cebe · samwilson · Jan 16, 2021 · Jan 16, 2021 · Jan 20, 2021 · Jan 20, 2021
diff --git a/MarkdownExtra.php b/MarkdownExtra.php
@@ -2,6 +2,7 @@
 
 namespace cebe\markdown;
 
+use cebe\markdown\block\FootnoteTrait;
 use cebe\markdown\block\TableTrait;
 
 // work around https://github.com/facebook/hhvm/issues/1120
@@ -19,6 +20,7 @@ class MarkdownExtra extends Markdown
 	// include block element parsing using traits
 	use block\TableTrait;
 	use block\FencedCodeTrait;
+	use block\FootnoteTrait;
 
 	// include inline element parsing using traits
 	// TODO
@@ -60,8 +62,6 @@ class MarkdownExtra extends Markdown
 
 	// TODO implement definition lists
 
-	// TODO implement footnotes
-
 	// TODO implement Abbreviations
 
 

diff --git a/block/FootnoteTrait.php b/block/FootnoteTrait.php
@@ -0,0 +1,217 @@
+<?php
+
+namespace cebe\markdown\block;
+
+trait FootnoteTrait
+{
+
+    /** @var string[][] Unordered array of footnotes. */
+    protected $footnotes = [];
+
+    /** @var int Incrementing counter of the footnote links. */
+    protected $footnoteLinkNum = 0;
+
+    /** @var string[] Ordered array of footnote links. */
+    protected $footnoteLinks = [];
+
+    /**
+     * @inheritDoc
+     */
+    abstract protected function parseBlocks($lines);
+
+    /**
+     * @inheritDoc
+     */
+    abstract protected function renderAbsy($blocks);
+
+    /**
+     * @param $text
+     * @return string
+     */
+    public function parse($text)
+    {
+        $html = parent::parse($text);
+
+        // If no footnotes found, do nothing more.
+        if (count($this->footnotes) === 0) {
+            return $html;
+        }
+
+        // Sort all found footnotes by the order in which they are linked in the text.
+        $footnotesSorted = [];
+        $footnoteNum = 0;
+        foreach ($this->footnoteLinks as $footnotePos => $footnoteLinkName) {
+            foreach ($this->footnotes as $footnoteName => $footnoteHtml) {
+                if ($footnoteLinkName === (string)$footnoteName) {
+                    // First time sorting this footnote.
+                    if (!isset($footnotesSorted[$footnoteName])) {
+                        $footnoteNum++;
+                        $footnotesSorted[$footnoteName] = [
+                            'html' => $footnoteHtml,
+                            'num' => $footnoteNum,
+                            'refs' => [1 => $footnotePos],
+                        ];
+                    } else {
+                        // Subsequent times sorting this footnote (i.e. every time it's referenced).
+                        $footnotesSorted[$footnoteName]['refs'][] = $footnotePos;
+                    }
+                }
+            }
+        }
+
+        // Replace the footnote substitution markers with their actual numbers.
+        $referencedHtml = preg_replace_callback('/\x1Afootnote-(refnum|num)(.*?)\x1A/', function ($match) use ($footnotesSorted) {
+            $footnoteName = $this->footnoteLinks[$match[2]];
+            // Replace only the footnote number.
+            if ($match[1] === 'num') {
+                return $footnotesSorted[$footnoteName]['num'];
+            }
+            // For backlinks, some have a footnote number and an additional link number.
+            if (count($footnotesSorted[$footnoteName]['refs']) > 1) {
+                // If this footnote is referenced more than once, use the `-x` suffix.
+                $linkNum = array_search($match[2], $footnotesSorted[$footnoteName]['refs']);
+                return $footnotesSorted[$footnoteName]['num'] . '-' . $linkNum;
+            } else {
+                // Otherwise, just the number.
+                return $footnotesSorted[$footnoteName]['num'];
+            }
+        }, $html);
+
+        // Get the footnote HTML and add it to the end of the document.
+        return $referencedHtml . $this->getFootnotesHtml($footnotesSorted);
+    }
+
+    /**
+     * @param mixed[] $footnotesSorted Array with 'html', 'num', and 'refs' keys.
+     * @return string
+     */
+    protected function getFootnotesHtml(array $footnotesSorted)
+    {
+        $hr = $this->html5 ? "<hr>\n" : "<hr />\n";
+        $footnotesHtml = "\n<div class=\"footnotes\" role=\"doc-endnotes\">\n$hr<ol>\n\n";
+        foreach ($footnotesSorted as $footnoteInfo) {
+            $backLinks = [];
+            foreach ($footnoteInfo['refs'] as $refIndex => $refNum) {
+                $fnref = count($footnoteInfo['refs']) > 1
+                    ? $footnoteInfo['num'] . '-' . $refIndex
+                    : $footnoteInfo['num'];
+                $backLinks[] = '<a href="#fnref'.'-'.$fnref.'" role="doc-backlink">&#8617;&#xFE0E;</a>';
+            }
+            $linksPara = '<p class="footnote-backrefs">'.join("\n", $backLinks)."</p>";
+            $footnotesHtml .= "<li id=\"fn-{$footnoteInfo['num']}\" role=\"doc-endnote\">\n{$footnoteInfo['html']}$linksPara\n</li>\n\n";
+        }
+        $footnotesHtml .= "</ol>\n</div>\n";
+        return $footnotesHtml;
+    }
+
+    /**
+     * Parses a footnote link indicated by `[^`.
+     * @marker [^
+     * @param $text
+     * @return array
+     */
+    protected function parseFootnoteLink($text)
+    {
+        if (preg_match('/^\[\^(.+?)]/', $text, $matches)) {
+            $footnoteName = $matches[1];
+
+            // We will later order the footnotes according to the order that the footnote links appear in.
+            $this->footnoteLinkNum++;
+            $this->footnoteLinks[$this->footnoteLinkNum] = $footnoteName;
+
+            // To render a footnote link, we only need to know its link-number,
+            // which will later be turned into its footnote-number (after sorting).
+            return [
+                ['footnoteLink', 'num' => $this->footnoteLinkNum],
+                strlen($matches[0])
+            ];
+        }
+        return [['text', $text[0]], 1];
+    }
+
+    /**
+     * @param string[] $block Array with 'num' key.
+     * @return string
+     */
+    protected function renderFootnoteLink($block)
+    {
+        $substituteRefnum = "\x1Afootnote-refnum".$block['num']."\x1A";
+        $substituteNum = "\x1Afootnote-num".$block['num']."\x1A";
+        return '<sup id="fnref-' . $substituteRefnum . '" class="footnote-ref">'
+            .'<a href="#fn-' . $substituteNum . '" role="doc-noteref">' . $substituteNum . '</a>'
+            .'</sup>';
+    }
+
+    /**
+     * identify a line as the beginning of a footnote block
+     *
+     * @param $line
+     * @return false|int
+     */
+    protected function identifyFootnoteList($line)
+    {
+        return preg_match('/^\[\^(.+?)]:/', $line);
+    }
+
+    /**
+     * Consume lines for a footnote
+     * @return array Array of two elements, the first element contains the block,
+     * the second contains the next line index to be parsed.
+     */
+    protected function consumeFootnoteList($lines, $current)
+    {
+        $name = '';
+        $footnotes = [];
+        $count = count($lines);
+        $nextLineIndent = null;
+        for ($i = $current; $i < $count; $i++) {
+            $line = $lines[$i];
+            $startsFootnote = preg_match('/^\[\^(.+?)]:[ \t]*/', $line, $matches);
+            if ($startsFootnote) {
+                // Current line starts a footnote.
+                $name = $matches[1];
+                $str = substr($line, strlen($matches[0]));
+                $footnotes[$name] = [ trim($str) ];
+            } else if (strlen(trim($line)) === 0) {
+                // Current line is empty and ends this list of footnotes unless the next line is indented.
+                if (isset($lines[$i+1])) {
+                    $nextLineIndented = preg_match('/^(\t| {4})/', $lines[$i + 1], $matches);
+                    if ($nextLineIndented) {
+                        // If the next line is indented, keep this empty line.
+                        $nextLineIndent = $matches[1];
+                        $footnotes[$name][] = $line;
+                    } else {
+                        // Otherwise, end the current footnote.
+                        break;
+                    }
+                }
+            } elseif (!$startsFootnote && isset($footnotes[$name])) {
+                // Current line continues the current footnote.
+                $footnotes[$name][] = $nextLineIndent
+                    ? substr($line, strlen($nextLineIndent))
+                    : trim($line);
+            }
+        }
+
+        // Parse all collected footnotes.
+        $parsedFootnotes = [];
+        foreach ($footnotes as $footnoteName => $footnoteLines) {
+            $parsedFootnotes[$footnoteName] = $this->parseBlocks($footnoteLines);
+        }
+
+        return [['footnoteList', 'content' => $parsedFootnotes], $i];
+    }
+
+    /**
+     * @param array $block
+     * @return string
+     */
+    protected function renderFootnoteList($block)
+    {
+        foreach ($block['content'] as $footnoteName => $footnote) {
+            $this->footnotes[$footnoteName] = $this->renderAbsy($footnote);
+        }
+        // Render nothing, because all footnote lists will be concatenated at the end of the text.
+        return '';
+    }
+}
diff --git a/tests/extra-data/footnotes.html b/tests/extra-data/footnotes.html
@@ -0,0 +1,63 @@
+<p>A <em>simple</em> footnote<sup id="fnref-1" class="footnote-ref"><a href="#fn-1" role="doc-noteref">1</a></sup> and one with a label.<sup id="fnref-2" class="footnote-ref"><a href="#fn-2" role="doc-noteref">2</a></sup> Labels can be anything.<sup id="fnref-3" class="footnote-ref"><a href="#fn-3" role="doc-noteref">3</a></sup></p>
+<p>Footnotes can be defined out of order<sup id="fnref-4" class="footnote-ref"><a href="#fn-4" role="doc-noteref">4</a></sup> (both where they're called and defined).</p>
+<p>Block elements such as…</p>
+<h2>…headers…<sup id="fnref-5-1" class="footnote-ref"><a href="#fn-5" role="doc-noteref">5</a></sup></h2>
+<ul>
+<li>…lists…<sup id="fnref-5-2" class="footnote-ref"><a href="#fn-5" role="doc-noteref">5</a></sup></li>
+</ul>
+<blockquote><p>…and quotes…<sup id="fnref-6" class="footnote-ref"><a href="#fn-6" role="doc-noteref">6</a></sup></p>
+</blockquote>
+<p>…can contain footnotes, and footnotes can contain block elements.
+One footnote can be referenced multiple times.</p>
+<p>End of test.</p>
+
+<div class="footnotes" role="doc-endnotes">
+<hr />
+<ol>
+
+<li id="fn-1" role="doc-endnote">
+<p>The <em>first</em> footnote, with <a href="https://example.org/">inline</a> formatting.</p>
+<p class="footnote-backrefs"><a href="#fnref-1" role="doc-backlink">&#8617;&#xFE0E;</a></p>
+</li>
+
+<li id="fn-2" role="doc-endnote">
+<p>Labelled footnote (number 2)
+also with
+multiple lines.</p>
+<p class="footnote-backrefs"><a href="#fnref-2" role="doc-backlink">&#8617;&#xFE0E;</a></p>
+</li>
+
+<li id="fn-3" role="doc-endnote">
+<p>Any characters are allowed.</p>
+<p class="footnote-backrefs"><a href="#fnref-3" role="doc-backlink">&#8617;&#xFE0E;</a></p>
+</li>
+
+<li id="fn-4" role="doc-endnote">
+<p>Out of order
+and with
+multiple lines.</p>
+<p class="footnote-backrefs"><a href="#fnref-4" role="doc-backlink">&#8617;&#xFE0E;</a></p>
+</li>
+
+<li id="fn-5" role="doc-endnote">
+<p>A footnote (number 5) with block elements.</p>
+<p>The blocks must be <em>intented</em></p>
+<ul>
+<li>by the same <em>amount</em>, and</li>
+<li>with a tab or four spaces.</li>
+</ul>
+<p>They can also contain</p>
+<pre><code>code blocks.
+</code></pre>
+<p class="footnote-backrefs"><a href="#fnref-5-1" role="doc-backlink">&#8617;&#xFE0E;</a>
+<a href="#fnref-5-2" role="doc-backlink">&#8617;&#xFE0E;</a></p>
+</li>
+
+<li id="fn-6" role="doc-endnote">
+<p>Block footnotes can start</p>
+<p>on or after the first line.</p>
+<p class="footnote-backrefs"><a href="#fnref-6" role="doc-backlink">&#8617;&#xFE0E;</a></p>
+</li>
+
+</ol>
+</div>
diff --git a/tests/extra-data/footnotes.md b/tests/extra-data/footnotes.md
@@ -0,0 +1,41 @@
+A *simple* footnote[^1] and one with a label.[^label] Labels can be anything.[^✳&|^"]
+
+[^1]: The *first* footnote, with [inline](https://example.org/) formatting.
+[^third]: Out of order
+    and with
+    multiple lines.
+[^label]: Labelled footnote (number 2)
+also with
+multiple lines.
+[^✳&|^"]: Any characters are allowed.
+
+Footnotes can be defined out of order[^third] (both where they're called and defined).
+
+Block elements such as…
+
+## …headers…[^block]
+
+* …lists…[^block]
+
+> …and quotes…[^block2]
+
+…can contain footnotes, and footnotes can contain block elements.
+One footnote can be referenced multiple times.
+
+[^block]: A footnote (number 5) with block elements.
+
+    The blocks must be _intented_
+
+    * by the same *amount*, and
+    * with a tab or four spaces.
+
+    They can also contain
+
+        code blocks.
+
+[^block2]:
+    Block footnotes can start
+
+    on or after the first line.
+
+End of test.