var Promise = require("bluebird"),
    br = require('binary-reader'),
    nacl = require('js-nacl'),
    sha256 = require("sha256"),
    utf8 = require('utf-8'),
    base64 = require('base-64')

nacl = nacl.instantiate()

module.exports = function(app) {
    const CAPTCHA_BLOCK_LENGT = 32 + 32 + 125
    const CAPTCHA_COUNT = 1048576
    const POW_LIMIT = 1

    return {
        verify: function(post) {
            var sign, hash

            sign = nacl.from_hex(this.getSign(post))
            post = utf8.setBytesFromString(this.cutSign(post))
            hash = nacl.from_hex(sha256(post))

            if (this.checkPOW(hash))
                return this.getCaptchaByIdx(this.getCaptchaIdx(hash))
                    .then(function(data) {
                        return nacl.crypto_sign_verify_detached(sign, post, data.pk)
                    })
            else
                return Promise.resolve(false)
        },

        checkPOW: function(hash) {
            for (var result = false, i = 0, l = hash.length; i < l; i++)
                if (hash[i] <= POW_LIMIT && hash[i + 1] <= POW_LIMIT && hash[i + 2] <= POW_LIMIT) {
                    result = true
                    break
                }

            return result
        },

        cutSign: function(post) {
            return post.replace(/\[sign=.+\]/g, '')
        },

        getSign: function(post) {
            return post.match(/\[sign=(.+)\]/g)[0].replace(/\[sign=(.+)\]/g, '$1')
        },

        getCaptchaIdx: function(hash) {
            return (hash[0] + hash[1] * 256 + hash[2] * 256 * 256) % CAPTCHA_COUNT
        },

        getCaptchaByIdx: function(idx) {
            return this.getCaptchaBlockByIdx(idx)
                .then(this.parseCaptchaBlock)
        },

        getCaptchaBlockByIdx: function(idx) {
            return new Promise((resolve, reject) => br.open('./captcha.nbc')
                .on ("error", (err) => reject(err))
                .seek(CAPTCHA_BLOCK_LENGT * idx)
                .read(CAPTCHA_BLOCK_LENGT, (len, data) => resolve(data))
                .close ())
        },

        parseCaptchaBlock: function(data) {
            return Promise.resolve({
                pk: data.slice(0, 32),
                sk: data.slice(32, 64),
                c: data.slice(64).toString('base64')
            })
        }
    }
}