perf: optimise normalisePath (#2162)

lib/FsHelpers/FsHelpers.cpp

@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <cctype>
 #include <cstring>
+#include <string_view>
 #include <vector>
 
 namespace FsHelpers {
@@ -36,12 +37,14 @@ std::string decodeUriEscapes(const std::string& path) {
 }
 
 std::string normalisePath(const std::string& path) {
-  std::vector<std::string> components;
-  std::string component;
+  std::vector<std::string_view> components;
+  components.reserve(8);  // Eight nested folders is more than we might expect
 
-  for (const auto c : path) {
-    if (c == '/') {
-      if (!component.empty()) {
+  size_t start = 0;
+  for (size_t i = 0; i <= path.length(); ++i) {
+    if (i == path.length() || path[i] == '/') {
+      if (i > start) {
+        std::string_view component(path.data() + start, i - start);
         if (component == "..") {
           if (!components.empty()) {
             components.pop_back();
@@ -49,23 +52,28 @@ std::string normalisePath(const std::string& path) {
         } else {
           components.push_back(component);
         }
-        component.clear();
       }
-    } else {
-      component += c;
+      start = i + 1;
     }
   }
 
-  if (!component.empty()) {
-    components.push_back(component);
+  if (components.empty()) {
+    return "";
+  }
+
+  size_t total_len = 0;
+  for (const auto& c : components) {
+    total_len += c.length() + 1;
   }
 
   std::string result;
-  for (const auto& c : components) {
-    if (!result.empty()) {
-      result += "/";
+  result.reserve(total_len - 1);
+
+  for (size_t i = 0; i < components.size(); ++i) {
+    if (i > 0) {
+      result += '/';
     }
-    result += c;
+    result.append(components[i].data(), components[i].length());
   }
 
   return result;