import Foundation

public enum MIMEParser {

	// MARK: - Public API

	/// Parse a raw MIME message into a structured tree of parts
	public static func parse(_ rawMessage: String) -> MIMEMessage {
		let (headers, body) = splitHeadersAndBody(rawMessage)
		let contentType = headers["content-type"] ?? "text/plain"

		if contentType.lowercased().contains("multipart/") {
			guard let boundary = extractBoundary(contentType) else {
				// Malformed: multipart without boundary — treat as plain text
				return MIMEMessage(
					headers: headers,
					textBody: body.trimmingCharacters(in: .whitespacesAndNewlines)
				)
			}
			let parts = splitOnBoundary(body, boundary: boundary)
			let parsedParts = parts.enumerated().map { (index, partString) in
				parsePart(partString, sectionPrefix: "", index: index + 1)
			}

			var message = MIMEMessage(headers: headers, parts: parsedParts)
			extractBodiesAndAttachments(from: parsedParts, contentType: contentType, into: &message, sectionPrefix: "")
			return message
		} else {
			// Single-part message
			let transferEncoding = parseTransferEncoding(headers["content-transfer-encoding"])
			let decoded = decodeContent(body, encoding: transferEncoding)
			let charset = extractParameter(contentType, name: "charset")

			if contentType.lowercased().contains("text/html") {
				return MIMEMessage(headers: headers, htmlBody: decodeString(decoded, charset: charset))
			} else {
				return MIMEMessage(
					headers: headers,
					textBody: decodeString(decoded, charset: charset)?.trimmingCharacters(in: .whitespacesAndNewlines)
				)
			}
		}
	}

	/// Decode content based on Content-Transfer-Encoding
	public static func decodeContent(_ content: String, encoding: TransferEncoding) -> Data {
		switch encoding {
		case .base64:
			let cleaned = content.filter { !$0.isWhitespace }
			return Data(base64Encoded: cleaned) ?? Data(content.utf8)
		case .quotedPrintable:
			return decodeQuotedPrintable(content)
		case .sevenBit, .eightBit, .binary:
			return Data(content.utf8)
		}
	}

	/// Generate a unique MIME boundary string
	public static func generateBoundary() -> String {
		"=_MagnumOpus_\(UUID().uuidString)"
	}

	// MARK: - Header Parsing

	private static func splitHeadersAndBody(_ raw: String) -> ([String: String], String) {
		// Split on first blank line (CRLF CRLF or LF LF)
		let separator: String
		if raw.contains("\r\n\r\n") {
			separator = "\r\n\r\n"
		} else if raw.contains("\n\n") {
			separator = "\n\n"
		} else {
			return ([:], raw)
		}

		guard let range = raw.range(of: separator) else {
			return ([:], raw)
		}

		let headerSection = String(raw[raw.startIndex..<range.lowerBound])
		let bodySection = String(raw[range.upperBound...])
		return (parseHeaders(headerSection), bodySection)
	}

	private static func parseHeaders(_ section: String) -> [String: String] {
		var headers: [String: String] = [:]
		let lineBreak = section.contains("\r\n") ? "\r\n" : "\n"
		let lines = section.components(separatedBy: lineBreak)

		var currentKey: String?
		var currentValue: String = ""

		for line in lines {
			if line.isEmpty { continue }

			if line.first == " " || line.first == "\t" {
				// Continuation of previous header (folded)
				currentValue += " " + line.trimmingCharacters(in: .whitespaces)
			} else if let colonIndex = line.firstIndex(of: ":") {
				// Save previous header
				if let key = currentKey {
					headers[key.lowercased()] = currentValue
				}
				currentKey = String(line[..<colonIndex]).trimmingCharacters(in: .whitespaces)
				currentValue = String(line[line.index(after: colonIndex)...]).trimmingCharacters(in: .whitespaces)
			}
		}
		// Save last header
		if let key = currentKey {
			headers[key.lowercased()] = currentValue
		}

		return headers
	}

	// MARK: - Boundary / Part Splitting

	private static func extractBoundary(_ contentType: String) -> String? {
		// Look for boundary="value" or boundary=value
		let lower = contentType.lowercased()
		guard let boundaryRange = lower.range(of: "boundary=") else { return nil }
		var value = String(contentType[boundaryRange.upperBound...])

		// Strip leading quote
		if value.hasPrefix("\"") {
			value = String(value.dropFirst())
			if let endQuote = value.firstIndex(of: "\"") {
				value = String(value[..<endQuote])
			}
		} else {
			// Unquoted — stop at semicolon or whitespace
			if let end = value.firstIndex(where: { $0 == ";" || $0.isWhitespace }) {
				value = String(value[..<end])
			}
		}

		return value
	}

	private static func splitOnBoundary(_ body: String, boundary: String) -> [String] {
		let delimiter = "--\(boundary)"
		let terminator = "--\(boundary)--"
		let lineBreak = body.contains("\r\n") ? "\r\n" : "\n"

		var parts: [String] = []
		let lines = body.components(separatedBy: lineBreak)
		var currentPart: [String]? = nil

		for line in lines {
			let trimmed = line.trimmingCharacters(in: .whitespaces)
			if trimmed == terminator || trimmed.hasPrefix(terminator) {
				if let part = currentPart {
					parts.append(part.joined(separator: lineBreak))
				}
				break
			} else if trimmed == delimiter || trimmed.hasPrefix(delimiter) {
				if let part = currentPart {
					parts.append(part.joined(separator: lineBreak))
				}
				currentPart = []
			} else if currentPart != nil {
				currentPart!.append(line)
			}
		}

		return parts
	}

	// MARK: - Part Parsing

	private static func parsePart(_ partString: String, sectionPrefix: String, index: Int) -> MIMEPart {
		let (headers, body) = splitHeadersAndBody(partString)
		let contentType = headers["content-type"] ?? "text/plain"
		let transferEncoding = parseTransferEncoding(headers["content-transfer-encoding"])
		let charset = extractParameter(contentType, name: "charset")
		let disposition = parseDisposition(headers["content-disposition"])
		let contentId = extractContentId(headers["content-id"])

		var filename = extractParameter(headers["content-disposition"] ?? "", name: "filename")
		if filename == nil {
			filename = extractParameter(contentType, name: "name")
		}
		// Decode RFC 2047 encoded filenames
		if let encoded = filename {
			filename = RFC2047Decoder.decode(encoded)
		}

		let section = sectionPrefix.isEmpty ? "\(index)" : "\(sectionPrefix).\(index)"

		// Check for nested multipart
		if contentType.lowercased().contains("multipart/") {
			if let boundary = extractBoundary(contentType) {
				let subparts = splitOnBoundary(body, boundary: boundary)
				let parsedSubparts = subparts.enumerated().map { (i, s) in
					parsePart(s, sectionPrefix: section, index: i + 1)
				}
				return MIMEPart(
					headers: headers,
					contentType: contentType.components(separatedBy: ";").first?.trimmingCharacters(in: .whitespaces).lowercased() ?? contentType,
					charset: charset,
					transferEncoding: transferEncoding,
					disposition: disposition,
					filename: filename,
					contentId: contentId,
					body: Data(),
					subparts: parsedSubparts
				)
			}
		}

		let decodedBody = decodeContent(body, encoding: transferEncoding)
		let baseContentType = contentType.components(separatedBy: ";").first?.trimmingCharacters(in: .whitespaces).lowercased() ?? contentType

		return MIMEPart(
			headers: headers,
			contentType: baseContentType,
			charset: charset,
			transferEncoding: transferEncoding,
			disposition: disposition,
			filename: filename,
			contentId: contentId,
			body: decodedBody,
			subparts: []
		)
	}

	// MARK: - Body & Attachment Extraction

	private static func extractBodiesAndAttachments(
		from parts: [MIMEPart],
		contentType: String,
		into message: inout MIMEMessage,
		sectionPrefix: String
	) {
		let lowerType = contentType.lowercased()

		if lowerType.contains("multipart/alternative") {
			for part in parts {
				if !part.subparts.isEmpty {
					extractBodiesAndAttachments(from: part.subparts, contentType: part.contentType, into: &message, sectionPrefix: "")
				} else if part.contentType == "text/plain" && message.textBody == nil {
					message.textBody = decodeString(part.body, charset: part.charset)?.trimmingCharacters(in: .whitespacesAndNewlines)
				} else if part.contentType == "text/html" && message.htmlBody == nil {
					message.htmlBody = decodeString(part.body, charset: part.charset)?.trimmingCharacters(in: .whitespacesAndNewlines)
				}
			}
		} else if lowerType.contains("multipart/related") {
			// First part is the HTML body, rest are inline resources
			for (index, part) in parts.enumerated() {
				if index == 0 {
					if !part.subparts.isEmpty {
						extractBodiesAndAttachments(from: part.subparts, contentType: part.contentType, into: &message, sectionPrefix: "")
					} else if part.contentType == "text/html" {
						message.htmlBody = decodeString(part.body, charset: part.charset)?.trimmingCharacters(in: .whitespacesAndNewlines)
					} else if part.contentType == "text/plain" {
						message.textBody = decodeString(part.body, charset: part.charset)?.trimmingCharacters(in: .whitespacesAndNewlines)
					}
				} else {
					let sectionIndex = sectionPrefix.isEmpty ? "\(index + 1)" : "\(sectionPrefix).\(index + 1)"
					let attachment = MIMEAttachment(
						filename: part.filename ?? "inline-\(index)",
						mimeType: part.contentType,
						size: estimateDecodedSize(part),
						contentId: part.contentId,
						sectionPath: sectionIndex,
						isInline: true
					)
					message.inlineImages.append(attachment)
				}
			}
		} else {
			// multipart/mixed or unknown multipart
			var bodyFound = false
			for (index, part) in parts.enumerated() {
				let sectionIndex = sectionPrefix.isEmpty ? "\(index + 1)" : "\(sectionPrefix).\(index + 1)"

				if !part.subparts.isEmpty {
					// Nested multipart — recurse
					extractBodiesAndAttachments(from: part.subparts, contentType: part.contentType, into: &message, sectionPrefix: "")
					bodyFound = true
				} else if !bodyFound && part.disposition != .attachment && part.contentType.hasPrefix("text/") {
					if part.contentType == "text/html" {
						message.htmlBody = decodeString(part.body, charset: part.charset)?.trimmingCharacters(in: .whitespacesAndNewlines)
					} else {
						message.textBody = decodeString(part.body, charset: part.charset)?.trimmingCharacters(in: .whitespacesAndNewlines)
					}
					bodyFound = true
				} else if part.disposition == .attachment || part.filename != nil || !part.contentType.hasPrefix("text/") {
					let attachment = MIMEAttachment(
						filename: part.filename ?? "attachment-\(index + 1)",
						mimeType: part.contentType,
						size: estimateDecodedSize(part),
						contentId: part.contentId,
						sectionPath: sectionIndex,
						isInline: part.disposition == .inline
					)
					if part.disposition == .inline {
						message.inlineImages.append(attachment)
					} else {
						message.attachments.append(attachment)
					}
				}
			}
		}
	}

	// MARK: - String Decoding

	/// Decode Data to String using the specified charset, falling back to UTF-8.
	public static func decodeString(_ data: Data, charset: String?) -> String? {
		let encoding = charsetToEncoding(charset)
		if let result = String(data: data, encoding: encoding) {
			return result
		}
		// Fallback: try UTF-8, then Latin-1 (which never fails)
		return String(data: data, encoding: .utf8) ?? String(data: data, encoding: .isoLatin1)
	}

	private static func charsetToEncoding(_ charset: String?) -> String.Encoding {
		guard let charset = charset?.lowercased().trimmingCharacters(in: .whitespaces) else { return .utf8 }
		switch charset {
		case "utf-8", "utf8": return .utf8
		case "iso-8859-1", "latin1", "iso_8859-1": return .isoLatin1
		case "iso-8859-2", "latin2": return .isoLatin2
		case "iso-8859-15": return .isoLatin1 // close enough
		case "windows-1252", "cp1252": return .windowsCP1252
		case "windows-1251", "cp1251": return .windowsCP1251
		case "us-ascii", "ascii": return .ascii
		case "utf-16", "utf16": return .utf16
		case "utf-16be": return .utf16BigEndian
		case "utf-16le": return .utf16LittleEndian
		default: return .utf8
		}
	}

	// MARK: - Helper Functions

	private static func parseTransferEncoding(_ value: String?) -> TransferEncoding {
		guard let value = value?.trimmingCharacters(in: .whitespaces).lowercased() else { return .sevenBit }
		return TransferEncoding(rawValue: value) ?? .sevenBit
	}

	private static func parseDisposition(_ value: String?) -> ContentDisposition? {
		guard let value = value?.lowercased() else { return nil }
		if value.hasPrefix("inline") { return .inline }
		if value.hasPrefix("attachment") { return .attachment }
		return nil
	}

	private static func extractParameter(_ headerValue: String, name: String) -> String? {
		let lower = headerValue.lowercased()
		let search = "\(name.lowercased())="
		guard let range = lower.range(of: search) else { return nil }
		var value = String(headerValue[range.upperBound...])
		if value.hasPrefix("\"") {
			value = String(value.dropFirst())
			if let endQuote = value.firstIndex(of: "\"") {
				value = String(value[..<endQuote])
			}
		} else {
			if let end = value.firstIndex(where: { $0 == ";" || $0.isWhitespace }) {
				value = String(value[..<end])
			}
		}
		return value.isEmpty ? nil : value
	}

	private static func extractContentId(_ value: String?) -> String? {
		guard var cid = value?.trimmingCharacters(in: .whitespaces) else { return nil }
		if cid.hasPrefix("<") { cid = String(cid.dropFirst()) }
		if cid.hasSuffix(">") { cid = String(cid.dropLast()) }
		return cid.isEmpty ? nil : cid
	}

	private static func estimateDecodedSize(_ part: MIMEPart) -> Int {
		// Body is already decoded at this point, so use body.count directly
		return part.body.count
	}

	private static func decodeQuotedPrintable(_ input: String) -> Data {
		var data = Data()
		let lines = input.components(separatedBy: "\n")

		for (lineIndex, line) in lines.enumerated() {
			var processedLine = line
			if processedLine.hasSuffix("\r") {
				processedLine = String(processedLine.dropLast())
			}

			// Check for soft line break
			if processedLine.hasSuffix("=") {
				processedLine = String(processedLine.dropLast())
				data.append(contentsOf: decodeQPLine(processedLine))
			} else {
				data.append(contentsOf: decodeQPLine(processedLine))
				if lineIndex < lines.count - 1 {
					data.append(contentsOf: "\r\n".utf8)
				}
			}
		}

		return data
	}

	private static func decodeQPLine(_ line: String) -> Data {
		var data = Data()
		var i = line.startIndex
		while i < line.endIndex {
			if line[i] == "=" {
				let next1 = line.index(after: i)
				guard next1 < line.endIndex else {
					data.append(contentsOf: "=".utf8)
					break
				}
				let next2 = line.index(after: next1)
				guard next2 < line.endIndex else {
					data.append(contentsOf: String(line[i...]).utf8)
					break
				}
				let hex = String(line[next1...next2])
				if let byte = UInt8(hex, radix: 16) {
					data.append(byte)
					i = line.index(after: next2)
				} else {
					data.append(contentsOf: "=".utf8)
					i = next1
				}
			} else {
				data.append(contentsOf: String(line[i]).utf8)
				i = line.index(after: i)
			}
		}
		return data
	}
}