/**
 * `shareDownloader`: the download page's file list. It remembers, in this browser, which files the
 * recipient has downloaded, and where the browser can write to a folder (the File System Access
 * API: Chrome and Edge on a computer) it downloads every file into the folder the recipient picks,
 * one at a time, in its folder path.
 *
 * One request per file keeps every request short enough for a proxy in front of the server. A
 * request that breaks off is retried after 1, 2, 4, 8 and 16 seconds from the byte the file on
 * disk has reached (an HTTP Range request); a file already there at its full size is skipped, so
 * running it again after a stop picks up where it stopped. A file that still fails is listed for
 * the Retry button; a share that is gone (404) stops the run.
 */
const RETRY_DELAYS = [1000, 2000, 4000, 8000, 16000]

const UNITS = ['B', 'KB', 'MB', 'GB', 'TB']

function formatSize(bytes) {
    let unit = 0

    while (bytes >= 1024 && unit < UNITS.length - 1) {
        bytes /= 1024
        unit++
    }

    return `${bytes.toFixed(unit === 0 ? 0 : 1)} ${UNITS[unit]}`
}

function readStored(key) {
    try {
        return JSON.parse(window.localStorage.getItem(key)) ?? []
    } catch {
        return []
    }
}

/** The file's handle under the picked folder, creating its subfolders; never a path out of it. */
async function fileHandle(directory, path) {
    const segments = path.split('/').filter((segment) => segment !== '' && segment !== '.' && segment !== '..')
    const name = segments.pop()

    for (const segment of segments) {
        directory = await directory.getDirectoryHandle(segment, { create: true })
    }

    return directory.getFileHandle(name, { create: true })
}

document.addEventListener('alpine:init', () => {
    window.Alpine.data('shareDownloader', ({ token, files, messages }) => {
        const storageKey = `sealshare.downloaded.${token}`
        const total = files.reduce((sum, file) => sum + file.size, 0)

        // Kept out of Alpine's reactive data: a proxied native handle throws on its own methods.
        let directory = null
        let controller = null

        return {
            supported: window.isSecureContext && 'showDirectoryPicker' in window,

            /** The ids of the files this browser has downloaded. */
            done: readStored(storageKey),

            running: false,

            /** Bytes of the run's files on disk, for the progress bar. */
            received: 0,

            /** How many of the run's files are in the folder. */
            saved: 0,

            /** The file on its way. */
            current: null,

            /** The files that failed, kept for the Retry button. */
            failed: [],

            /** Why the run ended early, if it did. */
            error: null,

            finished: false,

            get progress() {
                return total === 0 ? 100 : (this.received / total) * 100
            },

            get status() {
                return messages.progress
                    .replace(':saved', this.saved)
                    .replace(':count', files.length)
                    .replace(':received', formatSize(this.received))
                    .replace(':total', formatSize(total))
            },

            isDone(id) {
                return this.done.includes(id)
            },

            markDone(id) {
                if (this.done.includes(id)) {
                    return
                }

                this.done.push(id)

                try {
                    window.localStorage.setItem(storageKey, JSON.stringify(this.done))
                } catch {
                    // Private mode or storage blocked: the marks last as long as the page.
                }
            },

            async downloadAll() {
                try {
                    directory = await window.showDirectoryPicker({ id: 'sealshare', mode: 'readwrite' })
                } catch {
                    return
                }

                this.run(files)
            },

            retry() {
                this.run(this.failed)
            },

            cancel() {
                controller?.abort()
            },

            async run(list) {
                controller = new AbortController()
                this.running = true
                this.finished = false
                this.error = null
                this.failed = []
                this.saved = files.length - list.length
                this.received = total - list.reduce((sum, file) => sum + file.size, 0)
                this.downloaded = true

                for (const file of list) {
                    const before = this.received
                    this.current = file.path

                    try {
                        await this.save(file, controller.signal)
                        this.saved++
                        this.markDone(file.id)
                    } catch (error) {
                        if (controller.signal.aborted) {
                            break
                        }

                        if (error.status === 404) {
                            this.error = messages.gone
                            break
                        }

                        this.failed.push(file)
                        this.received = before + file.size
                    }
                }

                this.running = false
                this.current = null
                this.finished = ! controller.signal.aborted
            },

            async save(file, signal) {
                const handle = await fileHandle(directory, file.path)
                const before = this.received
                // ponytail: a file is matched by name and size only, so a different file of the same name and size already in the picked folder is kept; compare a checksum if that ever bites.
                let written = (await handle.getFile()).size

                if (written > file.size) {
                    written = 0
                }

                for (let attempt = 0; written < file.size; attempt++) {
                    try {
                        written = await this.fetchInto(handle, file, written, signal, (bytes) => (this.received = before + bytes))
                    } catch (error) {
                        if (signal.aborted || error.status === 404 || attempt >= RETRY_DELAYS.length) {
                            throw error
                        }

                        await new Promise((resolve) => setTimeout(resolve, RETRY_DELAYS[attempt]))
                        written = (await handle.getFile()).size
                    }
                }

                this.received = before + file.size
            },

            /** Write the file from byte `offset` on; the bytes that arrived are kept even when the connection breaks. */
            async fetchInto(handle, file, offset, signal, onProgress) {
                const response = await fetch(file.url, {
                    headers: offset > 0 ? { Range: `bytes=${offset}-` } : {},
                    signal,
                })

                if (! response.ok) {
                    throw Object.assign(new Error(`HTTP ${response.status}`), { status: response.status })
                }

                if (response.status !== 206) {
                    offset = 0
                }

                const writable = await handle.createWritable({ keepExistingData: offset > 0 })
                const reader = response.body.getReader()

                try {
                    await writable.truncate(offset)
                    await writable.seek(offset)

                    for (;;) {
                        const { done, value } = await reader.read()

                        if (done) {
                            break
                        }

                        await writable.write(value)
                        offset += value.length
                        onProgress(offset)
                    }
                } finally {
                    // Closing commits what was written, so a retry continues from there.
                    await writable.close().catch(() => {})
                }

                if (offset < file.size) {
                    throw new Error('The download ended early')
                }

                return offset
            },

            warnBeforeLeaving(event) {
                if (this.running) {
                    event.preventDefault()
                    event.returnValue = ''
                }
            },
        }
    })
})