fed-prims: Phase C — dag-cbor encode/decode, pure OCaml, RFC 8949 vectors + determinism

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_cbor.ml

@@ -0,0 +1,142 @@
+(** dag-cbor encode / decode — pure OCaml, WASM-safe.
+
+    RFC 8949 deterministic subset as constrained by IPLD dag-cbor
+    (RFC 8742): unsigned/negative ints, text strings, arrays, maps
+    with keys sorted by **length-then-bytewise**, bool, null, and
+    tag 42 (CID link, decode-side passthrough). Floats are not
+    supported (no fed-sx shape needs them yet) — encoding a [Number]
+    or decoding a float head raises. Reference: RFC 8949 §3, §4.2. *)
+
+open Sx_types
+
+exception Cbor_error of string
+
+(* ---- Encoder ---- *)
+
+let write_head buf major v =
+  let m = major lsl 5 in
+  if v < 24 then
+    Buffer.add_char buf (Char.chr (m lor v))
+  else if v < 0x100 then begin
+    Buffer.add_char buf (Char.chr (m lor 24));
+    Buffer.add_char buf (Char.chr v)
+  end else if v < 0x10000 then begin
+    Buffer.add_char buf (Char.chr (m lor 25));
+    Buffer.add_char buf (Char.chr ((v lsr 8) land 0xFF));
+    Buffer.add_char buf (Char.chr (v land 0xFF))
+  end else if v < 0x100000000 then begin
+    Buffer.add_char buf (Char.chr (m lor 26));
+    for i = 3 downto 0 do
+      Buffer.add_char buf (Char.chr ((v lsr (8 * i)) land 0xFF))
+    done
+  end else begin
+    Buffer.add_char buf (Char.chr (m lor 27));
+    for i = 7 downto 0 do
+      Buffer.add_char buf (Char.chr ((v lsr (8 * i)) land 0xFF))
+    done
+  end
+
+(* dag-cbor map key order: shorter key first, then bytewise. *)
+let key_order a b =
+  let la = String.length a and lb = String.length b in
+  if la <> lb then compare la lb else compare a b
+
+let rec encode_into buf (v : value) : unit =
+  match v with
+  | Integer n ->
+    if n >= 0 then write_head buf 0 n
+    else write_head buf 1 (-1 - n)
+  | String s ->
+    write_head buf 3 (String.length s);
+    Buffer.add_string buf s
+  | Symbol s | Keyword s ->
+    write_head buf 3 (String.length s);
+    Buffer.add_string buf s
+  | Bool false -> Buffer.add_char buf '\xf4'
+  | Bool true  -> Buffer.add_char buf '\xf5'
+  | Nil        -> Buffer.add_char buf '\xf6'
+  | List items ->
+    write_head buf 4 (List.length items);
+    List.iter (encode_into buf) items
+  | Dict d ->
+    let keys = Hashtbl.fold (fun k _ acc -> k :: acc) d [] in
+    let keys = List.sort_uniq key_order keys in
+    write_head buf 5 (List.length keys);
+    List.iter (fun k ->
+      write_head buf 3 (String.length k);
+      Buffer.add_string buf k;
+      encode_into buf (Hashtbl.find d k)) keys
+  | Number _ ->
+    raise (Cbor_error "cbor-encode: floats unsupported (dag-cbor subset)")
+  | _ ->
+    raise (Cbor_error
+             ("cbor-encode: unencodable value " ^ type_of v))
+
+let encode (v : value) : string =
+  let buf = Buffer.create 64 in
+  encode_into buf v;
+  Buffer.contents buf
+
+(* ---- Decoder ---- *)
+
+let decode (s : string) : value =
+  let pos = ref 0 in
+  let len = String.length s in
+  let byte () =
+    if !pos >= len then raise (Cbor_error "cbor-decode: truncated");
+    let c = Char.code s.[!pos] in incr pos; c
+  in
+  let read_uint ai =
+    if ai < 24 then ai
+    else if ai = 24 then byte ()
+    else if ai = 25 then let a = byte () in let b = byte () in (a lsl 8) lor b
+    else if ai = 26 then begin
+      let v = ref 0 in
+      for _ = 0 to 3 do v := (!v lsl 8) lor byte () done; !v
+    end else if ai = 27 then begin
+      let v = ref 0 in
+      for _ = 0 to 7 do v := (!v lsl 8) lor byte () done; !v
+    end else raise (Cbor_error "cbor-decode: bad additional info")
+  in
+  let read_bytes n =
+    if !pos + n > len then raise (Cbor_error "cbor-decode: truncated");
+    let r = String.sub s !pos n in pos := !pos + n; r
+  in
+  let rec item () =
+    let b = byte () in
+    let major = b lsr 5 and ai = b land 0x1f in
+    match major with
+    | 0 -> Integer (read_uint ai)
+    | 1 -> Integer (-1 - read_uint ai)
+    | 2 -> String (read_bytes (read_uint ai))
+    | 3 -> String (read_bytes (read_uint ai))
+    | 4 ->
+      let n = read_uint ai in
+      List (List.init n (fun _ -> item ()))
+    | 5 ->
+      let n = read_uint ai in
+      let d = make_dict () in
+      for _ = 1 to n do
+        let k = match item () with
+          | String k -> k
+          | _ -> raise (Cbor_error "cbor-decode: non-string map key")
+        in
+        Hashtbl.replace d k (item ())
+      done;
+      Dict d
+    | 6 ->
+      (* Tag: tag-42 CID link → pass the inner item through. *)
+      ignore (read_uint ai); item ()
+    | 7 ->
+      (match ai with
+       | 20 -> Bool false
+       | 21 -> Bool true
+       | 22 -> Nil
+       | 23 -> Nil
+       | _ ->
+         raise (Cbor_error
+                  "cbor-decode: floats/simple unsupported (dag-cbor subset)"))
+    | _ -> raise (Cbor_error "cbor-decode: bad major type")
+  in
+  let v = item () in
+  v

hosts/ocaml/lib/sx_primitives.ml

@@ -4174,4 +4174,18 @@ let () =
   register "crypto-sha3-256" (fun args ->
     match args with
     | [String s] -> String (Sx_sha3.sha3_256_hex s)
-    | _ -> raise (Eval_error "crypto-sha3-256: (bytes)"))
+    | _ -> raise (Eval_error "crypto-sha3-256: (bytes)"));
+
+  register "cbor-encode" (fun args ->
+    match args with
+    | [v] ->
+      (try String (Sx_cbor.encode v)
+       with Sx_cbor.Cbor_error m -> raise (Eval_error m))
+    | _ -> raise (Eval_error "cbor-encode: (value)"));
+
+  register "cbor-decode" (fun args ->
+    match args with
+    | [String s] ->
+      (try Sx_cbor.decode s
+       with Sx_cbor.Cbor_error m -> raise (Eval_error m))
+    | _ -> raise (Eval_error "cbor-decode: (bytes)"))