[Template Expressions] changing ${ leader to $i18n{

ui/base/template_expressions.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/base/template_expressions.h"
 
 #include <stddef.h>
 
 #include "base/logging.h"
 
+namespace {
+const char kLeader[] = "$i18n{";
+const size_t kLeaderSize = arraysize(kLeader) - 1;
+const char kTail[] = "}";
+const size_t kTailSize = arraysize(kTail) - 1;
+}  // namespace
+
 namespace ui {
 
 std::string ReplaceTemplateExpressions(
     base::StringPiece format_string,
-    const std::map<base::StringPiece, std::string>& substitutions) {
+    const TemplateReplacements& substitutions) {
   std::string formatted;
   const size_t kValueLengthGuess = 16;
   formatted.reserve(format_string.length() +
                     substitutions.size() * kValueLengthGuess);
   base::StringPiece::const_iterator i = format_string.begin();
   while (i < format_string.end()) {
-    if (*i == '$' && i + 2 < format_string.end() && i[1] == '{' &&
-        i[2] != '}') {
-      size_t key_start = i + strlen("${") - format_string.begin();
-      size_t key_length = format_string.find('}', key_start);
+    if (base::StringPiece(i).starts_with(kLeader)) {
+      size_t key_start = i + kLeaderSize - format_string.begin();
+      size_t key_length = format_string.find(kTail, key_start);
       if (key_length == base::StringPiece::npos)
-        NOTREACHED() << "TemplateExpression missing ending brace '}'";
+        NOTREACHED() << "TemplateExpression missing ending tag";
       key_length -= key_start;
-      base::StringPiece key(format_string.begin() + key_start, key_length);
+      std::string key(format_string.begin() + key_start, key_length);
+      DCHECK(!key.empty());
       const auto& replacement = substitutions.find(key);
       if (replacement != substitutions.end()) {
         formatted.append(replacement->second);
-        i += strlen("${") + key_length + strlen("}");
+        i += kLeaderSize + key_length + kTailSize;
         continue;
       } else {
         NOTREACHED() << "TemplateExpression key not found: " << key;
       }
     }
     formatted.push_back(*i);
     ++i;
   }
   return formatted;
 }
 
 }  // namespace ui