diff --git a/implementations/rust/src/lib.rs b/implementations/rust/src/lib.rs
index c336852..46d58dd 100644
--- a/implementations/rust/src/lib.rs
+++ b/implementations/rust/src/lib.rs
@@ -172,3 +172,21 @@ mod value_tests {
         assert_eq!(r, s);
     }
 }
+
+#[cfg(test)]
+mod decoder_tests {
+    use crate::value::codec::Decoder;
+
+    #[test] fn read_123() {
+        let mut d = Decoder::new(&b"abc"[..], None);
+        assert_eq!(d.read().ok(), Some(97));
+        assert_eq!(d.read().ok(), Some(98));
+        assert_eq!(d.read().ok(), Some(99));
+        assert!(d.read().err().unwrap().is_eof())
+    }
+
+    #[test] fn read_samples() {
+        let mut d = Decoder::new(std::fs::File::open("../../tests/samples.bin").unwrap(), None);
+        println!("{:?}", d.next().ok().unwrap());
+    }
+}
diff --git a/implementations/rust/src/value/codec.rs b/implementations/rust/src/value/codec.rs
new file mode 100644
index 0000000..a9c0613
--- /dev/null
+++ b/implementations/rust/src/value/codec.rs
@@ -0,0 +1,267 @@
+use std::io::Read;
+use std::convert::TryInto;
+use crate::value::value::{Value, AValue, Set, Dictionary};
+use num::bigint::BigInt;
+
+pub type Result<V> = std::result::Result<V, Error>;
+
+#[derive(Debug)]
+pub enum Error {
+    Io(std::io::Error),
+    Syntax(&'static str),
+    Eof,
+}
+
+impl From<std::io::Error> for Error {
+    fn from(v: std::io::Error) -> Self {
+        Error::Io(v)
+    }
+}
+
+impl From<std::str::Utf8Error> for Error {
+    fn from(_v: std::str::Utf8Error) -> Self {
+        Error::Syntax("Invalid UTF-8")
+    }
+}
+
+impl Error {
+    pub fn is_io(&self) -> bool { if let Error::Io(_) = *self { true } else { false } }
+    pub fn is_syntax(&self) -> bool { if let Error::Syntax(_) = *self { true } else { false } }
+    pub fn is_eof(&self) -> bool { if let Error::Eof = *self { true } else { false } }
+}
+
+pub type PlaceholderMap = std::collections::BTreeMap<usize, Value>;
+
+pub struct Decoder<R: Read> {
+    read: R,
+    index: usize,
+    buf: Box<Option<Option<u8>>>,
+    placeholders: PlaceholderMap,
+}
+
+impl<R: Read> Decoder<R> {
+    pub fn new(read: R, placeholders: Option<PlaceholderMap>) -> Self {
+        Decoder{
+            read,
+            index: 0,
+            buf: Box::new(None),
+            placeholders: placeholders.unwrap_or(PlaceholderMap::new())
+        }
+    }
+
+    fn prime_if_possible(&mut self) -> Result<()> {
+        match *self.buf {
+            None => {
+                let bs = &mut [0];
+                *self.buf = Some(match self.read.read(bs)? {
+                    0 => None,
+                    1 => {
+                        self.index = self.index + 1;
+                        Some(bs[0])
+                    },
+                    _ => panic!("buffer overrun")
+                })
+            }
+            Some(_) => ()
+        }
+        Ok(())
+    }
+
+    pub fn skip(&mut self) -> Result<()> {
+        self.prime_if_possible()?;
+        *self.buf = None;
+        Ok(())
+    }
+
+    pub fn peek(&mut self) -> Result<u8> {
+        self.prime_if_possible()?;
+        match *self.buf {
+            Some(Some(v)) => Ok(v),
+            Some(None) => Err(Error::Eof),
+            None => panic!()
+        }
+    }
+
+    pub fn read(&mut self) -> Result<u8> {
+        let v = self.peek()?;
+        self.skip()?;
+        Ok(v)
+    }
+
+    pub fn readbytes(&mut self, n: usize) -> Result<Vec<u8>> {
+        if (*self.buf).is_some() {
+            panic!();
+        }
+        let mut bs = vec![0; n];
+        match self.read.read_exact(&mut bs) {
+            Ok(()) => Ok(bs),
+            Err(e) =>
+                if e.kind() == std::io::ErrorKind::UnexpectedEof {
+                    Err(Error::Eof)
+                } else {
+                    Err(Error::from(e))
+                }
+        }
+    }
+
+    pub fn readvalues(&mut self, mut count: usize) -> Result<Vec<AValue>> {
+        let mut pieces: Vec<AValue> = Vec::new();
+        while count > 0 {
+            pieces.push(self.next()?);
+            count = count - 1;
+        }
+        Ok(pieces)
+    }
+
+    pub fn nextop(&mut self) -> Result<(u8, u8, u8)> {
+        let b = self.read()?;
+        let major = b >> 6;
+        let minor = (b >> 4) & 3;
+        let arg = b & 15;
+        Ok((major, minor, arg))
+    }
+
+    pub fn varint(&mut self) -> Result<usize> {
+        let v = self.read()?;
+        if v < 128 {
+            Ok(usize::from(v))
+        } else {
+            Ok(self.varint()? * 128 + usize::from(v - 128))
+        }
+    }
+
+    pub fn wirelength(&mut self, arg: u8) -> Result<usize> {
+        if arg < 15 {
+            Ok(usize::from(arg))
+        } else {
+            self.varint()
+        }
+    }
+
+    pub fn peekend(&mut self) -> Result<bool> {
+        if self.peek()? == 4 {
+            self.skip()?;
+            Ok(true)
+        } else {
+            Ok(false)
+        }
+    }
+
+    pub fn decodeint(bs: &[u8]) -> BigInt {
+        BigInt::from_signed_bytes_be(bs)
+    }
+
+    pub fn decodebinary(minor: u8, bs: Vec<u8>) -> Result<AValue> {
+        Ok(match minor {
+            0 => Value::from(Self::decodeint(&bs)).wrap(),
+            1 => Value::from(std::str::from_utf8(&bs)?).wrap(),
+            2 => Value::from(bs).wrap(),
+            3 => Value::symbol(std::str::from_utf8(&bs)?).wrap(),
+            _ => panic!()
+        })
+    }
+
+    pub fn decodecompound(minor: u8, mut pieces: Vec<AValue>) -> Result<AValue> {
+        match minor {
+            0 =>
+                if pieces.len() == 0 {
+                    Err(Error::Syntax("Too few elements in encoded record"))
+                } else {
+                    let label = pieces.remove(0).rc();
+                    Ok(Value::record(&label, pieces).wrap())
+                },
+            1 => Ok(Value::from(pieces).wrap()),
+            2 => {
+                let mut s = Set::new();
+                while let Some(v) = pieces.pop() {
+                    s.insert(v);
+                }
+                Ok(Value::Set(s).wrap())
+            }
+            3 =>
+                if pieces.len() % 2 != 0 {
+                    Err(Error::Syntax("Missing dictionary value"))
+                } else {
+                    let mut d = Dictionary::new();
+                    while let Some(v) = pieces.pop() {
+                        let k = pieces.pop().unwrap();
+                        d.insert(k, v);
+                    }
+                    Ok(Value::Dictionary(d).wrap())
+                },
+            _ => panic!()
+        }
+    }
+
+    pub fn binarystream(&mut self, minor: u8) -> Result<AValue> {
+        let mut bs: Vec<u8> = Vec::new();
+        while !self.peekend()? {
+            match self.next()?.1.as_bytestring() {
+                Some(chunk) => bs.extend_from_slice(chunk),
+                None => return Err(Error::Syntax("Unexpected non-binary chunk")),
+            }
+        }
+        Self::decodebinary(minor, bs)
+    }
+
+    pub fn valuestream(&mut self, minor: u8) -> Result<AValue> {
+        let mut pieces: Vec<AValue> = Vec::new();
+        while !self.peekend()? {
+            pieces.push(self.next()?);
+        }
+        Self::decodecompound(minor, pieces)
+    }
+
+    pub fn next(&mut self) -> Result<AValue> {
+        match self.nextop()? {
+            (0, 0, 0) => Ok(Value::from(false).wrap()),
+            (0, 0, 1) => Ok(Value::from(true).wrap()),
+            (0, 0, 2) => {
+                let bs: &[u8] = &self.readbytes(4)?;
+                Ok(Value::from(f32::from_bits(u32::from_be_bytes(bs.try_into().unwrap()))).wrap())
+            }
+            (0, 0, 3) => {
+                let bs: &[u8] = &self.readbytes(8)?;
+                Ok(Value::from(f64::from_bits(u64::from_be_bytes(bs.try_into().unwrap()))).wrap())
+            }
+            (0, 0, 5) => {
+                let a = self.next()?;
+                let mut v = self.next()?;
+                v.annotations_mut().push(a);
+                Ok(v)
+            }
+            (0, 0, _) => Err(Error::Syntax("Invalid format A encoding")),
+            (0, 1, arg) => {
+                let n = self.wirelength(arg)?;
+                match self.placeholders.get(&n) {
+                    Some(v) => Ok(v.clone().wrap()),
+                    None => Err(Error::Syntax("Invalid Preserves placeholder")),
+                }
+            }
+            (0, 2, arg) => {
+                let t = arg >> 2;
+                let n = arg & 3;
+                match t {
+                    1 => self.binarystream(n),
+                    2 => self.valuestream(n),
+                    _ => Err(Error::Syntax("Invalid format C start byte")),
+                }
+            }
+            (0, 3, arg) => {
+                let n = if arg > 12 { i32::from(arg) - 16 } else { i32::from(arg) };
+                Ok(Value::from(n).wrap())
+            }
+            (0, _, _) => panic!(),
+            (1, minor, arg) => {
+                let n = self.wirelength(arg)?;
+                Self::decodebinary(minor, self.readbytes(n)?)
+            }
+            (2, minor, arg) => {
+                let n = self.wirelength(arg)?;
+                Self::decodecompound(minor, self.readvalues(n)?)
+            }
+            (3, _, _) => Err(Error::Syntax("Invalid lead byte (major 3)")),
+            (_, _, _) => panic!(),
+        }
+    }
+}
diff --git a/implementations/rust/src/value/mod.rs b/implementations/rust/src/value/mod.rs
index fc45e21..6ee32f0 100644
--- a/implementations/rust/src/value/mod.rs
+++ b/implementations/rust/src/value/mod.rs
@@ -1 +1,2 @@
 pub mod value;
+pub mod codec;
diff --git a/implementations/rust/src/value/value.rs b/implementations/rust/src/value/value.rs
index 54bdc07..28c7def 100644
--- a/implementations/rust/src/value/value.rs
+++ b/implementations/rust/src/value/value.rs
@@ -21,26 +21,30 @@ pub enum Value {
     Symbol(String),
     Record(Record),
     Sequence(Vec<AValue>),
-    Set(BTreeSet<AValue>),
-    Dictionary(BTreeMap<AValue, AValue>),
+    Set(Set),
+    Dictionary(Dictionary),
 }
 
 /// An possibly-annotated Value, with annotations (themselves
 /// possibly-annotated) in order of appearance.
 #[derive(Clone, Debug)]
-pub struct AValue(Vec<AValue>, Value);
+pub struct AValue(pub Vec<AValue>, pub Value);
 
 /// Single-precision IEEE 754 Value
 #[derive(Clone, Debug)]
-pub struct Float(f32);
+pub struct Float(pub f32);
 
 /// Double-precision IEEE 754 Value
 #[derive(Clone, Debug)]
-pub struct Double(f64);
+pub struct Double(pub f64);
 
 /// A Record `Value`
 pub type Record = (Rc<AValue>, Vec<AValue>);
 
+pub type Set = BTreeSet<AValue>;
+
+pub type Dictionary = BTreeMap<AValue, AValue>;
+
 impl From<f32> for Float {
     fn from(v: f32) -> Self {
         Float(v)
@@ -174,6 +178,8 @@ impl From<String> for Value { fn from(v: String) -> Self { Value::String(v) } }
 impl From<&[u8]> for Value { fn from(v: &[u8]) -> Self { Value::ByteString(Vec::from(v)) } }
 impl From<Vec<u8>> for Value { fn from(v: Vec<u8>) -> Self { Value::ByteString(v) } }
 
+impl From<Vec<AValue>> for Value { fn from(v: Vec<AValue>) -> Self { Value::Sequence(v) } }
+
 //---------------------------------------------------------------------------
 
 impl AValue {
@@ -395,7 +401,7 @@ impl Value {
         self.as_set().is_some()
     }
 
-    pub fn as_set(&self) -> Option<&BTreeSet<AValue>> {
+    pub fn as_set(&self) -> Option<&Set> {
         if let Value::Set(ref s) = *self {
             Some(s)
         } else {
@@ -403,7 +409,7 @@ impl Value {
         }
     }
 
-    pub fn as_set_mut(&mut self) -> Option<&mut BTreeSet<AValue>> {
+    pub fn as_set_mut(&mut self) -> Option<&mut Set> {
         if let Value::Set(ref mut s) = *self {
             Some(s)
         } else {
@@ -415,7 +421,7 @@ impl Value {
         self.as_dictionary().is_some()
     }
 
-    pub fn as_dictionary(&self) -> Option<&BTreeMap<AValue, AValue>> {
+    pub fn as_dictionary(&self) -> Option<&Dictionary> {
         if let Value::Dictionary(ref s) = *self {
             Some(s)
         } else {
@@ -423,7 +429,7 @@ impl Value {
         }
     }
 
-    pub fn as_dictionary_mut(&mut self) -> Option<&mut BTreeMap<AValue, AValue>> {
+    pub fn as_dictionary_mut(&mut self) -> Option<&mut Dictionary> {
         if let Value::Dictionary(ref mut s) = *self {
             Some(s)
         } else {