/// Representation of the HTTP headers associated with a `HTTPRequest` or `HTTPResponse`.
/// Headers are subscriptable using case-insensitive comparison or provide `Name` constants. eg.
/// ```swift
///    let contentLength = headers["content-length"]
/// ```
/// or
/// ```swift
///    let contentLength = headers[.contentLength]
/// ```
public struct HTTPHeaders {
    var original: [(name: Name, value: String)]?
    var storage: [Name: [String]] {
        didSet { original = nil }
    }

    /// :nodoc:
    public subscript(name: Name) -> String? {
        get {
            guard let value = storage[name] else { return nil }
            switch name {
                case Name.setCookie: // Exception, see note in [RFC7230, section 3.2.2]
                    return value.isEmpty ? nil : value[0]
                default:
                    return value.joined(separator: ",")
            }
        }
        set {
            storage[name] = newValue.map { [$0] }
        }
    }

    /// :nodoc:
    public subscript(valuesFor name: Name) -> [String] {
        get { return storage[name] ?? [] }
        set { storage[name] = newValue.isEmpty ? nil : newValue }
    }
}

extension HTTPHeaders: ExpressibleByDictionaryLiteral {
    /// Creates HTTP headers.
    public init(dictionaryLiteral: (Name, String)...) {
        storage = [:]
        for (name, value) in dictionaryLiteral {
#if swift(>=4.0)
            storage[name, default: []].append(value)
#else
            if storage[name] == nil {
                storage[name] = [value]
            } else {
                storage[name]!.append(value)
            }
#endif
        }
        original = dictionaryLiteral
    }
}

extension HTTPHeaders {
    // Used instead of HTTPHeaders to save CPU on dictionary construction
    /// :nodoc:
    public struct Literal: ExpressibleByDictionaryLiteral {
        let fields: [(name: Name, value: String)]

        public init(dictionaryLiteral: (Name, String)...) {
            fields = dictionaryLiteral
        }
    }

    /// Appends a header to the headers
    public mutating func append(_ literal: HTTPHeaders.Literal) {
        for (name, value) in literal.fields {
#if swift(>=4.0)
            storage[name, default: []].append(value)
#else
            if storage[name] == nil {
                storage[name] = [value]
            } else {
                storage[name]!.append(value)
            }
#endif
        }
    }

    /// Replaces a header in the headers
    public mutating func replace(_ literal: HTTPHeaders.Literal) {
        for (name, _) in literal.fields {
            storage[name] = []
        }
        for (name, value) in literal.fields {
            storage[name]!.append(value)
        }
    }
}

extension HTTPHeaders: Sequence {
    /// :nodoc:
    public func makeIterator() -> AnyIterator<(name: Name, value: String)> {
        if let original = original {
            return AnyIterator(original.makeIterator())
        } else {
            return AnyIterator(StorageIterator(storage.makeIterator()))
        }
    }

    struct StorageIterator: IteratorProtocol {
        var headers: DictionaryIterator<Name, [String]>
        var header: (name: Name, values: IndexingIterator<[String]>)?

        init(_ iterator: DictionaryIterator<Name, [String]>) {
            headers = iterator
            header = headers.next().map { (name: $0.key, values: $0.value.makeIterator()) }
        }

        mutating func next() -> (name: Name, value: String)? {
            while header != nil {
                if let value = header!.values.next() {
                    return (name: header!.name, value: value)
                } else {
                    header = headers.next().map { (name: $0.key, values: $0.value.makeIterator()) }
                }
            }
            return nil
        }
    }
}

/// HTTPHeaders structure.
extension HTTPHeaders {
    /// Type used for the name of a HTTP header in the `HTTPHeaders` storage.
    public struct Name: Hashable, ExpressibleByStringLiteral, CustomStringConvertible {
        let original: String
        let lowercased: String
        public let hashValue: Int

        /// Create a HTTP header name with the provided String.
        public init(_ name: String) {
            original = name
            lowercased = name.lowercased()
            hashValue = lowercased.hashValue
        }

        public init(stringLiteral: String) {
            self.init(stringLiteral)
        }

        public init(unicodeScalarLiteral: String) {
            self.init(unicodeScalarLiteral)
        }

        public init(extendedGraphemeClusterLiteral: String) {
            self.init(extendedGraphemeClusterLiteral)
        }

        /// :nodoc:
        public var description: String {
            return original
        }

        /// :nodoc:
        public static func == (lhs: Name, rhs: Name) -> Bool {
            return lhs.lowercased == rhs.lowercased
        }

        // https://www.iana.org/assignments/message-headers/message-headers.xhtml
        // Permanent Message Header Field Names

        /// A-IM header.
        public static let aIM = Name("A-IM")
        /// Accept header.
        public static let accept = Name("Accept")
        /// Accept-Additions header.
        public static let acceptAdditions = Name("Accept-Additions")
        /// Accept-Charset header.
        public static let acceptCharset = Name("Accept-Charset")
        /// Accept-Datetime header.
        public static let acceptDatetime = Name("Accept-Datetime")
        /// Accept-Encoding header.
        public static let acceptEncoding = Name("Accept-Encoding")
        /// Accept-Features header.
        public static let acceptFeatures = Name("Accept-Features")
        /// Accept-Language header.
        public static let acceptLanguage = Name("Accept-Language")
        /// Accept-Patch header.
        public static let acceptPatch = Name("Accept-Patch")
        /// Accept-Post header.
        public static let acceptPost = Name("Accept-Post")
        /// Accept-Ranges header.
        public static let acceptRanges = Name("Accept-Ranges")
        /// Accept-Age header.
        public static let age = Name("Age")
        /// Accept-Allow header.
        public static let allow = Name("Allow")
        /// ALPN header.
        public static let alpn = Name("ALPN")
        /// Alt-Svc header.
        public static let altSvc = Name("Alt-Svc")
        /// Alt-Used header.
        public static let altUsed = Name("Alt-Used")
        /// Alternatives header.
        public static let alternates = Name("Alternates")
        /// Apply-To-Redirect-Ref header.
        public static let applyToRedirectRef = Name("Apply-To-Redirect-Ref")
        /// Authentication-Control header.
        public static let authenticationControl = Name("Authentication-Control")
        /// Authentication-Info header.
        public static let authenticationInfo = Name("Authentication-Info")
        /// Authorization header.
        public static let authorization = Name("Authorization")
        /// C-Ext header.
        public static let cExt = Name("C-Ext")
        /// C-Man header.
        public static let cMan = Name("C-Man")
        /// C-Opt header.
        public static let cOpt = Name("C-Opt")
        /// C-PEP header.
        public static let cPEP = Name("C-PEP")
        /// C-PEP-Indo header.
        public static let cPEPInfo = Name("C-PEP-Info")
        /// Cache-Control header.
        public static let cacheControl = Name("Cache-Control")
        /// CalDav-Timezones header.
        public static let calDAVTimezones = Name("CalDAV-Timezones")
        /// Close header.
        public static let close = Name("Close")
        /// Connection header.
        public static let connection = Name("Connection")
        /// Content-Base.
        public static let contentBase = Name("Content-Base")
        /// Content-Disposition header.
        public static let contentDisposition = Name("Content-Disposition")
        /// Content-Encoding header.
        public static let contentEncoding = Name("Content-Encoding")
        /// Content-ID header.
        public static let contentID = Name("Content-ID")
        /// Content-Language header.
        public static let contentLanguage = Name("Content-Language")
        /// Content-Length header.
        public static let contentLength = Name("Content-Length")
        /// Content-Location header.
        public static let contentLocation = Name("Content-Location")
        /// Content-MD5 header.
        public static let contentMD5 = Name("Content-MD5")
        /// Content-Range header.
        public static let contentRange = Name("Content-Range")
        /// Content-Script-Type header.
        public static let contentScriptType = Name("Content-Script-Type")
        /// Content-Style-Type header.
        public static let contentStyleType = Name("Content-Style-Type")
        /// Content-Type header.
        public static let contentType = Name("Content-Type")
        /// Content-Version header.
        public static let contentVersion = Name("Content-Version")
        /// Content-Cookie header.
        public static let cookie = Name("Cookie")
        /// Content-Cookie2 header.
        public static let cookie2 = Name("Cookie2")
        /// DASL header.
        public static let dasl = Name("DASL")
        /// DASV header.
        public static let dav = Name("DAV")
        /// Date header.
        public static let date = Name("Date")
        /// Default-Style header.
        public static let defaultStyle = Name("Default-Style")
        /// Delta-Base header.
        public static let deltaBase = Name("Delta-Base")
        /// Depth header.
        public static let depth = Name("Depth")
        /// Derived-From header.
        public static let derivedFrom = Name("Derived-From")
        /// Destination header.
        public static let destination = Name("Destination")
        /// Differential-ID header.
        public static let differentialID = Name("Differential-ID")
        /// Digest header.
        public static let digest = Name("Digest")
        /// ETag header.
        public static let eTag = Name("ETag")
        /// Expect header.
        public static let expect = Name("Expect")
        /// Expires header.
        public static let expires = Name("Expires")
        /// Ext header.
        public static let ext = Name("Ext")
        /// Forwarded header.
        public static let forwarded = Name("Forwarded")
        /// From header.
        public static let from = Name("From")
        /// GetProfile header.
        public static let getProfile = Name("GetProfile")
        /// Hobareg header.
        public static let hobareg = Name("Hobareg")
        /// Host header.
        public static let host = Name("Host")
        /// HTTP2-Settings header.
        public static let http2Settings = Name("HTTP2-Settings")
        /// IM header.
        public static let im = Name("IM")
        /// If header.
        public static let `if` = Name("If")
        /// If-Match header.
        public static let ifMatch = Name("If-Match")
        /// If-Modified-Since header.
        public static let ifModifiedSince = Name("If-Modified-Since")
        /// If-None-Match header.
        public static let ifNoneMatch = Name("If-None-Match")
        /// If-Range header.
        public static let ifRange = Name("If-Range")
        /// If-Schedule-Tag-Match header.
        public static let ifScheduleTagMatch = Name("If-Schedule-Tag-Match")
        /// If-Unmodified-Since header.
        public static let ifUnmodifiedSince = Name("If-Unmodified-Since")
        /// Keep-Alive header.
        public static let keepAlive = Name("Keep-Alive")
        /// Label header.
        public static let label = Name("Label")
        /// Last-Modified header.
        public static let lastModified = Name("Last-Modified")
        /// Link header.
        public static let link = Name("Link")
        /// Location header.
        public static let location = Name("Location")
        /// Lock-Token header.
        public static let lockToken = Name("Lock-Token")
        /// Man header.
        public static let man = Name("Man")
        /// Max-Forwards header.
        public static let maxForwards = Name("Max-Forwards")
        /// Memento-Date header.
        public static let mementoDatetime = Name("Memento-Datetime")
        /// Meter header.
        public static let meter = Name("Meter")
        /// MIME-Version header.
        public static let mimeVersion = Name("MIME-Version")
        /// Negotiate header.
        public static let negotiate = Name("Negotiate")
        /// Opt header.
        public static let opt = Name("Opt")
        /// Optional-WWW-Authenticate header.
        public static let optionalWWWAuthenticate = Name("Optional-WWW-Authenticate")
        /// Ordering-Type header.
        public static let orderingType = Name("Ordering-Type")
        /// Origin header.
        public static let origin = Name("Origin")
        /// Overwrite header.
        public static let overwrite = Name("Overwrite")
        /// P3P header.
        public static let p3p = Name("P3P")
        /// PEP header.
        public static let pep = Name("PEP")
        /// PICS-Label header.
        public static let picsLabel = Name("PICS-Label")
        /// Pep-Info header.
        public static let pepInfo = Name("Pep-Info")
        /// Position header.
        public static let position = Name("Position")
        /// Pragma header.
        public static let pragma = Name("Pragma")
        /// Prefer header.
        public static let prefer = Name("Prefer")
        /// Preference-Applied header.
        public static let preferenceApplied = Name("Preference-Applied")
        /// ProfileObject header.
        public static let profileObject = Name("ProfileObject")
        /// Protocol header.
        public static let `protocol` = Name("Protocol")
        /// Protocol-Info header.
        public static let protocolInfo = Name("Protocol-Info")
        /// Protocol-Query header.
        public static let protocolQuery = Name("Protocol-Query")
        /// Protocol-Request header.
        public static let protocolRequest = Name("Protocol-Request")
        /// Proxy-Authenticate header.
        public static let proxyAuthenticate = Name("Proxy-Authenticate")
        /// Proxy-Authentication-Info header.
        public static let proxyAuthenticationInfo = Name("Proxy-Authentication-Info")
        /// Proxy-Authorization header.
        public static let proxyAuthorization = Name("Proxy-Authorization")
        /// Proxy-Features header.
        public static let proxyFeatures = Name("Proxy-Features")
        /// Proxy-Instruction header.
        public static let proxyInstruction = Name("Proxy-Instruction")
        /// Public header.
        public static let `public` = Name("Public")
        /// Public-Key-Pins header.
        public static let publicKeyPins = Name("Public-Key-Pins")
        /// Public-Key-Pins-Report-Only header.
        public static let publicKeyPinsReportOnly = Name("Public-Key-Pins-Report-Only")
        /// Range header.
        public static let range = Name("Range")
        /// Redirect-Ref header.
        public static let redirectRef = Name("Redirect-Ref")
        /// Referer header.
        public static let referer = Name("Referer")
        /// Retry-After header.
        public static let retryAfter = Name("Retry-After")
        /// Safe header.
        public static let safe = Name("Safe")
        /// Schedule-Reply header.
        public static let scheduleReply = Name("Schedule-Reply")
        /// Schedule-Tag header.
        public static let scheduleTag = Name("Schedule-Tag")
        /// Sec-WebSocket-Accept header.
        public static let secWebSocketAccept = Name("Sec-WebSocket-Accept")
        /// Sec-WebSocket-Extensions header.
        public static let secWebSocketExtensions = Name("Sec-WebSocket-Extensions")
        /// Sec-WebSocket-Key header.
        public static let secWebSocketKey = Name("Sec-WebSocket-Key")
        /// Sec-WebSocket-Protocol header.
        public static let secWebSocketProtocol = Name("Sec-WebSocket-Protocol")
        /// Sec-WebSocket-Version header.
        public static let secWebSocketVersion = Name("Sec-WebSocket-Version")
        /// Security-Scheme header.
        public static let securityScheme = Name("Security-Scheme")
        /// Server header.
        public static let server = Name("Server")
        /// Set-Cookie header.
        public static let setCookie = Name("Set-Cookie")
        /// Set-Cookie2 header.
        public static let setCookie2 = Name("Set-Cookie2")
        /// SetProfile header.
        public static let setProfile = Name("SetProfile")
        /// SLUG header.
        public static let slug = Name("SLUG")
        /// SoapAction header.
        public static let soapAction = Name("SoapAction")
        /// Status-URI header.
        public static let statusURI = Name("Status-URI")
        /// Strict-Transport-Security header.
        public static let strictTransportSecurity = Name("Strict-Transport-Security")
        /// Surrogate-Capability header.
        public static let surrogateCapability = Name("Surrogate-Capability")
        /// Surrogate-Control header.
        public static let surrogateControl = Name("Surrogate-Control")
        /// TCN header.
        public static let tcn = Name("TCN")
        /// TE header.
        public static let te = Name("TE")
        /// Timeout header.
        public static let timeout = Name("Timeout")
        /// Topic header.
        public static let topic = Name("Topic")
        /// Trailer header.
        public static let trailer = Name("Trailer")
        /// Transfer-Encoding header.
        public static let transferEncoding = Name("Transfer-Encoding")
        /// TTL header.
        public static let ttl = Name("TTL")
        /// Urgency header.
        public static let urgency = Name("Urgency")
        /// URI header.
        public static let uri = Name("URI")
        /// Upgrade header.
        public static let upgrade = Name("Upgrade")
        /// User-Agent header.
        public static let userAgent = Name("User-Agent")
        /// Variant-Vary header.
        public static let variantVary = Name("Variant-Vary")
        /// Vary header.
        public static let vary = Name("Vary")
        /// Via header.
        public static let via = Name("Via")
        /// WWW-Authenticate header.
        public static let wwwAuthenticate = Name("WWW-Authenticate")
        /// Want-Digest header.
        public static let wantDigest = Name("Want-Digest")
        /// Warning header.
        public static let warning = Name("Warning")
        /// X-Frame-Options header.
        public static let xFrameOptions = Name("X-Frame-Options")

        // https://www.iana.org/assignments/message-headers/message-headers.xhtml
        // Provisional Message Header Field Names
        /// Access-Control header.
        public static let accessControl = Name("Access-Control")
        /// Access-Control-Allow-Credentials header.
        public static let accessControlAllowCredentials = Name("Access-Control-Allow-Credentials")
        /// Access-Control-Allow-Headers header.
        public static let accessControlAllowHeaders = Name("Access-Control-Allow-Headers")
        /// Access-Control-Allow-Methods header.
        public static let accessControlAllowMethods = Name("Access-Control-Allow-Methods")
        /// Access-Control-Allow-Origin header.
        public static let accessControlAllowOrigin = Name("Access-Control-Allow-Origin")
        /// Access-Control-Max-Age header.
        public static let accessControlMaxAge = Name("Access-Control-Max-Age")
        /// Access-Control-Request-Method header.
        public static let accessControlRequestMethod = Name("Access-Control-Request-Method")
        /// Access-Control-Request-Headers header.
        public static let accessControlRequestHeaders = Name("Access-Control-Request-Headers")
        /// Compliance header.
        public static let compliance = Name("Compliance")
        /// Content-Transfer-Encoding header.
        public static let contentTransferEncoding = Name("Content-Transfer-Encoding")
        /// Cost header.
        public static let cost = Name("Cost")
        /// EDIINT-Features header.
        public static let ediintFeatures = Name("EDIINT-Features")
        /// Message-ID header.
        public static let messageID = Name("Message-ID")
        /// Method-Check header.
        public static let methodCheck = Name("Method-Check")
        /// Method-Check-Expires header.
        public static let methodCheckExpires = Name("Method-Check-Expires")
        /// Non-Compliance header.
        public static let nonCompliance = Name("Non-Compliance")
        /// Optional header.
        public static let optional = Name("Optional")
        /// Referer-Root header.
        public static let refererRoot = Name("Referer-Root")
        /// Resolution-Hint header.
        public static let resolutionHint = Name("Resolution-Hint")
        /// Resolver-Location header.
        public static let resolverLocation = Name("Resolver-Location")
        /// SubOK header.
        public static let subOK = Name("SubOK")
        /// Subst header.
        public static let subst = Name("Subst")
        /// Title header.
        public static let title = Name("Title")
        /// UA-Color header.
        public static let uaColor = Name("UA-Color")
        /// UA-Media header.
        public static let uaMedia = Name("UA-Media")
        /// UA-Pixels header.
        public static let uaPixels = Name("UA-Pixels")
        /// UA-Resolution header.
        public static let uaResolution = Name("UA-Resolution")
        /// UA-Windowpixels header.
        public static let uaWindowpixels = Name("UA-Windowpixels")
        /// Version header.
        public static let version = Name("Version")
        /// X-Device-Accept header.
        public static let xDeviceAccept = Name("X-Device-Accept")
        /// X-Device-Accept-Charset header.
        public static let xDeviceAcceptCharset = Name("X-Device-Accept-Charset")
        /// X-Device-Accept-Encoding header.
        public static let xDeviceAcceptEncoding = Name("X-Device-Accept-Encoding")
        /// X-Device-Accept-Language header.
        public static let xDeviceAcceptLanguage = Name("X-Device-Accept-Language")
        /// X-Device-User-Agent header.
        public static let xDeviceUserAgent = Name("X-Device-User-Agent")
    }
}