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

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

/* eslint-disable no-case-declarations */
import {type Diff, DIFF_DELETE, DIFF_EQUAL, DIFF_INSERT} from './diff.js'

/**
 * Given the original text1, and an encoded string which describes the
 * operations required to transform text1 into text2, compute the full diff.
 *
 * @param text1 - Source string for the diff.
 * @param delta - Delta text.
 * @returns Array of diff tuples.
 * @internal
 */
export function fromDelta(text1: string, delta: string): Diff[] {
  const diffs: Diff[] = []
  let diffsLength = 0 // Keeping our own length var is faster in JS.
  let pointer = 0 // Cursor in text1
  const tokens = delta.split(/\t/g)

  for (let x = 0; x < tokens.length; x++) {
    // Each token begins with a one character parameter which specifies the
    // operation of this token (delete, insert, equality).
    const param = tokens[x].substring(1)
    switch (tokens[x].charAt(0)) {
      case '+':
        try {
          diffs[diffsLength++] = [DIFF_INSERT, decodeURI(param)]
          // eslint-disable-next-line @typescript-eslint/no-unused-vars
        } catch (ex) {
          // Malformed URI sequence.
          throw new Error(`Illegal escape in fromDelta: ${param}`)
        }
        break
      case '-':
      // Fall through.
      case '=':
        const n = parseInt(param, 10)
        if (isNaN(n) || n < 0) {
          throw new Error(`Invalid number in fromDelta: ${param}`)
        }
        const text = text1.substring(pointer, (pointer += n))
        if (tokens[x].charAt(0) === '=') {
          diffs[diffsLength++] = [DIFF_EQUAL, text]
        } else {
          diffs[diffsLength++] = [DIFF_DELETE, text]
        }
        break
      default:
        // Blank tokens are ok (from a trailing \t).
        // Anything else is an error.
        if (tokens[x]) {
          throw new Error(`Invalid diff operation in fromDelta: ${tokens[x]}`)
        }
    }
  }
  if (pointer !== text1.length) {
    throw new Error(`Delta length (${pointer}) does not equal source text length (${text1.length})`)
  }
  return diffs
}