'use strict'

const { EventEmitter } = require('events')
const crypto = require('crypto')
const queryString = require('query-string')
const serverSecrets = require('./server-secrets')
const mapKeys = require('lodash.mapkeys')

const userTokenRateLimit = 12500

let githubUserTokens = []
// Ideally, we would want priority queues here.
const reqRemaining = new Map() // From token to requests remaining.
const reqReset = new Map() // From token to timestamp.

const emitter = new EventEmitter()

// Personal tokens allow access to GitHub private repositories.
// You can manage your personal GitHub token at
// <https://github.com/settings/tokens>.
if (serverSecrets && serverSecrets.gh_token) {
  addGithubToken(serverSecrets.gh_token)
}

// token: client token as a string.
// reqs: number of requests remaining.
// reset: timestamp when the number of remaining requests is reset.
function setReqRemaining(token, reqs, reset) {
  reqRemaining.set(token, reqs)
  reqReset.set(token, reset)
}

function rmReqRemaining(token) {
  reqRemaining.delete(token)
  reqReset.delete(token)
}

function utcEpochSeconds() {
  return (Date.now() / 1000) >>> 0
}

// Return false if the token cannot reasonably be expected to perform
// a GitHub request.
function isTokenUsable(token, now) {
  const reqs = reqRemaining.get(token)
  const reset = reqReset.get(token)
  // We don't want to empty more than 3/4 of a user's rate limit.
  const hasRemainingReqs = reqs > userTokenRateLimit / 4
  const isBeyondRateLimitReset = reset < now
  return hasRemainingReqs || isBeyondRateLimitReset
}

// Return a list of tokens (as strings) which can be used for a GitHub request,
// with a reasonable chance that the request will succeed.
function usableTokens() {
  const now = utcEpochSeconds()
  return githubUserTokens.filter(token => isTokenUsable(token, now))
}

// Retrieve a user token if there is one for which we believe there are requests
// remaining. Return undefined if we could not find one.
function getReqRemainingToken() {
  // Go through the user tokens.
  // Among usable ones, use the one with the highest number of remaining
  // requests.
  const tokens = usableTokens()
  let highestReq = -1
  let highestToken
  for (let i = 0; i < tokens.length; i++) {
    const token = tokens[i]
    const reqs = reqRemaining.get(token)
    if (reqs > highestReq) {
      highestReq = reqs
      highestToken = token
    }
  }
  return highestToken
}

function addGithubToken(token) {
  // A reset date of 0 has to be in the past.
  setReqRemaining(token, userTokenRateLimit, 0)
  // Insert it only if it is not registered yet.
  if (githubUserTokens.indexOf(token) === -1) {
    githubUserTokens.push(token)
  }
  emitter.emit('token-added', token)
}

function rmGithubToken(token) {
  rmReqRemaining(token)
  // Remove it only if it is in there.
  const idx = githubUserTokens.indexOf(token)
  if (idx >= 0) {
    githubUserTokens.splice(idx, 1)
  }
}

// Convert an ES6 Map to an object.
function mapToObject(map) {
  const result = {}
  for (const [k, v] of map) {
    result[k] = v
  }
  return result
}

// Compute a one-way hash of the input string.
function sha(str) {
  return crypto
    .createHash('sha256')
    .update(str, 'utf-8')
    .digest('hex')
}

function getAllTokenIds() {
  return githubUserTokens.slice()
}

function removeAllTokens() {
  githubUserTokens = []
}

function serializeDebugInfo(options) {
  // Apply defaults.
  const { sanitize } = Object.assign({ sanitize: true }, options)

  const unsanitized = {
    tokens: githubUserTokens,
    reqRemaining: mapToObject(reqRemaining),
    reqReset: mapToObject(reqReset),
    utcEpochSeconds: utcEpochSeconds(),
    sanitized: false,
  }

  if (sanitize) {
    return {
      tokens: unsanitized.tokens.map(k => sha(k)),
      reqRemaining: mapKeys(unsanitized.reqRemaining, (v, k) => sha(k)),
      reqReset: mapKeys(unsanitized.reqReset, (v, k) => sha(k)),
      utcEpochSeconds: unsanitized.utcEpochSeconds,
      sanitized: true,
    }
  } else {
    return unsanitized
  }
}

// When a global gh_token is configured, use that in place of our shields.io
// token-cycling logic. This produces more predictable behavior when a token
// is provided, and more predictable failures if that token is exhausted.
//
// You can manage your personal GitHub token at https://github.com/settings/tokens
const globalToken = (serverSecrets || {}).gh_token

// Act like request(), but tweak headers and query to avoid hitting a rate
// limit.
function githubRequest(request, url, query, cb) {
  query = query || {}
  // A special User-Agent is required:
  // http://developer.github.com/v3/#user-agent-required
  const headers = {
    'User-Agent': 'Shields.io',
    Accept: 'application/vnd.github.v3+json',
  }

  const githubToken =
    globalToken === undefined ? getReqRemainingToken() : globalToken

  if (githubToken != null) {
    // Typically, GitHub user tokens grants us 12500 req/hour.
    headers['Authorization'] = `token ${githubToken}`
  } else if (serverSecrets && serverSecrets.gh_client_id) {
    // Using our OAuth App secret grants us 5000 req/hour
    // instead of the standard 60 req/hour.
    query.client_id = serverSecrets.gh_client_id
    query.client_secret = serverSecrets.gh_client_secret
  }

  const qs = queryString.stringify(query)
  if (qs) {
    url += `?${qs}`
  }

  request(url, { headers }, (err, res, buffer) => {
    if (globalToken !== null && githubToken !== null && err === null) {
      if (res.statusCode === 401) {
        // Unauthorized.
        rmGithubToken(githubToken)
        emitter.emit('token-removed', githubToken)
      } else {
        const remaining = +res.headers['x-ratelimit-remaining']
        // reset is in UTC epoch seconds.
        const reset = +res.headers['x-ratelimit-reset']
        setReqRemaining(githubToken, remaining, reset)
        if (remaining === 0) {
          return
        } // Hope for the best in the cache.
      }
    }
    cb(err, res, buffer)
  })
}

module.exports = {
  request: githubRequest,
  serializeDebugInfo,
  addGithubToken,
  rmGithubToken,
  getAllTokenIds,
  removeAllTokens,
  emitter,
}