cmStringAlgorithms: Add faster cmJoin overloads for strings

cmJoin() is often used with std::string ranges. Generic implementation
uses std::ostringstream which is not optimal. With strings we can avoid
operator<<() and make much faster implementation. Additional 'initial'
argument is useful for cmStringCommand.cxx:HandleAppendCommand().

3 files changed, 69 insertions, 4 deletions

diff --git a/Source/cmStringAlgorithms.cxx b/Source/cmStringAlgorithms.cxx
index 71d28a49df..e0af28185a 100644
--- a/Source/cmStringAlgorithms.cxx
+++ b/Source/cmStringAlgorithms.cxx
@@ -7,6 +7,7 @@
 #include <cstddef> // IWYU pragma: keep
 #include <cstdio>
 #include <cstdlib>
+#include <iterator>
 
 std::string cmTrimWhitespace(cm::string_view str)
 {
@@ -323,3 +324,52 @@ bool cmStrToULong(std::string const& str, unsigned long* value)
 {
   return cmStrToULong(str.c_str(), value);
 }
+
+template <typename Range>
+std::size_t getJoinedLength(Range const& rng, cm::string_view separator)
+{
+  std::size_t rangeLength{};
+  for (auto const& item : rng) {
+    rangeLength += item.size();
+  }
+
+  auto const separatorsLength = (rng.size() - 1) * separator.size();
+
+  return rangeLength + separatorsLength;
+}
+
+template <typename Range>
+std::string cmJoinImpl(Range const& rng, cm::string_view separator,
+                       cm::string_view initial)
+{
+  if (rng.empty()) {
+    return { std::begin(initial), std::end(initial) };
+  }
+
+  std::string result;
+  result.reserve(initial.size() + getJoinedLength(rng, separator));
+  result.append(std::begin(initial), std::end(initial));
+
+  auto begin = std::begin(rng);
+  auto end = std::end(rng);
+  result += *begin;
+
+  for (++begin; begin != end; ++begin) {
+    result.append(std::begin(separator), std::end(separator));
+    result += *begin;
+  }
+
+  return result;
+}
+
+std::string cmJoin(std::vector<std::string> const& rng,
+                   cm::string_view separator, cm::string_view initial)
+{
+  return cmJoinImpl(rng, separator, initial);
+}
+
+std::string cmJoin(cmStringRange const& rng, cm::string_view separator,
+                   cm::string_view initial)
+{
+  return cmJoinImpl(rng, separator, initial);
+}
diff --git a/Source/cmStringAlgorithms.h b/Source/cmStringAlgorithms.h
index 4b0090b97d..c82a9495cc 100644
--- a/Source/cmStringAlgorithms.h
+++ b/Source/cmStringAlgorithms.h
@@ -81,6 +81,17 @@ std::string cmJoin(Range const& rng, cm::string_view separator)
   return os.str();
 }
 
+/**
+ * Faster overloads for std::string ranges.
+ * If @a initial is provided, it prepends the resulted string without
+ * @a separator between them.
+ */
+std::string cmJoin(std::vector<std::string> const& rng,
+                   cm::string_view separator, cm::string_view initial = {});
+
+std::string cmJoin(cmStringRange const& rng, cm::string_view separator,
+                   cm::string_view initial = {});
+
 /** Extract tokens that are separated by any of the characters in @a sep.  */
 std::vector<std::string> cmTokenize(cm::string_view str, cm::string_view sep);
 
diff --git a/Source/cmStringCommand.cxx b/Source/cmStringCommand.cxx
index 653b383ccf..4000a7de08 100644
--- a/Source/cmStringCommand.cxx
+++ b/Source/cmStringCommand.cxx
@@ -11,6 +11,7 @@
 #include <memory>
 
 #include <cm/iterator>
+#include <cm/string_view>
 #include <cmext/string_view>
 
 #include "cmsys/RegularExpression.hxx"
@@ -534,11 +535,14 @@ bool HandleAppendCommand(std::vector<std::string> const& args,
     return true;
   }
 
-  const std::string& variable = args[1];
+  auto const& variableName = args[1];
+
+  cm::string_view oldView{ status.GetMakefile().GetSafeDefinition(
+    variableName) };
+
+  auto const newValue = cmJoin(cmMakeRange(args).advance(2), {}, oldView);
+  status.GetMakefile().AddDefinition(variableName, newValue);
 
-  std::string value = status.GetMakefile().GetSafeDefinition(variable);
-  value += cmJoin(cmMakeRange(args).advance(2), std::string());
-  status.GetMakefile().AddDefinition(variable, value);
   return true;
 }