dart2js: Implement frequency based naming.

tests/compiler/dart2js/token_naming_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:compiler/src/js_backend/js_backend.dart" show TokenScope;
+
+import "package:expect/expect.dart";
+
+String forwardN(TokenScope scope, int N) {
+  for (int i = 1; i < N; ++i) {
+    scope.getNextName();
+  }
+
+  return scope.getNextName();
+}
+
+int main() {
+  // Test a normal scope.
+  TokenScope scope = new TokenScope();
+
+  // We start with 'a'.
+  Expect.equals("a", scope.getNextName());
+  // We have 24 lower case characters, as s and g are illegal.
+  Expect.equals("A", forwardN(scope, 24));
+  // Make it overflow by skipping all uppercase.
+  Expect.equals("a_", forwardN(scope, 26));
+  // Now numbers.
+  Expect.equals("a0", forwardN(scope, 1));
+  Expect.equals("a9", forwardN(scope, 9));
+  // Then lower case letters.
+  Expect.equals("aa", forwardN(scope, 1));
+  Expect.equals("az", forwardN(scope, 25));
+  // Then upper case letters
+  Expect.equals("aA", forwardN(scope, 1));
+  Expect.equals("aZ", forwardN(scope, 25));
+  // Overflow to first position.
+  Expect.equals("b_", forwardN(scope, 1));
+  // Make sure we skipe g. We have 1 + 10 + 26 + 26 = 63 digits.
+  Expect.equals("h_", forwardN(scope, 63 * 5));
+  // Likewise, ensure we skip s.
+  Expect.equals("t_", forwardN(scope, 63 * 11));
+  // And wrap around another digit. 
+  Expect.equals("a__", forwardN(scope, 63 * 33));
+
+  // Test a filtered scope.
+  Set<String> illegal = new Set.from(["b", "aa"]);
+  scope = new TokenScope(illegal);
+
+  // We start with 'a'.
+  Expect.equals("a", forwardN(scope, 1));
+  // Make sure 'b' is skipped.
+  Expect.equals("c", forwardN(scope, 1));
+  // We have 24 lower case characters, as s and g are illegal.
+  Expect.equals("A", forwardN(scope, 22));
+  // Make it overflow by skipping all uppercase.
+  Expect.equals("a_", forwardN(scope, 26));
+  // Now numbers.
+  Expect.equals("a0", forwardN(scope, 1));
+  Expect.equals("a9", forwardN(scope, 9));
+  // Make sure 'aa' is skipped on wrapping
+  Expect.equals("ab", forwardN(scope, 1));
+  Expect.equals("az", forwardN(scope, 24));
+}