`len` method for some collections. (#101)

* Basic len for vec and mappings.

* Fix link

* Slicify.

* Support collection types.

* Usize.

* Better usize.

* Update src/codec.rs

src/codec.rs

@@ -34,6 +34,8 @@ use arrayvec::ArrayVec;
 #[cfg(feature = "std")]
 use std::fmt;
 
+use core::convert::TryFrom;
+
 #[cfg_attr(feature = "std", derive(Debug))]
 #[derive(PartialEq)]
 #[cfg(feature = "std")]
@@ -223,6 +225,13 @@ pub trait EncodeAppend {
 	fn append(self_encoded: Vec<u8>, to_append: &[Self::Item]) -> Result<Vec<u8>, Error>;
 }
 
+/// Trait that allows the length of a collection to be read, without having
+/// to read and decode the entire elements.
+pub trait DecodeLength {
+	/// Return the number of elements in `self_encoded`.
+	fn len(self_encoded: &mut &[u8]) -> Result<usize, Error>;
+}
+
 /// Trait that allows zero-copy read of value-references from slices in LE format.
 pub trait Decode: Sized {
 	#[doc(hidden)]
@@ -677,6 +686,20 @@ impl Decode for () {
 	}
 }
 
+macro_rules! impl_len {
+	( $( $type:ident< $($g:ident),* > ),* ) => { $(
+		impl<$($g),*> DecodeLength for $type<$($g),*> {
+			fn len(self_encoded: &mut &[u8]) -> Result<usize, Error> {
+				usize::try_from(u32::from(Compact::<u32>::decode(self_encoded)?))
+					.map_err(|_| "Failed convert decded size into usize.".into())
+			}
+		}
+	)*}
+}
+
+// Collection types that support compact decode length.
+impl_len!(Vec<T>, BTreeSet<T>, BTreeMap<K, V>, VecDeque<T>, BinaryHeap<T>, LinkedList<T>);
+
 macro_rules! tuple_impl {
 	($one:ident,) => {
 		impl<$one: Encode> Encode for ($one,) {
@@ -992,6 +1015,37 @@ mod tests {
 		assert_eq!(decoded, expected);
 	}
 
+	fn test_encode_length<T: Encode + Decode + DecodeLength>(thing: &T, len: usize) {
+		assert_eq!(<T as DecodeLength>::len(&mut &thing.encode()[..]).unwrap(), len);
+	}
+
+	#[test]
+	fn len_works_for_decode_collection_types() {
+		let vector = vec![10; 10];
+		let mut btree_map: BTreeMap<u32, u32> = BTreeMap::new();
+		btree_map.insert(1, 1);
+		btree_map.insert(2, 2);
+		let mut btree_set: BTreeSet<u32> = BTreeSet::new();
+		btree_set.insert(1);
+		btree_set.insert(2);
+		let mut vd = VecDeque::new();
+		vd.push_front(1);
+		vd.push_front(2);
+		let mut bh = BinaryHeap::new();
+		bh.push(1);
+		bh.push(2);
+		let mut ll = LinkedList::new();
+		ll.push_back(1);
+		ll.push_back(2);
+
+		test_encode_length(&vector, 10);
+		test_encode_length(&btree_map, 2);
+		test_encode_length(&btree_set, 2);
+		test_encode_length(&vd, 2);
+		test_encode_length(&bh, 2);
+		test_encode_length(&ll, 2);
+	}
+
 	#[test]
 	fn vec_of_string_encoded_as_expected() {
 		let value = vec![