import { AuthError } from './errors.js'
import { Hex } from './hex.js'

/** Signs requests. Implement this to use another algorithm (e.g. secp256k1 via libauth). */
export interface RequestSigner {
  /** Algorithm name sent in `X-Signature-Algorithm`; the server must have a matching verifier. */
  readonly algorithm: string
  /** Public key as lowercase hex. Also the owner segment of the caller's keys (`/<publicKey>/...`). */
  readonly publicKey: string
  sign(payload: Uint8Array): Promise<Uint8Array>
}

/** Verifies signatures for one algorithm. */
export interface SignatureVerifier {
  readonly algorithm: string
  verify(publicKey: string, payload: Uint8Array, signature: Uint8Array): Promise<boolean>
}

export const AUTH_HEADERS = {
  publicKey: 'X-Public-Key',
  timestamp: 'X-Timestamp',
  signature: 'X-Signature',
  algorithm: 'X-Signature-Algorithm',
} as const

export interface RequestAuthOptions {
  /** Accepted difference between the request timestamp and the server clock. Default 60 000 ms. */
  maxSkewMs?: number
  now?: () => number
}

/**
 * Signed-request authentication. The client signs `${timestamp}:${path}:${body}` (path = URL
 * pathname, body = the exact request body) and sends the public key, timestamp, signature (hex)
 * and algorithm as headers. The server checks the timestamp is recent, verifies the signature
 * with the verifier for that algorithm, and rejects a signature it has already seen within the
 * timestamp window (replay protection).
 */
export class RequestAuth {
  private readonly verifiers: Map<string, SignatureVerifier>
  private readonly maxSkewMs: number
  private readonly now: () => number
  /** Signatures accepted within the window → when they expire; insertion order ≈ expiry order. */
  private readonly seen = new Map<string, number>()

  constructor(verifiers: SignatureVerifier[], options: RequestAuthOptions = {}) {
    if (verifiers.length === 0) throw new TypeError('RequestAuth needs at least one verifier')
    this.verifiers = new Map(verifiers.map((v) => [v.algorithm, v]))
    this.maxSkewMs = options.maxSkewMs ?? 60_000
    this.now = options.now ?? Date.now
  }

  static payload(timestamp: string, path: string, body: string): Uint8Array {
    return new TextEncoder().encode(`${timestamp}:${path}:${body}`)
  }

  /** Auth headers for a request to `path` with `body`. */
  static async headers(signer: RequestSigner, path: string, body: string, now: number = Date.now()): Promise<Record<string, string>> {
    const timestamp = String(now)
    const signature = await signer.sign(RequestAuth.payload(timestamp, path, body))
    return {
      [AUTH_HEADERS.publicKey]: signer.publicKey,
      [AUTH_HEADERS.timestamp]: timestamp,
      [AUTH_HEADERS.signature]: Hex.encode(signature),
      [AUTH_HEADERS.algorithm]: signer.algorithm,
    }
  }

  /** Returns the caller's public key, or throws `AuthError` (401). */
  async verify(headers: Headers, path: string, body: string): Promise<string> {
    const publicKey = headers.get(AUTH_HEADERS.publicKey)
    const timestamp = headers.get(AUTH_HEADERS.timestamp)
    const signature = headers.get(AUTH_HEADERS.signature)
    const algorithm = headers.get(AUTH_HEADERS.algorithm)
    if (!publicKey || !timestamp || !signature || !algorithm) throw new AuthError(401, 'Missing authentication headers')
    const verifier = this.verifiers.get(algorithm)
    if (!verifier) throw new AuthError(401, `Unsupported signature algorithm "${algorithm}"`)
    if (!/^[0-9a-f]+$/.test(publicKey)) throw new AuthError(401, 'Public key must be lowercase hex')
    const now = this.now()
    const time = Number(timestamp)
    if (!Number.isInteger(time) || Math.abs(now - time) > this.maxSkewMs) throw new AuthError(401, 'Timestamp outside the accepted window')
    this.forgetExpired(now)
    if (this.seen.has(signature)) throw new AuthError(401, 'Replayed request')
    let valid = false
    try {
      valid = await verifier.verify(publicKey, RequestAuth.payload(timestamp, path, body), Hex.decode(signature))
    } catch {
      valid = false
    }
    if (!valid) throw new AuthError(401, 'Invalid signature')
    this.seen.set(signature, time + this.maxSkewMs)
    return publicKey
  }

  private forgetExpired(now: number): void {
    for (const [signature, expires] of this.seen) {
      if (expires >= now) break
      this.seen.delete(signature)
    }
  }
}

/** Ed25519 signer using WebCrypto (Node 22+ and current browsers; no dependencies). */
export class Ed25519Signer implements RequestSigner {
  readonly algorithm = 'ed25519'

  private constructor(
    private readonly privateKey: CryptoKey,
    readonly publicKey: string,
  ) {}

  /** A new random key pair. */
  static async generate(): Promise<Ed25519Signer> {
    const pair = (await crypto.subtle.generateKey({ name: 'Ed25519' }, true, ['sign', 'verify'])) as CryptoKeyPair
    return new Ed25519Signer(pair.privateKey, Hex.encode(new Uint8Array(await crypto.subtle.exportKey('raw', pair.publicKey))))
  }

  /** Restores a key exported with `exportPrivateKey()` (PKCS #8). */
  static async fromPrivateKey(pkcs8: Uint8Array): Promise<Ed25519Signer> {
    const privateKey = await crypto.subtle.importKey('pkcs8', pkcs8 as BufferSource, { name: 'Ed25519' }, true, ['sign'])
    // WebCrypto can't derive the public key directly; the JWK export of a private key includes it (`x`).
    const jwk = await crypto.subtle.exportKey('jwk', privateKey)
    const publicKey = Uint8Array.from(atob(jwk.x!.replace(/-/g, '+').replace(/_/g, '/')), (c) => c.charCodeAt(0))
    return new Ed25519Signer(privateKey, Hex.encode(publicKey))
  }

  async exportPrivateKey(): Promise<Uint8Array> {
    return new Uint8Array(await crypto.subtle.exportKey('pkcs8', this.privateKey))
  }

  async sign(payload: Uint8Array): Promise<Uint8Array> {
    return new Uint8Array(await crypto.subtle.sign({ name: 'Ed25519' }, this.privateKey, payload as BufferSource))
  }
}

/** Ed25519 verifier using WebCrypto. */
export class Ed25519Verifier implements SignatureVerifier {
  readonly algorithm = 'ed25519'
  private readonly keys = new Map<string, Promise<CryptoKey>>()

  async verify(publicKey: string, payload: Uint8Array, signature: Uint8Array): Promise<boolean> {
    let key = this.keys.get(publicKey)
    if (!key) {
      key = crypto.subtle.importKey('raw', Hex.decode(publicKey) as BufferSource, { name: 'Ed25519' }, false, ['verify'])
      this.keys.set(publicKey, key)
      if (this.keys.size > 10_000) this.keys.delete(this.keys.keys().next().value!)
    }
    return crypto.subtle.verify({ name: 'Ed25519' }, await key, signature as BufferSource, payload as BufferSource)
  }
}