From 71dc9c0ad46f79afef069026b4ba1bb34ac0e4a8 Mon Sep 17 00:00:00 2001
From: Gav Wood <i@gavwood.com>
Date: Sat, 28 Nov 2015 03:09:36 +0100
Subject: [PATCH] OVerlayDB draft.

---
 src/overlaydb.rs | 70 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 src/overlaydb.rs

diff --git a/src/overlaydb.rs b/src/overlaydb.rs
new file mode 100644
index 000000000..99d66cd5f
--- /dev/null
+++ b/src/overlaydb.rs
@@ -0,0 +1,70 @@
+//! Disk-backed HashDB implementation.
+
+use hash::*;
+use bytes::*;
+use sha3::*;
+use hashdb::*;
+use memorydb::*;
+use std::ops::*;
+use rocksdb::{DB, Writable};
+
+#[derive(Clone)]
+pub struct OverlayDB {
+	overlay: MemoryDB,
+	backing: DB,
+}
+
+impl OverlayDB {
+	/// Create a new instance of OverlayDB given a `backing` database.
+	fn new(backing: DB) {
+		self.backing = backing;
+		overlay = MemoryDB::new();
+	}
+
+	/// Commit all memory operations to the backing database.
+	fn commit(&mut self) {
+		unimplemented!();
+	}
+
+	/// Get the refs and value of the given key.
+	fn payload(&self, key: &H256) -> Option<(Bytes, i32)> {
+		unimplemented!();
+	}
+}
+
+impl HashDB for OverlayDB {
+	fn lookup(&self, key: &H256) -> Option<Bytes> {
+		// TODO: return ok if positive; if negative, check backing - might be enough references there to make
+		// it positive again.
+		let k = self.overlay.data.get(key);
+		match k {
+			Some(&(ref d, rc)) if rc > 0 => Some(d.clone()),
+			_ => {
+				let memrc = k.map(|&(_, rc)| rc).unwrap_or(0);
+				match self.payload(key) {
+					Some((d, rc)) if rc + memrc > 0 => Some(d),
+					_ => None,
+				}
+			}
+		}
+	}
+	fn exists(&self, key: &H256) -> bool {
+		// TODO: copy and adapt code above.
+		m_overlay.exists(key)
+	}
+	fn insert(&mut self, value: &[u8]) -> H256 { m_overlay.insert(value) }
+	fn kill(&mut self, key: &H256) { m_overlay.kill(key); }
+}
+
+#[test]
+fn playpen() {
+	let mut db: DB = DB::open_default("/tmp/test").unwrap();
+	db.put(b"test", b"test2");
+	match db.get(b"test") {
+		Ok(Some(value)) => println!("Got value {:?}", value.deref()),
+		Ok(None) => println!("No value for that key"),
+		Err(e) => println!("Gah"),
+	}
+	db.delete(b"test");
+	assert!(false);
+}
\ No newline at end of file