Reduce amount of clone() going on when applying data to MMRs (#2046)

* less cloning when pushing to PMMR

* pass a ref to data when applying to the MMR and appending to the backend

* cleanup

* rustfmt

* fixup tests

core/src/core/block.rs

@@ -162,7 +162,7 @@ impl Default for BlockHeader {
 impl PMMRable for BlockHeader {
 	type E = Hash;
 
-	fn as_elmt(self) -> Self::E {
+	fn as_elmt(&self) -> Self::E {
 		self.hash()
 	}
 }

core/src/core/pmmr/backend.rs

@@ -27,7 +27,7 @@ pub trait Backend<T: PMMRable> {
 	/// associated data element to flatfile storage (for leaf nodes only). The
 	/// position of the first element of the Vec in the MMR is provided to
 	/// help the implementation.
-	fn append(&mut self, data: T, hashes: Vec<Hash>) -> Result<(), String>;
+	fn append(&mut self, data: &T, hashes: Vec<Hash>) -> Result<(), String>;
 
 	/// Rewind the backend state to a previous position, as if all append
 	/// operations after that had been canceled. Expects a position in the PMMR

core/src/core/pmmr/pmmr.rs

@@ -171,7 +171,7 @@ where
 
 	/// Push a new element into the MMR. Computes new related peaks at
 	/// the same time if applicable.
-	pub fn push(&mut self, elmt: T) -> Result<u64, String> {
+	pub fn push(&mut self, elmt: &T) -> Result<u64, String> {
 		let elmt_pos = self.last_pos + 1;
 		let mut current_hash = elmt.hash_with_index(elmt_pos - 1);
 

core/src/core/transaction.rs

@@ -192,8 +192,8 @@ impl Readable for TxKernel {
 impl PMMRable for TxKernel {
 	type E = TxKernelEntry;
 
-	fn as_elmt(self) -> Self::E {
-		self.into()
+	fn as_elmt(&self) -> TxKernelEntry {
+		TxKernelEntry::from_kernel(self)
 	}
 }
 
@@ -260,6 +260,9 @@ impl TxKernel {
 }
 
 /// Wrapper around a tx kernel used when maintaining them in the MMR.
+/// These will be useful once we implement relative lockheights via relative kernels
+/// as a kernel may have an optional rel_kernel but we will not want to store these
+/// directly in the kernel MMR.
 #[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct TxKernelEntry {
 	/// The underlying tx kernel.
@@ -305,6 +308,13 @@ impl TxKernelEntry {
 	pub fn verify(&self) -> Result<(), Error> {
 		self.kernel.verify()
 	}
+
+	/// Build a new tx kernel entry from a kernel.
+	pub fn from_kernel(kernel: &TxKernel) -> TxKernelEntry {
+		TxKernelEntry {
+			kernel: kernel.clone(),
+		}
+	}
 }
 
 impl From<TxKernel> for TxKernelEntry {
@@ -1151,8 +1161,8 @@ impl Readable for Output {
 impl PMMRable for Output {
 	type E = OutputIdentifier;
 
-	fn as_elmt(self) -> Self::E {
-		self.into()
+	fn as_elmt(&self) -> OutputIdentifier {
+		OutputIdentifier::from_output(self)
 	}
 }
 

core/src/ser.rs

@@ -515,8 +515,8 @@ impl FixedLength for RangeProof {
 impl PMMRable for RangeProof {
 	type E = Self;
 
-	fn as_elmt(self) -> Self::E {
-		self
+	fn as_elmt(&self) -> Self::E {
+		self.clone()
 	}
 }
 
@@ -695,7 +695,7 @@ pub trait PMMRable: Writeable + Clone + Debug {
 	type E: FixedLength + Readable + Writeable;
 
 	/// Convert the pmmrable into the element to be stored in the MMR data file.
-	fn as_elmt(self) -> Self::E;
+	fn as_elmt(&self) -> Self::E;
 }
 
 /// Generic trait to ensure PMMR elements can be hashed with an index