// test.wasm is the following:

// (module
//   (func $div_s (param i32) (param i32)  (result i32)
//    (get_local 0)
//    (get_local 1)
//    (i32.div_s)
//   )
//   (func $div_u (param i32) (param i32)  (result i32)
//    (get_local 0)
//    (get_local 1)
//    (i32.div_u)
//   )
//   (func $echo (param i32) (result i32)
//    (get_local 0)
//   )
//   (export "echo" (func $echo))
//   (export "div_s" (func $div_s))
//   (export "div_u" (func $div_u))
// )


fetch('test.wasm').then(response =>
  response.arrayBuffer()
).then(bytes =>
  WebAssembly.instantiate(bytes)
).then(module => {
  
  const {div_s, div_u, echo} = module.instance.exports;
  
  // As per the wasm spec, the ToWebAssemblyValue algoritm coerces a JS value
  // into a wasm value
  // https://webassembly.github.io/spec/js-api/index.html#exported-function-exotic-objects
  // for i32, this is the ToInt32 as described here:
  // https://tc39.github.io/ecma262/#sec-toint32
  const positiveMod = (a, n) => {
    const remain = a % n;
    return remain > 0 ? remain : remain + n;
  };
  const toWebAssemblyValue = value => {
    const sign = value > 0 ? 1 : -1;
    const ints = sign * Math.floor(Math.abs(value));
    const int32bit = positiveMod(ints, Math.pow(2, 32));
    return int32bit >= Math.pow(2, 31) ? int32bit - Math.pow(2, 32) : int32bit;
  };
  
  // The inverse algorithm, ToJSValue, is slighty more vaguelly described 
  // as " the Number value for i32"
  // my guess is that this is the following conversion
  const toJSValue = value => value | 0;
  
  // testing the conversion to wasm and back to JS, via a simple echo function
  // that simply returns the input argument
  const value = 335991768761;
  console.log("echo wasm", m.exports.echo(value));
  console.log("echo js", toJSValue(toWebAssemblyValue(value)));
  
  // this works just fine!
  
  // unsigned division - here's a problem case (found through random number testing)
  const a = 335991768761;
  const b = 77022195821; 

  console.log("div_u wasm", m.exports.div_u(a, b));
  console.log("div_u js",
    toJSValue(div_u(toWebAssemblyValue(a), toWebAssemblyValue(b)))
  );
  
  // these give a different result :-(
});