Add source code.

include/LineEditor.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "Completion.h"
+
+// Reads one line with cursor editing, history (up/down) and tab completion.
+// Falls back to plain std::getline when stdin/stdout are not a terminal.
+class LineEditor {
+public:
+    using CompletionFn = std::function<Completion(const std::string& textBeforeCursor)>;
+
+    LineEditor();
+
+    void setCompleter(const CompletionFn& completer);
+    void setHistoryFile(const std::string& path);  // loads it and appends new entries
+    void addHistory(const std::string& line);
+
+    // Returns false on end of input (Ctrl-D on an empty line, or closed stdin).
+    bool readLine(const std::string& prompt, std::string& line);
+
+private:
+    bool readPlain(const std::string& prompt, std::string& line);
+    bool readInteractive(const std::string& prompt, std::string& line);
+
+    CompletionFn completer_;
+    std::vector<std::string> history_;
+    std::string historyFile_;
+};