(cursor)
    }
    
    // ===Primitive Operations===
    async fn write_bytes(&mut self, bytes: &[u8]) -> Result<usize> {
        Ok(self.file.write(bytes).await?)
    }

    // ===Store Header Manipulation===
    async fn increment_total_count(&mut self) -> Result<()> 
        where T: Send
    {
        self.seek_to_start().await?;
        self.seek_to(StoreHeader::TOTAL_COUNT_OFFSET as u64).await?;
        let new_count = self.header.increment_total_count();
        self.write_bytes(&encode::<usize>(&new_count)?).await?;
        Ok(())
    }

    async fn increment_deleted_count(&mut self) -> Result<()> 
        where T: Send
    {
        self.seek_to_start().await?;
        self.seek_to(StoreHeader::DELETED_COUNT_OFFSET as u64).await?;
        let new_count = self.header.increment_deleted_count();
        self.write_bytes(&encode::<usize>(&new_count)?).await?;
        Ok(())
    }

    // ===Entry Header Manipulation===
    // assumes we are at the start of the valid entry.
    async fn set_entry_is_deleted_to(&mut self, is_deleted: bool) -> Result<()> 
        where T: Send
    {
        self.seek_to(EntryHeaderWithDataSize::IS_DELETED_OFFSET as u64).await?;
        self.write_bytes(&encode::<bool>(&is_deleted)?).await?;
        Ok(())
    }

    // ===Append Entry===

    // Moves cursor to the end.
    // Returns file position to the start of the new entry.
    pub async fn append_entry(&mut self, entry: &Entry<T>) -> Result<FilePosition> 
        where T: Encode + Send
    {
        self.increment_total_count().await?;

        let encoded_entry: Vec<u8> = entry.encode()?;
        self.seek_to_end().await?;
        let file_position: FilePosition = self.current_file_position().await?;
        self.write_bytes(&encoded_entry).await?;

        let eof_file_position: FilePosition = self.current_file_position().await?;
        self.eof_file_position = eof_file_position;

        Ok(file_position)
    }

    // ===Deletion===
    pub async fn mark_deleted_at(&mut self, file_position: FilePosition) -> Result<()> 
        where T: Send
    {
        self.seek_to(file_position).await?;
        let entry_header = self.read_entry_header().await?;
        if entry_header.is_deleted {
            Ok(())
        } else {
            self.increment_deleted_count().await?;
            self.seek_to(file_position).await?;
            self.set_entry_is_deleted_to(true).await?;

            self.attempt_garbage_collection_if_necessary().await?;
            Ok(())
        }
    }

    async fn attempt_garbage_collection_if_necessary(&mut self) -> Result<()> {
        // TODO: What should be the policy? Counting size of garbage? Counting how many entries are
        // garbage?
        if self.header.deleted_count > 100 {
            todo!()
        } else {
            Ok(())
        }
    }
}



// TODO
    // pub async fn search_for(&mut self, index: T) -> Result<()> 
    //     where T: Send
    // {
    //     // let index = self.primary_index.borrow_mut();
    //     // let x = index.lookup(self, 123).await?;
    //     todo!()
    // }
    // pub async fn search_for_entry_with_id(&mut self, id: T) -> Result<Option<EntryDetailed<T>>> {
    //     // TODO: make call to the primary index
    //     todo!()
    // }

    // // TODO: This needs to be some sort of an iterator
    // pub async fn get_all_eq(&self, column: Column, value: T) -> Result<Option<EntryDetailed<T>>> {
    //     todo!()
    // }