feat: implement looping around

utils/jumpToPattern.ts

@@ -5,6 +5,12 @@ import { shim as matchAllShim } from "string.prototype.matchall";
 // Polyfill for String.prototype.matchAll, in case it's not available (pre-ES2020)
 matchAllShim();
 
+/**
+ * Returns the position of the repeat-th instance of a pattern from a given starting position, in
+ * the given direction; looping to the other end of the document when reaching one end.
+ *
+ * Under the hood, we avoid repeated loops around the document by using modulo arithmetic.
+ */
 export function jumpToPattern({
   cm,
   oldPosition,
@@ -31,8 +37,11 @@ export function jumpToPattern({
 }
 
 /**
- * Returns the index of (up to) the n-th next instance of a pattern in a string after a given
- * starting index. If the pattern is not found at all, returns undefined.
+ * Returns the index (from the start of the content) of the nth-next instance of a pattern after a
+ * given starting index.
+ *
+ * "Loops" to the top of the document when the bottom is reached; under the hood, we avoid repeated
+ * loops by using modulo arithmetic.
  */
 function findNthNextRegexMatch({
   content,
@@ -46,22 +55,23 @@ function findNthNextRegexMatch({
   n: number;
 }): number | undefined {
   const globalRegex = makeGlobalRegex(regex);
-  globalRegex.lastIndex = startingIdx + 1;
-  let currMatch, lastMatch;
-  let numMatchesFound = 0;
-  while (
-    numMatchesFound < n &&
-    (currMatch = globalRegex.exec(content)) != null
-  ) {
-    lastMatch = currMatch;
-    numMatchesFound++;
+  const allMatches = [...content.matchAll(globalRegex)];
+  const previousMatches = allMatches.filter((match) => match.index <= startingIdx);
+  const nextMatches = allMatches.filter((match) => match.index > startingIdx);
+  const nModulo = n % allMatches.length;
+  const effectiveN = nModulo === 0 ? allMatches.length : nModulo;
+  if (effectiveN <= nextMatches.length) {
+    return nextMatches[effectiveN - 1].index;
   }
-  return lastMatch?.index;
+  return previousMatches[effectiveN - nextMatches.length - 1].index;
 }
 
 /**
- * Returns the index of (up to) the nth-previous instance of a pattern in a string before a given
- * starting index. If the pattern is not found at all, returns undefined.
+ * Returns the index (from the start of the content) of the nth-previous instance of a pattern
+ * before a given starting index.
+ *
+ * "Loops" to the bottom of the document when the top is reached; under the hood, we avoid repeated
+ * loops by using modulo arithmetic.
  */
 function findNthPreviousRegexMatch({
   content,
@@ -75,12 +85,11 @@ function findNthPreviousRegexMatch({
   n: number;
 }): number | undefined {
   const globalRegex = makeGlobalRegex(regex);
-  const contentToSearch = content.substring(0, startingIdx);
-  const previousMatches = [...contentToSearch.matchAll(globalRegex)];
-  if (previousMatches.length < n) {
-    return previousMatches[0]?.index;
-  }
-  return previousMatches[previousMatches.length - n].index;
+  const allMatches = [...content.matchAll(globalRegex)];
+  const previousMatches = allMatches.filter((match) => match.index < startingIdx);
+  const nextMatches = allMatches.filter((match) => match.index >= startingIdx);
+  const match = getNthPreviousMatch(previousMatches, nextMatches, n);
+  return match?.index;
 }
 
 function makeGlobalRegex(regex: RegExp): RegExp {
@@ -92,3 +101,29 @@ function getGlobalFlags(regex: RegExp): string {
   const { flags } = regex;
   return flags.includes("g") ? flags : `${flags}g`;
 }
+
+function getNthPreviousMatch(
+  previousMatches: RegExpExecArray[],
+  nextMatches: RegExpExecArray[],
+  n: number
+): RegExpExecArray | undefined {
+  const numMatches = previousMatches.length + nextMatches.length;
+  const effectiveN = n % numMatches; // every `numMatches` is a full loop
+  if (effectiveN <= previousMatches.length) {
+    return getNthItemFromEnd(previousMatches, effectiveN);
+  }
+  return getNthItemFromEnd(nextMatches, effectiveN - previousMatches.length);
+}
+
+/**
+ * Returns the nth (1-indexed) item from the end of an array. Expects 1 <= n <= items.length, but
+ * just returns undefined if n is out of bounds.
+ */
+function getNthItemFromEnd<T>(items: T[], n: number): T | undefined {
+  const numItems = items.length;
+  if (n < 1 || n > numItems) {
+    console.warn(`Invalid n: ${n} for array of length ${numItems}`);
+    return undefined;
+  }
+  return items[numItems - n];
+}