Restore external RS compiler

While webengine is faster that duktape, the external compiler
still beats it.

src/calibre/utils/rapydscript.py

@@ -10,6 +10,7 @@ import re
 import subprocess
 from io import BytesIO
 
+from calibre import force_unicode
 from calibre.constants import FAKE_HOST, FAKE_PROTOCOL, __appname__, __version__
 from calibre.ptempfile import TemporaryDirectory
 from calibre.utils.filenames import atomic_rename
@@ -214,8 +215,51 @@ def compile_pyj(data, filename='<stdin>', beautify=True, private_scope=True, lib
 has_external_compiler = None
 
 
+def detect_external_compiler():
+    from calibre.utils.filenames import find_executable_in_path
+    rs = find_executable_in_path('rapydscript')
+    try:
+        raw = subprocess.check_output([rs, '--version'])
+    except Exception:
+        raw = b''
+    if raw.startswith(b'rapydscript-ng '):
+        ver = raw.partition(b' ')[-1]
+        try:
+            ver = tuple(map(int, ver.split(b'.')))
+        except Exception:
+            ver = (0, 0, 0)
+        if ver >= (0, 7, 5):
+            return rs
+    return False
+
+
 def compile_fast(data, filename=None, beautify=True, private_scope=True, libdir=None, omit_baselib=False, js_version=None):
-    return compile_pyj(data, filename or '<stdin>', beautify, private_scope, libdir, omit_baselib, js_version or 6)
+    global has_external_compiler
+    if has_external_compiler is None:
+        has_external_compiler = detect_external_compiler()
+    if not has_external_compiler:
+        return compile_pyj(data, filename or '<stdin>', beautify, private_scope, libdir, omit_baselib, js_version or 6)
+    args = ['--cache-dir', module_cache_dir()]
+    if libdir:
+        args += ['--import-path', libdir]
+    if not beautify:
+        args.append('--uglify')
+    if not private_scope:
+        args.append('--bare')
+    if omit_baselib:
+        args.append('--omit-baselib')
+    if js_version:
+        args.append('--js-version=' + str(js_version))
+    if not isinstance(data, bytes):
+        data = data.encode('utf-8')
+    if filename:
+        args.append('--filename-for-stdin'), args.append(filename)
+    p = subprocess.Popen([has_external_compiler, 'compile'] + args,
+            stdout=subprocess.PIPE, stdin=subprocess.PIPE, stderr=subprocess.PIPE)
+    js, stderr = p.communicate(data)
+    if p.wait() != 0:
+        raise CompileFailure(force_unicode(stderr, 'utf-8'))
+    return js.decode('utf-8')
 
 
 def create_manifest(html):