// Borrowed from Vapor (Date Middleware)
// https://github.com/vapor/vapor/blob/master/Sources/Vapor/Middleware/DateMiddleware.swift

import Foundation
#if os(Linux)
import Glibc
#else
import Darwin.C
#endif

fileprivate let DAY_NAMES = [
    "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
]

fileprivate let MONTH_NAMES = [
    "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
]

fileprivate let NUMBERS = [
    "00", "01", "02", "03", "04", "05", "06", "07", "08", "09",
    "10", "11", "12", "13", "14", "15", "16", "17", "18", "19",
    "20", "21", "22", "23", "24", "25", "26", "27", "28", "29",
    "30", "31", "32", "33", "34", "35", "36", "37", "38", "39",
    "40", "41", "42", "43", "44", "45", "46", "47", "48", "49",
    "50", "51", "52", "53", "54", "55", "56", "57", "58", "59",
    "60", "61", "62", "63", "64", "65", "66", "67", "68", "69",
    "70", "71", "72", "73", "74", "75", "76", "77", "78", "79",
    "80", "81", "82", "83", "84", "85", "86", "87", "88", "89",
    "90", "91", "92", "93", "94", "95", "96", "97", "98", "99"
]

fileprivate var cachedTimeComponents: (key: time_t, components: tm)?

let secondsInDay = 60 * 60 * 24
let accuracy = 1 // seconds

final class RFC1123DateFormatter {
    var cachedTimestamp: (timestamp: String, createdAt: time_t)?

    /// Gets the current RFC 1123 date string.
    func getDate() -> String {
        var date = time(nil)

        if let (timestamp, createdAt) = cachedTimestamp, (createdAt...(createdAt + accuracy)).contains(date) {
            return timestamp
        }

        // generate a key used for caching.
        // this key is a unique id for each day
        let key = date / secondsInDay

        // get time components
        let dateComponents: tm
        if let cached = cachedTimeComponents, cached.key == key {
            dateComponents = cached.components
        } else {
            let tc = gmtime(&date).pointee
            dateComponents = tc
            cachedTimeComponents = (key: key, components: tc)
        }

        // parse components
        let year: Int = numericCast(dateComponents.tm_year) + 1900 // years since 1900
        let month: Int = numericCast(dateComponents.tm_mon) // months since January [0-11]
        let monthDay: Int = numericCast(dateComponents.tm_mday) // day of the month [1-31]
        let weekDay: Int = numericCast(dateComponents.tm_wday) // days since Sunday [0-6]

        // get basic time info
        let timeX: Int = date % secondsInDay
        let hours: Int = numericCast(timeX / 3600)
        let minutes: Int = numericCast((timeX / 60) % 60)
        let seconds: Int = numericCast(timeX % 60)

        var rfc1123 = ""
        rfc1123.reserveCapacity(30)

        rfc1123.append(DAY_NAMES[weekDay])
        rfc1123.append(", ")
        rfc1123.append(NUMBERS[monthDay])
        rfc1123.append(" ")
        rfc1123.append(MONTH_NAMES[month])
        rfc1123.append(" ")
        rfc1123.append(NUMBERS[year / 100])
        rfc1123.append(NUMBERS[year % 100])
        rfc1123.append(" ")
        rfc1123.append(NUMBERS[hours])
        rfc1123.append(":")
        rfc1123.append(NUMBERS[minutes])
        rfc1123.append(":")
        rfc1123.append(NUMBERS[seconds])
        rfc1123.append(" GMT")

        cachedTimestamp = (rfc1123, date)

        return rfc1123
    }
}