feat(code): add sst iterator / day 2

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm/src/table/builder.rs

@@ -0,0 +1,75 @@
+use anyhow::Result;
+use bytes::BufMut;
+use std::path::Path;
+
+use super::{BlockMeta, FileObject, SsTable};
+use crate::block::BlockBuilder;
+
+pub struct SsTableBuilder {
+    builder: BlockBuilder,
+    first_key: Vec<u8>,
+    data: Vec<u8>,
+    pub(super) meta: Vec<BlockMeta>,
+    target_size: usize,
+    block_size: usize,
+}
+
+impl SsTableBuilder {
+    pub fn new(target_size: usize, block_size: usize) -> Self {
+        Self {
+            data: Vec::new(),
+            meta: Vec::new(),
+            first_key: Vec::new(),
+            target_size,
+            block_size,
+            builder: BlockBuilder::new(block_size),
+        }
+    }
+
+    #[must_use]
+    pub fn add(&mut self, key: &[u8], value: &[u8]) -> bool {
+        if self.data.len() > self.target_size {
+            return false;
+        }
+
+        if self.first_key.is_empty() {
+            self.first_key = key.to_vec();
+        }
+
+        if self.builder.add(key, value) {
+            return true;
+        }
+        // create a new block builder and append block data
+        self.finish_block();
+
+        // add the key-value pair to the next block
+        assert!(self.builder.add(key, value));
+        self.first_key = key.to_vec();
+
+        true
+    }
+
+    fn finish_block(&mut self) {
+        let builder = std::mem::replace(&mut self.builder, BlockBuilder::new(self.block_size));
+        let encoded_block = builder.build().encode();
+        self.meta.push(BlockMeta {
+            offset: self.data.len(),
+            first_key: std::mem::take(&mut self.first_key).into(),
+        });
+        self.data.extend(encoded_block);
+    }
+
+    pub fn build(mut self, path: impl AsRef<Path>) -> Result<SsTable> {
+        self.finish_block();
+        let mut buf = self.data;
+        let meta_offset = buf.len();
+        BlockMeta::encode_block_meta(&self.meta, &mut buf);
+        buf.put_u32(meta_offset as u32);
+        let file = FileObject::create(path.as_ref(), buf)?;
+        Ok(SsTable {
+            file,
+            block_metas: self.meta,
+            block_meta_offset: meta_offset,
+        })
+    }
+}