getPotentialIndex.ts

// Copied from https://github.com/vercel/ai/blob/main/packages/ai/core/util/get-potential-start-index.ts

/**
 * Returns the index of the start of the searchedText in the text, or null if it
 * is not found.
 */
export function getPotentialStartIndex(text: string, searchedText: string): number | null {
  // Return null immediately if searchedText is empty.
  if (searchedText.length === 0) {
    return null
  }

  // Check if the searchedText exists as a direct substring of text.
  const directIndex = text.indexOf(searchedText)

  if (directIndex !== -1) {
    return directIndex
  }

  // Otherwise, look for the largest suffix of "text" that matches
  // a prefix of "searchedText". We go from the end of text inward.
  for (let i = text.length - 1; i >= 0; i--) {
    const suffix = text.substring(i)

    if (searchedText.startsWith(suffix)) {
      return i
    }
  }

  return null
}