libutil/strings: add partitionString

This partitions a string into two parts based on the first occurrence of
the separator.

This function handles the case when the separator never occur in the
target string by returning an optional<T> in the second argument of the
pair.

Change-Id: I9ad44375dbaa137a4d21d2408bc2ffa34fab62a7
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/libutil/strings.cc

@@ -36,6 +36,16 @@ template Strings tokenizeString(std::string_view s, std::string_view separators)
 template StringSet tokenizeString(std::string_view s, std::string_view separators);
 template std::vector<std::string> tokenizeString(std::string_view s, std::string_view separators);
 
+std::pair<std::string_view, std::optional<std::string_view>>
+partitionString(std::string_view s, char separator)
+{
+    auto pos = s.find_first_of(separator);
+    if (pos == std::string::npos) {
+        return {s, std::nullopt};
+    } else {
+        return {s.substr(0, pos), s.substr(pos + 1)};
+    }
+}
 
 std::string chomp(std::string_view s)
 {

lix/libutil/strings.hh

@@ -35,6 +35,15 @@ MakeError(FormatError, Error);
  */
 template<class C> C tokenizeString(std::string_view s, std::string_view separators = " \t\n\r");
 
+/**
+ * Returns a pair that partitions the string when the separator is encountered for the first time in the
+ * string. e.g. `a=b` with separator `=` would split into {a, b} If the string passed does not contain the
+ * separator, the returned pair will set the second element to nullopt and put the whole string in the first
+ * element.
+ */
+std::pair<std::string_view, std::optional<std::string_view>>
+partitionString(std::string_view s, char separator);
+
 /**
  * Concatenate the given strings with a separator between the
  * elements.

tests/unit/libutil/tests.cc

@@ -729,6 +729,71 @@ namespace nix {
         ASSERT_EQ(tokenizeString<Strings>(s, ","), expected);
     }
 
+    /* ----------------------------------------------------------------------------
+     * partitionString
+     * --------------------------------------------------------------------------*/
+
+    TEST(partitionString, emptyString)
+    {
+        auto result = partitionString("", '=');
+
+        ASSERT_EQ(result.first, "");
+        ASSERT_FALSE(result.second.has_value());
+    }
+
+    TEST(partitionString, noSeparator)
+    {
+        auto result = partitionString("foobar", '=');
+
+        ASSERT_EQ(result.first, "foobar");
+        ASSERT_FALSE(result.second.has_value());
+    }
+
+    TEST(partitionString, separatorInMiddle)
+    {
+        auto result = partitionString("a=b", '=');
+
+        ASSERT_EQ(result.first, "a");
+        ASSERT_TRUE(result.second.has_value());
+        ASSERT_EQ(result.second.value(), "b");
+    }
+
+    TEST(partitionString, separatorAtBeginning)
+    {
+        auto result = partitionString("=abc", '=');
+
+        ASSERT_EQ(result.first, "");
+        ASSERT_TRUE(result.second.has_value());
+        ASSERT_EQ(result.second.value(), "abc");
+    }
+
+    TEST(partitionString, separatorAtEnd)
+    {
+        auto result = partitionString("abc=", '=');
+
+        ASSERT_EQ(result.first, "abc");
+        ASSERT_TRUE(result.second.has_value());
+        ASSERT_EQ(result.second.value(), "");
+    }
+
+    TEST(partitionString, multipleSeparatorsOnlyFirstIsUsed)
+    {
+        auto result = partitionString("a=b=c", '=');
+
+        ASSERT_EQ(result.first, "a");
+        ASSERT_TRUE(result.second.has_value());
+        ASSERT_EQ(result.second.value(), "b=c");
+    }
+
+    TEST(partitionString, differentSeparator)
+    {
+        auto result = partitionString("key:value", ':');
+
+        ASSERT_EQ(result.first, "key");
+        ASSERT_TRUE(result.second.has_value());
+        ASSERT_EQ(result.second.value(), "value");
+    }
+
     /* ----------------------------------------------------------------------------
      * get
      * --------------------------------------------------------------------------*/