path: root/base/src/main/java/bjc/utils/gui/MarkdownSplitPreview.java

package bjc.utils.gui;

import javax.swing.*;
import javax.swing.event.CaretEvent;
import javax.swing.event.CaretListener;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.*;
import java.awt.*;
import java.awt.event.*;
import java.net.URI;

public final class MarkdownSplitPreview extends JPanel {

	private static final int RENDER_DEBOUNCE_MS = 150;
	private static final int CARET_SYNC_DEBOUNCE_MS = 75;

	// If true: when the user scrolls the preview, automatically turn off "Follow
	// caret".
	private static final boolean AUTO_DISABLE_FOLLOW_ON_PREVIEW_SCROLL = true;

	private final JTextArea editor = new JTextArea();
	private final JTextPane preview = new JTextPane();

	private final MarkdownEditorKit previewKit = new MarkdownEditorKit();

	private final Timer renderTimer;
	private final Timer caretSyncTimer;

	private String lastRenderedMarkdown = null;

	// Reentrancy guard for programmatic scrollbar updates
	private boolean adjustingScrollbars = false;

	// Follow-caret UI state
	private final JToggleButton followCaretToggle = new JToggleButton("Follow caret", true);

	public MarkdownSplitPreview() {
		super(new BorderLayout());

		// Toolbar
		JToolBar tb = new JToolBar();
		tb.setFloatable(false);

		followCaretToggle.setFocusable(false);
		followCaretToggle.setToolTipText("When enabled, the preview tracks where you are editing.");
		tb.add(followCaretToggle);

		JButton renderNowButton = new JButton("Render now");
		renderNowButton.setFocusable(false);
		renderNowButton.setToolTipText("Force an immediate preview update (Ctrl+Enter / Ctrl+R).");
		tb.addSeparator();
		tb.add(renderNowButton);

		add(tb, BorderLayout.NORTH);

		// Editor (left)
		editor.setFont(new Font(Font.MONOSPACED, Font.PLAIN, 13));
		editor.setTabSize(4);
		editor.setLineWrap(true);
		editor.setWrapStyleWord(true);

		// Preview (right)
		preview.setEditable(false);
		preview.setEditorKit(previewKit);

		final JScrollPane leftScroll = new JScrollPane(editor);
		final JScrollPane rightScroll = new JScrollPane(preview);

		JSplitPane split = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, leftScroll, rightScroll);
		split.setResizeWeight(0.5);
		add(split, BorderLayout.CENTER);

		// Debounced render (EDT)
		renderTimer = new Timer(RENDER_DEBOUNCE_MS, e -> renderPreview(rightScroll, leftScroll));
		renderTimer.setRepeats(false);

		// Debounced caret-follow (EDT)
		caretSyncTimer = new Timer(CARET_SYNC_DEBOUNCE_MS, e -> maybeSyncPreviewToCaret(leftScroll, rightScroll));
		caretSyncTimer.setRepeats(false);

		// Typing/editing triggers render + (optional) caret-follow
		editor.getDocument().addDocumentListener(new DocumentListener() {
			@Override
			public void insertUpdate(DocumentEvent e) {
				schedule();
			}

			@Override
			public void removeUpdate(DocumentEvent e) {
				schedule();
			}

			@Override
			public void changedUpdate(DocumentEvent e) {
				schedule();
			}

			private void schedule() {
				renderTimer.restart();
				caretSyncTimer.restart();
			}
		});

		// Caret movement should also move preview (if follow enabled)
		editor.addCaretListener(new CaretListener() {
			@Override
			public void caretUpdate(CaretEvent e) {
				if (!editor.isFocusOwner())
					return;
				caretSyncTimer.restart();
			}
		});

		// Button triggers immediate render
		renderNowButton.addActionListener(e -> renderNow(rightScroll, leftScroll));

		installRenderNowHotkeys(rightScroll, leftScroll);
		installFollowCaretHotkey(); // Ctrl+L
		installLinkHandling(preview);
		installScrollSync(leftScroll, rightScroll);
		installPreviewManualScrollDetection(rightScroll);

		// Initial render
		renderPreview(rightScroll, leftScroll);
	}

	public void setMarkdown(String markdown) {
		editor.setText(markdown == null ? "" : markdown);
		renderTimer.stop();
		caretSyncTimer.stop();
		JScrollPane rightScroll = (JScrollPane) SwingUtilities.getAncestorOfClass(JScrollPane.class, preview);
		JScrollPane leftScroll = (JScrollPane) SwingUtilities.getAncestorOfClass(JScrollPane.class, editor);
		if (rightScroll != null && leftScroll != null) {
			renderPreview(rightScroll, leftScroll);
		} else {
			previewKit.setMarkdown(preview, markdown == null ? "" : markdown);
			lastRenderedMarkdown = markdown;
		}
	}

	public String getMarkdown() {
		return editor.getText();
	}

	public boolean isFollowCaretEnabled() {
		return followCaretToggle.isSelected();
	}

	public void setFollowCaretEnabled(boolean enabled) {
		followCaretToggle.setSelected(enabled);
	}

	// -------------------------------------------------------------------------
	// Render logic
	// -------------------------------------------------------------------------

	private void renderPreview(JScrollPane previewScroll, JScrollPane editorScroll) {
		String md = editor.getText();
		if (md == null)
			md = "";

		// Skip rerender if unchanged
		if (md.equals(lastRenderedMarkdown)) {
			maybeSyncPreviewToCaret(editorScroll, previewScroll);
			return;
		}

		lastRenderedMarkdown = md;

		try {
			previewKit.setMarkdown(preview, md);
		} catch (RuntimeException ex) {
			preview.setDocument(new DefaultStyledDocument());
			preview.setText("Markdown render error:\n" + ex.getMessage());
		}

		SwingUtilities.invokeLater(() -> maybeSyncPreviewToCaret(editorScroll, previewScroll));
	}

	private void renderNow(JScrollPane previewScroll, JScrollPane editorScroll) {
		renderTimer.stop();
		renderPreview(previewScroll, editorScroll);
	}

	// -------------------------------------------------------------------------
	// Hotkeys
	// -------------------------------------------------------------------------

	private void installRenderNowHotkeys(JScrollPane previewScroll, JScrollPane editorScroll) {
		InputMap im = editor.getInputMap(JComponent.WHEN_FOCUSED);
		ActionMap am = editor.getActionMap();

		Action renderNowAction = new AbstractAction() {
			@Override
			public void actionPerformed(ActionEvent e) {
				renderNow(previewScroll, editorScroll);
			}
		};

		// Ctrl+Enter
		im.put(KeyStroke.getKeyStroke(KeyEvent.VK_ENTER, InputEvent.CTRL_DOWN_MASK), "md.renderNow");
		// Ctrl+R
		im.put(KeyStroke.getKeyStroke(KeyEvent.VK_R, InputEvent.CTRL_DOWN_MASK), "md.renderNow");

		am.put("md.renderNow", renderNowAction);
	}

	private void installFollowCaretHotkey() {
		InputMap im = editor.getInputMap(JComponent.WHEN_FOCUSED);
		ActionMap am = editor.getActionMap();

		im.put(KeyStroke.getKeyStroke(KeyEvent.VK_L, InputEvent.CTRL_DOWN_MASK), "md.toggleFollowCaret");
		am.put("md.toggleFollowCaret", new AbstractAction() {
			@Override
			public void actionPerformed(ActionEvent e) {
				followCaretToggle.setSelected(!followCaretToggle.isSelected());
			}
		});
	}

	// -------------------------------------------------------------------------
	// Clickable links in preview
	// -------------------------------------------------------------------------

	private void installLinkHandling(JTextPane pane) {
		pane.addMouseMotionListener(new MouseMotionAdapter() {
			@Override
			public void mouseMoved(MouseEvent e) {
				String href = hrefAtPoint(pane, e.getPoint());
				pane.setCursor(
						href != null ? Cursor.getPredefinedCursor(Cursor.HAND_CURSOR) : Cursor.getDefaultCursor());
			}
		});

		pane.addMouseListener(new MouseAdapter() {
			@Override
			public void mouseClicked(MouseEvent e) {
				if (e.getButton() != MouseEvent.BUTTON1 || e.getClickCount() != 1)
					return;
				String href = hrefAtPoint(pane, e.getPoint());
				if (href != null)
					openLink(href, pane);
			}
		});
	}

	private static String hrefAtPoint(JTextPane pane, Point p) {
		int pos = pane.viewToModel(p); // Java 8 compatible
		if (pos < 0)
			return null;

		Document d = pane.getDocument();
		if (!(d instanceof StyledDocument))
			return null;

		StyledDocument sd = (StyledDocument) d;
		Element elem = sd.getCharacterElement(pos);
		if (elem == null)
			return null;

		AttributeSet attrs = elem.getAttributes();
		Object href = attrs.getAttribute(MarkdownEditorKit.MarkdownDocument.ATTR_LINK_HREF);
		return (href instanceof String) ? (String) href : null;
	}

	private static void openLink(String href, Component parent) {
		try {
			if (!Desktop.isDesktopSupported()) {
				Toolkit.getDefaultToolkit().beep();
				return;
			}
			Desktop.getDesktop().browse(new URI(href));
		} catch (Exception ex) {
			JOptionPane.showMessageDialog(parent, "Could not open link:\n" + href + "\n\n" + ex.getMessage(),
					"Open Link Failed", JOptionPane.ERROR_MESSAGE);
		}
	}

	// -------------------------------------------------------------------------
	// Manual preview scroll detection (optionally disables follow-caret)
	// -------------------------------------------------------------------------

	private void installPreviewManualScrollDetection(JScrollPane previewScroll) {
		JScrollBar bar = previewScroll.getVerticalScrollBar();

		// Mouse wheel in preview => manual
		MouseWheelListener wheel = e -> onPreviewManualScroll();
		previewScroll.addMouseWheelListener(wheel);
		previewScroll.getViewport().addMouseWheelListener(wheel);

		// Scrollbar drag/click => manual
		bar.addMouseListener(new MouseAdapter() {
			@Override
			public void mousePressed(MouseEvent e) {
				onPreviewManualScroll();
			}

			@Override
			public void mouseReleased(MouseEvent e) {
				onPreviewManualScroll();
			}
		});

		// Any user adjustment (not programmatic) => manual
		bar.addAdjustmentListener(e -> {
			if (adjustingScrollbars)
				return;
			if (e.getValueIsAdjusting())
				onPreviewManualScroll();
		});
	}

	private void onPreviewManualScroll() {
		if (AUTO_DISABLE_FOLLOW_ON_PREVIEW_SCROLL && followCaretToggle.isSelected()) {
			followCaretToggle.setSelected(false);
		}
	}

	// -------------------------------------------------------------------------
	// Scroll sync (proportional, bidirectional)
	// -------------------------------------------------------------------------

	private void installScrollSync(JScrollPane editorScroll, JScrollPane previewScroll) {
		JScrollBar eBar = editorScroll.getVerticalScrollBar();
		JScrollBar pBar = previewScroll.getVerticalScrollBar();

		// Editor scrolled => move preview only if follow is enabled
		eBar.addAdjustmentListener(ev -> {
			if (adjustingScrollbars)
				return;
			if (!followCaretToggle.isSelected())
				return;
			syncFromTo(eBar, pBar);
		});

		// Preview scrolled => move editor always (user intent)
		pBar.addAdjustmentListener(ev -> {
			if (adjustingScrollbars)
				return;
			syncFromTo(pBar, eBar);
		});
	}

	private void syncFromTo(JScrollBar from, JScrollBar to) {
		int fromRange = scrollRange(from);
		int toRange = scrollRange(to);
		if (fromRange <= 0 || toRange <= 0)
			return;

		double frac = from.getValue() / (double) fromRange;
		int target = (int) Math.round(frac * toRange);
		target = clamp(target, 0, toRange);

		if (to.getValue() == target)
			return;

		adjustingScrollbars = true;
		try {
			to.setValue(target);
		} finally {
			adjustingScrollbars = false;
		}
	}

	private static int scrollRange(JScrollBar bar) {
		int range = bar.getMaximum() - bar.getVisibleAmount();
		return Math.max(0, range);
	}

	private static int clamp(int v, int lo, int hi) {
		if (v < lo)
			return lo;
		if (v > hi)
			return hi;
		return v;
	}

	// -------------------------------------------------------------------------
	// Caret-follow preview syncing (when enabled)
	// -------------------------------------------------------------------------

	private void maybeSyncPreviewToCaret(JScrollPane editorScroll, JScrollPane previewScroll) {
		if (!followCaretToggle.isSelected())
			return;
		if (!editor.isFocusOwner())
			return;

		syncPreviewToCaret(editorScroll, previewScroll);
	}

	private void syncPreviewToCaret(JScrollPane editorScroll, JScrollPane previewScroll) {
		JScrollBar pBar = previewScroll.getVerticalScrollBar();
		int pRange = scrollRange(pBar);
		if (pRange <= 0)
			return;

		int caret = editor.getCaretPosition();
		Rectangle caretRect;
		try {
			caretRect = editor.modelToView(caret); // Java 8
		} catch (BadLocationException e) {
			return;
		}
		if (caretRect == null)
			return;

		int prefHeight = editor.getPreferredSize().height;
		int visibleHeight = editorScroll.getViewport().getExtentSize().height;

		int eRange = Math.max(0, prefHeight - visibleHeight);
		if (eRange <= 0)
			return;

		double frac = caretRect.y / (double) eRange;
		if (frac < 0)
			frac = 0;
		if (frac > 1)
			frac = 1;

		int target = (int) Math.round(frac * pRange);
		target = clamp(target, 0, pRange);

		if (pBar.getValue() == target)
			return;

		adjustingScrollbars = true;
		try {
			pBar.setValue(target);
		} finally {
			adjustingScrollbars = false;
		}
	}

	// -------------------------------------------------------------------------
	// Demo
	// -------------------------------------------------------------------------

	public static void main(String[] args) {
		SwingUtilities.invokeLater(() -> {
			JFrame f = new JFrame("Markdown Editor + Preview");
			f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

			MarkdownSplitPreview panel = new MarkdownSplitPreview();
			panel.setMarkdown("# Hello\n\n" + "Type Markdown on the left.\n\n"
					+ "A link: [example](https://example.com)\n\n" + "| Col A | Col B |\n" + "|------:|:-----|\n"
					+ "| **bold** | `code` |\n" + "| [link](https://example.com) | *italic* |\n\n"
					+ "If you scroll the preview, Follow caret will turn off.\n"
					+ "Toggle it back on with the button or Ctrl+L.\n");

			f.setContentPane(panel);
			f.setSize(1100, 700);
			f.setLocationRelativeTo(null);
			f.setVisible(true);
		});
	}
}