Init merkle-mountain-range

The MMR code is copy from https://github.com/nervosnetwork/ckb/pull/1598

src/util.rs

@@ -0,0 +1,73 @@
+use crate::{MMRStore, Merge, Result, MMR};
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::fmt::Debug;
+use std::marker::PhantomData;
+
+#[derive(Clone)]
+pub struct MemStore<T>(RefCell<HashMap<u64, T>>);
+
+impl<T> Default for MemStore<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T> MemStore<T> {
+    fn new() -> Self {
+        MemStore(RefCell::new(HashMap::new()))
+    }
+}
+
+impl<T: Clone> MMRStore<T> for &MemStore<T> {
+    fn get_elem(&self, pos: u64) -> Result<Option<T>> {
+        Ok(self.0.borrow().get(&pos).cloned())
+    }
+
+    fn append(&mut self, pos: u64, elems: Vec<T>) -> Result<()> {
+        let mut store = self.0.borrow_mut();
+        for (i, elem) in elems.into_iter().enumerate() {
+            store.insert(pos + i as u64, elem);
+        }
+        Ok(())
+    }
+}
+
+pub struct MemMMR<T, M> {
+    store: MemStore<T>,
+    mmr_size: u64,
+    merge: PhantomData<M>,
+}
+
+impl<T: Clone + Debug + PartialEq, M: Merge<Item = T>> Default for MemMMR<T, M> {
+    fn default() -> Self {
+        Self::new(0, Default::default())
+    }
+}
+
+impl<T: Clone + Debug + PartialEq, M: Merge<Item = T>> MemMMR<T, M> {
+    pub fn new(mmr_size: u64, store: MemStore<T>) -> Self {
+        MemMMR {
+            mmr_size,
+            store,
+            merge: PhantomData,
+        }
+    }
+
+    pub fn store(&self) -> &MemStore<T> {
+        &self.store
+    }
+
+    pub fn get_root(&self) -> Result<T> {
+        let mmr = MMR::<T, M, &MemStore<T>>::new(self.mmr_size, &self.store);
+        mmr.get_root()
+    }
+
+    pub fn push(&mut self, elem: T) -> Result<u64> {
+        let mut mmr = MMR::<T, M, &MemStore<T>>::new(self.mmr_size, &self.store);
+        let pos = mmr.push(elem)?;
+        self.mmr_size = mmr.mmr_size();
+        mmr.commit()?;
+        Ok(pos)
+    }
+}