import { TextDecoder } from "node:util";

import { HashlineError, TEXT_ERROR_MESSAGES } from "./errors.ts";

/** Why text cannot be written as editable UTF-8 (NUL or unpaired surrogates); undefined when it can. */
export function unwritableTextError(text: string): HashlineError | undefined {
  if (text.includes("\0"))
    return new HashlineError("UNSUPPORTED_TEXT", TEXT_ERROR_MESSAGES.UNSUPPORTED_TEXT);
  if (!text.isWellFormed())
    return new HashlineError("INVALID_UNICODE", TEXT_ERROR_MESSAGES.INVALID_UNICODE);
  return undefined;
}

export type Utf8Decoding = "strict" | "lossy";

/**
 * Confirmed malformed UTF-8, distinct from decoder input or resource failures.
 * The decoder's own error states the same fact, so it is not reported as a cause.
 */
export class Utf8DecodingError extends HashlineError<"UNSUPPORTED_ENCODING"> {
  constructor() {
    super("UNSUPPORTED_ENCODING", TEXT_ERROR_MESSAGES.UNSUPPORTED_ENCODING);
  }
}

/** Streaming UTF-8 decoding preserves BOM; lossy mode replaces malformed bytes with U+FFFD. */
export function createUtf8Decoder(mode: Utf8Decoding = "strict") {
  const decoder = new TextDecoder("utf-8", { fatal: mode === "strict", ignoreBOM: true });
  if (mode === "lossy") {
    return (bytes?: Uint8Array, stream = false): string => decoder.decode(bytes, { stream });
  }
  return (bytes?: Uint8Array, stream = false): string => {
    try {
      return decoder.decode(bytes, { stream });
    } catch (error) {
      if (
        error instanceof TypeError &&
        "code" in error &&
        error.code === "ERR_ENCODING_INVALID_ENCODED_DATA"
      ) {
        throw new Utf8DecodingError();
      }
      throw error;
    }
  };
}

const decode = createUtf8Decoder();

/**
 * Reject byte-oriented/binary content before entering a text mutation pipeline; malformed bytes
 * are not replaced and a leading BOM stays for byte-stable rewrites.
 */
export function decodeEditableText(bytes: Uint8Array): string {
  if (bytes.includes(0))
    throw new HashlineError("UNSUPPORTED_TEXT", TEXT_ERROR_MESSAGES.UNSUPPORTED_TEXT);
  return decode(bytes);
}