Add cached summarization to PhamePost

Summary: Restore summarization. Use the remarkup cache, and try to do it somewhat-intelligently (pick the first paragraph that looks like it's text).

Test Plan:
{F21323}

{F21324}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1373

Differential Revision: https://secure.phabricator.com/D3715

src/infrastructure/markup/PhabricatorMarkupEngine.php

@@ -495,4 +495,54 @@ final class PhabricatorMarkupEngine {
     return $mentions;
   }
 
+
+  /**
+   * Produce a corpus summary, in a way that shortens the underlying text
+   * without truncating it somewhere awkward.
+   *
+   * TODO: We could do a better job of this.
+   *
+   * @param string  Remarkup corpus to summarize.
+   * @return string Summarized corpus.
+   */
+  public static function summarize($corpus) {
+
+    // Major goals here are:
+    //  - Don't split in the middle of a character (utf-8).
+    //  - Don't split in the middle of, e.g., **bold** text, since
+    //    we end up with hanging '**' in the summary.
+    //  - Try not to pick an image macro, header, embedded file, etc.
+    //  - Hopefully don't return too much text. We don't explicitly limit
+    //    this right now.
+
+    $blocks = preg_split("/\n *\n\s*/", trim($corpus));
+
+    $best = null;
+    foreach ($blocks as $block) {
+      // This is a test for normal spaces in the block, i.e. a heuristic to
+      // distinguish standard paragraphs from things like image macros. It may
+      // not work well for non-latin text. We prefer to summarize with a
+      // paragraph of normal words over an image macro, if possible.
+      $has_space = preg_match('/\w\s\w/', $block);
+
+      // This is a test to find embedded images and headers. We prefer to
+      // summarize with a normal paragraph over a header or an embedded object,
+      // if possible.
+      $has_embed = preg_match('/^[{=]/', $block);
+
+      if ($has_space && !$has_embed) {
+        // This seems like a good summary, so return it.
+        return $block;
+      }
+
+      if (!$best) {
+        // This is the first block we found; if everything is garbage just
+        // use the first block.
+        $best = $block;
+      }
+    }
+
+    return $best;
+  }
+
 }