[Bug] Fix missing subtitle text in manually downloaded *.SRT files. (issue #10030)

app/src/main/java/org/schabi/newpipe/streams/SrtFromTtmlWriter.java

@@ -15,7 +15,11 @@
 import java.nio.charset.StandardCharsets;
 
 /**
- * @author kapodamy
+ * Converts TTML subtitles to SRT format.
+ *
+ * References:
+ *  - TTML 2.0 (W3C): https://www.w3.org/TR/ttml2/
+ *  - SRT format: https://en.wikipedia.org/wiki/SubRip
  */
 public class SrtFromTtmlWriter {
     private static final String NEW_LINE = "\r\n";
@@ -54,6 +58,226 @@ private void writeString(final String text) throws IOException {
         out.write(text.getBytes(charset));
     }
 
+    /**
+     * Decode XML or HTML entities into their actual (literal) characters.
+     *
+     * TTML is XML-based, so text nodes may contain escaped entities
+     * instead of direct characters. For example:
+     *
+     *   "&"          → "&"
+     *   "&lt;"           → "<"
+     *   "&gt;"           → ">"
+     *   "&#x9;"          → "\t" (TAB)
+     *   "&#xA;" (&#10;)  → "\n" (LINE FEED)
+     *
+     * XML files cannot contain characters like "<", ">", "&" directly,
+     * so they must be represented using their entity-encoded forms.
+     *
+     * Jsoup sometimes leaves nested or encoded entities unresolved
+     * (e.g. inside <p> text nodes in TTML files), so this function
+     * acts as a final “safety net” to ensure all entities are decoded
+     * before further normalization.
+     *
+     * Character representation layers for reference:
+     *   - Literal characters: <, >, &
+     *       → appear in runtime/output text (e.g. final SRT output)
+     *   - Escaped entities: &lt;, &gt;, &amp;
+     *       → appear in XML/HTML/TTML source files
+     *   - Numeric entities: &#xA0;, &#x9;, &#xD;
+     *       → appear mainly in XML/TTML files (also valid in HTML)
+     *         for non-printable or special characters
+     *   - Unicode escapes: \u00A0 (Java/Unicode internal form)
+     *       → appear only in Java source code (NOT valid in XML)
+     *
+     * XML entities include both named (&amp;, &lt;) and numeric
+     * (&#xA0;, &#160;) forms.
+     *
+     * @param encodedEntities The raw text fragment possibly containing
+     *                        encoded XML entities.
+     * @return A decoded string where all entities are replaced by their
+     *         actual (literal) characters.
+     */
+    private String decodeXmlEntities(final String encodedEntities) {
+        return Parser.unescapeEntities(encodedEntities, true);
+    }
+
+    /**
+     * Handle rare XML entity characters like LF: &#xA;(`\n`),
+     * CR: &#xD;(`\r`) and CRLF: (`\r\n`).
+     *
+     * These are technically valid in TTML (XML allows them)
+     * but unusual in practice, since most TTML line breaks
+     * are represented as <br/> tags instead.
+     * As a defensive approach, we normalize them:
+     *
+     * - Windows (\r\n), macOS (\r), and Unix (\n) → unified SRT NEW_LINE (\r\n)
+     *
+     * Although well-formed TTML normally encodes line breaks
+     * as <br/> tags, some auto-generated or malformed TTML files
+     * may embed literal newline entities (&#xA;, &#xD;). This
+     * normalization ensures these cases render properly in SRT
+     * players instead of breaking the subtitle structure.
+     *
+     * @param text To be normalized text with actual characters.
+     * @return Unified SRT NEW_LINE converted from all kinds of line breaks.
+     */
+    private String normalizeLineBreakForSrt(final String text) {
+        String cleaned = text;
+
+        // NOTE:
+        // The order of newline replacements must NOT change,
+        // or duplicated line breaks (e.g. \r\n → \n\n) will occur.
+        cleaned = cleaned.replace("\r\n", "\n")
+                         .replace("\r", "\n");
+
+        cleaned = cleaned.replace("\n", NEW_LINE);
+
+        return cleaned;
+    }
+
+    private String normalizeForSrt(final String actualText) {
+        String cleaned = actualText;
+
+        // Replace NBSP "non-breaking space" (\u00A0) with regular space ' '(\u0020).
+        //
+        // Why:
+        // - Some viewers render NBSP(\u00A0) incorrectly:
+        //   * MPlayer 1.5: shown as “??”
+        //   * Linux command `cat -A`: displayed as control-like markers
+        //     (M-BM-)
+        //   * Acode (Android editor): displayed as visible replacement
+        //     glyphs (red dots)
+        // - Other viewers show it as a normal space (e.g., VS Code 1.104.0,
+        //   vlc 3.0.20, mpv 0.37.0, Totem 43.0)
+        // → Mixed rendering creates inconsistency and may confuse users.
+        //
+        // Details:
+        // - YouTube TTML subtitles use both regular spaces (\u0020)
+        //   and non-breaking spaces (\u00A0).
+        // - SRT subtitles only support regular spaces (\u0020),
+        //   so \u00A0 may cause display issues.
+        // - \u00A0 and \u0020 are visually identical (i.e., they both
+        //   appear as spaces ' '), but they differ in Unicode encoding,
+        //   and NBSP (\u00A0) renders differently in different viewers.
+        // - SRT is a plain-text format and does not interpret
+        //   "non-breaking" behavior.
+        //
+        // Conclusion:
+        // - Ensure uniform behavior, so replace it to a regular space
+        //   without "non-breaking" behavior.
+        //
+        // References:
+        //   - Unicode U+00A0 NBSP (Latin-1 Supplement):
+        //     https://unicode.org/charts/PDF/U0080.pdf
+        cleaned = cleaned.replace('\u00A0', ' ') // Non-breaking space
+                 .replace('\u202F', ' ') // Narrow no-break space
+                 .replace('\u205F', ' ') // Medium mathematical space
+                 .replace('\u3000', ' ') // Ideographic space
+                 // \u2000 ~ \u200A are whitespace characters (e.g.,
+                 // en space, em space), replaced with regular space (\u0020).
+                 .replaceAll("[\\u2000-\\u200A]", " "); // Whitespace characters
+
+        // \u200B ~ \u200F are a range of non-spacing characters
+        // (e.g., zero-width space, zero-width non-joiner, etc.),
+        // which have no effect in *.SRT files and may cause
+        // display issues.
+        // These characters are invisible to the human eye, and
+        // they still exist in the encoding, so they need to be
+        // removed.
+        // After removal, the actual content becomes completely
+        // empty "", meaning there are no characters left, just
+        // an empty space, which helps avoid formatting issues
+        // in subtitles.
+        cleaned = cleaned.replaceAll("[\\u200B-\\u200F]", ""); // Non-spacing characters
+
+        // Remove control characters (\u0000 ~ \u001F, except
+        // \n, \r, \t).
+        // - These are ASCII C0 control codes (e.g. \u0001 SOH,
+        //   \u0008 BS, \u001F US), invisible and irrelevant in
+        //   subtitles, may cause square boxes (?) in players.
+        // - Reference:
+        //   Unicode Basic Latin (https://unicode.org/charts/PDF/U0000.pdf)
+        //   ASCII Control (https://en.wikipedia.org/wiki/ASCII#Control_characters)
+        cleaned = cleaned.replaceAll("[\\u0000-\\u0008\\u000B\\u000C\\u000E-\\u001F]", "");
+
+        // Reasoning:
+        // - subtitle files generally don't require tabs for alignment.
+        // - Tabs can be displayed with varying widths across different
+        //   editors or platforms, which may cause display issues.
+        // - Replace it with a single space for consistent display
+        //   across different editors or platforms.
+        cleaned = cleaned.replace('\t', ' ');
+
+        cleaned = normalizeLineBreakForSrt(cleaned);
+
+        return cleaned;
+    }
+
+    private String sanitizeFragment(final String raw) {
+        if (null == raw) {
+            return "";
+        }
+
+        final String actualCharacters = decodeXmlEntities(raw);
+
+        final String srtSafeText = normalizeForSrt(actualCharacters);
+
+        return srtSafeText;
+    }
+
+    // Recursively process all child nodes to ensure text inside
+    // nested tags (e.g., <span>) is also extracted.
+    private void traverseChildNodesForNestedTags(final Node parent,
+                                                 final StringBuilder text) {
+        for (final Node child : parent.childNodes()) {
+            extractText(child, text);
+        }
+    }
+
+    // CHECKSTYLE:OFF checkstyle:JavadocStyle
+    // checkstyle does not understand that span tags are inside a code block
+    /**
+     * <p>Recursive method to extract text from all nodes.</p>
+     * <p>
+     *   This method processes {@link TextNode}s and {@code <br>} tags,
+     *   recursively extracting text from nested tags
+     *   (e.g. extracting text from nested {@code <span>} tags).
+     *   Newlines are added for {@code <br>} tags.
+     * </p>
+     * @param node the current node to process
+     * @param text the {@link StringBuilder} to append the extracted text to
+     */
+    // --------------------------------------------------------------------
+    // [INTERNAL NOTE] TTML text layer explanation
+    //
+    // TTML parsing involves multiple text "layers":
+    //   1. Raw XML entities (e.g., &lt;, &#xA0;) are decoded by Jsoup.
+    //   2. extractText() works on DOM TextNodes (already parsed strings).
+    //   3. sanitizeFragment() decodes remaining entities and fixes
+    //      Unicode quirks.
+    //   4. normalizeForSrt() ensures literal text is safe for SRT output.
+    //
+    // In short:
+    //   Jsoup handles XML-level syntax,
+    //   our code handles text-level normalization for subtitles.
+    // --------------------------------------------------------------------
+    private void extractText(final Node node, final StringBuilder text) {
+        if (node instanceof TextNode textNode) {
+            String rawTtmlFragment = textNode.getWholeText();
+            String srtContent = sanitizeFragment(rawTtmlFragment);
+            text.append(srtContent);
+        } else if (node instanceof Element element) {
+            // <br> is a self-closing HTML tag used to insert a line break.
+            if (element.tagName().equalsIgnoreCase("br")) {
+                // Add a newline for <br> tags
+                text.append(NEW_LINE);
+            }
+        }
+
+        traverseChildNodesForNestedTags(node, text);
+    }
+    // CHECKSTYLE:ON
+
     public void build(final SharpStream ttml) throws IOException {
         /*
          * TTML parser with BASIC support
@@ -74,21 +298,15 @@ public void build(final SharpStream ttml) throws IOException {
         final Elements paragraphList = doc.select("body > div > p");
 
         // check if has frames
-        if (paragraphList.size() < 1) {
+        if (paragraphList.isEmpty()) {
             return;
         }
 
         for (final Element paragraph : paragraphList) {
             text.setLength(0);
 
-            for (final Node children : paragraph.childNodes()) {
-                if (children instanceof TextNode) {
-                    text.append(((TextNode) children).text());
-                } else if (children instanceof Element
-                        && ((Element) children).tagName().equalsIgnoreCase("br")) {
-                    text.append(NEW_LINE);
-                }
-            }
+            // Recursively extract text from all child nodes
+            extractText(paragraph, text);
 
             if (ignoreEmptyFrames && text.length() < 1) {
                 continue;