import { Transform } from "node:stream";
import type { TransformCallback } from "node:stream";

const JSON_LD_OPENING_TAG = Buffer.from('<script type="application/ld+json">');

export function nonceJsonLdScripts(nonce: string): Transform {
  const nonceOpeningTag = Buffer.from(
    `<script nonce="${nonce}" type="application/ld+json">`,
  );
  const retainedBytes = JSON_LD_OPENING_TAG.length - 1;
  let pending = Buffer.alloc(0);

  return new Transform({
    transform(chunk: Buffer, _encoding: BufferEncoding, callback: TransformCallback) {
      const data = Buffer.concat([pending, chunk]);
      const safeLimit = Math.max(0, data.length - retainedBytes);
      let cursor = 0;
      let match = data.indexOf(JSON_LD_OPENING_TAG, cursor);

      while (match !== -1 && match < safeLimit) {
        this.push(data.subarray(cursor, match));
        this.push(nonceOpeningTag);
        cursor = match + JSON_LD_OPENING_TAG.length;
        match = data.indexOf(JSON_LD_OPENING_TAG, cursor);
      }

      const safeEnd = Math.max(cursor, safeLimit);
      if (safeEnd > cursor) this.push(data.subarray(cursor, safeEnd));
      pending = Buffer.from(data.subarray(safeEnd));
      callback();
    },
    flush(callback: TransformCallback) {
      let cursor = 0;
      let match = pending.indexOf(JSON_LD_OPENING_TAG, cursor);

      while (match !== -1) {
        this.push(pending.subarray(cursor, match));
        this.push(nonceOpeningTag);
        cursor = match + JSON_LD_OPENING_TAG.length;
        match = pending.indexOf(JSON_LD_OPENING_TAG, cursor);
      }

      this.push(pending.subarray(cursor));
      callback();
    },
  });
}