Split Cursor into ReadCursor and WriteCursor

storage_engine/src/cursor.rs

@@ -0,0 +1,302 @@
+use tokio::io::{AsyncReadExt, AsyncWriteExt, AsyncSeekExt, SeekFrom};
+use tokio::fs::{File, OpenOptions, DirBuilder};
+use std::path::Path;
+use std::marker::PhantomData;
+
+use async_trait::async_trait;
+
+use bincode;
+use bincode::{Decode, Encode};
+use crate::binary_coding::{encode, decode, encode_sequence, encode_sequence_with_sizes, decode_sequence};
+use tokio::fs;
+
+use crate::error::{Error, DecodeErrorKind};
+use crate::storage_engine::{Store, StoreHeader, FilePosition, Result, ROWS_FILE_NAME, EntryDetailed, EntryHeaderWithDataSize, Entry};
+
+#[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]
+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))
+    }
+
+    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(())
+    }
+}
+
+
+// ===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)
+    }
+}
+
+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(())
+        }
+    }
+}