// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
//
// SPDX-License-Identifier: MIT

package mail

import (
	"bytes"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"mime"
	"mime/multipart"
	"mime/quotedprintable"
	netmail "net/mail"
	"os"
	"strings"
)

// EMLToMsgFromString parses a given EML string and returns a pre-filled Msg pointer.
//
// This function takes an EML formatted string, converts it into a bytes buffer, and then
// calls EMLToMsgFromReader to parse the buffer and create a Msg object. This provides a
// convenient way to convert EML strings directly into Msg objects.
//
// Parameters:
//   - emlString: A string containing the EML formatted message.
//
// Returns:
//   - A pointer to the Msg object populated with the parsed data, and an error if parsing
//     fails.
func EMLToMsgFromString(emlString string) (*Msg, error) {
	eb := bytes.NewBufferString(emlString)
	return EMLToMsgFromReader(eb)
}

// EMLToMsgFromReader parses a reader that holds EML content and returns a pre-filled Msg pointer.
//
// This function reads EML content from the provided io.Reader and populates a Msg object
// with the parsed data. It initializes the Msg and extracts headers and body parts from
// the EML content. Any errors encountered during parsing are returned.
//
// Parameters:
//   - reader: An io.Reader containing the EML formatted message.
//
// Returns:
//   - A pointer to the Msg object populated with the parsed data, and an error if parsing
//     fails.
func EMLToMsgFromReader(reader io.Reader) (*Msg, error) {
	msg := &Msg{
		addrHeader:    make(map[AddrHeader][]*netmail.Address),
		genHeader:     make(map[Header][]string),
		preformHeader: make(map[Header]string),
		mimever:       MIME10,
	}

	parsedMsg, bodybuf, err := readEMLFromReader(reader)
	if err != nil || parsedMsg == nil {
		return msg, fmt.Errorf("failed to parse EML from reader: %w", err)
	}

	if err = parseEML(parsedMsg, bodybuf, msg); err != nil {
		return msg, fmt.Errorf("failed to parse EML contents: %w", err)
	}

	return msg, nil
}

// EMLToMsgFromFile opens and parses a .eml file at a provided file path and returns a
// pre-filled Msg pointer.
//
// This function attempts to read and parse an EML file located at the specified file path.
// It initializes a Msg object and populates it with the parsed headers and body. Any errors
// encountered during the file operations or parsing are returned.
//
// Parameters:
//   - filePath: The path to the .eml file to be parsed.
//
// Returns:
//   - A pointer to the Msg object populated with the parsed data, and an error if parsing
//     fails.
func EMLToMsgFromFile(filePath string) (*Msg, error) {
	msg := &Msg{
		addrHeader:    make(map[AddrHeader][]*netmail.Address),
		genHeader:     make(map[Header][]string),
		preformHeader: make(map[Header]string),
		mimever:       MIME10,
	}

	parsedMsg, bodybuf, err := readEML(filePath)
	if err != nil || parsedMsg == nil {
		return msg, fmt.Errorf("failed to parse EML file: %w", err)
	}

	if err = parseEML(parsedMsg, bodybuf, msg); err != nil {
		return msg, fmt.Errorf("failed to parse EML contents: %w", err)
	}

	return msg, nil
}

// parseEML parses the EML's headers and body and inserts the parsed values into the Msg.
//
// This function extracts relevant header fields and body content from the parsed EML message
// and stores them in the provided Msg object. It handles various header types and body
// parts, ensuring that the Msg is correctly populated with all necessary information.
//
// Parameters:
//   - parsedMsg: A pointer to the netmail.Message containing the parsed EML data.
//   - bodybuf: A bytes.Buffer containing the body content of the EML message.
//   - msg: A pointer to the Msg object to be populated with the parsed data.
//
// Returns:
//   - An error if any issues occur during the parsing process; otherwise, returns nil.
func parseEML(parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *Msg) error {
	if err := parseEMLHeaders(&parsedMsg.Header, msg); err != nil {
		return fmt.Errorf("failed to parse EML headers: %w", err)
	}
	if err := parseEMLBodyParts(parsedMsg, bodybuf, msg); err != nil {
		return fmt.Errorf("failed to parse EML body parts: %w", err)
	}
	return nil
}

// readEML opens an EML file and uses net/mail to parse the header and body.
//
// This function opens the specified EML file for reading and utilizes the net/mail package
// to parse the message's headers and body. It returns the parsed message and a buffer
// containing the body content, along with any errors encountered during the process.
//
// Parameters:
//   - filePath: The path to the EML file to be opened and parsed.
//
// Returns:
//   - A pointer to the parsed netmail.Message, a bytes.Buffer containing the body, and an
//     error if any issues occur during file operations or parsing.
func readEML(filePath string) (*netmail.Message, *bytes.Buffer, error) {
	fileHandle, err := os.Open(filePath)
	if err != nil {
		return nil, nil, fmt.Errorf("failed to open EML file: %w", err)
	}
	defer func() {
		_ = fileHandle.Close()
	}()
	return readEMLFromReader(fileHandle)
}

// readEMLFromReader uses net/mail to parse the header and body from a given io.Reader.
//
// This function reads the EML content from the provided io.Reader and uses the net/mail
// package to parse the message's headers and body. It returns the parsed netmail.Message
// along with a bytes.Buffer containing the body content. Any errors encountered during
// the parsing process are returned.
//
// Parameters:
//   - reader: An io.Reader containing the EML formatted message.
//
// Returns:
//   - A pointer to the parsed netmail.Message, a bytes.Buffer containing the body, and an
//     error if any issues occur during parsing.
func readEMLFromReader(reader io.Reader) (*netmail.Message, *bytes.Buffer, error) {
	parsedMsg, err := netmail.ReadMessage(reader)
	if err != nil {
		return parsedMsg, nil, fmt.Errorf("failed to parse EML: %w", err)
	}

	buf := bytes.Buffer{}
	if _, err = buf.ReadFrom(parsedMsg.Body); err != nil {
		return nil, nil, err
	}

	return parsedMsg, &buf, nil
}

// parseEMLHeaders parses the EML's headers and populates the Msg with relevant information.
//
// This function checks the EML headers for common headers and sets the corresponding fields
// in the Msg object. It extracts address headers, content types, and other relevant data
// for further processing.
//
// Parameters:
//   - mailHeader: A pointer to the netmail.Header containing the EML headers.
//   - msg: A pointer to the Msg object to be populated with parsed header information.
//
// Returns:
//   - An error if parsing the headers fails; otherwise, returns nil.
func parseEMLHeaders(mailHeader *netmail.Header, msg *Msg) error {
	commonHeaders := []Header{
		HeaderContentType, HeaderImportance, HeaderInReplyTo, HeaderListUnsubscribe,
		HeaderListUnsubscribePost, HeaderMessageID, HeaderMIMEVersion, HeaderOrganization,
		HeaderPrecedence, HeaderPriority, HeaderReferences, HeaderSubject, HeaderUserAgent,
		HeaderXMailer, HeaderXMSMailPriority, HeaderXPriority,
	}

	// Extract content type, charset and encoding first
	parseEMLEncoding(mailHeader, msg)
	parseEMLContentTypeCharset(mailHeader, msg)

	// Extract address headers
	if value := mailHeader.Get(HeaderFrom.String()); value != "" {
		if err := msg.From(value); err != nil {
			return fmt.Errorf(`failed to parse %q header: %w`, HeaderFrom, err)
		}
	}
	addrHeaders := map[AddrHeader]func(...string) error{
		HeaderTo:  msg.To,
		HeaderCc:  msg.Cc,
		HeaderBcc: msg.Bcc,
	}
	for addrHeader, addrFunc := range addrHeaders {
		if v := mailHeader.Get(addrHeader.String()); v != "" {
			var addrStrings []string
			parsedAddrs, err := netmail.ParseAddressList(v)
			if err != nil {
				return fmt.Errorf(`failed to parse address list: %w`, err)
			}
			for _, addr := range parsedAddrs {
				addrStrings = append(addrStrings, addr.String())
			}
			// We can skip the error checking here since netmail.ParseAddressList already performed the
			// same address checking that the msg methods do.
			_ = addrFunc(addrStrings...)
		}
	}

	// Extract date from message
	date, err := mailHeader.Date()
	if err != nil {
		switch {
		case errors.Is(err, netmail.ErrHeaderNotPresent):
			msg.SetDate()
		default:
			return fmt.Errorf("failed to parse EML date: %w", err)
		}
	}
	if err == nil {
		msg.SetDateWithValue(date)
	}

	// Extract common headers
	for _, header := range commonHeaders {
		if value := mailHeader.Get(header.String()); value != "" {
			if strings.EqualFold(header.String(), HeaderContentType.String()) &&
				strings.HasPrefix(value, TypeMultipartMixed.String()) {
				continue
			}
			msg.SetGenHeader(header, value)
		}
	}

	return nil
}

// parseEMLBodyParts parses the body of an EML based on the different content types and encodings.
//
// This function examines the content type of the parsed EML message and processes the body
// parts accordingly. It handles both plain text and multipart types, ensuring that the
// Msg object is populated with the appropriate body content.
//
// Parameters:
//   - parsedMsg: A pointer to the netmail.Message containing the parsed EML data.
//   - bodybuf: A bytes.Buffer containing the body content of the EML message.
//   - msg: A pointer to the Msg object to be populated with the parsed body content.
//
// Returns:
//   - An error if any issues occur during the body parsing process; otherwise, returns nil.
func parseEMLBodyParts(parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *Msg) error {
	// Extract the transfer encoding of the body
	mediatype, params, err := mime.ParseMediaType(parsedMsg.Header.Get(HeaderContentType.String()))
	if err != nil {
		switch {
		// If no Content-Type header is found, we assume that this is a plain text, 7bit, US-ASCII mail
		case strings.EqualFold(err.Error(), "mime: no media type"):
			mediatype = TypeTextPlain.String()
			params = make(map[string]string)
			params["charset"] = CharsetASCII.String()
		default:
			return fmt.Errorf("failed to extract content type: %w", err)
		}
	}
	if value, ok := params["charset"]; ok {
		msg.SetCharset(Charset(value))
	}

	switch {
	case strings.EqualFold(mediatype, TypeTextPlain.String()),
		strings.EqualFold(mediatype, TypeTextHTML.String()):
		if err = parseEMLBodyPlain(mediatype, parsedMsg, bodybuf, msg); err != nil {
			return fmt.Errorf("failed to parse plain body: %w", err)
		}
	case strings.EqualFold(mediatype, TypeMultipartAlternative.String()),
		strings.EqualFold(mediatype, TypeMultipartMixed.String()),
		strings.EqualFold(mediatype, TypeMultipartRelated.String()):
		if err = parseEMLMultipart(params, bodybuf, msg); err != nil {
			return fmt.Errorf("failed to parse multipart body: %w", err)
		}
	default:
		return fmt.Errorf("failed to parse body, unknown content type: %s", mediatype)
	}
	return nil
}

// parseEMLBodyPlain parses the mail body of plain type messages.
//
// This function handles the parsing of plain text messages based on their encoding. It
// identifies the content transfer encoding and decodes the body content accordingly,
// storing the result in the provided Msg object.
//
// Parameters:
//   - mediatype: The media type of the message (e.g., text/plain).
//   - parsedMsg: A pointer to the netmail.Message containing the parsed EML data.
//   - bodybuf: A bytes.Buffer containing the body content of the EML message.
//   - msg: A pointer to the Msg object to be populated with the parsed body content.
//
// Returns:
//   - An error if any issues occur during the parsing of the plain body; otherwise, returns nil.
func parseEMLBodyPlain(mediatype string, parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *Msg) error {
	contentTransferEnc := parsedMsg.Header.Get(HeaderContentTransferEnc.String())
	// If no Content-Transfer-Encoding is set, we can imply 7bit US-ASCII encoding
	// https://datatracker.ietf.org/doc/html/rfc2045#section-6.1
	if contentTransferEnc == "" || strings.EqualFold(contentTransferEnc, EncodingUSASCII.String()) {
		msg.SetEncoding(EncodingUSASCII)
		msg.SetBodyString(ContentType(mediatype), bodybuf.String())
		return nil
	}
	if strings.EqualFold(contentTransferEnc, NoEncoding.String()) {
		msg.SetEncoding(NoEncoding)
		msg.SetBodyString(ContentType(mediatype), bodybuf.String())
		return nil
	}
	if strings.EqualFold(contentTransferEnc, EncodingQP.String()) {
		msg.SetEncoding(EncodingQP)
		qpReader := quotedprintable.NewReader(bodybuf)
		qpBuffer := bytes.Buffer{}
		if _, err := qpBuffer.ReadFrom(qpReader); err != nil {
			return fmt.Errorf("failed to read quoted-printable body: %w", err)
		}
		msg.SetBodyString(ContentType(mediatype), qpBuffer.String())
		return nil
	}
	if strings.EqualFold(contentTransferEnc, EncodingB64.String()) {
		msg.SetEncoding(EncodingB64)
		b64Decoder := base64.NewDecoder(base64.StdEncoding, bodybuf)
		b64Buffer := bytes.Buffer{}
		if _, err := b64Buffer.ReadFrom(b64Decoder); err != nil {
			return fmt.Errorf("failed to read base64 body: %w", err)
		}
		msg.SetBodyString(ContentType(mediatype), b64Buffer.String())
		return nil
	}
	return fmt.Errorf("unsupported Content-Transfer-Encoding")
}

// parseEMLMultipart parses a multipart body part of an EML message.
//
// This function handles the parsing of multipart messages, extracting the individual parts
// and determining their content types. It processes each part according to its content type
// and ensures that all relevant data is stored in the Msg object.
//
// Parameters:
//   - params: A map containing the parameters from the multipart content type.
//   - bodybuf: A bytes.Buffer containing the body content of the EML message.
//   - msg: A pointer to the Msg object to be populated with the parsed body parts.
//
// Returns:
//   - An error if any issues occur during the parsing of the multipart body; otherwise,
//     returns nil.
func parseEMLMultipart(params map[string]string, bodybuf *bytes.Buffer, msg *Msg) error {
	boundary, ok := params["boundary"]
	if !ok {
		return fmt.Errorf("no boundary tag found in multipart body")
	}
	multipartReader := multipart.NewReader(bodybuf, boundary)
ReadNextPart:
	multiPart, err := multipartReader.NextPart()
	defer func() {
		if multiPart != nil {
			_ = multiPart.Close()
		}
	}()
	if err != nil && !errors.Is(err, io.EOF) {
		return fmt.Errorf("failed to get next part of multipart message: %w", err)
	}
	for err == nil {
		// Multipart/related and Multipart/alternative parts need to be parsed separately
		if contentTypeSlice, ok := multiPart.Header[HeaderContentType.String()]; ok && len(contentTypeSlice) == 1 {
			contentType, _ := parseMultiPartHeader(contentTypeSlice[0])
			if strings.EqualFold(contentType, TypeMultipartRelated.String()) ||
				strings.EqualFold(contentType, TypeMultipartAlternative.String()) {
				relatedPart := &netmail.Message{
					Header: netmail.Header(multiPart.Header),
					Body:   multiPart,
				}
				relatedBuf := &bytes.Buffer{}
				if _, err = relatedBuf.ReadFrom(multiPart); err != nil {
					return fmt.Errorf("failed to read related multipart message to buffer: %w", err)
				}
				if err := parseEMLBodyParts(relatedPart, relatedBuf, msg); err != nil {
					return fmt.Errorf("failed to parse related multipart body: %w", err)
				}
			}
		}

		// Content-Disposition header means we have an attachment or embed
		if contentDisposition, ok := multiPart.Header[HeaderContentDisposition.String()]; ok {
			if err = parseEMLAttachmentEmbed(contentDisposition, multiPart, msg); err != nil {
				return fmt.Errorf("failed to parse attachment/embed: %w", err)
			}
			goto ReadNextPart
		}

		multiPartData, mperr := io.ReadAll(multiPart)
		if mperr != nil {
			_ = multiPart.Close()
			return fmt.Errorf("failed to read multipart: %w", err)
		}

		multiPartContentType, ok := multiPart.Header[HeaderContentType.String()]
		if !ok {
			return fmt.Errorf("failed to get content-type from part")
		}
		contentType, optional := parseMultiPartHeader(multiPartContentType[0])
		if strings.EqualFold(contentType, TypeMultipartRelated.String()) {
			goto ReadNextPart
		}
		part := msg.newPart(ContentType(contentType))
		if charset, ok := optional["charset"]; ok {
			part.SetCharset(Charset(charset))
		}

		mutliPartTransferEnc, ok := multiPart.Header[HeaderContentTransferEnc.String()]
		if !ok {
			// If CTE is empty we can assume that it's a quoted-printable CTE since the
			// GO stdlib multipart packages deletes that header
			// See: https://cs.opensource.google/go/go/+/refs/tags/go1.22.0:src/mime/multipart/multipart.go;l=161
			mutliPartTransferEnc = []string{EncodingQP.String()}
		}

		switch {
		case strings.EqualFold(mutliPartTransferEnc[0], EncodingUSASCII.String()):
			part.SetEncoding(EncodingUSASCII)
			part.SetContent(string(multiPartData))
		case strings.EqualFold(mutliPartTransferEnc[0], NoEncoding.String()):
			part.SetEncoding(NoEncoding)
			part.SetContent(string(multiPartData))
		case strings.EqualFold(mutliPartTransferEnc[0], EncodingB64.String()):
			part.SetEncoding(EncodingB64)
			if err = handleEMLMultiPartBase64Encoding(multiPartData, part); err != nil {
				return fmt.Errorf("failed to handle multipart base64 transfer-encoding: %w", err)
			}
		case strings.EqualFold(mutliPartTransferEnc[0], EncodingQP.String()):
			part.SetEncoding(EncodingQP)
			part.SetContent(string(multiPartData))
		default:
			return fmt.Errorf("unsupported Content-Transfer-Encoding: %s", mutliPartTransferEnc[0])
		}

		msg.parts = append(msg.parts, part)
		multiPart, err = multipartReader.NextPart()
	}
	if !errors.Is(err, io.EOF) {
		return fmt.Errorf("failed to read multipart: %w", err)
	}
	return nil
}

// parseEMLEncoding parses and determines the encoding of the message.
//
// This function extracts the content transfer encoding from the EML headers and sets the
// corresponding encoding in the Msg object. It ensures that the correct encoding is used
// for further processing of the message content.
//
// Parameters:
//   - mailHeader: A pointer to the netmail.Header containing the EML headers.
//   - msg: A pointer to the Msg object to be updated with the encoding information.
func parseEMLEncoding(mailHeader *netmail.Header, msg *Msg) {
	if value := mailHeader.Get(HeaderContentTransferEnc.String()); value != "" {
		switch {
		case strings.EqualFold(value, EncodingQP.String()):
			msg.SetEncoding(EncodingQP)
		case strings.EqualFold(value, EncodingB64.String()):
			msg.SetEncoding(EncodingB64)
		default:
			msg.SetEncoding(NoEncoding)
		}
	}
}

// parseEMLContentTypeCharset parses and determines the charset and content type of the message.
//
// This function extracts the content type and charset from the EML headers, setting them
// appropriately in the Msg object. It ensures that the Msg object is configured with the
// correct content type for further processing.
//
// Parameters:
//   - mailHeader: A pointer to the netmail.Header containing the EML headers.
//   - msg: A pointer to the Msg object to be updated with content type and charset information.
func parseEMLContentTypeCharset(mailHeader *netmail.Header, msg *Msg) {
	if value := mailHeader.Get(HeaderContentType.String()); value != "" {
		contentType, optional := parseMultiPartHeader(value)
		if charset, ok := optional["charset"]; ok {
			msg.SetCharset(Charset(charset))
		}
		msg.setEncoder()
		if contentType != "" && !strings.EqualFold(contentType, TypeMultipartMixed.String()) {
			msg.SetGenHeader(HeaderContentType, contentType)
		}
	}
}

// handleEMLMultiPartBase64Encoding sets the content body of a base64 encoded Part.
//
// This function decodes the base64 encoded content of a multipart part and stores the
// resulting content in the provided Part object. It handles any errors that occur during
// the decoding process.
//
// Parameters:
//   - multiPartData: A byte slice containing the base64 encoded data.
//   - part: A pointer to the Part object where the decoded content will be stored.
//
// Returns:
//   - An error if the base64 decoding fails; otherwise, returns nil.
func handleEMLMultiPartBase64Encoding(multiPartData []byte, part *Part) error {
	part.SetEncoding(EncodingB64)
	content, err := base64.StdEncoding.DecodeString(string(multiPartData))
	if err != nil {
		return fmt.Errorf("failed to decode base64 part: %w", err)
	}
	part.SetContent(string(content))
	return nil
}

// parseMultiPartHeader parses a multipart header and returns the value and optional parts as a map.
//
// This function splits a multipart header into its main value and any optional parameters,
// returning them separately. It helps in processing multipart messages by extracting
// relevant information from headers.
//
// Parameters:
//   - multiPartHeader: A string representing the multipart header to be parsed.
//
// Returns:
//   - The main header value as a string and a map of optional parameters.
func parseMultiPartHeader(multiPartHeader string) (header string, optional map[string]string) {
	optional = make(map[string]string)
	headerSplit := strings.SplitN(multiPartHeader, ";", 2)
	header = headerSplit[0]
	if len(headerSplit) == 2 {
		optString := strings.TrimLeft(headerSplit[1], " ")
		optSplit := strings.SplitN(optString, "=", 2)
		if len(optSplit) == 2 {
			optional[optSplit[0]] = optSplit[1]
		}
	}
	return
}

// parseEMLAttachmentEmbed parses a multipart that is an attachment or embed.
//
// This function handles the parsing of multipart sections that are marked as attachments or
// embedded content. It processes the content disposition and sets the appropriate fields in
// the Msg object based on the parsed data.
//
// Parameters:
//   - contentDisposition: A slice of strings containing the content disposition header.
//   - multiPart: A pointer to the multipart.Part to be parsed.
//   - msg: A pointer to the Msg object to be populated with the attachment or embed data.
//
// Returns:
//   - An error if any issues occur during the parsing of attachments or embeds; otherwise,
//     returns nil.
func parseEMLAttachmentEmbed(contentDisposition []string, multiPart *multipart.Part, msg *Msg) error {
	cdType, optional := parseMultiPartHeader(contentDisposition[0])
	filename := "generic.attachment"
	if name, ok := optional["filename"]; ok {
		filename = name[1 : len(name)-1]
	}

	var dataReader io.Reader
	dataReader = multiPart
	contentTransferEnc, _ := parseMultiPartHeader(multiPart.Header.Get(HeaderContentTransferEnc.String()))
	b64Decoder := base64.NewDecoder(base64.StdEncoding, multiPart)
	if strings.EqualFold(contentTransferEnc, EncodingB64.String()) {
		dataReader = b64Decoder
	}

	switch strings.ToLower(cdType) {
	case "attachment":
		if err := msg.AttachReader(filename, dataReader); err != nil {
			return fmt.Errorf("failed to attach multipart body: %w", err)
		}
	case "inline":
		if contentID, _ := parseMultiPartHeader(multiPart.Header.Get(HeaderContentID.String())); contentID != "" {
			if err := msg.EmbedReader(filename, dataReader, WithFileContentID(contentID)); err != nil {
				return fmt.Errorf("failed to embed multipart body: %w", err)
			}
			return nil
		}
		if err := msg.EmbedReader(filename, dataReader); err != nil {
			return fmt.Errorf("failed to embed multipart body: %w", err)
		}
	default:
		return errors.New("unsupported content disposition type")
	}
	return nil
}
