@sanity/diff-match-patch/src/diff/commonOverlap.ts

Robust diff, match and patch algorithms to perform operations required for synchronizing plain text

/**
 * Determine if the suffix of one string is the prefix of another.
 *
 * @param textA - First string.
 * @param textB - Second string.
 * @returns Number of characters common to the end of the first string
 *   and the start of the second string.
 * @internal
 */
export function getCommonOverlap(textA: string, textB: string): number {
  let text1 = textA
  let text2 = textB

  // Cache the text lengths to prevent multiple calls.
  const text1Length = text1.length
  const text2Length = text2.length

  // Eliminate the null case.
  if (text1Length === 0 || text2Length === 0) {
    return 0
  }

  // Truncate the longer string.
  if (text1Length > text2Length) {
    text1 = text1.substring(text1Length - text2Length)
  } else if (text1Length < text2Length) {
    text2 = text2.substring(0, text1Length)
  }
  const textLength = Math.min(text1Length, text2Length)

  // Quick check for the worst case.
  if (text1 === text2) {
    return textLength
  }

  // Start by looking for a single character match
  // and increase length until no match is found.
  // Performance analysis: http://neil.fraser.name/news/2010/11/04/
  let best = 0
  let length = 1

  for (let found = 0; found !== -1; ) {
    const pattern = text1.substring(textLength - length)
    found = text2.indexOf(pattern)
    if (found === -1) {
      return best
    }
    length += found
    if (found === 0 || text1.substring(textLength - length) === text2.substring(0, length)) {
      best = length
      length++
    }
  }

  // Only for typescript, never reached
  return best
}