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

use async_trait::async_trait;

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

use crate::error::{Error, DecodeErrorKind};
use crate::segments::entry::{Entry, EntryDetailed};
use crate::segments::entry_header::{EntryHeaderWithDataSize, EntryHeader};
use crate::segments::store_header::StoreHeader;
use crate::storage_engine::{Store, FilePosition, Column, Result, ROWS_FILE_NAME, GARBAGE_COLLECTION_INTERMEDIATE_ROWS_FILE_NAME};
use crate::index::Index;

const GARBAGE_COLLECTION_TRIGGER: usize = 100;

// ===Concrete Cursors===
pub struct ReadCursor<'a, T> {
    header: StoreHeader,
    indexes: &'a [Option<Index<T, FilePosition>>],
    file: File,

    eof_file_position: FilePosition,
}

pub struct WriteCursor<'a, T> {
    header: &'a mut StoreHeader,
    indexes: &'a mut [Option<Index<T, FilePosition>>],
    file: File,

    eof_file_position: FilePosition,
}

// This is used as a cursor to temporary file during Garbage Collection
pub struct AppendOnlyCursor<T> {
    header: StoreHeader,
    file: File,
    data_type: PhantomData<T>,

    eof_file_position: FilePosition,
}


// ===Traits===
#[async_trait]
pub(crate) 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<FilePosition> {
        let file_position = self.file().seek(SeekFrom::Start(file_position)).await?;
        Ok(file_position)
    }

    // Start of the file i.e. the Header, not the entries.
    async fn seek_to_start(&mut self) -> Result<FilePosition> {
        let file_position = self.file().seek(SeekFrom::Start(0)).await?;
        Ok(file_position)
    }

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

    // Seeks from current position by offset and returns new file position
    async fn seek_by(&mut self, offset: i64) -> Result<FilePosition> {
        let file_position = self.file().seek(SeekFrom::Current(offset)).await?;
        Ok(file_position)
    }

    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> {
        let current_file_position = self.current_file_position().await?;
        let eof_file_position = self.eof_file_position();
        Ok(current_file_position == eof_file_position)
    }
}

#[async_trait]
pub(crate) trait PrimitiveWriteCursor<T>: PrimitiveCursor<T> {
    async fn write_bytes(&mut self, bytes: &[u8]) -> Result<usize> {
        Ok(self.file().write(bytes).await?)
    }

}

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

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

    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===
    // The following functions assume that the current file position is at a valid entry or EOF.


    // WARNING: This moves the file_position to start of the data, so you can't just call
    //    next_entry_header() a bunch of times. You must move the cursor!
    async fn next_entry_header(&mut self) -> Result<Option<EntryHeaderWithDataSize>> {
        if self.is_at_eof().await? {
            return Ok(None)
        }

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

        Ok(Some(entry_header))
    }

    // This is meant to be used after next_entry_header() is called.
    async fn jump_from_start_of_entry_data_to_next_entry(&mut self, entry_header: &EntryHeaderWithDataSize) -> Result<FilePosition>{
        let file_position = self.seek_by(entry_header.size_of_data() as i64).await?;
        Ok(file_position)
    }

    async fn next(&mut self) -> Result<Option<EntryDetailed<T>>> 
        where T: Decode
    {
        let file_position = self.current_file_position().await?;
        let Some(entry_header) = self.next_entry_header().await? else { return Ok(None) };

        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, file_position, self.header().number_of_columns, &mut data_bytes)?;

        Ok(Some(entry))
    }

    // Like next, but only reads the column, not the whole entry. 
    async fn next_at_column(&mut self, column: Column) -> Result<Option<(EntryHeaderWithDataSize, FilePosition, T)>> 
        where T: Decode + Send
    {
        let file_position = self.current_file_position().await?;
        let Some(entry_header) = self.next_entry_header().await? else { return Ok(None) };
        let file_position_at_start_of_data = self.current_file_position().await?;

        // figuring out how much to decode
        let column_offset = entry_header.offset_of_column(column);
        self.seek_by(column_offset as i64).await?;

        // reading and decoding
        let mut bytes: Vec<u8> = vec![0; entry_header.data_sizes[column as usize]];
        self.read_bytes(&mut bytes).await?;
        let (value, _) =
            decode::<T>(&bytes[..])
                .map_err(|e| Error::DecodeError(DecodeErrorKind::EntryIsDeleted, e))?;

        // jumping to next entry
        self.seek_to(file_position_at_start_of_data).await?;
        self.jump_from_start_of_entry_data_to_next_entry(&entry_header).await?;

        Ok(Some((entry_header, file_position, value)))
    }

    async fn next_alive_at_column(&mut self, column: Column) -> Result<Option<(EntryHeaderWithDataSize, FilePosition, T)>> 
        where T: Decode + Send
    {
        while let Some((header, file_position, t)) = self.next_at_column(column).await? {
            if !header.is_deleted  {
                return Ok(Some((header, file_position, t)))
            }
        }
        Ok(None)
    } 

    async fn next_alive(&mut self) -> Result<Option<EntryDetailed<T>>> 
        where T: Decode
    {
        while let Some(entry) = self.next().await? {
            if !entry.header.is_deleted  {
                return Ok(Some(entry))
            }
        }
        Ok(None)
    }

    // ===Search===
    async fn find_first_eq_bruteforce(&mut self, column: Column, t0: &T) -> Result<Option<EntryDetailed<T>>> 
        where T: Decode + PartialEq + Send + Sync
    {
        let mut file_position = self.current_file_position().await?;
        while let Some((_, _, t)) = self.next_alive_at_column(column).await? {
            if &t == t0  {
                // go back and decode the whole entry
                self.seek_to(file_position).await?;
                return self.next().await
            } else {
                file_position = self.current_file_position().await?;
            }
        }
        Ok(None)
    }

    async fn find_all_eq_bruteforce(&mut self, column: Column, t0: &T) -> Result<Vec<EntryDetailed<T>>> 
        where T: Decode + PartialEq + Send + Sync
    {
        let mut entries = vec![];
        while let Some(entry) = self.find_first_eq_bruteforce(column, t0).await? {
            entries.push(entry)
        }
        Ok(entries)
    }

    // ===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.seek_to_start().await.map_err(|e| e.to_io_or_panic())?;
        self.file().read_to_end(&mut bytes).await?;
        Ok(bytes)
    }
}

#[async_trait]
pub trait CursorWithWriteStoreHeader<T>: CursorWithStoreHeader<T> + PrimitiveWriteCursor<T> {
    fn header_mut(&mut self) -> &mut StoreHeader;
    fn set_eof_file_position(&mut self, new_file_position: FilePosition);

    // ===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_mut().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_mut().increment_deleted_count();
        self.write_bytes(&encode::<usize>(&new_count)?).await?;
        Ok(())
    }

    async fn set_header(&mut self, header: &StoreHeader) -> Result<()> 
        where T: Send
    {
        self.seek_to_start().await?;
        let encoded_header: Vec<u8> = header.encode()?;
        self.write_bytes(&encoded_header).await?;

        Ok(())
    }

    // ===Append Entry===

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

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

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

        Ok(file_position)
    }
}

#[async_trait]
pub trait CursorWithAccessToIndex<T>: CursorWithStoreHeader<T> {
    fn indexes(&mut self) -> &[Option<Index<T, FilePosition>>];

    async fn index_lookup(&mut self, column: Column, value: &T) -> Result<Vec<EntryDetailed<T>>> 
        where T: Encode + Decode + Ord + Send + Sync
    {
        match &self.indexes()[column as usize] {
            Some(index) => {
                let file_positions = index.lookup(value).await?.unwrap_or_else(|| HashSet::new());
                let mut entries: Vec<EntryDetailed<T>> = vec![];
                for &file_position in file_positions.iter() {
                    match self.read_entry_at(file_position).await? {
                        Some(entry) => {
                            entries.push(entry)
                        },
                        None => {
                            return Err(Error::IndexIsStoringEofFilePosition(column))
                        }
                    }
                }

                Ok(entries)
            },
            None =>
                Err(Error::AttemptToIndexNonIndexableColumn(column))
        }
    }

    async fn select_entries_where_eq(&mut self, column: Column, value: &T) -> Result<Vec<EntryDetailed<T>>> 
        where T: Encode + Decode + Ord + Send + Sync
    {
        if self.header().is_column_indexed(column) {
            println!("INDEXED LOOKUP");
            self.index_lookup(column, value).await
        } else {
            println!("BRUTE-FORCE LOOKUP");
            self.find_all_eq_bruteforce(column, value).await
        }
    }
}

#[async_trait]
pub trait CursorWithWriteAccessToIndex<T>: CursorWithAccessToIndex<T> + CursorWithWriteStoreHeader<T> {
    fn indexes_mut(&mut self) -> &mut [Option<Index<T, FilePosition>>];

    // Assumes that the column is indexable.
    fn mut_index_at(&mut self, column: Column) -> &mut Index<T, FilePosition> {
        match &mut self.indexes_mut()[column as usize] {
            Some(index) => {
                index
            },
            None => {
                unreachable!()
            }
        }
    }

    // Assumes that the column is indexable.
    async fn insert_into_index(&mut self, column: Column, value: T, file_position: FilePosition) -> Result<()> 
        where T: Encode + Decode + Ord + Send + Sync + 'async_trait
    {
        let index = self.mut_index_at(column as Column);
        index.insert(value, file_position).await?;
        Ok(())
    }

    // Assumes that the column is indexable.
    async fn delete_from_index(&mut self, column: Column, value: T, file_position: FilePosition) -> Result<()> 
        where T: Encode + Decode + Ord + Send + Sync + 'async_trait
    {
        let index = self.mut_index_at(column as Column);
        index.delete(value, file_position).await?;
        Ok(())
    }
        
    async fn insert_entry(&mut self, entry: Entry<T>) -> Result<FilePosition> 
        where T: Encode + Decode + Ord + Send + Sync + 'async_trait
    {
        let file_position = self.append_entry_no_indexing(&entry).await?;

        // insert the indexable values of the entry into corresponding indexes.
        for (column, (value, should_index)) in entry.data.into_iter().zip(self.header().indexed_columns.clone()).enumerate() {
            if should_index {
                // SAFETY: If should_index is true, then the column is indexable.
                self.insert_into_index(column as Column, value, file_position).await?
            }
        }

        Ok(file_position)
    }

    async fn delete_entry_values_from_indexes(&mut self, entry: EntryDetailed<T>) -> Result<()>
        where T: Encode + Decode + Ord + Send + Sync + 'async_trait
    {
        for (column, (value, should_index)) in entry.data.into_iter().zip(self.header().indexed_columns.clone()).enumerate() {
            if should_index {
                // SAFETY: If should_index is true, then the column is indexable.
                self.delete_from_index(column as Column, value, entry.file_position).await?
            }
        }
        Ok(())
    }
}


// ===========Implementations=============
// ===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
    }
}

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

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

// ===PrimitiveCursor===
impl <T>PrimitiveWriteCursor<T> for WriteCursor<'_, T> {}
impl <T>PrimitiveWriteCursor<T> for AppendOnlyCursor<T> {}


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

// ===CursorWithWriteStoreHeader===
impl <T>CursorWithWriteStoreHeader<T> for WriteCursor<'_, T> {
    fn header_mut(&mut self) -> &mut StoreHeader { self.header }
    fn set_eof_file_position(&mut self, new_file_position: FilePosition) { self.eof_file_position = new_file_position } 
}

impl <T>CursorWithWriteStoreHeader<T> for AppendOnlyCursor<T> {
    fn header_mut(&mut self) -> &mut StoreHeader { &mut self.header }
    fn set_eof_file_position(&mut self, new_file_position: FilePosition) { self.eof_file_position = new_file_position } 
}

// ===CursorWithAccessToIndex===
impl <T>CursorWithAccessToIndex<T> for ReadCursor<'_, T> {
    fn indexes(&mut self) -> &[Option<Index<T, FilePosition>>] { &self.indexes }
}

impl <T>CursorWithAccessToIndex<T> for WriteCursor<'_, T> {
    fn indexes(&mut self) -> &[Option<Index<T, FilePosition>>] { &self.indexes }
}

// ===CursorWithWriteAccessToIndex===
impl <T>CursorWithWriteAccessToIndex<T> for WriteCursor<'_, T> {
    fn indexes_mut(&mut self) -> &mut [Option<Index<T, FilePosition>>] { self.indexes }
}


impl <'cursor, T> ReadCursor<'cursor, T> {
    pub async fn new<'store: 'cursor>(store: &'store Store<T>) -> Result<Self> 
        where T: Send + Sync
    {
            let path_to_rows = Path::new(&store.header.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,
                indexes: &store.indexes,

                eof_file_position: 0,
            };
            let eof_file_position: FilePosition = cursor.seek_to_end().await?;
            cursor.eof_file_position = eof_file_position;

            cursor.seek_to_start_of_data().await?;

            Ok(cursor)
    }
}



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.header.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,
                indexes: &mut store.indexes,

                eof_file_position: 0,
            };
            let eof_file_position: FilePosition = cursor.seek_to_end().await?;
            cursor.eof_file_position = eof_file_position;

            cursor.seek_to_start_of_data().await?;

            Ok(cursor)
    }

    // ===Entry Header Manipulation===
    // assumes we are at the start of valid entry.
    async fn set_new_entry_header(&mut self, entry_header: EntryHeader) -> Result<()> 
        where T: Send
    {
        let bytes: Vec<u8> = entry_header.encode()?;
        self.write_bytes(&bytes).await?;
        Ok(())
    }

    // ===Deletion===
    pub async fn mark_deleted_at(&mut self, file_position: FilePosition, enable_garbage_collector: bool) -> Result<()> 
        where T: Encode + Decode + Ord + Send + Sync + Clone + Ord
    {
        println!("MARKING {} as DELETED", file_position);
        self.seek_to(file_position).await?;
        let mut entry_header = self.read_entry_header().await?;
        if entry_header.is_deleted {
            Ok(())
        } else {
            // Update store and entry headers
            self.increment_deleted_count().await?;
            self.seek_to(file_position).await?;

            entry_header.is_deleted = true;
            self.set_new_entry_header(entry_header.into()).await?;

            // Update index
            self.seek_to(file_position).await?;
            match self.next().await? {
                Some(entry) => {
                    self.delete_entry_values_from_indexes(entry).await?
                },
                None => {
                    // SAFETY: We just modified its header, so it must exist.
                    unreachable!()
                }
            }

            if enable_garbage_collector {
                self.attempt_garbage_collection_if_necessary().await?;
            }
            Ok(())
        }
    }

    async fn find_first_eq_bruteforce_and_delete(&mut self, column: Column, t0: &T, enable_garbage_collector: bool) -> Result<Option<EntryDetailed<T>>> 
        where T: Encode + Decode + Ord + Send + Sync + Clone
    {
        let maybe_entry = self.find_first_eq_bruteforce(column, t0).await?;
        if let Some(entry) = maybe_entry {
            self.mark_deleted_at(entry.file_position, enable_garbage_collector).await?;
            Ok(Some(entry))
        } else {
            Ok(maybe_entry)
        }
    }

    // Doesn't update indexes.
    async fn find_all_eq_bruteforce_and_delete(&mut self, column: Column, t0: &T) -> Result<usize> 
        where T: Encode + Decode + Ord + Send + Sync + Clone
    {
        let mut count = 0;
        while let Some(_) = self.find_first_eq_bruteforce_and_delete(column, t0, false).await? {
            count += 1;
        }
        Ok(count)
    }

    pub async fn delete_entries_where_eq(&mut self, column: Column, value: &T, enable_garbage_collector: bool) -> Result<usize> 
        where T: Encode + Decode + Ord + Send + Sync + Clone
    {
        let count = 
            if self.header().is_column_indexed(column) {
                println!("DELETION: INDEXED LOOKUP");
                let entries = self.index_lookup(column, value).await?;
                let count = entries.len();
                for entry in entries {
                    self.mark_deleted_at(entry.file_position, false).await?
                }
                count
            } else {
                println!("DELETION: BRUTE-FORCE LOOKUP");
                let count = self.find_all_eq_bruteforce_and_delete(column, value).await?;
                count
            };
        if enable_garbage_collector {
            self.attempt_garbage_collection_if_necessary().await?;
        }
        Ok(count)
    }

    // ===Indexing===
    // WARNING: Assumes the column is NOT indexable.
    pub async fn attach_index(&mut self, column: Column) -> Result<()> 
        where T: Ord + Decode + Encode + Send + Sync
    {
        // New Index
        let index = Store::create_empty_index_at(&self.header, column).await?;
        self.indexes[column as usize] = Some(index);

        // Mark column as indexed
        self.header.make_column_indexed(column);
        self.set_header(&self.header.clone()).await?;

        // Build index
        self.seek_to_start_of_data().await?;
        while let Some((_, file_position, value)) = self.next_alive_at_column(column).await? {
            self.insert_into_index(column, value, file_position).await?
        }

        Ok(())
    }

    // ===Garbage Collection===
    async fn attempt_garbage_collection_if_necessary(&mut self) -> Result<()> 
        where T: Send + Sync + Decode + Encode + Clone + Ord
    {
        if self.header.deleted_count > GARBAGE_COLLECTION_TRIGGER {
            println!("=======START GARBAGE COLLETOR====");
            self.initiate_garbage_collection().await?;
        }
        Ok(())
    }

    pub async fn initiate_garbage_collection(&mut self) -> Result<usize> 
        where T: Send + Sync + Decode + Encode + Clone + Ord
    {
        let mut cursor_to_intermediate = self.spawn_cursor_to_intermediate_file().await?;
        // Since garbage collection changes FilePositions of live entries, we need to update the
        // indexes too.

        let mut in_memory_indexes: Vec<Option<BTreeMap<T, HashSet<FilePosition>>>> = Vec::with_capacity(self.header.number_of_columns);
        for column in 0..self.header.number_of_columns {
            if self.header.is_column_indexed(column as Column) {
                let in_memory_index = BTreeMap::new();
                in_memory_indexes.push(Some(in_memory_index))
            } else {
                in_memory_indexes.push(None)
            }
        }

        // We'll dump all alive entries into a new file.
        let mut entries_deleted = 0;
        self.seek_to_start_of_data().await?;
        {
            while let Some(live_entry) = self.next_alive().await? {
                entries_deleted += 1;
                let file_position = cursor_to_intermediate.append_entry_no_indexing(&live_entry.forget()).await?;

                // Update index. (Wouldn't it be nice if we had `for let ...`?)
                for (maybe_in_memory_index, value) in in_memory_indexes.iter_mut().zip(&live_entry.data) {
                    if let Some(in_memory_index) = maybe_in_memory_index {
                        in_memory_index.entry(value.clone()).or_insert_with(HashSet::new).insert(file_position);
                    }
                }
            }
        }

        // ===swap===
        // swapping indexes
        // Update indexes on disk.
        for (column, maybe_in_memory_index) in in_memory_indexes.into_iter().enumerate() {
            if let Some(in_memory_index) = maybe_in_memory_index {
                let index = self.mut_index_at(column as Column);
                index.reset(in_memory_index).await?;
            }
        }

        // swapping headers
        self.header.deleted_count = 0;
        self.header.total_count = cursor_to_intermediate.header.total_count;

        self.file = cursor_to_intermediate.file;
        self.eof_file_position = cursor_to_intermediate.eof_file_position;

        // swap files on disk
        // current file
        let path_to_table = Path::new(&self.header.table_folder);
        let path_to_rows = path_to_table.join(ROWS_FILE_NAME);
        let path_to_intermediate_rows = path_to_table.join(GARBAGE_COLLECTION_INTERMEDIATE_ROWS_FILE_NAME);
        fs::remove_file(path_to_rows.clone()).await?;
        fs::rename(path_to_intermediate_rows, path_to_rows).await?;

        Ok(entries_deleted)
    }

    async fn spawn_cursor_to_intermediate_file(&self) -> Result<AppendOnlyCursor<T>> 
        where T: Send
    {
        let table_folder = self.header.table_folder.to_string();
        let path_to_table = Path::new(&table_folder);
        let path_to_rows = path_to_table.join(GARBAGE_COLLECTION_INTERMEDIATE_ROWS_FILE_NAME);

        let intermediate_file: File = Store::<T>::create_empty_rows_file(path_to_rows, &self.header).await?;

        let intermediate_header: StoreHeader = StoreHeader {
            table_folder,
            number_of_columns: self.header.number_of_columns,
            deleted_count: 0,
            total_count: 0,
            primary_column: self.header.primary_column,
            indexed_columns: self.header.indexed_columns.clone(),
        };

        // Creates a new (append) cursor to the intermediate file in which we'll dump the live entries.
        let mut cursor_to_intermediate = AppendOnlyCursor {
            header: intermediate_header,
            file: intermediate_file,
            data_type: PhantomData::<T>,

            eof_file_position: 0,
        };
        let eof_file_position: FilePosition = cursor_to_intermediate.seek_to_end().await?;
        cursor_to_intermediate.eof_file_position = eof_file_position;

        Ok(cursor_to_intermediate)
    }
}