cbor

Minimal CBOR (RFC 8949) codec for the rpcv2Cbor protocol.

AWS’s rpcv2Cbor wire form is the canonical / deterministic CBOR subset: definite-length arrays + maps, no tags, no indefinite-length items, sort keys lexicographically when encoding maps. The decoder is more permissive — it accepts indefinite-length items too because real services have been observed shipping them.

Major types this implementation covers:

0 unsigned int — 1/2/3/5/9-byte head encoding
1 negative int — 1/2/3/5/9-byte head
2 byte string — length-prefixed
3 text string — length-prefixed UTF-8
4 array — definite-length on encode, both on decode
5 map — definite-length on encode, both on decode
7 simple values + floats — false/true/null/float64

Not yet covered: tags (major type 6), undefined (simple 23), half-float (0xF9), 32-bit float (0xFA). Half- and 32-bit floats would surface only from CBOR senders explicitly downcasting; the AWS rpcv2Cbor services we’ve seen always send float64. Tags are reserved for date-time / bignum encodings the rpcv2Cbor spec excludes from request/response bodies.

Types

Value

</>

pub type Value {
  CInt(value: Int)
  CFloat(value: Float)
  CBool(value: Bool)
  CNull
  CString(value: String)
  CBytes(value: BitArray)
  CList(items: List(Value))
  CMap(entries: List(#(Value, Value)))
}

Constructors

```
CInt(value: Int)
```
```
CFloat(value: Float)
```
```
CBool(value: Bool)
```
```
CNull
```
```
CString(value: String)
```
```
CBytes(value: BitArray)
```
```
CList(items: List(Value))
```
```
CMap(entries: List(#(Value, Value)))
```
Map values are stored as a List rather than a Dict so the encoder can preserve insertion order — rpcv2Cbor only cares about lexicographic key order on encode, which the encoder enforces, but on decode the original order is observable for downstream consumers (e.g. for diffing against a deterministic reference).

Values

decode

</>

pub fn decode(
  bytes: BitArray,
) -> Result(#(Value, BitArray), String)

Decode a CBOR byte stream into a Value. Returns the leftover bytes on success so callers can decode multiple items from one buffer; rpcv2Cbor request bodies are single items, so most callers can just drop the leftover.

decode_value

</>

pub fn decode_value(bytes: BitArray) -> Result(Value, String)

Convenience helper for callers that just want the decoded value and don’t care about the trailing bytes (the common rpcv2Cbor request/response body case).

encode

</>

pub fn encode(v: Value) -> BitArray

Encode a Value to its canonical CBOR byte stream.

get_field

</>

pub fn get_field(map: Value, key: String) -> option.Option(Value)

option.None (None) when looking up a key that’s not present in a CMap. Used by hand-written decoders that pluck specific fields out of a CBOR-decoded map.