Implement compact encoding scheme for partial tries.

test-support/reference-trie/src/lib.rs

@@ -33,8 +33,9 @@ use std::borrow::Borrow;
 use keccak_hasher::KeccakHasher;
 
 pub use trie_db::{
-	Trie, TrieError, TrieMut, TrieIterator, TrieDBNodeIterator, NibbleSlice, NibbleVec, Recorder,
-	NodeCodec,
+	Trie, TrieMut, TrieDB, TrieDBMut, TrieError, TrieIterator, TrieDBNodeIterator,
+	NibbleSlice, NibbleVec, NodeCodec, Recorder,
+	encode_compact, decode_compact,
 };
 pub use trie_db::{Record, TrieLayout, TrieConfiguration, nibble_ops};
 pub use trie_root::TrieStream;

trie-db/src/lib.rs

@@ -85,6 +85,7 @@ mod iterator;
 mod lookup;
 mod nibble;
 mod node_codec;
+mod trie_codec;
 
 pub use hash_db::{HashDB, HashDBRef, Hasher};
 pub use self::triedb::{TrieDB, TrieDBIterator};
@@ -100,6 +101,7 @@ pub use node_codec::{NodeCodec, Partial};
 pub use iter_build::{trie_visit, ProcessEncodedNode,
 	 TrieBuilder, TrieRoot, TrieRootUnhashed};
 pub use iterator::TrieDBNodeIterator;
+pub use trie_codec::{decode_compact, encode_compact};
 
 #[cfg(feature = "std")]
 pub use iter_build::TrieRootPrint;

trie-db/src/nibble/nibblevec.rs

@@ -204,6 +204,24 @@ impl NibbleVec {
 		}
 	}
 
+	/// Do we start with the same nibbles as the whole of `them`?
+	pub fn starts_with(&self, other: &Self) -> bool {
+		if self.len() < other.len() {
+			return false;
+		}
+		let byte_len = other.len() / nibble_ops::NIBBLE_PER_BYTE;
+		if &self.inner[..byte_len] != &other.inner[..byte_len] {
+			return false;
+		}
+		for pad in 0..(other.len() - byte_len * nibble_ops::NIBBLE_PER_BYTE) {
+			let self_nibble = nibble_ops::at_left(pad as u8, self.inner[byte_len]);
+			let other_nibble = nibble_ops::at_left(pad as u8, other.inner[byte_len]);
+			if self_nibble != other_nibble {
+				return false;
+			}
+		}
+		true
+	}
 }
 
 impl<'a> From<NibbleSlice<'a>> for NibbleVec {