[NFC] Factor out and simplify minified name generation (#3909)

Simplifies the public API to not unnecessarily take an index and simplifies the
implementation to use a single integer as state rather than a vector of indices.

1 files changed, 77 insertions, 0 deletions

diff --git a/src/ir/names.cpp b/src/ir/names.cpp
new file mode 100644
index 000000000..cbc703438
--- /dev/null
+++ b/src/ir/names.cpp
@@ -0,0 +1,77 @@
+/*
+ * Copyright 2021 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.
+ */
+
+#include "ir/names.h"
+#include <sstream>
+
+namespace wasm {
+
+namespace Names {
+
+// Reserved words in JS that we will not emit up to size 4 - size 5 and above
+// would mean we use an astronomical number of symbols, which is not realistic
+// anyhow.
+static std::unordered_set<std::string> reserved = {"do",
+                                                   "if",
+                                                   "in",
+                                                   "for",
+                                                   "new",
+                                                   "try",
+                                                   "var",
+                                                   "env",
+                                                   "let",
+                                                   "case",
+                                                   "else",
+                                                   "enum",
+                                                   "void",
+                                                   "this",
+                                                   "with"};
+
+// Possible initial letters.
+static std::string validInitialChars =
+  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_$";
+
+// Possible later letters.
+static std::string validLaterChars =
+  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_$0123456789";
+
+std::string MinifiedNameGenerator::getName() {
+  std::string name;
+  do {
+    size_t n = state++;
+    std::stringstream ss;
+    ss << validInitialChars[n % validInitialChars.size()];
+    n /= validInitialChars.size();
+    // `m` is the number of `state` counts each of the `n` represents.
+    size_t m = validInitialChars.size();
+    while (n) {
+      if (n % (validLaterChars.size() + 1) == 0) {
+        // Skip states that contain zeros in later positions.
+        state += m;
+        ++n;
+      }
+      ss << validLaterChars[(n % (validLaterChars.size() + 1)) - 1];
+      n /= (validLaterChars.size() + 1);
+      m *= (validLaterChars.size() + 1);
+    }
+    name = ss.str();
+  } while (reserved.count(name));
+  return name;
+}
+
+} // namespace Names
+
+} // namespace wasm