See #301: Speed-up js2python string conversion

src/hiwire.c

@@ -60,7 +60,7 @@ EM_JS(int, hiwire_string_ucs4, (int ptr, int len), {
   var jsstr = "";
   var idx = ptr / 4;
   for (var i = 0; i < len; ++i) {
-    jsstr += String.fromCharCode(Module.HEAPU32[idx + i]);
+    jsstr += String.fromCodePoint(Module.HEAPU32[idx + i]);
   }
   return Module.hiwire_new_value(jsstr);
 });

src/js2python.c

@@ -9,9 +9,15 @@
 // bubble out to Python
 
 int
-_js2python_string(char* val)
+_js2python_allocate_string(int size, int max_code_point)
 {
-  return (int)PyUnicode_FromString(val);
+  return (int)PyUnicode_New(size, max_code_point);
+}
+
+int
+_js2python_get_ptr(int obj)
+{
+  return (int)PyUnicode_DATA((PyObject*)obj);
 }
 
 int
@@ -68,9 +74,46 @@ EM_JS(int, __js2python, (int id), {
   var value = Module.hiwire_get_value(id);
   var type = typeof value;
   if (type === 'string') {
-    var charptr = allocate(intArrayFromString(value), 'i8', ALLOC_NORMAL);
+    // The general idea here is to allocate a Python string and then
-    var result = __js2python_string(charptr);
+    // have Javascript write directly into its buffer.  We first need
-    _free(charptr);
+    // to determine if is needs to be a 1-, 2- or 4-byte string, since
+    // Python handles all 3.
+    var max_code_point = 0;
+    for (var i = 0; i < value.length; i++) {
+      code_point = value.codePointAt(i);
+      max_code_point = Math.max(max_code_point, code_point);
+      if (max_code_point > 0xffff) {
+        // If we're dealing with UTF-16 surrogate pairs, convert the string
+        // to an array of each of its characters, so we correctly count the
+        // number of characters.
+        value = Array.from(value[Symbol.iterator]());
+        // We can short circuit here -- we already know we need a 4-byte output.
+        break;
+      }
+    }
+
+    var result = __js2python_allocate_string(value.length, max_code_point);
+    if (result == 0) {
+      return 0;
+    }
+
+    var ptr = __js2python_get_ptr(result);
+    if (max_code_point > 0xffff) {
+      ptr = ptr / 4;
+      for (var i = 0; i < value.length; i++) {
+        Module.HEAPU32[ptr + i] = value[i].codePointAt(0);
+      }
+    } else if (max_code_point > 0xff) {
+      ptr = ptr / 2;
+      for (var i = 0; i < value.length; i++) {
+        Module.HEAPU16[ptr + i] = value.codePointAt(i);
+      }
+    } else {
+      for (var i = 0; i < value.length; i++) {
+        Module.HEAPU8[ptr + i] = value.codePointAt(i);
+      }
+    }
+
     return result;
   } else if (type === 'number') {
     return __js2python_number(value);

test/test_python.py

@@ -34,6 +34,8 @@ def test_python2js(selenium):
         'return pyodide.runPython("\'ιωδιούχο\'") === "ιωδιούχο"')
     assert selenium.run_js(
         'return pyodide.runPython("\'碘化物\'") === "碘化物"')
+    assert selenium.run_js(
+        'return pyodide.runPython("\'🐍\'") === "🐍"')
     assert selenium.run_js(
         'let x = pyodide.runPython("b\'bytes\'");\n'
         'return (x instanceof window.Uint8ClampedArray) && '
@@ -156,7 +158,9 @@ def test_pythonexc2js(selenium):
 def test_js2python(selenium):
     selenium.run_js(
         """
-        window.jsstring = "碘化物";
+        window.jsstring_ucs1 = "pyodidé";
+        window.jsstring_ucs2 = "碘化物";
+        window.jsstring_ucs4 = "🐍";
         window.jsnumber0 = 42;
         window.jsnumber1 = 42.5;
         window.jsundefined = undefined;
@@ -170,8 +174,14 @@ def test_js2python(selenium):
         """
     )
     assert selenium.run(
-        'from js import jsstring\n'
+        'from js import jsstring_ucs1\n'
-        'jsstring == "碘化物"')
+        'jsstring_ucs1 == "pyodidé"')
+    assert selenium.run(
+        'from js import jsstring_ucs2\n'
+        'jsstring_ucs2 == "碘化物"')
+    assert selenium.run(
+        'from js import jsstring_ucs4\n'
+        'jsstring_ucs4 == "🐍"')
     assert selenium.run(
         'from js import jsnumber0\n'
         'jsnumber0 == 42')