more work on the demo, looks pretty good

.gitattributes

@@ -9,3 +9,5 @@
 *.sh text
 *.txt text
 *.y text
+demo/primer.css binary
+demo/libwasm.js binary

demo/custom.css

@@ -0,0 +1,19 @@
+textarea {
+  width: 100%;
+  height: 400px;
+  min-height: 400px;
+  font-family: monospace;
+  resize: vertical;
+}
+
+.two-fifths {
+  width:40%;
+}
+
+.three-fifths {
+  width:60%;
+}
+
+.container {
+  width: 1200px;
+}

demo/demo.js

@@ -0,0 +1,205 @@
+/*
+ * Copyright 2016 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+var defaultIndent = '  ';
+var input = document.getElementById('input');
+var output = document.getElementById('output');
+var select = document.getElementById('select');
+
+function debounce(f, wait) {
+  var lastTime = 0;
+  var timeoutId = -1;
+  var wrapped = function() {
+    var time = +new Date();
+    if (time - lastTime < wait) {
+      if (timeoutId == -1)
+        timeoutId = setTimeout(wrapped, (lastTime + wait) - time);
+      return;
+    }
+    if (timeoutId != -1)
+      clearTimeout(timeoutId);
+    timeoutId = -1;
+    lastTime = time;
+    f.apply(null, arguments);
+  };
+  return wrapped;
+}
+
+function insertTextAtSelection(input, src) {
+  var selectionStart = input.selectionStart;
+  var selectionEnd = input.selectionEnd;
+  var oldValue = input.value;
+  input.value = oldValue.slice(0, selectionStart) + src +
+                oldValue.slice(selectionEnd);
+  input.selectionStart = input.selectionEnd = selectionStart + src.length;
+}
+
+function onInputKeyDown(e) {
+  if (e.keyCode == 9) {  // tab
+    insertTextAtSelection(this, defaultIndent);
+    e.preventDefault();
+  } else if (e.keyCode == 13) {  // newline
+    // count nesting depth
+    var parens = 0;
+    var lastOpen = -1;
+    var indent = '';
+    for (var i = this.selectionStart - 1; i >= 0; --i) {
+      var c = this.value[i];
+      if (c == '(') {
+        if (--parens < 0) {
+          if (lastOpen != -1)
+            i = lastOpen;
+          else
+            indent = defaultIndent;
+          break;
+        // find first sibling "(", if any
+        } else if (parens == 0 && lastOpen == -1) {
+          lastOpen = i;
+        }
+      } else if (c == ')') {
+        parens++;
+      }
+    }
+    // get column of current nesting
+    var col = 0;
+    for (; i > 0; --i) {
+      var c = this.value[i];
+      if (c == '\n') {
+        col--;  // went back too far
+        break;
+      } else {
+        col++;
+      }
+    }
+    // write newline, plus indentation
+    insertTextAtSelection(this, '\n' + ' '.repeat(col) + indent);
+    e.preventDefault();
+  }
+}
+
+function onError(loc, error, sourceLine, sourceLineColumnOffset) {
+  var lines = [
+    loc.filename + ':' + loc.line + ':' + loc.firstColumn,
+    error
+  ];
+  if (sourceLine.length > 0) {
+    var numSpaces = loc.firstColumn - 1 - sourceLineColumnOffset;
+    var numCarets = loc.lastColumn - loc.firstColumn;
+    lines.push(sourceLine);
+    lines.push(' '.repeat(numSpaces) + '^'.repeat(numCarets));
+  }
+  output.textContent += lines.join('\n') + '\n';
+}
+
+var allocator = wasm.LibcAllocator;
+var eh = new wasm.SourceErrorHandler(onError, 80);
+
+function compile(text) {
+  output.textContent = '';
+  try {
+    var buf = wasm.Buffer.fromString(text);
+    var lexer = wasm.Lexer.fromBuffer(allocator, 'test.wast', buf);
+    var script = wasm.parse(lexer, eh);
+    wasm.checkAst(lexer, script, eh);
+    var memoryWriter = new wasm.MemoryWriter(allocator);
+    var jsWriter = new wasm.JSStringWriter();
+    var logStream = new wasm.Stream(jsWriter.writer);
+    var options = new wasm.WriteBinaryOptions({logStream: logStream});
+    wasm.writeBinaryScript(allocator, memoryWriter.base, script, options);
+    output.textContent = jsWriter.string;
+  } catch (e) {
+    output.textContent += e.toString();
+  } finally {
+    if (options)
+      options.$destroy();
+    if (logStream)
+      logStream.$destroy();
+    if (script)
+      script.$destroy();
+    if (jsWriter)
+      jsWriter.$destroy();
+    if (memoryWriter)
+      memoryWriter.$destroy();
+    if (lexer)
+      lexer.$destroy();
+    if (buf)
+      buf.$destroy();
+  }
+}
+
+var compileInput = debounce(function() { compile(input.value); }, 100);
+
+function onInputInput(e) {
+  compileInput();
+}
+
+var examples = [
+  {
+    name: 'empty',
+    contents: '(module)'
+  },
+
+  {
+    name: 'simple',
+    contents:
+      '(module\n' +
+      '  (func $addTwo (param i32 i32) (result i32)\n' +
+      '    (i32.add\n' +
+      '      (get_local 0)\n' +
+      '      (get_local 1)))\n' +
+      '  (export "addTwo" $addTwo))\n'
+  },
+
+  {
+    name: 'factorial',
+    contents:
+      '(module\n' +
+      '  (func $fac (param i64) (result i64)\n' +
+      '    (if (i64.lt_s (get_local 0) (i64.const 0))\n' +
+      '      (then (i64.const 1))\n' +
+      '      (else\n' +
+      '        (i64.mul\n' +
+      '          (get_local 0)\n' +
+      '          (call $fac\n' +
+      '            (i64.sub\n' +
+      '              (get_local 0)\n' +
+      '              (i64.const 1)))))))\n' +
+      '  (export "fac" $fac))\n'
+  }
+];
+
+function setExample(index) {
+  var contents = examples[index].contents;
+  input.value = contents;
+  compileInput();
+}
+
+function onSelectChanged(e) {
+  setExample(this.selectedIndex);
+}
+
+input.addEventListener('keydown', onInputKeyDown);
+input.addEventListener('input', onInputInput);
+select.addEventListener('change', onSelectChanged);
+
+for (var i = 0; i < examples.length; ++i) {
+  var example = examples[i];
+  var option = document.createElement('option');
+  option.textContent = example.name;
+  select.appendChild(option);
+}
+select.selectedIndex = 1;
+setExample(select.selectedIndex);