Shell: Tab completion for programs in PATH

This patch adds a function to LineEditor that takes the current shell
buffer, searches PATH for the first program that starts with that
buffer and then compares that to any other programs starting with the
buffer to remove any mismatching characters off the end. The result is
appended to the buffer.

This may be faster with a data structure but that seems overkill.

Shell/LineEditor.cpp

@@ -37,6 +37,60 @@ void LineEditor::append(const String& string)
     m_cursor = m_buffer.size();
 }
 
+void LineEditor::tab_complete_first_token()
+{
+    auto input = String::copy(m_buffer);
+
+    String path = getenv("PATH");
+    if (path.is_empty())
+        return;
+    auto directories = path.split(':');
+
+    String match;
+
+    // Go through the files in PATH.
+    for (const auto& directory : directories) {
+        CDirIterator programs(directory.characters(), CDirIterator::SkipDots);
+        while (programs.has_next()) {
+            String program = programs.next_path();
+            if (!program.starts_with(input))
+                continue;
+
+            // Check that the file is an executable program.
+            struct stat program_status;
+            StringBuilder program_path;
+            program_path.append(directory.characters());
+            program_path.append('/');
+            program_path.append(program.characters());
+            int stat_error = stat(program_path.to_string().characters(), &program_status);
+            if (stat_error || !(program_status.st_mode & S_IXUSR))
+                continue;
+
+            // Set `match` to the first one that starts with `input`.
+            if (match.is_empty()) {
+                match = program;
+            } else {
+                // Remove characters from the end of `match` if they're
+                // different from another `program` starting with `input`.
+                int i = input.length();
+                while (i < match.length() && i < program.length() && match[i] == program[i])
+                    ++i;
+                match = match.substring(0, i);
+            }
+
+            if (match.length() == input.length())
+                return;
+        }
+    }
+
+    if (match.is_empty())
+        return;
+
+    // Then append `match` to the buffer, excluding the `input` part which is
+    // already in the buffer.
+    append(match.substring(input.length(), match.length() - input.length()).characters());
+}
+
 String LineEditor::get_line(const String& prompt)
 {
     fputs(prompt.characters(), stdout);
@@ -174,7 +228,21 @@ String LineEditor::get_line(const String& prompt)
             }
 
             if (ch == '\t') {
-                // FIXME: Implement tab-completion.
+                if (m_buffer.is_empty())
+                    continue;
+
+                bool is_first_token = true;
+                for (const auto& character : m_buffer) {
+                    if (isspace(character)) {
+                        is_first_token = false;
+                        break;
+                    }
+                }
+
+                // FIXME: Implement tab-completion for other tokens (paths).
+                if (is_first_token)
+                    tab_complete_first_token();
+
                 continue;
             }