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Merge pull request #787 from pipermerriam/piper/add-sos-style-offset-serialization-to-ssz

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Update SSZ spec to use SOS style offset based layout.
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Danny Ryan 2019-04-24 12:39:33 -06:00 committed by GitHub
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specs/simple-serialize.md
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@ -13,7 +13,7 @@ This is a **work in progress** describing typing, serialization and Merkleizatio
- [Serialization](#serialization)
- [`"uintN"`](#uintn)
- [`"bool"`](#bool)
- [Vectors, containers, lists](#vectors-containers-lists)
- [Containers, vectors, lists](#containers-vectors-lists)
- [Deserialization](#deserialization)
- [Merkleization](#merkleization)
- [Self-signed containers](#self-signed-containers)
@ -23,8 +23,9 @@ This is a **work in progress** describing typing, serialization and Merkleizatio
| Name | Value | Description |
|-|-|-|
| `BYTES_PER_CHUNK` | `32` | Number of bytes per chunk.
| `BYTES_PER_LENGTH_PREFIX` | `4` | Number of bytes per serialized length prefix. |
| `BYTES_PER_CHUNK` | `32` | Number of bytes per chunk. |
| `BYTES_PER_LENGTH_OFFSET` | `4` | Number of bytes per serialized length offset. |
| `BITS_PER_BYTE` | `8` | Number of bits per byte. |
## Typing
### Basic types
@ -59,7 +60,8 @@ The default value of a type upon initialization is recursively defined using `0`
We recursively define the `serialize` function which consumes an object `value` (of the type specified) and returns a bytestring of type `"bytes"`.
*Note*: In the function definitions below (`serialize`, `hash_tree_root`, `signing_root`, etc.) objects implicitly carry their type.
> *Note*: In the function definitions below (`serialize`, `hash_tree_root`, `signing_root`, `is_variable_size`, etc.) objects implicitly carry their type.
### `"uintN"`
@ -75,21 +77,24 @@ assert value in (True, False)
return b"\x01" if value is True else b"\x00"
```
### Vectors, containers, lists
If `value` is fixed-size:
### Containers, vectors, lists
```python
return "".join([serialize(element) for element in value])
```
# Reccursively serialize
fixed_parts = [serialize(element) if not is_variable_size(element) else None for element in value]
variable_parts = [serialize(element) if is_variable_size(element) else b"" for element in value]
If `value` is variable-size:
# Compute and check lengths
fixed_lengths = [len(part) if part != None else BYTES_PER_LENGTH_OFFSET for part in fixed_parts]
variable_lengths = [len(part) for part in variable_parts]
assert sum(fixed_lengths + variable_lengths) < 2**(BYTES_PER_LENGTH_OFFSET * BITS_PER_BYTE)
```python
serialized_bytes = "".join([serialize(element) for element in value])
assert len(serialized_bytes) < 2**(8 * BYTES_PER_LENGTH_PREFIX)
serialized_length = len(serialized_bytes).to_bytes(BYTES_PER_LENGTH_PREFIX, "little")
return serialized_length + serialized_bytes
# Interleave offsets of variable-size parts with fixed-size parts
variable_offsets = [serialize(sum(fixed_lengths + variable_lengths[:i])) for i in range(len(value))]
fixed_parts = [part if part != None else variable_offsets[i] for i, part in enumerate(fixed_parts)]
# Return the concatenation of the fixed-size parts (offsets interleaved) with the variable-size parts
return b"".join(fixed_parts + variable_parts)
```
## Deserialization