reverse-engineering-anything/src/domain/jsonStringSourceSpans.ts

/** Ordered, nonoverlapping UTF-16 spans in a decoded JSON string. */
export interface JsonStringSpan {
  readonly start: number;
  readonly end: number;
}

/** Map parsed string facts back to their original, validated JSON spelling.
 * `encoded` excludes the quotes; callers own JSON decoding and span validity.
 */
export function* jsonStringSourceSpans(
  encoded: string,
  offset: number,
  spans: Iterable<JsonStringSpan>,
): Generator<JsonStringSpan> {
  let sourceCursor = 0;
  let decodedCursor = 0;
  const sourceOffset = (boundary: number): number => {
    while (decodedCursor < boundary) {
      sourceCursor +=
        encoded[sourceCursor] === "\\"
          ? encoded[sourceCursor + 1] === "u"
            ? 6
            : 2
          : 1;
      decodedCursor++;
    }
    return offset + sourceCursor;
  };
  for (const span of spans)
    yield { start: sourceOffset(span.start), end: sourceOffset(span.end) };
}