import { deflateSync, inflateSync, strToU8, strFromU8 } from 'fflate'

// The document lives in the URL hash as: '#' + base64url(version byte + raw
// DEFLATE of the UTF-8 text). The hash fragment is never sent to the server,
// and base64url needs no percent-escaping, so the link survives copy/paste
// through chat clients and email intact.

const VERSION = 1

function toBase64Url(bytes: Uint8Array): string {
  let bin = ''
  const chunk = 0x8000
  for (let i = 0; i < bytes.length; i += chunk) {
    bin += String.fromCharCode(...bytes.subarray(i, i + chunk))
  }
  return btoa(bin).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '')
}

function fromBase64Url(s: string): Uint8Array {
  const b64 = s.replace(/-/g, '+').replace(/_/g, '/')
  const bin = atob(b64)
  const bytes = new Uint8Array(bin.length)
  for (let i = 0; i < bin.length; i++) bytes[i] = bin.charCodeAt(i)
  return bytes
}

export function encodeDocument(text: string): string {
  const compressed = deflateSync(strToU8(text), { level: 9 })
  const payload = new Uint8Array(compressed.length + 1)
  payload[0] = VERSION
  payload.set(compressed, 1)
  return toBase64Url(payload)
}

export function decodeDocument(encoded: string): string {
  const payload = fromBase64Url(encoded)
  if (payload.length === 0 || payload[0] !== VERSION) {
    throw new Error(`unsupported document encoding (version ${payload[0]})`)
  }
  return strFromU8(inflateSync(payload.subarray(1)))
}