HackStudio: Basic C++ autocomplete logic

CppAutoComplete gets a string of code and a position within it, and
returns a Vector of auto-complete suggestions that are relevant for the
given position.

Currently, it's very naive - it uses our CppLexer to find identifiers
in the code which the auto-complete target is a prefix of.

DevTools/HackStudio/CppAutoComplete.cpp

@@ -0,0 +1,92 @@
+/*
+ * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "CppAutoComplete.h"
+#include <AK/HashTable.h>
+#include <LibGUI/CppLexer.h>
+
+// #define DEBUG_AUTOCOMPLETE
+
+namespace HackStudio {
+Vector<String> CppAutoComplete::get_suggestions(const String& code, GUI::TextPosition autocomplete_position)
+{
+    auto lines = code.split('\n', true);
+    GUI::CppLexer lexer(code);
+    auto tokens = lexer.lex();
+
+    auto index_of_target_token = token_in_position(tokens, autocomplete_position);
+    if (!index_of_target_token.has_value())
+        return {};
+
+    auto suggestions = identifier_prefixes(lines, tokens, index_of_target_token.value());
+
+#ifdef DEBUG_AUTOCOMPLETE
+    for (auto& suggestion : suggestions) {
+        dbg() << "suggestion: " << suggestion;
+    }
+#endif
+
+    return suggestions;
+}
+
+String CppAutoComplete::text_of_token(const Vector<String> lines, const GUI::CppToken& token)
+{
+    return lines[token.m_start.line].substring(token.m_start.column, token.m_end.column - token.m_start.column + 1);
+}
+
+Optional<size_t> CppAutoComplete::token_in_position(const Vector<GUI::CppToken>& tokens, GUI::TextPosition position)
+{
+    for (size_t token_index = 0; token_index < tokens.size(); ++token_index) {
+        auto& token = tokens[token_index];
+        if (token.m_start.line != position.line())
+            continue;
+        if (token.m_start.column > position.column() || token.m_end.column < position.column())
+            continue;
+        return token_index;
+    }
+    return {};
+}
+
+Vector<String> CppAutoComplete::identifier_prefixes(const Vector<String> lines, const Vector<GUI::CppToken>& tokens, size_t target_token_index)
+{
+    auto partial_input = text_of_token(lines, tokens[target_token_index]);
+    Vector<String> suggestions;
+
+    HashTable<String> suggestions_lookup; // To avoid duplicate results
+
+    for (size_t i = 0; i < target_token_index; ++i) {
+        auto& token = tokens[i];
+        if (token.m_type != GUI::CppToken::Type::Identifier)
+            continue;
+        auto text = text_of_token(lines, token);
+        if (text.starts_with(partial_input) && !suggestions_lookup.contains(text)) {
+            suggestions_lookup.set(text);
+            suggestions.append(text);
+        }
+    }
+    return suggestions;
+}
+};