'use strict'

const STANDARD_ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
const URL_SAFE_ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_'

/**
 * 纯 JS Base64 编码，默认输出带填充的标准 Base64。
 * 协议调用方必须显式声明 URL-safe 与是否保留填充，避免现场字符串替换。
 */
function encodeBase64(value, options = {}) {
  if (!(value instanceof Uint8Array)) throw new TypeError('Base64 value must be Uint8Array')
  const alphabet = options.urlSafe ? URL_SAFE_ALPHABET : STANDARD_ALPHABET
  const padding = options.padding !== false
  let output = ''
  for (let offset = 0; offset < value.length; offset += 3) {
    const first = value[offset]
    const hasSecond = offset + 1 < value.length
    const hasThird = offset + 2 < value.length
    const second = hasSecond ? value[offset + 1] : 0
    const third = hasThird ? value[offset + 2] : 0
    output += alphabet[first >>> 2]
    output += alphabet[((first & 0x03) << 4) | (second >>> 4)]
    if (hasSecond) {
      output += alphabet[((second & 0x0f) << 2) | (third >>> 6)]
    } else if (padding) {
      output += '='
    }
    if (hasThird) {
      output += alphabet[third & 0x3f]
    } else if (padding) {
      output += '='
    }
  }
  return output
}

/**
 * 严格解码标准 Base64 或 Base64URL，不依赖浏览器/Hermes 的 atob。
 * 接受有填充或无填充形式，但拒绝混合字母表、非法长度、非法填充和非零尾随位。
 */
function decodeBase64(value) {
  if (typeof value !== 'string') throw new TypeError('Base64 value must be a string')
  if (value.length === 0) return new Uint8Array(0)
  if (/[^A-Za-z0-9+/_=-]/.test(value)) throw new Error('Base64 contains invalid characters')
  if (/[+\/]/.test(value) && /[-_]/.test(value)) {
    throw new Error('Base64 cannot mix standard and URL-safe alphabets')
  }

  const firstPadding = value.indexOf('=')
  const data = firstPadding === -1 ? value : value.slice(0, firstPadding)
  const padding = firstPadding === -1 ? '' : value.slice(firstPadding)
  if (padding !== '' && padding !== '=' && padding !== '==') {
    throw new Error('Base64 padding is invalid')
  }
  if (data.includes('=')) throw new Error('Base64 padding is invalid')
  const remainder = data.length % 4
  if (remainder === 1) throw new Error('Base64 length is invalid')
  const expectedPadding = remainder === 0 ? 0 : 4 - remainder
  if (padding.length !== 0 && padding.length !== expectedPadding) {
    throw new Error('Base64 padding length is invalid')
  }

  const normalized = data.replace(/-/g, '+').replace(/_/g, '/')
  const outputLength = Math.floor((normalized.length * 6) / 8)
  const output = new Uint8Array(outputLength)
  let accumulator = 0
  let bitCount = 0
  let outputIndex = 0
  for (const character of normalized) {
    const sextet = STANDARD_ALPHABET.indexOf(character)
    if (sextet < 0) throw new Error('Base64 contains invalid characters')
    accumulator = (accumulator << 6) | sextet
    bitCount += 6
    if (bitCount >= 8) {
      bitCount -= 8
      output[outputIndex] = (accumulator >>> bitCount) & 0xff
      outputIndex += 1
      accumulator &= (1 << bitCount) - 1
    }
  }
  if (accumulator !== 0) throw new Error('Base64 has non-zero trailing bits')
  return output
}

module.exports = { decodeBase64, encodeBase64 }