feat(perf): cache iconv decoder (#2391)

* Cache with Map.

* Cache with lru-cache.

* Require default explicitly.

lib/parsers/string.js

@@ -1,21 +1,42 @@
 'use strict';
 
 const Iconv = require('iconv-lite');
+const LRU = require('lru-cache').default;
 
-exports.decode = function(buffer, encoding, start, end, options) {
+const decoderCache = new LRU({
+  max: 500,
+});
+
+exports.decode = function (buffer, encoding, start, end, options) {
   if (Buffer.isEncoding(encoding)) {
     return buffer.toString(encoding, start, end);
   }
 
-  const decoder = Iconv.getDecoder(encoding, options || {});
+  // Optimize for common case: encoding="short_string", options=undefined.
+  let decoder;
+  if (!options) {
+    decoder = decoderCache.get(encoding);
+    if (!decoder) {
+      decoder = Iconv.getDecoder(encoding);
+      decoderCache.set(encoding, decoder);
+    }
+  } else {
+    const decoderArgs = { encoding, options };
+    const decoderKey = JSON.stringify(decoderArgs);
+    decoder = decoderCache.get(decoderKey);
+    if (!decoder) {
+      decoder = Iconv.getDecoder(decoderArgs.encoding, decoderArgs.options);
+      decoderCache.set(decoderKey, decoder);
+    }
+  }
 
   const res = decoder.write(buffer.slice(start, end));
   const trail = decoder.end();
 
   return trail ? res + trail : res;
 };
 
-exports.encode = function(string, encoding, options) {
+exports.encode = function (string, encoding, options) {
   if (Buffer.isEncoding(encoding)) {
     return Buffer.from(string, encoding);
   }