Add ssz examples, decoder struct

eth2/utils/ssz2/examples/struct_definition.rs

@@ -0,0 +1,58 @@
+use ssz2::{Decodable, DecodeError, Encodable, SszDecoderBuilder, SszStream};
+
+pub struct Foo {
+    a: u16,
+    b: Vec<u8>,
+    c: u16,
+}
+
+impl Encodable for Foo {
+    fn is_ssz_fixed_len() -> bool {
+        <u16 as Encodable>::is_ssz_fixed_len() && <Vec<u16> as Encodable>::is_ssz_fixed_len()
+    }
+
+    fn as_ssz_bytes(&self) -> Vec<u8> {
+        let mut stream = SszStream::new();
+
+        stream.append(&self.a);
+        stream.append(&self.b);
+        stream.append(&self.c);
+
+        stream.drain()
+    }
+}
+
+impl Decodable for Foo {
+    fn is_ssz_fixed_len() -> bool {
+        <u16 as Decodable>::is_ssz_fixed_len() && <Vec<u16> as Decodable>::is_ssz_fixed_len()
+    }
+
+    fn from_ssz_bytes(bytes: &[u8]) -> Result<Self, DecodeError> {
+        let mut builder = SszDecoderBuilder::new(bytes);
+
+        builder.register_type::<u16>()?;
+        builder.register_type::<Vec<u8>>()?;
+        builder.register_type::<u16>()?;
+
+        let mut decoder = builder.build()?;
+
+        Ok(Self {
+            a: decoder.decode_next()?,
+            b: decoder.decode_next()?,
+            c: decoder.decode_next()?,
+        })
+    }
+}
+
+fn main() {
+    let foo = Foo {
+        a: 42,
+        b: vec![0, 1, 2, 3],
+        c: 11,
+    };
+
+    assert_eq!(
+        foo.as_ssz_bytes(),
+        vec![42, 0, 8, 0, 0, 0, 11, 0, 0, 1, 2, 3]
+    );
+}

eth2/utils/ssz2/src/decode.rs

@@ -36,6 +36,144 @@ pub trait Decodable: Sized {
     fn from_ssz_bytes(bytes: &[u8]) -> Result<Self, DecodeError>;
 }
 
+#[derive(Copy, Clone)]
+pub struct Offset {
+    position: usize,
+    offset: usize,
+}
+
+pub struct SszDecoderBuilder<'a> {
+    bytes: &'a [u8],
+    items: Vec<&'a [u8]>,
+    offsets: Vec<Offset>,
+    items_index: usize,
+}
+
+impl<'a> SszDecoderBuilder<'a> {
+    pub fn new(bytes: &'a [u8]) -> Self {
+        Self {
+            bytes,
+            items: vec![],
+            offsets: vec![],
+            items_index: 0,
+        }
+    }
+
+    pub fn register_type<T: Decodable>(&mut self) -> Result<(), DecodeError> {
+        if T::is_ssz_fixed_len() {
+            let start = self.items_index;
+            self.items_index += T::ssz_fixed_len();
+
+            let slice = self.bytes.get(start..self.items_index).ok_or_else(|| {
+                DecodeError::InvalidByteLength {
+                    len: self.bytes.len(),
+                    expected: self.items_index,
+                }
+            })?;
+
+            self.items.push(slice);
+        } else {
+            let offset = read_offset(&self.bytes[self.items_index..])?;
+
+            let previous_offset = self
+                .offsets
+                .last()
+                .and_then(|o| Some(o.offset))
+                .unwrap_or_else(|| BYTES_PER_LENGTH_OFFSET);
+
+            if previous_offset > offset {
+                return Err(DecodeError::OutOfBoundsByte { i: offset });
+            } else if offset >= self.bytes.len() {
+                return Err(DecodeError::OutOfBoundsByte { i: offset });
+            }
+
+            self.offsets.push(Offset {
+                position: self.items.len(),
+                offset,
+            });
+
+            self.items_index += BYTES_PER_LENGTH_OFFSET;
+        }
+
+        Ok(())
+    }
+
+    fn apply_offsets(&mut self) -> Result<(), DecodeError> {
+        if !self.offsets.is_empty() {
+            let mut insertions = 0;
+            let mut running_offset = self.offsets[0].offset;
+
+            for i in 1..self.offsets.len() {
+                let (slice_option, position) = if i == self.offsets.len() {
+                    (self.bytes.get(running_offset..), self.offsets.len())
+                } else {
+                    let offset = self.offsets[i];
+                    let start = running_offset;
+                    running_offset = offset.offset;
+
+                    (self.bytes.get(start..running_offset), offset.position)
+                };
+
+                let slice = slice_option
+                    .ok_or_else(|| DecodeError::OutOfBoundsByte { i: running_offset })?;
+
+                self.items.insert(position + insertions, slice);
+                insertions += 1;
+            }
+        }
+
+        Ok(())
+    }
+
+    pub fn build(mut self) -> Result<SszDecoder<'a>, DecodeError> {
+        self.apply_offsets()?;
+
+        Ok(SszDecoder { items: self.items })
+    }
+}
+
+pub struct SszDecoder<'a> {
+    items: Vec<&'a [u8]>,
+}
+
+impl<'a> SszDecoder<'a> {
+    /// Decodes the next item.
+    ///
+    /// # Panics
+    ///
+    /// Panics when attempting to decode more items than actually exist.
+    pub fn decode_next<T: Decodable>(&mut self) -> Result<T, DecodeError> {
+        T::from_ssz_bytes(self.items.remove(0))
+    }
+}
+
+/// Reads a `BYTES_PER_LENGTH_OFFSET`-byte length from `bytes`, where `bytes.len() >=
+/// BYTES_PER_LENGTH_OFFSET`.
+fn read_offset(bytes: &[u8]) -> Result<usize, DecodeError> {
+    decode_offset(bytes.get(0..BYTES_PER_LENGTH_OFFSET).ok_or_else(|| {
+        DecodeError::InvalidLengthPrefix {
+            len: bytes.len(),
+            expected: BYTES_PER_LENGTH_OFFSET,
+        }
+    })?)
+}
+
+/// Decode bytes as a little-endian usize, returning an `Err` if `bytes.len() !=
+/// BYTES_PER_LENGTH_OFFSET`.
+fn decode_offset(bytes: &[u8]) -> Result<usize, DecodeError> {
+    let len = bytes.len();
+    let expected = BYTES_PER_LENGTH_OFFSET;
+
+    if len != expected {
+        Err(DecodeError::InvalidLengthPrefix { len, expected })
+    } else {
+        let mut array: [u8; BYTES_PER_LENGTH_OFFSET] = std::default::Default::default();
+        array.clone_from_slice(bytes);
+
+        Ok(u32::from_le_bytes(array) as usize)
+    }
+}
+
 /*
 
 /// Decode the given bytes for the given type

eth2/utils/ssz2/src/decode/impls.rs

@@ -28,6 +28,7 @@ macro_rules! impl_decodable_for_uint {
     };
 }
 
+impl_decodable_for_uint!(u8, 8);
 impl_decodable_for_uint!(u16, 16);
 impl_decodable_for_uint!(u32, 32);
 impl_decodable_for_uint!(u64, 64);
@@ -94,33 +95,6 @@ impl<T: Decodable> Decodable for Vec<T> {
     }
 }
 
-/// Reads a `BYTES_PER_LENGTH_OFFSET`-byte length from `bytes`, where `bytes.len() >=
-/// BYTES_PER_LENGTH_OFFSET`.
-fn read_offset(bytes: &[u8]) -> Result<usize, DecodeError> {
-    decode_offset(bytes.get(0..BYTES_PER_LENGTH_OFFSET).ok_or_else(|| {
-        DecodeError::InvalidLengthPrefix {
-            len: bytes.len(),
-            expected: BYTES_PER_LENGTH_OFFSET,
-        }
-    })?)
-}
-
-/// Decode bytes as a little-endian usize, returning an `Err` if `bytes.len() !=
-/// BYTES_PER_LENGTH_OFFSET`.
-pub fn decode_offset(bytes: &[u8]) -> Result<usize, DecodeError> {
-    let len = bytes.len();
-    let expected = BYTES_PER_LENGTH_OFFSET;
-
-    if len != expected {
-        Err(DecodeError::InvalidLengthPrefix { len, expected })
-    } else {
-        let mut array: [u8; BYTES_PER_LENGTH_OFFSET] = std::default::Default::default();
-        array.clone_from_slice(bytes);
-
-        Ok(u32::from_le_bytes(array) as usize)
-    }
-}
-
 /*
 use super::decode::decode_ssz_list;
 use super::ethereum_types::{Address, H256};

eth2/utils/ssz2/src/encode.rs

@@ -17,19 +17,26 @@ pub trait Encodable {
     }
 }
 
+pub struct VariableLengths {
+    pub fixed_bytes_position: usize,
+    pub variable_bytes_length: usize,
+}
+
 /// Provides a buffer for appending SSZ values.
 #[derive(Default)]
 pub struct SszStream {
-    fixed: Vec<u8>,
-    variable: Vec<u8>,
+    fixed_bytes: Vec<u8>,
+    variable_bytes: Vec<u8>,
+    variable_lengths: Vec<VariableLengths>,
 }
 
 impl SszStream {
     /// Create a new, empty stream for writing SSZ values.
     pub fn new() -> Self {
         SszStream {
-            fixed: vec![],
-            variable: vec![],
+            fixed_bytes: vec![],
+            variable_bytes: vec![],
+            variable_lengths: vec![],
         }
     }
 
@@ -38,18 +45,42 @@ impl SszStream {
         let mut bytes = item.as_ssz_bytes();
 
         if T::is_ssz_fixed_len() {
-            self.fixed.append(&mut bytes);
+            self.fixed_bytes.append(&mut bytes);
         } else {
-            self.fixed.append(&mut encode_length(bytes.len()));
-            self.variable.append(&mut bytes);
+            self.variable_lengths.push(VariableLengths {
+                fixed_bytes_position: self.fixed_bytes.len(),
+                variable_bytes_length: bytes.len(),
+            });
+
+            self.fixed_bytes
+                .append(&mut vec![0; BYTES_PER_LENGTH_OFFSET]);
+            self.variable_bytes.append(&mut bytes);
+        }
+    }
+
+    /// Update the offsets (if any) in the fixed-length bytes to correctly point to the values in
+    /// the variable length part.
+    pub fn apply_offsets(&mut self) {
+        let mut running_offset = self.fixed_bytes.len();
+
+        for v in &self.variable_lengths {
+            let offset = running_offset;
+            running_offset += v.variable_bytes_length;
+
+            self.fixed_bytes.splice(
+                v.fixed_bytes_position..v.fixed_bytes_position + BYTES_PER_LENGTH_OFFSET,
+                encode_length(offset),
+            );
         }
     }
 
     /// Append the variable-length bytes to the fixed-length bytes and return the result.
     pub fn drain(mut self) -> Vec<u8> {
-        self.fixed.append(&mut self.variable);
+        self.apply_offsets();
 
-        self.fixed
+        self.fixed_bytes.append(&mut self.variable_bytes);
+
+        self.fixed_bytes
     }
 }
 

eth2/utils/ssz2/src/lib.rs

@@ -16,7 +16,7 @@ pub mod decode;
 mod decode;
 mod encode;
 
-pub use decode::{Decodable, DecodeError};
+pub use decode::{Decodable, DecodeError, SszDecoderBuilder};
 pub use encode::{Encodable, SszStream};
 
 pub const BYTES_PER_LENGTH_OFFSET: usize = 4;