Include @mentioned names in search results

ts/util/search.ts

@@ -4,3 +4,97 @@
 export const SNIPPET_LEFT_PLACEHOLDER = '<<left>>';
 export const SNIPPET_RIGHT_PLACEHOLDER = '<<right>>';
 export const SNIPPET_TRUNCATION_PLACEHOLDER = '<<truncation>>';
+
+/**
+ * Generate a snippet suitable for rendering search results, in the style returned from
+ * FTS's snippet() function.
+ *
+ * @param approxSnippetLength - If generating a snippet from a mention, the approximate
+ * length of snippet (not including any hydrated mentions that might occur when rendering)
+ * @param maxCharsBeforeHighlight - Max chars to show before the highlight, to ensure the
+ * highlight is visible even at narrow search result pane widths
+ *
+ * If generating a snippet from a mention, will not truncate in the middle of a word.
+ *
+ * @returns Return a snippet suitable for rendering search results, e.g.
+ * `<<truncation>>some text with a <<left>>highlight<<right>>.`
+ */
+export function generateSnippetAroundMention({
+  body,
+  mentionStart,
+  mentionLength = 1,
+  approxSnippetLength = 50,
+  maxCharsBeforeHighlight = 30,
+}: {
+  body: string;
+  mentionStart: number;
+  mentionLength: number;
+  approxSnippetLength?: number;
+  maxCharsBeforeHighlight?: number;
+}): string {
+  const segmenter = new Intl.Segmenter([], { granularity: 'word' });
+
+  // Grab a substring of the body around the mention, larger than the desired snippet
+  const bodyAroundMention = body.substring(
+    mentionStart - 2 * approxSnippetLength,
+    mentionStart + mentionLength + 2 * approxSnippetLength
+  );
+
+  const words = [...segmenter.segment(bodyAroundMention)].filter(
+    word => word.isWordLike
+  );
+
+  let snippetStartIdx = 0;
+  let snippetEndIdx = body.length;
+
+  let leftWordIdx = 0;
+  let rightWordIdx = words.length - 1;
+
+  // Gradually narrow the substring, word by word, until a snippet of appropriate length
+  // is found
+  while (leftWordIdx <= rightWordIdx) {
+    const leftWord = words[leftWordIdx];
+    const rightWord = words[rightWordIdx];
+
+    snippetStartIdx = Math.min(leftWord.index, mentionStart);
+    snippetEndIdx = Math.max(
+      rightWord.index + rightWord.segment.length,
+      mentionStart + mentionLength
+    );
+
+    const lengthBeforeMention = mentionStart - snippetStartIdx;
+    const lengthAfterMention = snippetEndIdx - mentionStart - mentionLength;
+
+    if (
+      lengthBeforeMention + lengthAfterMention <= approxSnippetLength &&
+      lengthBeforeMention <= maxCharsBeforeHighlight
+    ) {
+      break;
+    }
+
+    if (lengthBeforeMention > maxCharsBeforeHighlight) {
+      leftWordIdx += 1;
+    } else if (lengthBeforeMention > lengthAfterMention) {
+      leftWordIdx += 1;
+    } else {
+      rightWordIdx -= 1;
+    }
+  }
+
+  const mentionStartInSnippet = mentionStart - snippetStartIdx;
+  const snippedBody = body.substring(snippetStartIdx, snippetEndIdx);
+
+  const snippedBodyWithPlaceholders =
+    (snippetStartIdx > 0 ? SNIPPET_TRUNCATION_PLACEHOLDER : '') +
+    snippedBody.substring(0, mentionStartInSnippet) +
+    SNIPPET_LEFT_PLACEHOLDER +
+    snippedBody.substring(
+      mentionStartInSnippet,
+      mentionStartInSnippet + mentionLength
+    ) +
+    SNIPPET_RIGHT_PLACEHOLDER +
+    snippedBody.substring(mentionStartInSnippet + mentionLength) +
+    (snippetEndIdx < body.length ? SNIPPET_TRUNCATION_PLACEHOLDER : '');
+
+  return snippedBodyWithPlaceholders;
+}