[ui/base;css] adding string template expression replacement

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 "base/logging.h"
+
+namespace {
+
+template <class FormatStringType, class OutStringType>

[Nico, 2015/07/09 19:57:45]

This is only used with one string type, no need for the template

[dschuyler, 2015/07/14 00:56:17]

The intention was to have more than one.  I've added more than one now.

+OutStringType DoReplaceTemplateExpressions(
+    const FormatStringType& format_string,
+    const std::map<std::string, OutStringType>& substitutions) {
+  OutStringType formatted;
+  const size_t kValueLengthGuess = 16;
+  formatted.reserve(format_string.length() +
+                    substitutions.size() * kValueLengthGuess);
+  typename FormatStringType::const_iterator i = format_string.begin();
+  while (i < format_string.end()) {
+    if (*i == '$') {
+      ++i;
+      if (i < format_string.end()) {
+        if (*i == '{') {
+          ++i;
+          std::string index;
+          while (i < format_string.end()) {

[Nico, 2015/07/09 19:57:45]

nit: I feel this can probably be simplified by using stuff like strchr, or
string.find('}') + substr

There would be way fewer allocations if you used StringPiece instead of string
for index (and I suppose as map keys, but even if you keep the key type you can
just .as_string() at lookup time)

[dschuyler, 2015/07/14 00:56:17]

I've mocked up an alternate version that leans more on the string function
members.  Which version is preferred?

[dschuyler, 2015/07/15 21:40:44]

Since I went back to the other version of the loop, I revisited these
suggestions.  The index/key is now a StringPiece that uses find() to locate the
'}'.

+            if (*i == '}') {
+              ++i;
+              break;
+            }
+            index.push_back(*i);
+            ++i;
+          }
+          const auto& replacement = substitutions.find(index);
+          if (replacement != substitutions.end()) {
+            formatted.append(replacement->second);
+          }

[Nico, 2015/07/09 19:57:46]

should it be an error if you say ${foo} and foo doesn't exist? Silently
replacing ${foo} with nothing is probably never the right thing?

[dschuyler, 2015/07/14 00:56:17]

Done.

+        } else if (*i == '$') {
+          while (i < format_string.end() && *i == '$') {
+            formatted.push_back('$');
+            ++i;
+          }
+        }
+      }
+    } else {
+      formatted.push_back(*i);
+      ++i;
+    }
+  }
+  return formatted;
+}
+
+}  // namespace
+
+namespace ui {
+
+std::string ReplaceTemplateExpressions(
+    const std::string& format_string,
+    const std::map<std::string, std::string>& substitutions) {
+  return DoReplaceTemplateExpressions(format_string, substitutions);
+}
+
+}  // namespace ui