fix(js): utilize JSOO externals to convert between uint8array & bytes

js/dune

@@ -5,6 +5,9 @@
  (libraries js_of_ocaml)
  (preprocess
   (pps js_of_ocaml-ppx))
+ (foreign_stubs
+  (language c)
+  (names stubs))
  (js_of_ocaml
   (flags --no-sourcemap)
   (javascript_files binaryen.js postlude.js)))

js/module.ml

@@ -1,28 +1,12 @@
-open Js_of_ocaml
 open Js_of_ocaml.Js
 open Js_of_ocaml.Js.Unsafe
 
-let make_encoder () = new_obj global ##. TextEncoder [||]
+external u8a_to_bytes : 'a -> bytes = "caml_bytes_of_array"
 
-let encode encoder (value : string) =
-  meth_call encoder "encode" [| inject (string value) |]
+(* TODO: Verify this converts to bytes correctly? *)
+external bytes_to_u8a : bytes -> 'a = "caml_array_of_bytes"
 
-let make_decoder () = new_obj global ##. TextDecoder [||]
-
-let decode decoder u8a = to_string (meth_call decoder "decode" [| inject u8a |])
-
-(* Uint8Array/Byte utilities *)
-let u8a_to_bytes u8a =
-  let decoder = make_decoder () in
-  Bytes.of_string (decode decoder u8a)
-
-let bytes_to_u8a byts =
-  let encoder = make_encoder () in
-  encode encoder (Bytes.to_string byts)
-
-let string_to_u8a str =
-  let encoder = make_encoder () in
-  encode encoder str
+external string_to_u8a : string -> 'a = "caml_array_of_string"
 
 type t
 
@@ -108,7 +92,6 @@ let run_passes wasm_mod passes =
 
 let auto_drop wasm_mod = meth_call wasm_mod "autoDrop" [||]
 
-(* TODO: This returns the wrong type, need to convert from `Uint8Array` to `bytes` *)
 let write wasm_mod sourcemap_url =
   match sourcemap_url with
   | Some url ->
@@ -124,7 +107,6 @@ let write_text wasm_mod =
   let text = meth_call wasm_mod "emitText" [||] in
   to_string text
 
-(* TODO: This doesn't handle `bytes` correctly *)
 let read byts =
   let data = bytes_to_u8a byts in
   meth_call global##.binaryen "readBinary" [| inject data |]

js/stubs.c

@@ -0,0 +1,15 @@
+// Ref https://github.com/ocsigen/js_of_ocaml/issues/804#issuecomment-495826520
+#include <stdlib.h>
+#include <stdio.h>
+void caml_bytes_of_array () {
+  fprintf(stderr, "Unimplemented Javascript primitive caml_bytes_of_array!\n");
+  exit(1);
+}
+void caml_array_of_bytes () {
+  fprintf(stderr, "Unimplemented Javascript primitive caml_array_of_bytes!\n");
+  exit(1);
+}
+void caml_array_of_string () {
+  fprintf(stderr, "Unimplemented Javascript primitive caml_array_of_string!\n");
+  exit(1);
+}