import { randomBytes } from "node:crypto";
import { readdirSync, rmSync } from "node:fs";
import { open, rename, rm } from "node:fs/promises";
import type { FileHandle } from "node:fs/promises";
import { dirname, join } from "node:path";
import { Unzip, UnzipInflate } from "fflate";
import {
    chunkHashFinal,
    chunkHashInit,
    chunkHashUpdate,
    fromBase64,
    initStreamPull,
    pullStreamChunk,
    STREAM_CHUNK_OVERHEAD,
    STREAM_CHUNK_SIZE,
    streamTagFinal,
} from "../crypto/index.js";
import { TruncatedStreamError } from "../errors.js";
import { safeExtension, sanitizeFileName, withExtension } from "../filename.js";
import { withRetry } from "../retry.js";
import type { ApiClient } from "../api/client.js";
import type { EnteFile } from "../model/types.js";

export interface DownloadResult {
    // Where the file was written. A live photo is written as two files, its
    // image here and its video at `videoPath` (see `decryptLivePhoto`).
    path: string;
    // The decrypted length; for a live photo, that of the ZIP it arrives as.
    bytesWritten: number;
    videoPath?: string;
}

// Fired as decrypted plaintext accumulates, with the running total of
// plaintext bytes recovered so far. Within one download it is non-decreasing
// and its last value equals the final `bytesWritten`. A retry restarts the
// file from byte zero (see `fetchAndDecrypt`), so a fresh attempt begins its
// own count from zero.
export type ProgressCallback = (bytesDone: number) => void;

const ENC_CHUNK_SIZE = STREAM_CHUNK_SIZE + STREAM_CHUNK_OVERHEAD;

// Decrypt a secretstream body, handing each plaintext chunk to `sink` as it is
// produced rather than accumulating the whole file. Peak memory is one
// ciphertext chunk of network buffer plus one plaintext chunk — bounded by
// `STREAM_CHUNK_SIZE` regardless of the file's size — so a multi-gigabyte video
// no longer needs its size again in RAM. Returns the total plaintext length.
//
// The truncation contract is exactly the buffered version's, only the sink is
// new: a body cut short still decrypts and authenticates up to its last whole
// chunk, so the absence of TAG_FINAL is the sole evidence it was cut short, and
// this throws rather than let a caller keep a short file. The sink has already
// seen those chunks by then; the callers (`decryptToTemp`, `decryptLivePhoto`)
// stage them in temp files that are renamed into place only on a clean return,
// so a throw leaves nothing on disk.
const streamDecrypt = async (
    stream: ReadableStream<Uint8Array>,
    header: Uint8Array,
    key: Uint8Array,
    sink: (plaintext: Uint8Array) => Promise<void>,
    onProgress?: ProgressCallback,
): Promise<number> => {
    const state = initStreamPull(header, key);
    const reader = stream.getReader();
    // Incoming reads are held as-is and only stitched into a contiguous chunk
    // at each `ENC_CHUNK_SIZE` boundary, so every received byte is copied once.
    // Concatenating on each read instead — reallocating the whole accumulator
    // per read — is O(n^2) in the bytes buffered, and for a 4 MiB chunk that
    // memory churn dwarfs the libsodium decryption itself.
    const pending: Uint8Array[] = [];
    let pendingBytes = 0;
    let totalPlain = 0;
    let chunksPulled = 0;
    let lastTag = -1;

    // Remove the first `size` bytes from `pending` as one contiguous buffer.
    // A read that straddles the boundary is split with `subarray` (a view, no
    // copy); its tail stays queued for the next chunk. `size` never exceeds
    // `pendingBytes`, so the queue always holds enough.
    const takeContiguous = (size: number): Uint8Array => {
        const out = new Uint8Array(size);
        let offset = 0;
        while (offset < size) {
            const piece = pending[0]!;
            const need = size - offset;
            if (piece.length <= need) {
                out.set(piece, offset);
                offset += piece.length;
                pending.shift();
            } else {
                out.set(piece.subarray(0, need), offset);
                pending[0] = piece.subarray(need);
                offset += need;
            }
        }
        pendingBytes -= size;
        return out;
    };

    const consume = async (
        plaintext: Uint8Array,
        tag: number,
    ): Promise<void> => {
        await sink(plaintext);
        totalPlain += plaintext.length;
        chunksPulled++;
        lastTag = tag;
        onProgress?.(totalPlain);
    };

    try {
        for (;;) {
            const { done, value } = await reader.read();
            if (value && value.length > 0) {
                pending.push(value);
                pendingBytes += value.length;
            }

            while (pendingBytes >= ENC_CHUNK_SIZE) {
                const encChunk = takeContiguous(ENC_CHUNK_SIZE);
                // A whole chunk that fails to authenticate while the stream
                // carries on is corruption, not truncation; that error
                // propagates unchanged.
                const { plaintext, tag } = pullStreamChunk(state, encChunk);
                await consume(plaintext, tag);
            }

            if (done) {
                if (pendingBytes > 0) {
                    const buffer = takeContiguous(pendingBytes);
                    // Whatever is left over once every whole chunk has been
                    // consumed must be the stream's final chunk, and a final
                    // chunk that actually arrived in full authenticates. If
                    // it does not, the body stopped part-way through a chunk
                    // — the ordinary shape of a dropped connection. Poly1305
                    // cannot tell a partial chunk from a corrupt one, so this
                    // is reported as the truncation it almost always is, with
                    // the authentication failure kept as the error's cause.
                    // Only the pull is guarded: a sink failure on a chunk
                    // that did authenticate is a disk error, not a
                    // truncation.
                    let pulled;
                    try {
                        pulled = pullStreamChunk(state, buffer);
                    } catch (err) {
                        throw new TruncatedStreamError(
                            `download: stream truncated: response body ended with ${buffer.length} trailing bytes that did not authenticate as a final chunk (transfer stopped mid-chunk, or the data is corrupt)`,
                            { cause: err },
                        );
                    }
                    await consume(pulled.plaintext, pulled.tag);
                }
                break;
            }
        }
    } finally {
        reader.releaseLock();
    }

    // Only the last chunk of a secretstream carries TAG_FINAL. Everything a
    // dropped connection did deliver still decrypts and authenticates, so the
    // absence of TAG_FINAL is the only evidence that the body was cut short.
    if (chunksPulled === 0) {
        throw new TruncatedStreamError(
            "download: stream truncated: response body contained no secretstream chunks",
        );
    }
    const tagFinal = streamTagFinal();
    if (lastTag !== tagFinal) {
        throw new TruncatedStreamError(
            `download: stream truncated: last chunk tag ${lastTag}, expected TAG_FINAL (${tagFinal})`,
        );
    }
    return totalPlain;
};

// Fsync a file or a directory, so its contents (for a directory, its entries)
// are on stable storage. Exported for the backup tree's copy, which needs the
// same durability as the writer below.
export const fsyncPath = async (path: string): Promise<void> => {
    const handle = await open(path, "r");
    try {
        await handle.sync();
    } finally {
        await handle.close();
    }
};

// A process-ID check: signal 0 delivers nothing and only reports whether the
// process exists. EPERM means it exists but belongs to another user.
const isRunning = (pid: number): boolean => {
    try {
        process.kill(pid, 0);
        return true;
    } catch (err) {
        return (err as NodeJS.ErrnoException).code === "EPERM";
    }
};

// Delete the temp files a killed process left in `dir`: the writer's
// `.quak-<pid>-<random>.tmp` and the backup copy's
// `.quak-backup-<name>-<pid>-<random>.tmp`. Only files whose process is no
// longer running are removed, so another process writing into the same
// directory keeps its own. A reused process ID can only keep a leftover a while
// longer, never remove a live one.
export const removeLeftoverTempFiles = (dir: string): void => {
    let names: string[];
    try {
        names = readdirSync(dir);
    } catch {
        return;
    }
    for (const name of names) {
        const match = /^\.quak-(?:.*-)?(\d+)-[0-9a-z]*\.tmp$/.exec(name);
        if (match && !isRunning(Number(match[1]))) {
            rmSync(join(dir, name), { force: true });
        }
    }
};

// A new temp file name in `dir`. The random suffix keeps concurrent downloads
// of the same destination from stepping on each other's temporary file; the
// process ID lets `removeLeftoverTempFiles` tell a leftover from a write in
// progress.
const tempPathIn = (dir: string): string =>
    join(dir, `.quak-${process.pid}-${randomBytes(16).toString("hex")}.tmp`);

// Stage a write to `destination` atomically and durably, then rename it into
// place. `fill` writes the contents into the open temp file handle — either the
// whole buffer at once (`writeAtomic`) or chunk by chunk as they decrypt
// (`decryptToTemp`). The temp file is a sibling of the destination (same
// directory, so the rename cannot cross a filesystem boundary), so callers
// never observe a partially written destination, and a pre-existing file is
// replaced only once the new contents are complete on disk.
//
// Durability against a power cut needs two fsyncs. Without them the write can
// return while the data or the rename is still only in the kernel's page
// cache, and a crash then resurrects an empty renamed file — exactly the
// corruption a later backup run treats as a complete download. So the temp
// file's contents are fsynced before the rename, and the containing directory
// is fsynced after it, so both the bytes and the new directory entry are on
// stable storage before this returns.
//
// On any failure — including a `fill` that throws because the stream was
// truncated — the temp file is removed, so the destination is untouched and no
// scratch file is left to fill the disk on repeated failures.
const stageAtomic = async (
    destination: string,
    fill: (handle: FileHandle) => Promise<void>,
): Promise<void> => {
    const dir = dirname(destination);
    const tmpPath = tempPathIn(dir);
    try {
        const handle = await open(tmpPath, "w");
        try {
            await fill(handle);
            await handle.sync();
        } finally {
            await handle.close();
        }
        // `rename` replaces the destination's directory entry rather than
        // writing through it: an existing symlink at `destination` is
        // replaced, not followed, and the new file has the temp file's
        // permissions, not those of the file it replaced.
        await rename(tmpPath, destination);
        // Fsync the directory so the rename itself survives a crash: renaming
        // over a synced temp file still leaves the new directory entry in the
        // page cache until the directory is synced.
        await fsyncPath(dir);
    } catch (err) {
        // Best-effort cleanup. A failure to remove the temporary file must
        // never replace the error that actually explains what went wrong.
        await rm(tmpPath, { force: true }).catch(() => undefined);
        throw err;
    }
};

// Write `plaintext` to `destination` atomically and durably. Exported so the
// metadata store can reuse the same durable write for small whole-buffer
// payloads; originals go through `decryptToTemp` instead so they never buffer.
export const writeAtomic = async (
    destination: string,
    plaintext: Uint8Array,
): Promise<void> =>
    stageAtomic(destination, (handle) => handle.writeFile(plaintext));

// Refuse an original whose bytes do not hash to what its uploader recorded.
// The error is not retried.
const checkHash = (file: EnteFile, actual: string): void => {
    if (actual !== file.metadata.hash) {
        throw new Error(
            `download: file ${file.id}: content hash ${actual} does not match the hash its uploader recorded, ${file.metadata.hash}`,
        );
    }
};

// Decrypt `stream` straight to `destination`, one plaintext chunk at a time,
// under the atomic writer's temp-then-rename discipline. Memory stays bounded
// by the chunk size: each decrypted chunk is written to the temp file and
// dropped. The rename happens only after the stream authenticates as terminated
// on TAG_FINAL; a truncated stream throws and leaves the destination untouched.
// Returns the plaintext length written.
//
// `original` is the file whose original this is (none for a thumbnail, which
// has no recorded hash). When its metadata has a hash, the decrypted bytes are
// hashed as they stream and must match it, or nothing is stored.
const decryptToTemp = async (
    destination: string,
    stream: ReadableStream<Uint8Array>,
    header: Uint8Array,
    key: Uint8Array,
    onProgress?: ProgressCallback,
    original?: EnteFile,
): Promise<number> => {
    const hash =
        original?.metadata.hash === undefined ? undefined : chunkHashInit();
    let bytesWritten = 0;
    await stageAtomic(destination, async (handle) => {
        bytesWritten = await streamDecrypt(
            stream,
            header,
            key,
            async (plaintext) => {
                if (hash !== undefined) chunkHashUpdate(hash, plaintext);
                await handle.write(plaintext);
            },
            onProgress,
        );
        if (original !== undefined && hash !== undefined) {
            checkHash(original, chunkHashFinal(hash));
        }
    });
    return bytesWritten;
};

// One of the two parts of a live photo being unpacked: the ZIP entry whose
// name starts with `kind`, written to its own temp file.
interface LivePhotoPart {
    kind: "image" | "video";
    tmpPath: string;
    handle: FileHandle;
    hash: ReturnType<typeof chunkHashInit>;
    // Decompressed bytes not yet written.
    pending: Uint8Array[];
    // The entry's extension, set once all of the entry has been read.
    ext?: string;
}

const openPart = async (
    kind: "image" | "video",
    dir: string,
): Promise<LivePhotoPart> => {
    const tmpPath = tempPathIn(dir);
    const handle = await open(tmpPath, "w");
    return { kind, tmpPath, handle, hash: chunkHashInit(), pending: [] };
};

// A live photo arrives as a ZIP of its image and its video. Ente's clients
// name the entries `image.<ext>` and `video.<ext>`. This takes the entry whose
// name starts with `image` as the image and the one whose name starts with
// `video` as the video, and refuses a ZIP holding a second of either. It is
// written unpacked: each part is named `destination` with the extension
// replaced by its own entry's, and the two must differ ignoring case. When the
// file records a hash, `<imageHash>:<videoHash>` must match it, each over that
// part's own bytes. Only then are the image, then the video, renamed into
// place; on any failure neither is stored.
//
// The ZIP is chosen by its uploader and may expand enormously, so each part is
// written as it decompresses and never held, and the ZIP is refused once the
// two together come to more than 20 times its size plus 16 MiB, the limit
// Ente's mobile client and CLI set. The ZIP's size is known only at its end,
// so until then the limit is taken over the part of it decrypted so far.
// fflate's `Unzip` inflates each push in one piece before `push` returns, and
// deflate expands at most about 1000-fold, so the ZIP is pushed in 4 KiB
// slices, keeping each decompressed piece near 4 MiB, one plaintext chunk, and
// each piece is checked and written before the next slice is pushed. Every
// entry is started, even one that is not kept, because fflate keeps an
// unstarted entry's data in memory.
const decryptLivePhoto = async (
    destination: string,
    stream: ReadableStream<Uint8Array>,
    header: Uint8Array,
    key: Uint8Array,
    onProgress: ProgressCallback | undefined,
    file: EnteFile,
): Promise<DownloadResult> => {
    const sliceSize = 4096;
    const dir = dirname(destination);
    const fail = (message: string, cause?: unknown): Error =>
        new Error(`download: file ${file.id}: ${message}`, { cause });
    const parts: LivePhotoPart[] = [];
    try {
        const image = await openPart("image", dir);
        parts.push(image);
        const video = await openPart("video", dir);
        parts.push(video);

        const claimed = new Set<LivePhotoPart>();
        // Set to a part the ZIP holds a second entry for; the ZIP is then
        // refused.
        let repeated: LivePhotoPart | undefined;
        const unzip = new Unzip((entry) => {
            const part = parts.find((p) => entry.name.startsWith(p.kind));
            if (part !== undefined) {
                if (claimed.has(part)) repeated = part;
                claimed.add(part);
            }
            entry.ondata = (err, data, final) => {
                if (err) throw err;
                if (part === undefined) return;
                chunkHashUpdate(part.hash, data);
                part.pending.push(data);
                if (final) part.ext = safeExtension(entry.name);
            };
            entry.start();
        });
        unzip.register(UnzipInflate);
        // The bytes of the ZIP pushed so far, and of the two parts they have
        // decompressed to.
        let zipBytes = 0;
        let expanded = 0;
        const push = async (
            data: Uint8Array,
            final: boolean,
        ): Promise<void> => {
            zipBytes += data.length;
            // fflate reports a bad ZIP by throwing, sometimes a TypeError,
            // which the retry would take for a network failure. A bad ZIP is
            // never retried, and nor are the refusals below.
            try {
                unzip.push(data, final);
            } catch (err) {
                throw fail("live photo is not a readable ZIP", err);
            }
            if (repeated !== undefined) {
                throw fail(
                    `live photo ZIP holds more than one ${repeated.kind}`,
                );
            }
            for (const part of parts) {
                for (const piece of part.pending) expanded += piece.length;
            }
            if (expanded > 20 * zipBytes + 16 * 1024 * 1024) {
                throw fail(
                    "live photo ZIP expands to more than 20 times its size plus 16 MiB",
                );
            }
            for (const part of parts) {
                for (const piece of part.pending)
                    await part.handle.write(piece);
                part.pending = [];
            }
        };

        const bytesWritten = await streamDecrypt(
            stream,
            header,
            key,
            async (plaintext) => {
                for (let i = 0; i < plaintext.length; i += sliceSize) {
                    await push(plaintext.subarray(i, i + sliceSize), false);
                }
            },
            onProgress,
        );
        await push(new Uint8Array(0), true);

        if (image.ext === undefined || video.ext === undefined) {
            throw fail(
                "live photo ZIP does not hold both an image and a video",
            );
        }
        if (file.metadata.hash !== undefined) {
            checkHash(
                file,
                `${chunkHashFinal(image.hash)}:${chunkHashFinal(video.hash)}`,
            );
        }
        if (image.ext.toLowerCase() === video.ext.toLowerCase()) {
            throw fail(
                `live photo's image and video have the same extension, ${video.ext}`,
            );
        }
        const path = withExtension(destination, image.ext);
        const videoPath = withExtension(destination, video.ext);
        for (const part of parts) {
            await part.handle.sync();
            await part.handle.close();
        }
        await rename(image.tmpPath, path);
        try {
            await rename(video.tmpPath, videoPath);
        } catch (err) {
            await rm(path, { force: true }).catch(() => undefined);
            throw err;
        }
        await fsyncPath(dir);
        return { path, bytesWritten, videoPath };
    } catch (err) {
        // Best-effort cleanup, as in `stageAtomic`.
        for (const part of parts) {
            await part.handle.close().catch(() => undefined);
            await rm(part.tmpPath, { force: true }).catch(() => undefined);
        }
        throw err;
    }
};

// Fetch a stream and decrypt it to `destination`, retrying the whole sequence.
//
// The request is only the first third of a download. `getXStream` returns as
// soon as headers arrive, and the bytes are pulled here, so a socket reset
// mid-body — the dominant failure mode for multi-megabyte photos over a CDN —
// throws in `streamDecrypt` and never reaches `ApiClient` at all. Retrying the
// request alone would miss it entirely.
//
// The client's own retry is therefore switched off for these two calls: with
// both layers active the budgets would multiply, and the library default of
// four attempts would mean sixteen requests for one file. The policy comes
// from the client so a caller that configured one gets it here too.
//
// Because the plaintext is streamed to disk rather than buffered, the atomic
// write is part of the retried unit. A retry starts the file over from byte
// zero — the secretstream pull state is not resumable and there is no Range
// support — staging into a fresh temp file each time: a failed attempt writes
// and then removes its own temp file, and only the attempt that reaches
// TAG_FINAL renames one into place, so a download that needed three tries still
// performs exactly one rename over the destination.
const fetchAndDecrypt = async (
    api: ApiClient,
    openStream: () => Promise<ReadableStream<Uint8Array>>,
    header: Uint8Array,
    key: Uint8Array,
    destination: string,
    onProgress?: ProgressCallback,
    original?: EnteFile,
): Promise<DownloadResult> =>
    withRetry(async () => {
        const stream = await openStream();
        try {
            if (original?.metadata.fileType === "livePhoto") {
                return await decryptLivePhoto(
                    destination,
                    stream,
                    header,
                    key,
                    onProgress,
                    original,
                );
            }
            const bytesWritten = await decryptToTemp(
                destination,
                stream,
                header,
                key,
                onProgress,
                original,
            );
            return { path: destination, bytesWritten };
        } catch (err) {
            // Cancel the body so its connection is closed now rather than held
            // until the stream is garbage collected. A backup run carries on
            // past a failed file, so without this every failure would hold a
            // socket. This covers every failure, including a temp file that
            // cannot be opened and a header that is rejected before the body
            // is read.
            await stream.cancel(err).catch(() => undefined);
            throw err;
        }
    }, api.getRetryOptions());

// Write `file`'s original to `outPath`. A live photo is written as its image
// and its video beside `outPath` instead (see `decryptLivePhoto`).
export const downloadFile = async (
    api: ApiClient,
    file: EnteFile,
    outPath?: string,
    onProgress?: ProgressCallback,
): Promise<DownloadResult> => {
    // `outPath` is the caller's and is used as is; the title is the server's
    // and is sanitized so it can only name a file in the current directory.
    const resolvedPath =
        outPath ?? sanitizeFileName(file.metadata.title, `file-${file.id}`);
    const header = fromBase64(file.file.decryptionHeader);
    return fetchAndDecrypt(
        api,
        () => api.getFileStream(file.id, { retry: false }),
        header,
        file.key,
        resolvedPath,
        onProgress,
        file,
    );
};

export const downloadThumbnail = async (
    api: ApiClient,
    file: EnteFile,
    outPath?: string,
    onProgress?: ProgressCallback,
): Promise<DownloadResult> => {
    const resolvedPath =
        outPath ??
        `thumb_${sanitizeFileName(file.metadata.title, `file-${file.id}`)}`;
    const header = fromBase64(file.thumbnail.decryptionHeader);
    return fetchAndDecrypt(
        api,
        () => api.getThumbnailStream(file.id, { retry: false }),
        header,
        file.key,
        resolvedPath,
        onProgress,
    );
};