Break decode out into own file

ssz/src/decode.rs

@@ -0,0 +1,135 @@
+use super::{
+    LENGTH_BYTES,
+};
+
+#[derive(Debug)]
+pub enum DecodeError {
+    OutOfBounds,
+    TooShort,
+    TooLong,
+}
+
+pub trait Decodable: Sized {
+    fn ssz_decode(bytes: &[u8]) -> Result<Self, DecodeError>;
+}
+
+
+pub fn decode_ssz_list_element<T>(ssz_bytes: &[u8], n: usize)
+    -> Result<T, DecodeError>
+    where T: Decodable
+{
+    T::ssz_decode(nth_value(ssz_bytes, n)?)
+}
+
+fn nth_value(ssz_bytes: &[u8], n: usize)
+    -> Result<&[u8], DecodeError>
+{
+    let mut c: usize = 0;
+    for i in 0..(n + 1) {
+        let length = decode_length(&ssz_bytes[c..], LENGTH_BYTES)?;
+        let next = c + LENGTH_BYTES + length;
+
+        if i == n {
+            return Ok(&ssz_bytes[c + LENGTH_BYTES..next]);
+        } else {
+            if next >= ssz_bytes.len() {
+                return Err(DecodeError::OutOfBounds);
+            } else {
+                c = next;
+            }
+        }
+    }
+    Err(DecodeError::OutOfBounds)
+}
+
+fn decode_length(bytes: &[u8], length_bytes: usize)
+    -> Result<usize, DecodeError>
+{
+    if bytes.len() < length_bytes {
+        return Err(DecodeError::TooShort);
+    };
+    let mut len: usize = 0;
+    for i in 0..length_bytes {
+        let offset = (length_bytes - i - 1) * 8;
+        len = ((bytes[i] as usize) << offset) | len;
+    };
+    Ok(len)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use super::super::encode_length;
+
+    #[test]
+    fn test_ssz_decode_length() {
+        let decoded = decode_length(
+            &vec![0, 0, 0, 1],
+            LENGTH_BYTES);
+        assert_eq!(decoded.unwrap(), 1);
+
+        let decoded = decode_length(
+            &vec![0, 0, 1, 0],
+            LENGTH_BYTES);
+        assert_eq!(decoded.unwrap(), 256);
+
+        let decoded = decode_length(
+            &vec![0, 0, 1, 255],
+            LENGTH_BYTES);
+        assert_eq!(decoded.unwrap(), 511);
+
+        let decoded = decode_length(
+            &vec![255, 255, 255, 255],
+            LENGTH_BYTES);
+        assert_eq!(decoded.unwrap(), 4294967295);
+    }
+
+    #[test]
+    fn test_encode_decode_length() {
+        let params: Vec<usize> = vec![
+            0, 1, 2, 3, 7, 8, 16,
+            2^8,  2^8  + 1,
+            2^16, 2^16 + 1,
+            2^24, 2^24 + 1,
+            2^32,
+        ];
+        for i in params {
+            let decoded = decode_length(
+                &encode_length(i, LENGTH_BYTES),
+                LENGTH_BYTES).unwrap();
+            assert_eq!(i, decoded);
+        }
+    }
+
+    #[test]
+    fn test_ssz_nth_value() {
+        let ssz = vec![0, 0, 0, 1, 0];
+        let result = nth_value(&ssz, 0).unwrap();
+        assert_eq!(result, vec![0].as_slice());
+
+        let ssz = vec![0, 0, 0, 4, 1, 2, 3, 4];
+        let result = nth_value(&ssz, 0).unwrap();
+        assert_eq!(result, vec![1, 2, 3, 4].as_slice());
+
+        let ssz = vec![0, 0, 0, 1, 0, 0, 0, 0, 1, 1];
+        let result = nth_value(&ssz, 1).unwrap();
+        assert_eq!(result, vec![1].as_slice());
+
+        let mut ssz = vec![0, 0, 1, 255];
+        ssz.append(&mut vec![42; 511]);
+        let result = nth_value(&ssz, 0).unwrap();
+        assert_eq!(result, vec![42; 511].as_slice());
+    }
+
+    /*
+    #[test]
+    fn test_ssz_decode_u16() {
+        let x: u16 = 100;
+        let mut s = SszStream::new();
+        s.append(&x);
+        let y: u16 = u16::ssz_decode(s.nth_value(0).unwrap())
+            .unwrap();
+        assert_eq!(x, y);
+    }
+    */
+}