import { z } from "zod";
import { randomBytes } from "crypto";

export namespace Identifier {
  const prefixes = {
    session: "ses",
    message: "msg",
  } as const;

  export function schema(prefix: keyof typeof prefixes) {
    return z.string().startsWith(prefixes[prefix]);
  }

  const LENGTH = 26;

  // State for monotonic ID generation
  let lastTimestamp = 0;
  let counter = 0;

  export function ascending(prefix: keyof typeof prefixes, given?: string) {
    return generateID(prefix, false, given);
  }

  export function descending(prefix: keyof typeof prefixes, given?: string) {
    return generateID(prefix, true, given);
  }

  function generateID(
    prefix: keyof typeof prefixes,
    descending: boolean,
    given?: string,
  ): string {
    if (!given) {
      return generateNewID(prefix, descending);
    }
    
    if (!given.startsWith(prefixes[prefix])) {
      throw new Error(`ID ${given} does not start with ${prefixes[prefix]}`);
    }
    return given;
  }

  function generateNewID(
    prefix: keyof typeof prefixes,
    descending: boolean,
  ): string {
    const currentTimestamp = Date.now();

    if (currentTimestamp !== lastTimestamp) {
      lastTimestamp = currentTimestamp;
      counter = 0;
    }
    counter++;

    let now = BigInt(currentTimestamp) * BigInt(0x1000) + BigInt(counter);

    now = descending ? ~now : now;

    const timeBytes = Buffer.alloc(6);
    for (let i = 0; i < 6; i++) {
      timeBytes[i] = Number((now >> BigInt(40 - 8 * i)) & BigInt(0xff));
    }

    const randLength = (LENGTH - 12) / 2;
    const random = randomBytes(randLength);

    return (
      prefixes[prefix] +
      "_" +
      timeBytes.toString("hex") +
      random.toString("hex")
    );
  }
}