Refactor: Add starts/ends_with functions

Adds `starts_with` and `ends_with` string functions to `util.hpp`.

This will hopefully prevent regressions caused by incorrect ad-hoc implementations, such as the one fixed by
sass/libsass@ef56d81

src/file.cpp

@@ -22,6 +22,7 @@
 #include "utf8_string.hpp"
 #include "sass_functions.hpp"
 #include "error_handling.hpp"
+#include "util.hpp"
 #include "sass2scss.h"
 
 #ifdef _WIN32

src/fn_colors.cpp

@@ -3,20 +3,17 @@
 #include "ast.hpp"
 #include "fn_utils.hpp"
 #include "fn_colors.hpp"
+#include "util.hpp"
 
 namespace Sass {
 
   namespace Functions {
 
     bool special_number(String_Constant_Ptr s) {
-      if (s) {
-        static const char* const calc = "calc(";
-        static const char* const var = "var(";
-        const std::string& str = s->value();
-        return str.compare(0, strlen(calc), calc) == 0 ||
-               str.compare(0, strlen(var), var) == 0;
-      }
-      return false;
+      if (s == nullptr) return false;
+      const std::string& str = s->value();
+      return starts_with(str, "calc(") ||
+             starts_with(str, "var(");
     }
 
     Signature rgb_sig = "rgb($red, $green, $blue)";

src/output.cpp

@@ -1,6 +1,7 @@
 #include "sass.hpp"
 #include "ast.hpp"
 #include "output.hpp"
+#include "util.hpp"
 
 namespace Sass {
 

src/output.hpp

@@ -11,13 +11,6 @@
 namespace Sass {
   class Context;
 
-  // Refactor to make it generic to find linefeed (look behind)
-  inline bool ends_with(std::string const & value, std::string const & ending)
-  {
-    if (ending.size() > value.size()) return false;
-    return std::equal(ending.rbegin(), ending.rend(), value.rbegin());
-  }
-
   class Output : public Inspect {
   protected:
     using Inspect::operator();

src/plugins.cpp

@@ -2,6 +2,7 @@
 #include <iostream>
 #include "output.hpp"
 #include "plugins.hpp"
+#include "util.hpp"
 
 #ifdef _WIN32
 #include <windows.h>

src/util.hpp

@@ -1,6 +1,7 @@
 #ifndef SASS_UTIL_H
 #define SASS_UTIL_H
 
+#include <cstring>
 #include <vector>
 #include <string>
 #include <assert.h>
@@ -34,6 +35,34 @@ namespace Sass {
 
   bool peek_linefeed(const char* start);
 
+  // C++20 `starts_with` equivalent.
+  // See https://en.cppreference.com/w/cpp/string/basic_string/starts_with
+  inline bool starts_with(const std::string& str, const char* prefix, size_t prefix_len) {
+    return str.compare(0, prefix_len, prefix) == 0;
+  }
+
+  inline bool starts_with(const std::string& str, const char* prefix) {
+    return starts_with(str, prefix, std::strlen(prefix));
+  }
+
+  // C++20 `ends_with` equivalent.
+  // See https://en.cppreference.com/w/cpp/string/basic_string/ends_with
+  inline bool ends_with(const std::string& str, const std::string& suffix) {
+    return suffix.size() <= str.size() && std::equal(suffix.rbegin(), suffix.rend(), str.rbegin());
+  }
+
+  inline bool ends_with(const std::string& str, const char* suffix, size_t suffix_len) {
+    if (suffix_len > str.size()) return false;
+    const char* suffix_it = suffix + suffix_len;
+    const char* str_it = str.c_str() + str.size();
+    while (suffix_it != suffix) if (*(--suffix_it) != *(--str_it)) return false;
+    return true;
+  }
+
+  inline bool ends_with(const std::string& str, const char* suffix) {
+    return ends_with(str, suffix, std::strlen(suffix));
+  }
+
   namespace Util {
 
     std::string rtrim(const std::string& str);