Bunch of Vec<Row> translated to Iterator<Item=Row>

There were several places where `Vec<Row>` was created just to iterate
it later and throw away, thus allocating memory unnecessarily. This
translates those `Vec`s to `Iterator`s in order to improve the
efficiency.

src/app.rs

@@ -27,7 +27,7 @@ impl App {
         }))
     }
 
-    fn write_store(&self) -> &store::WriteStore {
+    fn write_store(&self) -> &impl store::WriteStore {
         &self.store
     }
     // TODO: use index for queries.

src/fake.rs

@@ -13,7 +13,7 @@ impl ReadStore for FakeStore {
 }
 
 impl WriteStore for FakeStore {
-    fn write(&self, _rows: Vec<Row>) {}
+    fn write<I: IntoIterator<Item=Row>>(&self, _rows: I) {}
     fn flush(&self) {}
 }
 

src/index.rs

@@ -170,38 +170,35 @@ pub fn compute_script_hash(data: &[u8]) -> FullHash {
     hash
 }
 
-pub fn index_transaction(txn: &Transaction, height: usize, rows: &mut Vec<Row>) {
+pub fn index_transaction<'a>(txn: &'a Transaction, height: usize) -> impl 'a + Iterator<Item=Row> {
     let null_hash = Sha256dHash::default();
     let txid: Sha256dHash = txn.txid();
-    for input in &txn.input {
+
+    let inputs = txn.input.iter().filter_map(move |input| {
         if input.previous_output.txid == null_hash {
-            continue;
+            None
+        } else {
+            Some(TxInRow::new(&txid, &input).to_row())
         }
-        rows.push(TxInRow::new(&txid, &input).to_row());
-    }
-    for output in &txn.output {
-        rows.push(TxOutRow::new(&txid, &output).to_row());
-    }
+    });
+    let outputs = txn.output.iter().map(move |output| TxOutRow::new(&txid, &output).to_row());
+
     // Persist transaction ID and confirmed height
-    rows.push(TxRow::new(&txid, height as u32).to_row());
+    inputs.chain(outputs).chain(std::iter::once(TxRow::new(&txid, height as u32).to_row()))
 }
 
-pub fn index_block(block: &Block, height: usize) -> Vec<Row> {
-    let mut rows = vec![];
-    for txn in &block.txdata {
-        index_transaction(&txn, height, &mut rows);
-    }
+pub fn index_block<'a>(block: &'a Block, height: usize) -> impl 'a + Iterator<Item=Row> {
     let blockhash = block.bitcoin_hash();
     // Persist block hash and header
-    rows.push(Row {
+    let row = Row {
         key: bincode::serialize(&BlockKey {
             code: b'B',
             hash: full_hash(&blockhash[..]),
         })
         .unwrap(),
         value: serialize(&block.header),
-    });
-    rows
+    };
+    block.txdata.iter().flat_map(move |txn| index_transaction(&txn, height)).chain(std::iter::once(row))
 }
 
 pub fn last_indexed_block(blockhash: &Sha256dHash) -> Row {
@@ -361,7 +358,7 @@ impl Index {
             .cloned()
     }
 
-    pub fn update(&self, store: &WriteStore, waiter: &Waiter) -> Result<Sha256dHash> {
+    pub fn update(&self, store: &impl WriteStore, waiter: &Waiter) -> Result<Sha256dHash> {
         let daemon = self.daemon.reconnect()?;
         let tip = daemon.getbestblockhash()?;
         let new_headers: Vec<HeaderEntry> = {
@@ -401,20 +398,19 @@ impl Index {
                 break;
             }
 
-            let mut rows = vec![];
-            for block in &batch {
+            let rows = batch.iter().flat_map(|block| {
                 let blockhash = block.bitcoin_hash();
                 let height = *height_map
                     .get(&blockhash)
                     .unwrap_or_else(|| panic!("missing header for block {}", blockhash));
 
                 let timer = self.stats.start_timer("index");
-                let mut block_rows = index_block(block, height);
-                block_rows.push(last_indexed_block(&blockhash));
-                rows.extend(block_rows);
+                let block_rows = index_block(block, height);
                 timer.observe_duration();
                 self.stats.update(block, height);
-            }
+                block_rows.chain(std::iter::once(last_indexed_block(&blockhash)))
+            });
+
             let timer = self.stats.start_timer("write");
             store.write(rows);
             timer.observe_duration();

src/mempool.rs

@@ -29,8 +29,7 @@ impl MempoolStore {
     }
 
     fn add(&mut self, tx: &Transaction) {
-        let mut rows = vec![];
-        index_transaction(tx, 0, &mut rows);
+        let rows = index_transaction(tx, 0);
         for row in rows {
             let (key, value) = row.into_pair();
             self.map.entry(key).or_insert_with(|| vec![]).push(value);
@@ -38,8 +37,7 @@ impl MempoolStore {
     }
 
     fn remove(&mut self, tx: &Transaction) {
-        let mut rows = vec![];
-        index_transaction(tx, 0, &mut rows);
+        let rows = index_transaction(tx, 0);
         for row in rows {
             let (key, value) = row.into_pair();
             let no_values_left = {

src/store.rs

@@ -21,7 +21,7 @@ pub trait ReadStore: Sync {
 }
 
 pub trait WriteStore: Sync {
-    fn write(&self, rows: Vec<Row>);
+    fn write<I: IntoIterator<Item=Row>>(&self, rows: I);
     fn flush(&self);
 }
 
@@ -148,7 +148,7 @@ impl ReadStore for DBStore {
 }
 
 impl WriteStore for DBStore {
-    fn write(&self, rows: Vec<Row>) {
+    fn write<I: IntoIterator<Item=Row>>(&self, rows: I) {
         let mut batch = rocksdb::WriteBatch::default();
         for row in rows {
             batch.put(row.key.as_slice(), row.value.as_slice()).unwrap();