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flexmark-java extension for GFM strikethrough using ~~ (GitHub Flavored Markdown)

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package com.vladsch.flexmark.ext.gfm.strikethrough.internal;

import com.vladsch.flexmark.ext.gfm.strikethrough.Strikethrough;
import com.vladsch.flexmark.ext.gfm.strikethrough.Subscript;
import com.vladsch.flexmark.parser.InlineParser;
import com.vladsch.flexmark.parser.core.delimiter.Delimiter;
import com.vladsch.flexmark.parser.delimiter.DelimiterProcessor;
import com.vladsch.flexmark.parser.delimiter.DelimiterRun;
import com.vladsch.flexmark.util.ast.DelimitedNode;
import com.vladsch.flexmark.util.ast.Node;
import com.vladsch.flexmark.util.sequence.BasedSequence;

public class StrikethroughSubscriptDelimiterProcessor implements DelimiterProcessor {
  @Override
  public char getOpeningCharacter() {
    return '~';
  }

  @Override
  public char getClosingCharacter() {
    return '~';
  }

  @Override
  public int getMinLength() {
    return 1;
  }

  @Override
  public boolean canBeOpener(
      String before,
      String after,
      boolean leftFlanking,
      boolean rightFlanking,
      boolean beforeIsPunctuation,
      boolean afterIsPunctuation,
      boolean beforeIsWhitespace,
      boolean afterIsWhiteSpace) {
    return leftFlanking;
  }

  @Override
  public boolean canBeCloser(
      String before,
      String after,
      boolean leftFlanking,
      boolean rightFlanking,
      boolean beforeIsPunctuation,
      boolean afterIsPunctuation,
      boolean beforeIsWhitespace,
      boolean afterIsWhiteSpace) {
    return rightFlanking;
  }

  @Override
  public boolean skipNonOpenerCloser() {
    return false;
  }

  @Override
  public int getDelimiterUse(DelimiterRun opener, DelimiterRun closer) {
    // "multiple of 3" rule for internal delimiter runs
    if ((opener.canClose() || closer.canOpen()) && (opener.length() + closer.length()) % 3 == 0) {
      return 0;
    }
    // calculate actual number of delimiters used from this closer
    if (opener.length() < 3 || closer.length() < 3) {
      return closer.length() <= opener.length() ? closer.length() : opener.length();
    }

    return closer.length() % 2 == 0 ? 2 : 1;
  }

  @Override
  public Node unmatchedDelimiterNode(InlineParser inlineParser, DelimiterRun delimiter) {
    return null;
  }

  @Override
  public void process(Delimiter opener, Delimiter closer, int delimitersUsed) {
    // wrap nodes between delimiters in strikethrough.
    DelimitedNode emphasis =
        delimitersUsed == 1
            ? new Subscript(
                opener.getTailChars(delimitersUsed),
                BasedSequence.NULL,
                closer.getLeadChars(delimitersUsed))
            : new Strikethrough(
                opener.getTailChars(delimitersUsed),
                BasedSequence.NULL,
                closer.getLeadChars(delimitersUsed));

    opener.moveNodesBetweenDelimitersTo(emphasis, closer);
  }
}