package design.model.undo;

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.List;
import java.util.stream.Collectors;

// Singleton caretaker that handles undo and redo stacks.
public final class UndoManager {

    private static final UndoManager INSTANCE = new UndoManager();

    public static UndoManager instance() {
        return INSTANCE;
    }

    private UndoManager() {
    }

    // Stack entry
    private static class Entry {
        final Originator originator;
        final Memento memento;
        final String label;

        Entry(Originator originator, Memento memento, String label) {
            this.originator = originator;
            this.memento = memento;
            this.label = label;
        }
    }

    private final Deque<Entry> undoStack = new ArrayDeque<>();
    private final Deque<Entry> redoStack = new ArrayDeque<>();

    // Capture state
    public void capture(Originator originator, String label) {
        undoStack.push(new Entry(originator, originator.createMemento(), label));
        redoStack.clear();
    }

    // Undo / Redo ops
    public boolean canUndo() {
        return !undoStack.isEmpty();
    }

    public boolean canRedo() {
        return !redoStack.isEmpty();
    }

    public String peekUndoLabel() {
        return canUndo() ? undoStack.peek().label : null;
    }

    public String peekRedoLabel() {
        return canRedo() ? redoStack.peek().label : null;
    }

    public List<String> getUndoHistoryLabels() {
        return undoStack.stream()
                .map(e -> e.label)
                .collect(Collectors.toList());
    }

    public void undo() {
        if (!canUndo()) {
            System.out.println("Nothing to undo.");
            return;
        }

        Entry entry = undoStack.pop();
        redoStack.push(new Entry(entry.originator, entry.originator.createMemento(), entry.label));
        entry.originator.restore(entry.memento);
    }

    public void redo() {
        if (!canRedo()) {
            System.out.println("Nothing to redo.");
            return;
        }

        Entry entry = redoStack.pop();
        undoStack.push(new Entry(entry.originator, entry.originator.createMemento(), entry.label));
        entry.originator.restore(entry.memento);
    }

    public void purge() {
        undoStack.clear();
        redoStack.clear();
    }
}