WASM binding

init-wasm.js

const WASM_FILE = "./main.wasm";

const W = 640;
const H = 360;

let mem_in, mem_out;
let mem_bg_img;
let instance = null;
let memoryStates = new WeakMap();

// basic system calls from the wasm module

function syscall(instance, n, args) {
  switch (n) {
    default:
      // console.log("Syscall " + n + " NYI.");
      break;
    case /* brk */ 45:
      return 0;
    case /* writev */ 146:
      return instance.exports.writev_c(args[0], args[1], args[2]);
    case /* mmap2 */ 192:
      debugger;
      const memory = instance.exports.memory;
      let memoryState = memoryStates.get(instance);
      const requested = args[1];
      if (!memoryState) {
        memoryState = {
          object: memory,
          currentPosition: memory.buffer.byteLength,
        };
        memoryStates.set(instance, memoryState);
      }
      let cur = memoryState.currentPosition;
      if (cur + requested > memory.buffer.byteLength) {
        const need = Math.ceil(
          (cur + requested - memory.buffer.byteLength) / 65536
        );
        memory.grow(need);
      }
      memoryState.currentPosition += requested;
      return cur;
  }
}

const initWASM = () =>
  fetch(WASM_FILE)
    .then((response) => response.arrayBuffer())
    .then((bytes) =>
      WebAssembly.instantiate(bytes, {
        env: {
          __syscall0: function __syscall0(n) {
            return syscall(instance, n, []);
          },
          __syscall1: function __syscall1(n, a) {
            return syscall(instance, n, [a]);
          },
          __syscall2: function __syscall2(n, a, b) {
            return syscall(instance, n, [a, b]);
          },
          __syscall3: function __syscall3(n, a, b, c) {
            return syscall(instance, n, [a, b, c]);
          },
          __syscall4: function __syscall4(n, a, b, c, d) {
            return syscall(instance, n, [a, b, c, d]);
          },
          __syscall5: function __syscall5(n, a, b, c, d, e) {
            return syscall(instance, n, [a, b, c, d, e]);
          },
          __syscall6: function __syscall6(n, a, b, c, d, e, f) {
            return syscall(instance, n, [a, b, c, d, e, f]);
          }
        }
      })
    )
    .then((results) => {
      instance = results.instance;
      
      // instruct the module to allocate the shared buffers
      const in_p = instance.exports.init_in(W, H);
      const out_p = instance.exports.init_out();
      const bg_p = instance.exports.init_bg_img();
      
      // create byte type view buffers
      mem_in = new Uint8Array(
        instance.exports.memory.buffer,
        in_p,
        W * H * 1.5
      );
      mem_out = new Uint8Array(
        instance.exports.memory.buffer,
        out_p,
        W * H * 4
      );
      mem_bg_img = new Uint8Array(
        instance.exports.memory.buffer,
        bg_p,
        W * H * 4
      );
    })
    .catch(console.error);