add async reader

lib/src/async_db.rs

@@ -1,21 +1,18 @@
 use std::{marker::PhantomData, path::Path, sync::Arc};
 
-use async_stream::stream;
-use tokio::pin;
-
 use async_compression::tokio::bufread::ZstdDecoder;
 use async_compression::tokio::write::ZstdEncoder;
 use async_compression::Level;
-use futures::stream::{self, StreamExt};
+use futures::stream::StreamExt;
 use futures_core::stream::Stream;
 use futures_util::pin_mut;
 
 use tokio::{
     fs,
-    io::{self, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt},
+    io::{self, AsyncReadExt, AsyncWriteExt},
 };
 
-use fmmap::tokio::{AsyncMmapFile, AsyncOptions};
+use fmmap::tokio::{AsyncMmapFile, AsyncMmapFileExt, AsyncOptions};
 
 type LSize = u32;
 const LEN_SIZE: usize = std::mem::size_of::<LSize>();
@@ -137,9 +134,97 @@ where
     }
 }
 
+pub struct Reader<T>
+where
+    T: bincode::Decode,
+{
+    mmap: AsyncMmapFile,
+    count: usize,
+    first_pos: LSize,
+    _t: Option<Arc<T>>, // PhantomData replacement
+}
+
+impl<T> Reader<T>
+where
+    T: bincode::Decode,
+{
+    pub async fn new<P: AsRef<Path>>(path: P, _buf_size: usize) -> Result<Self, String> {
+        let mmap = AsyncOptions::new()
+            .read(true)
+            .open_mmap_file(path)
+            .await
+            .str_err()?;
+        mmap.try_lock_shared().str_err()?;
+
+        // read first pos and records count
+        let first_data: [u8; LEN_SIZE] = mmap.bytes(0, LEN_SIZE).str_err()?.try_into().str_err()?;
+        let first_pos = LSize::from_le_bytes(first_data);
+        let tab_len = (first_pos as usize) / LEN_SIZE;
+        let count = tab_len - 1;
+
+        Ok(Self {
+            mmap,
+            count,
+            first_pos,
+            _t: None,
+        })
+    }
+
+    pub fn len(&self) -> usize {
+        self.count
+    }
+
+    pub fn is_empty(&self) -> bool {
+        0 == self.len()
+    }
+
+    pub async fn get(&self, index: usize) -> Result<T, String> {
+        if index >= self.len() {
+            return Err("index out of range".into());
+        }
+
+        let next_pos: usize = (index + 1) * LEN_SIZE;
+
+        // read item data pos
+        let data_pos = if 0 == index {
+            self.first_pos
+        } else {
+            let tab_pos: usize = index * LEN_SIZE;
+            let pos_curr_data: [u8; LEN_SIZE] = self
+                .mmap
+                .bytes(tab_pos, LEN_SIZE)
+                .str_err()?
+                .try_into()
+                .str_err()?;
+            LSize::from_le_bytes(pos_curr_data)
+        } as usize;
+
+        // read next item pos
+        let pos_next_data: [u8; LEN_SIZE] = self
+            .mmap
+            .bytes(next_pos, LEN_SIZE)
+            .str_err()?
+            .try_into()
+            .str_err()?;
+        let data_pos_next = LSize::from_le_bytes(pos_next_data) as usize;
+        let data_len = data_pos_next - data_pos;
+
+        // read & unpack item data
+        let mut decoder = ZstdDecoder::new(self.mmap.range_reader(data_pos, data_len).str_err()?);
+        let mut data = Vec::<u8>::new();
+        decoder.read_to_end(&mut data).await.str_err()?;
+
+        // decode item
+        let item: (T, usize) = bincode::decode_from_slice(&data, BINCODE_CFG).str_err()?;
+
+        Ok(item.0)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;
+    use async_stream::stream;
     use tempfile::tempdir;
 
     #[derive(bincode::Encode, bincode::Decode, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
@@ -185,4 +270,32 @@ mod test {
         writer.load(src).await.expect("load");
         writer.finish().await.expect("finish write");
     }
+
+    #[tokio::test]
+    async fn test_write_read() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            compress_lvl: Level::Default,
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = stream_iter(items.clone().into_iter());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+
+        let reader: Reader<TestData> = Reader::new(&tmpfile, 2048).await.expect("new reader");
+        assert_eq!(items.len(), reader.len());
+
+        for (idx, item) in items.iter().enumerate() {
+            let ritem = reader.get(idx).await.expect("get");
+            assert_eq!(*item, ritem);
+        }
+    }
 }