Add line_iterator_t

Adds support for splitting a collection into lines.

src/common.h

@@ -379,6 +379,42 @@ wcstring_list_t split_string(const wcstring &val, wchar_t sep);
 /// Join a list of strings by a separator character.
 wcstring join_strings(const wcstring_list_t &vals, wchar_t sep);
 
+/// Support for iterating over a newline-separated string.
+template <typename Collection>
+class line_iterator_t {
+    // Storage for each line.
+    Collection storage;
+
+    // The collection we're iterating. Note we hold this by reference.
+    const Collection &coll;
+
+    // The current location in the iteration.
+    typename Collection::const_iterator current;
+
+public:
+    /// Construct from a collection (presumably std::string or std::wcstring).
+    line_iterator_t(const Collection &coll) : coll(coll), current(coll.cbegin()) {}
+
+    /// Access the storage in which the last line was stored.
+    const Collection &line() const {
+        return storage;
+    }
+
+    /// Advances to the next line. \return true on success, false if we have exhausted the string.
+    bool next() {
+        if (current == coll.end())
+            return false;
+        auto newline_or_end = std::find(current, coll.cend(), '\n');
+        storage.assign(current, newline_or_end);
+        current = newline_or_end;
+
+        // Skip the newline.
+        if (current != coll.cend())
+            ++current;
+        return true;
+    }
+};
+
 enum fuzzy_match_type_t {
     // We match the string exactly: FOOBAR matches FOOBAR.
     fuzzy_match_exact = 0,

src/fish_tests.cpp

@@ -2809,6 +2809,22 @@ static void test_input() {
     }
 }
 
+static void test_line_iterator() {
+    say(L"Testing line iterator");
+
+    std::string text1 = "Alpha\nBeta\nGamma\n\nDelta\n";
+    std::vector<std::string> lines1;
+    line_iterator_t<std::string> iter1(text1);
+    while (iter1.next()) lines1.push_back(iter1.line());
+    do_test((lines1 == std::vector<std::string>{"Alpha", "Beta", "Gamma", "", "Delta"}));
+
+    wcstring text2 = L"\n\nAlpha\nBeta\nGamma\n\nDelta";
+    std::vector<wcstring> lines2;
+    line_iterator_t<wcstring> iter2(text2);
+    while (iter2.next()) lines2.push_back(iter2.line());
+    do_test((lines2 == std::vector<wcstring>{L"", L"", L"Alpha", L"Beta", L"Gamma", L"", L"Delta"}));
+}
+
 #define UVARS_PER_THREAD 8
 #define UVARS_TEST_PATH L"test/fish_uvars_test/varsfile.txt"
 
@@ -4805,6 +4821,7 @@ int main(int argc, char **argv) {
     if (should_test_function("colors")) test_colors();
     if (should_test_function("complete")) test_complete();
     if (should_test_function("input")) test_input();
+    if (should_test_function("line_iterator")) test_line_iterator();
     if (should_test_function("universal")) test_universal();
     if (should_test_function("universal")) test_universal_callbacks();
     if (should_test_function("notifiers")) test_universal_notifiers();