/** Known CPython source-format identity, independent of decoder coverage. */
export interface PythonBytecodeIdentity {
  readonly pythonVersion: string;
  readonly headerBytes: 12 | 16;
}

const identities = (
  values: readonly number[],
  pythonVersion: string,
  headerBytes: 12 | 16 = 16,
): ReadonlyArray<readonly [number, PythonBytecodeIdentity]> =>
  values.map((magic) => [magic, { pythonVersion, headerBytes }]);

const range = (start: number, end: number): readonly number[] =>
  Array.from({ length: end - start + 1 }, (_, offset) => start + offset);

/** Pinned identities include archive reconstruction and standalone decoder inputs. */
const PYTHON_BYTECODE_IDENTITIES: ReadonlyMap<number, PythonBytecodeIdentity> =
  new Map([
    ...identities([3390, 3391], "3.7", 12),
    ...identities([3392, 3393, 3394], "3.7"),
    ...identities([3400, 3401, 3410, 3411, 3412, 3413], "3.8"),
    ...identities(range(3420, 3425), "3.9"),
    ...identities(range(3430, 3439), "3.10"),
    ...identities([3494, 3495], "3.11"),
    ...identities(range(3500, 3531), "3.12"),
    ...identities(range(3550, 3571), "3.13"),
    ...identities([3627], "3.14"),
  ]);

export const pythonBytecodeIdentity = (
  magic: number,
): PythonBytecodeIdentity | null =>
  PYTHON_BYTECODE_IDENTITIES.get(magic) ?? null;

export type PythonBytecodePrefix =
  | {
      readonly state: "incomplete";
      readonly prefix: Uint8Array;
      readonly magic: number | null;
      readonly identity: null;
    }
  | {
      readonly state: "invalid-crlf";
      readonly prefix: Uint8Array;
      readonly magic: number;
      readonly identity: null;
    }
  | {
      readonly state: "framed";
      readonly prefix: Uint8Array;
      readonly magic: number;
      readonly identity: PythonBytecodeIdentity | null;
    };

/** Borrow the source prefix; a CR LF trailer establishes framing even for unknown magic. */
export const observePythonBytecodePrefix = (
  bytes: Uint8Array,
): PythonBytecodePrefix => {
  const prefix = bytes.subarray(0, 4);
  const magic =
    prefix.byteLength < 2
      ? null
      : new DataView(
          prefix.buffer,
          prefix.byteOffset,
          prefix.byteLength,
        ).getUint16(0, true);
  if (prefix.byteLength < 4 || magic === null)
    return { state: "incomplete", prefix, magic, identity: null };
  if (prefix[2] !== 0x0d || prefix[3] !== 0x0a)
    return { state: "invalid-crlf", prefix, magic, identity: null };
  return {
    state: "framed",
    prefix,
    magic,
    identity: pythonBytecodeIdentity(magic),
  };
};