import type { FileHandle } from "node:fs/promises";
import { Readable } from "node:stream";

/** Cancellation observed by the descriptor read, retaining the caller's exact reason. */
export class FileReadCancelled extends Error {
  constructor(readonly reason: unknown) {
    super("File read cancelled", { cause: reason });
    this.name = "FileReadCancelled";
  }
}

export interface FileHandleChunkOptions {
  readonly start?: number | bigint;
  readonly end?: number | bigint;
  readonly signal?: AbortSignal | undefined;
  readonly cancellationFailure?: (reason: unknown) => Error;
}

/** Physical descriptor coverage is separate from delivery of buffered stream bytes. */
export type FileHandleReadCoverage =
  | { readonly kind: "incomplete"; readonly bytes: bigint }
  | {
      readonly kind: "complete";
      readonly bytes: bigint;
      readonly boundary: "eof" | "range";
    };

export type FileHandleChunkStream = Readable & {
  readCoverage(): FileHandleReadCoverage;
};

/** Stream positional reads while leaving descriptor cleanup to its owner. */
export const readFileHandleChunks = (
  handle: Pick<FileHandle, "read">,
  options: FileHandleChunkOptions = {},
): FileHandleChunkStream => {
  const start = fileOffset(options.start ?? 0);
  const unlimited = options.end === undefined || options.end === Infinity;
  const end = unlimited ? undefined : fileOffset(options.end);
  if (
    start === undefined ||
    (!unlimited && end === undefined) ||
    (end !== undefined && end < start)
  )
    throw new RangeError("Invalid file read range");
  let readPending = false;
  let totalBytes = 0n;
  let coverage:
    | Extract<FileHandleReadCoverage, { kind: "complete" }>
    | undefined;
  const cancellationFailure = (): Error =>
    options.cancellationFailure?.(options.signal?.reason) ??
    new FileReadCancelled(options.signal?.reason);
  const assertActive = (): void => {
    if (options.signal?.aborted) throw cancellationFailure();
  };
  const source = Readable.from(
    (async function* () {
      let position = start;
      while (end === undefined || position <= end) {
        assertActive();
        const remaining = end === undefined ? undefined : end - position + 1n;
        const chunk = Buffer.allocUnsafe(
          remaining === undefined || remaining > 64n * 1024n
            ? 64 * 1024
            : Number(remaining),
        );
        readPending = true;
        let bytesRead: number;
        try {
          ({ bytesRead } = await handle.read(chunk, 0, chunk.length, position));
        } finally {
          readPending = false;
        }
        totalBytes += BigInt(bytesRead);
        if (bytesRead === 0)
          coverage = Object.freeze({
            kind: "complete",
            bytes: totalBytes,
            boundary: "eof",
          });
        else if (end !== undefined && position + BigInt(bytesRead) > end)
          coverage = Object.freeze({
            kind: "complete",
            bytes: totalBytes,
            boundary: "range",
          });
        assertActive();
        if (bytesRead === 0) return;
        position += BigInt(bytesRead);
        yield chunk.subarray(0, bytesRead);
      }
    })(),
    {
      objectMode: false,
    },
  );
  const onAbort = (): void => {
    // A native descriptor read cannot be interrupted. Its rejection remains
    // primary; a fulfilled read observes cancellation before yielding bytes.
    if (!readPending && !source.destroyed)
      source.destroy(cancellationFailure());
  };
  options.signal?.addEventListener("abort", onAbort, { once: true });
  source.once("close", () =>
    options.signal?.removeEventListener("abort", onAbort),
  );
  if (options.signal?.aborted) onAbort();
  return Object.assign(source, {
    readCoverage: (): FileHandleReadCoverage =>
      coverage ?? Object.freeze({ kind: "incomplete", bytes: totalBytes }),
  });
};

const fileOffset = (value: number | bigint): bigint | undefined => {
  if (typeof value === "bigint") return value >= 0n ? value : undefined;
  return Number.isSafeInteger(value) && value >= 0 ? BigInt(value) : undefined;
};