/**
 * Browse backend of the directory-picker seam: registers `ctx.directoryPicker`
 * with the `browse` capability — one-level directory listing and child-directory
 * creation over the host filesystem via Node's stdlib (which already carries
 * the per-OS adaptation). Nothing renders on the host display, so this backend
 * serves remote clients the dialog backend cannot. Policy decisions (hidden
 * entries flagged but returned, symlinks followed, whole-filesystem scope) are
 * recorded in the directory-picker seam Agent Note.
 * @module @deepseek-ai/dsh-host-directory-picker-browse
 */

import { mkdir, opendir, stat } from 'node:fs/promises'
import { homedir } from 'node:os'
import { basename, dirname, join, posix, resolve, win32 } from 'node:path'
import type { Context } from 'cordis'
import z from 'schemastery'
import {
  DirectoryPicker, DirectoryPickerError,
} from '@deepseek-ai/dsh-host-directory-picker'
import type {
  DirectoryEntry, DirectoryListing, DirectoryPickerCapability,
} from '@deepseek-ai/dsh-host-directory-picker'

/**
 * Ancestor chain from the filesystem root to `target` inclusive — the
 * breadcrumb rows of a listing, every one a jump target.
 */
function ancestryCrumbs(target: string): DirectoryEntry[] {
  const crumbs: DirectoryEntry[] = []
  let current = target
  for (;;) {
    const parent = dirname(current)
    // basename of a root is '' — label the root crumb by its full path ('/', 'C:\').
    crumbs.unshift({ name: parent === current ? current : basename(current), path: current, hidden: false })
    if (parent === current) return crumbs
    current = parent
  }
}

/**
 * True when the path names one fixed filesystem location regardless of
 * process state: POSIX-absolute on POSIX; on Windows only drive-qualified
 * (`C:\…`) or complete UNC (`\\server\share…`) forms. Rooted drive-less
 * forms (`\foo`, `/foo`) and incomplete UNC prefixes (`\\`, `\\server`)
 * pass `isAbsolute` yet still resolve against the process's current drive.
 * @param path - candidate path.
 * @param platform - replaces `process.platform` for deterministic tests.
 * @returns whether the path is fully qualified on the platform.
 */
export function fullyQualified(path: string, platform: NodeJS.Platform = process.platform): boolean {
  return platform === 'win32'
    ? win32.isAbsolute(path) && /^(?:[A-Za-z]:[\\/]|[\\/]{2}[^\\/]+[\\/]+[^\\/]+)/.test(path)
    : posix.isAbsolute(path)
}

/** One streamed listing candidate: the dirent facts a row needs, nothing else retained. */
export interface ListingCandidate {
  /** Base name within the streamed level. */
  name: string
  /** Dirent says directory (no probe needed). */
  isDirectory: boolean
  /** Dirent says symlink (enterability needs a stat probe). */
  isSymbolicLink: boolean
}

/**
 * Insert a streamed candidate into the name-sorted bounded window, evicting
 * the name-largest candidate when the window exceeds `keep`. Memory over an
 * arbitrarily large level therefore stays O(keep) regardless of how many
 * children the directory holds.
 * @param window - the name-ascending window, mutated in place.
 * @param candidate - the streamed candidate to place.
 * @param keep - the window bound.
 * @returns true when an eviction happened (the level has candidates beyond the window).
 */
export function boundedInsert(window: ListingCandidate[], candidate: ListingCandidate, keep: number): boolean {
  const at = window.findIndex(existing => candidate.name.localeCompare(existing.name) < 0)
  if (at === -1) window.push(candidate)
  else window.splice(at, 0, candidate)
  if (window.length <= keep) return false
  window.pop()
  return true
}

/** Message text of an unknown thrown value. */
function messageOf(error: unknown): string {
  /* v8 ignore next -- node:fs rejects with Error instances; the String arm only satisfies the unknown narrowing. */
  return error instanceof Error ? error.message : String(error)
}

/**
 * One listing row for a dirent, following symlinks to directories; null for
 * non-directories and broken/cyclic links (skipped silently — the browser
 * shows what can be entered, and a broken link cannot).
 */
async function directoryRow(parent: string, name: string, isDirectory: boolean, isSymbolicLink: boolean): Promise<DirectoryEntry | null> {
  const path = join(parent, name)
  let enterable = isDirectory
  if (!enterable && isSymbolicLink) {
    try {
      enterable = (await stat(path)).isDirectory()
    } catch {
      // Broken or cyclic symlink: stat is the probe, failure means "not enterable".
      return null
    }
  }
  if (!enterable) return null
  // POSIX hidden convention; Windows' hidden attribute is not exposed by
  // dirents (Known Limitations). The client owns whether hidden rows show.
  return { name, path, hidden: name.startsWith('.') }
}

/** Validated plugin configuration. */
export interface Config {
  /** Complete-result bound of one listing level; see {@link BrowseDirectoryPicker.Config}. */
  maxEntries: number
}

/** The `ctx.directoryPicker` browse implementation (stable capability object per service life). */
export default class BrowseDirectoryPicker extends DirectoryPicker {
  /**
   * `maxEntries` bounds the complete listing level a single `list` call may
   * materialize and put on the wire: at most this many child-directory rows
   * (hidden rows included), with `truncated` flagging a cut level. The
   * default follows GitHub's web UI, which truncates directory listings at
   * 1,000 entries.
   */
  static Config: z<Config> = z.object({
    maxEntries: z.natural().min(1).default(1000),
  })

  private readonly browseCapability: DirectoryPickerCapability = {
    kind: 'browse',
    list: path => this.list(path),
    createDirectory: (path, name) => this.createDirectory(path, name),
  }

  constructor(ctx: Context, private readonly config: Config) {
    super(ctx)
  }

  /**
   * The browse interaction capability.
   * @returns the stable `browse` capability object.
   */
  capability(): DirectoryPickerCapability {
    return this.browseCapability
  }

  private async list(path?: string): Promise<DirectoryListing> {
    const home = homedir()
    // The seam contract takes fully qualified paths only; resolve() would
    // silently rebase a relative or empty wire value under the host process
    // cwd (or, for rooted drive-less Windows forms, its current drive).
    if (path !== undefined && !fullyQualified(path)) {
      throw new DirectoryPickerError('directory-unreadable', path, `cannot list "${path}": not a fully qualified path`)
    }
    const target = resolve(path ?? home)
    // Stream the level (opendir, one dirent at a time) into a name-sorted
    // window of maxEntries + 1 candidates: memory stays bounded no matter how
    // many children the directory holds, the window keeps the name-sorted
    // head, and the +1 slot lets an in-window extra row prove the cut. A
    // window candidate that turns out non-enterable (broken symlink) is not
    // backfilled from beyond the window — an eviction already marks the
    // level truncated, which stays the honest answer.
    const keep = this.config.maxEntries + 1
    const window: ListingCandidate[] = []
    let evicted = false
    try {
      const level = await opendir(target)
      for await (const dirent of level) {
        // Only rows a browser could enter contend for the window; dirent
        // says "directory" outright, a symlink needs the later stat probe.
        if (!dirent.isDirectory() && !dirent.isSymbolicLink()) continue
        const candidate = { name: dirent.name, isDirectory: dirent.isDirectory(), isSymbolicLink: dirent.isSymbolicLink() }
        if (boundedInsert(window, candidate, keep)) evicted = true
      }
    } catch (error: unknown) {
      throw new DirectoryPickerError('directory-unreadable', target, `cannot list ${target}: ${messageOf(error)}`)
    }
    const entries: DirectoryEntry[] = []
    let truncated = evicted
    for (const candidate of window) {
      const row = await directoryRow(target, candidate.name, candidate.isDirectory, candidate.isSymbolicLink)
      if (row === null) continue
      if (entries.length === this.config.maxEntries) {
        truncated = true
        break
      }
      entries.push(row)
    }
    return { path: target, home, crumbs: ancestryCrumbs(target), entries, truncated }
  }

  private async createDirectory(path: string, name: string): Promise<string> {
    // Same fully-qualified fence as list: never rebase a parent under the
    // cwd or the current drive.
    if (!fullyQualified(path)) {
      throw new DirectoryPickerError('directory-create-failed', path, `cannot create under "${path}": not a fully qualified parent path`)
    }
    const parent = resolve(path)
    // The backend owns segment validation (the wire schema also refuses these,
    // but direct service consumers must hit the same fence).
    if (name.trim() === '' || name === '.' || name === '..' || /[/\\]/.test(name)) {
      throw new DirectoryPickerError('directory-create-failed', join(parent, name), `"${name}" is not a single path segment`)
    }
    const target = join(parent, name)
    try {
      // Non-recursive: the parent is the directory the browser is showing, so
      // a missing parent is a real failure, not a level to invent.
      await mkdir(target)
      return target
    } catch (error: unknown) {
      if (typeof error === 'object' && error !== null && 'code' in error && error.code === 'EEXIST') {
        throw new DirectoryPickerError('directory-exists', target, `${target} already exists`)
      }
      throw new DirectoryPickerError('directory-create-failed', target, `cannot create ${target}: ${messageOf(error)}`)
    }
  }
}