util/StringUtil: add StringAfterPrefix()

src/util/StringUtil.cxx

@@ -18,6 +18,7 @@
  */
 
 #include "StringUtil.hxx"
+#include "StringAPI.hxx"
 #include "CharUtil.hxx"
 #include "ASCII.hxx"
 
@@ -44,6 +45,21 @@ StringEndsWith(const char *haystack, const char *needle)
 		       needle, needle_length) == 0;
 }
 
+const char *
+StringAfterPrefix(const char *string, const char *prefix)
+{
+#if !CLANG_CHECK_VERSION(3,6)
+	/* disabled on clang due to -Wtautological-pointer-compare */
+	assert(string != nullptr);
+	assert(prefix != nullptr);
+#endif
+
+	size_t prefix_length = strlen(prefix);
+	return StringIsEqual(string, prefix, prefix_length)
+		? string + prefix_length
+		: nullptr;
+}
+
 const char *
 FindStringSuffix(const char *p, const char *suffix)
 {

src/util/StringUtil.hxx

@@ -36,6 +36,15 @@ gcc_pure
 bool
 StringEndsWith(const char *haystack, const char *needle);
 
+/**
+ * Returns the portion of the string after a prefix.  If the string
+ * does not begin with the specified prefix, this function returns
+ * nullptr.
+ */
+gcc_pure gcc_nonnull_all
+const char *
+StringAfterPrefix(const char *string, const char *prefix);
+
 /**
  * Check if the given string ends with the specified suffix.  If yes,
  * returns the position of the suffix, and nullptr otherwise.

src/util/WStringUtil.cxx

@@ -42,6 +42,21 @@ StringEndsWith(const wchar_t *haystack, const wchar_t *needle)
 		StringIsEqual(haystack + haystack_length - needle_length, needle);
 }
 
+const wchar_t *
+StringAfterPrefix(const wchar_t *string, const wchar_t *prefix)
+{
+#if !CLANG_CHECK_VERSION(3,6)
+	/* disabled on clang due to -Wtautological-pointer-compare */
+	assert(string != nullptr);
+	assert(prefix != nullptr);
+#endif
+
+	size_t prefix_length = StringLength(prefix);
+	return StringIsEqual(string, prefix, prefix_length)
+		? string + prefix_length
+		: nullptr;
+}
+
 const wchar_t *
 FindStringSuffix(const wchar_t *p, const wchar_t *suffix)
 {

src/util/WStringUtil.hxx

@@ -32,6 +32,15 @@ gcc_pure
 bool
 StringEndsWith(const wchar_t *haystack, const wchar_t *needle);
 
+/**
+ * Returns the portion of the string after a prefix.  If the string
+ * does not begin with the specified prefix, this function returns
+ * nullptr.
+ */
+gcc_nonnull_all
+const wchar_t *
+StringAfterPrefix(const wchar_t *string, const wchar_t *prefix);
+
 /**
  * Check if the given string ends with the specified suffix.  If yes,
  * returns the position of the suffix, and nullptr otherwise.