consenus_encoding: Add a SliceEncoder

Add an encoder that can be used to encode a slice of `T: Encodable`
types. Will be used by `Transaction`. 

Add an example usage as well as an integration test.

The `Encoder` impl was written during review by Andrew, props.

Co-developed-by: Andrew Poelstra <apoelstra@wpsoftware.net>

Author: tcharding

consensus_encoding/Cargo.toml

@@ -25,6 +25,10 @@ internals = { package = "bitcoin-internals", path = "../internals" }
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
+[[example]]
+name = "encoder"
+required-features = ["alloc"]
+
 [lints.rust]
 unexpected_cfgs = { level = "deny", check-cfg = [] }
 

consensus_encoding/contrib/test_vars.sh

@@ -11,4 +11,4 @@ FEATURES_WITH_STD=""
 FEATURES_WITHOUT_STD="alloc"
 
 # Run these examples.
-EXAMPLES=""
+EXAMPLES="encoder:alloc"

consensus_encoding/examples/encoder.rs

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: CC0-1.0
+
+//! Example of creating an encoder that encodes a slice of encodable objects.
+
+use consensus_encoding as encoding;
+use encoding::{ArrayEncoder, BytesEncoder, Encodable, Encoder2, SliceEncoder};
+
+fn main() {
+    let v = vec![Inner::new(0xcafe_babe), Inner::new(0xdead_beef)];
+    let b = vec![0xab, 0xcd];
+
+    let adt = Adt::new(v, b);
+    let encoded = encoding::encode_to_vec(&adt);
+
+    let want = [0x02, 0xca, 0xfe, 0xba, 0xbe, 0xde, 0xad, 0xbe, 0xef, 0xab, 0xcd];
+    assert_eq!(encoded, want);
+}
+
+/// Some abstract data type.
+struct Adt {
+    v: Vec<Inner>,
+    b: Vec<u8>,
+}
+
+impl Adt {
+    /// Constructs a new `Adt`.
+    pub fn new(v: Vec<Inner>, b: Vec<u8>) -> Self { Self { v, b } }
+}
+
+encoding::encoder_newtype! {
+    /// The encoder for the [`Adt`] type.
+    pub struct AdtEncoder<'e>(Encoder2<SliceEncoder<'e, Inner>, BytesEncoder<'e>>);
+}
+
+impl Encodable for Adt {
+    type Encoder<'a>
+        = AdtEncoder<'a>
+    where
+        Self: 'a;
+
+    fn encoder(&self) -> Self::Encoder<'_> {
+        let a = SliceEncoder::with_length_prefix(&self.v);
+        let b = BytesEncoder::without_length_prefix(self.b.as_ref());
+
+        AdtEncoder(Encoder2::new(a, b))
+    }
+}
+
+/// A simple data type to use as list item.
+#[derive(Debug, Default, Clone)]
+pub struct Inner(u32);
+
+impl Inner {
+    /// Constructs a new `Inner`.
+    pub fn new(x: u32) -> Self { Self(x) }
+
+    /// Returns some meaningful 4 byte array for this type.
+    pub fn to_array(&self) -> [u8; 4] { self.0.to_be_bytes() }
+}
+
+encoding::encoder_newtype! {
+    /// The encoder for the [`Inner`] type.
+    pub struct InnerEncoder(ArrayEncoder<4>);
+}
+
+impl Encodable for Inner {
+    type Encoder<'e> = InnerEncoder;
+    fn encoder(&self) -> Self::Encoder<'_> {
+        InnerEncoder(ArrayEncoder::without_length_prefix(self.to_array()))
+    }
+}

consensus_encoding/src/encode/encoders.rs

@@ -14,7 +14,7 @@
 use internals::array_vec::ArrayVec;
 use internals::compact_size;
 
-use super::Encoder;
+use super::{Encodable, Encoder};
 
 /// The maximum length of a compact size encoding.
 const SIZE: usize = compact_size::MAX_ENCODING_SIZE;
@@ -76,6 +76,78 @@ impl<const N: usize> Encoder<'_> for ArrayEncoder<N> {
     }
 }
 
+/// An encoder for a list of encodable types.
+pub struct SliceEncoder<'e, T: Encodable> {
+    /// The list of references to the objects we are encoding.
+    ///
+    /// This is **never** mutated. All accesses are done by array accesses because
+    /// of lifetimes and the borrow checker.
+    sl: &'e [T],
+    /// The length prefix.
+    compact_size: Option<ArrayVec<u8, SIZE>>,
+    /// Index into `sl` of the element we are currently encoding.
+    cur_idx: usize,
+    /// Current encoder (for `sl[self.cur_idx]`).
+    cur_enc: Option<T::Encoder<'e>>,
+}
+
+impl<'e, T: Encodable> SliceEncoder<'e, T> {
+    /// Constructs an encoder which encodes the slice with a length prefix.
+    pub fn with_length_prefix(sl: &'e [T]) -> Self {
+        let len = sl.len();
+        let compact_size = Some(compact_size::encode(len));
+
+        // In this `map` call we cannot remove the closure. Seems to be a bug in the compiler.
+        // Perhaps https://github.com/rust-lang/rust/issues/102540 which is 3 years old with
+        // no replies or even an acknowledgement. We will not bother filing our own issue.
+        Self { sl, compact_size, cur_idx: 0, cur_enc: sl.first().map(|x| T::encoder(x)) }
+    }
+}
+
+impl<'e, T: Encodable> Encoder<'e> for SliceEncoder<'e, T> {
+    fn current_chunk(&self) -> Option<&[u8]> {
+        if let Some(compact_size) = self.compact_size.as_ref() {
+            return Some(compact_size);
+        }
+
+        // `advance` sets `cur_enc` to `None` once the slice encoder is completely exhausted.
+        // `current_chunk` is required to return `None` if called after the encoder is exhausted.
+        self.cur_enc.as_ref().and_then(T::Encoder::current_chunk)
+    }
+
+    fn advance(&mut self) -> bool {
+        let Some(cur) = self.cur_enc.as_mut() else {
+            return false;
+        };
+
+        loop {
+            if self.compact_size.is_some() {
+                // On the first call to advance(), just mark the compact_size as already
+                // yielded and leave self.cur_idx at 0.
+                self.compact_size = None;
+            } else {
+                // On subsequent calls, attempt to advance the current encoder and return
+                // success if this succeeds.
+                if cur.advance() {
+                    return true;
+                }
+                self.cur_idx += 1;
+            }
+
+            // If advancing the current encoder failed, attempt to move to the next encoder.
+            if let Some(x) = self.sl.get(self.cur_idx) {
+                *cur = x.encoder();
+                if cur.current_chunk().is_some() {
+                    return true;
+                }
+            } else {
+                self.cur_enc = None; // shortcut the next call to advance()
+                return false;
+            }
+        }
+    }
+}
+
 /// An encoder which encodes two objects, one after the other.
 pub struct Encoder2<A, B> {
     enc_idx: usize,
@@ -239,4 +311,4 @@ mod tests {
         let want = [0u8];
         assert_eq!(got, want);
     }
-}
+ }

consensus_encoding/src/lib.rs

@@ -31,6 +31,6 @@ pub use self::encode::encode_to_vec;
 #[cfg(feature = "std")]
 pub use self::encode::encode_to_writer;
 pub use self::encode::encoders::{
-    ArrayEncoder, BytesEncoder, Encoder2, Encoder3, Encoder4, Encoder6,
+    ArrayEncoder, BytesEncoder, Encoder2, Encoder3, Encoder4, Encoder6, SliceEncoder,
 };
 pub use self::encode::{encode_to_hash_engine, Encodable, Encoder};

consensus_encoding/tests/wrappers.rs

@@ -3,7 +3,7 @@
 #![cfg(feature = "std")]
 
 use consensus_encoding as encoding;
-use encoding::{ArrayEncoder, BytesEncoder, Encodable, Encoder2};
+use encoding::{ArrayEncoder, BytesEncoder, Encodable, Encoder2, SliceEncoder};
 
 encoding::encoder_newtype! {
     /// An encoder that uses an inner `ArrayEncoder`.
@@ -109,3 +109,47 @@ fn two_encoder() {
 
     assert_eq!(got, want);
 }
+
+#[test]
+fn slice_encoder() {
+    #[derive(Debug, Default, Clone)]
+    pub struct Test(Vec<Inner>);
+
+    encoding::encoder_newtype! {
+        /// An encoder that uses an inner `SliceEncoder`.
+        pub struct TestEncoder<'e>(SliceEncoder<'e, Inner>);
+    }
+
+    impl Encodable for Test {
+        type Encoder<'a>
+            = TestEncoder<'a>
+        where
+            Self: 'a;
+
+        fn encoder(&self) -> Self::Encoder<'_> {
+            TestEncoder(SliceEncoder::with_length_prefix(&self.0))
+        }
+    }
+
+    #[derive(Debug, Default, Clone)]
+    pub struct Inner(u32);
+
+    encoding::encoder_newtype! {
+        /// The encoder for the [`Inner`] type.
+        pub struct InnerArrayEncoder(ArrayEncoder<4>);
+    }
+
+    impl Encodable for Inner {
+        type Encoder<'e> = InnerArrayEncoder;
+        fn encoder(&self) -> Self::Encoder<'_> {
+            // Big-endian to make reading the test assertion easier.
+            InnerArrayEncoder(ArrayEncoder::without_length_prefix(self.0.to_be_bytes()))
+        }
+    }
+
+    let t = Test(vec![Inner(0xcafe_babe), Inner(0xdead_beef)]);
+    let encoded = encoding::encode_to_vec(&t);
+
+    let want = [0x02, 0xca, 0xfe, 0xba, 0xbe, 0xde, 0xad, 0xbe, 0xef];
+    assert_eq!(encoded, want);
+}