// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package textproto

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"io"
	"math"
	"strconv"
	"strings"
	"sync"
	_ "unsafe" // for linkname
)

// TODO: This should be a distinguishable error (ErrMessageTooLarge)
// to allow mime/multipart to detect it.
var errMessageTooLarge = errors.New("message too large")

// A Reader implements convenience methods for reading requests
// or responses from a text protocol network connection.
type Reader struct {
	R   *bufio.Reader
	dot *dotReader
	buf []byte // a re-usable buffer for readContinuedLineSlice
}

// NewReader returns a new [Reader] reading from r.
//
// To avoid denial of service attacks, the provided [bufio.Reader]
// should be reading from an [io.LimitReader] or similar Reader to bound
// the size of responses.
func NewReader(r *bufio.Reader) *Reader {
	return &Reader{R: r}
}

// ReadLine reads a single line from r,
// eliding the final \n or \r\n from the returned string.
func (r *Reader) ReadLine() (string, error) {
	line, err := r.readLineSlice(-1)
	return string(line), err
}

// ReadLineBytes is like [Reader.ReadLine] but returns a []byte instead of a string.
func (r *Reader) ReadLineBytes() ([]byte, error) {
	line, err := r.readLineSlice(-1)
	if line != nil {
		line = bytes.Clone(line)
	}
	return line, err
}

// readLineSlice reads a single line from r,
// up to lim bytes long (or unlimited if lim is less than 0),
// eliding the final \r or \r\n from the returned string.
func (r *Reader) readLineSlice(lim int64) ([]byte, error) {
	r.closeDot()
	var line []byte
	for {
		l, more, err := r.R.ReadLine()
		if err != nil {
			return nil, err
		}
		if lim >= 0 && int64(len(line))+int64(len(l)) > lim {
			return nil, errMessageTooLarge
		}
		// Avoid the copy if the first call produced a full line.
		if line == nil && !more {
			return l, nil
		}
		line = append(line, l...)
		if !more {
			break
		}
	}
	return line, nil
}

// ReadContinuedLine reads a possibly continued line from r,
// eliding the final trailing ASCII white space.
// Lines after the first are considered continuations if they
// begin with a space or tab character. In the returned data,
// continuation lines are separated from the previous line
// only by a single space: the newline and leading white space
// are removed.
//
// For example, consider this input:
//
//	Line 1
//	  continued...
//	Line 2
//
// The first call to ReadContinuedLine will return "Line 1 continued..."
// and the second will return "Line 2".
//
// Empty lines are never continued.
func (r *Reader) ReadContinuedLine() (string, error) {
	line, err := r.readContinuedLineSlice(-1, noValidation)
	return string(line), err
}

// trim returns s with leading and trailing spaces and tabs removed.
// It does not assume Unicode or UTF-8.
func trim(s []byte) []byte {
	i := 0
	for i < len(s) && (s[i] == ' ' || s[i] == '\t') {
		i++
	}
	s = s[i:]
	n := len(s) - 1
	for n >= 0 && (s[n] == ' ' || s[n] == '\t') {
		n--
	}
	return s[:n+1]
}

// ReadContinuedLineBytes is like [Reader.ReadContinuedLine] but
// returns a []byte instead of a string.
func (r *Reader) ReadContinuedLineBytes() ([]byte, error) {
	line, err := r.readContinuedLineSlice(-1, noValidation)
	if line != nil {
		line = bytes.Clone(line)
	}
	return line, err
}

// readContinuedLineSlice reads continued lines from the reader buffer,
// returning a byte slice with all lines. The validateFirstLine function
// is run on the first read line, and if it returns an error then this
// error is returned from readContinuedLineSlice.
// It reads up to lim bytes of data (or unlimited if lim is less than 0).
func (r *Reader) readContinuedLineSlice(lim int64, validateFirstLine func([]byte) error) ([]byte, error) {
	if validateFirstLine == nil {
		return nil, fmt.Errorf("missing validateFirstLine func")
	}

	// Read the first line.
	line, err := r.readLineSlice(lim)
	if err != nil {
		return nil, err
	}
	if len(line) == 0 { // blank line - no continuation
		return line, nil
	}

	if err := validateFirstLine(line); err != nil {
		return nil, err
	}

	// Optimistically assume that we have started to buffer the next line
	// and it starts with an ASCII letter (the next header key), or a blank
	// line, so we can avoid copying that buffered data around in memory
	// and skipping over non-existent whitespace.
	if r.R.Buffered() > 1 {
		peek, _ := r.R.Peek(2)
		if len(peek) > 0 && (isASCIILetter(peek[0]) || peek[0] == '\n') ||
			len(peek) == 2 && peek[0] == '\r' && peek[1] == '\n' {
			return trim(line), nil
		}
	}

	// ReadByte or the next readLineSlice will flush the read buffer;
	// copy the slice into buf.
	r.buf = append(r.buf[:0], trim(line)...)

	if lim < 0 {
		lim = math.MaxInt64
	}
	lim -= int64(len(r.buf))

	// Read continuation lines.
	for r.skipSpace() > 0 {
		r.buf = append(r.buf, ' ')
		if int64(len(r.buf)) >= lim {
			return nil, errMessageTooLarge
		}
		line, err := r.readLineSlice(lim - int64(len(r.buf)))
		if err != nil {
			break
		}
		r.buf = append(r.buf, trim(line)...)
	}
	return r.buf, nil
}

// skipSpace skips R over all spaces and returns the number of bytes skipped.
func (r *Reader) skipSpace() int {
	n := 0
	for {
		c, err := r.R.ReadByte()
		if err != nil {
			// Bufio will keep err until next read.
			break
		}
		if c != ' ' && c != '\t' {
			r.R.UnreadByte()
			break
		}
		n++
	}
	return n
}

func (r *Reader) readCodeLine(expectCode int) (code int, continued bool, message string, err error) {
	line, err := r.ReadLine()
	if err != nil {
		return
	}
	return parseCodeLine(line, expectCode)
}

func parseCodeLine(line string, expectCode int) (code int, continued bool, message string, err error) {
	if len(line) < 4 || line[3] != ' ' && line[3] != '-' {
		err = ProtocolError("short response: " + line)
		return
	}
	continued = line[3] == '-'
	code, err = strconv.Atoi(line[0:3])
	if err != nil || code < 100 {
		err = ProtocolError("invalid response code: " + line)
		return
	}
	message = line[4:]
	if 1 <= expectCode && expectCode < 10 && code/100 != expectCode ||
		10 <= expectCode && expectCode < 100 && code/10 != expectCode ||
		100 <= expectCode && expectCode < 1000 && code != expectCode {
		err = &Error{code, message}
	}
	return
}

// ReadCodeLine reads a response code line of the form
//
//	code message
//
// where code is a three-digit status code and the message
// extends to the rest of the line. An example of such a line is:
//
//	220 plan9.bell-labs.com ESMTP
//
// If the prefix of the status does not match the digits in expectCode,
// ReadCodeLine returns with err set to &Error{code, message}.
// For example, if expectCode is 31, an error will be returned if
// the status is not in the range [310,319].
//
// If the response is multi-line, ReadCodeLine returns an error.
//
// An expectCode <= 0 disables the check of the status code.
func (r *Reader) ReadCodeLine(expectCode int) (code int, message string, err error) {
	code, continued, message, err := r.readCodeLine(expectCode)
	if err == nil && continued {
		err = ProtocolError("unexpected multi-line response: " + message)
	}
	return
}

// ReadResponse reads a multi-line response of the form:
//
//	code-message line 1
//	code-message line 2
//	...
//	code message line n
//
// where code is a three-digit status code. The first line starts with the
// code and a hyphen. The response is terminated by a line that starts
// with the same code followed by a space. Each line in message is
// separated by a newline (\n).
//
// See page 36 of RFC 959 (https://www.ietf.org/rfc/rfc959.txt) for
// details of another form of response accepted:
//
//	code-message line 1
//	message line 2
//	...
//	code message line n
//
// If the prefix of the status does not match the digits in expectCode,
// ReadResponse returns with err set to &Error{code, message}.
// For example, if expectCode is 31, an error will be returned if
// the status is not in the range [310,319].
//
// An expectCode <= 0 disables the check of the status code.
func (r *Reader) ReadResponse(expectCode int) (code int, message string, err error) {
	code, continued, first, err := r.readCodeLine(expectCode)
	multi := continued
	var messageBuilder strings.Builder
	messageBuilder.WriteString(first)
	for continued {
		line, err := r.ReadLine()
		if err != nil {
			return 0, "", err
		}

		var code2 int
		var moreMessage string
		code2, continued, moreMessage, err = parseCodeLine(line, 0)
		if err != nil || code2 != code {
			messageBuilder.WriteByte('\n')
			messageBuilder.WriteString(strings.TrimRight(line, "\r\n"))
			continued = true
			continue
		}
		messageBuilder.WriteByte('\n')
		messageBuilder.WriteString(moreMessage)
	}
	message = messageBuilder.String()
	if err != nil && multi && message != "" {
		// replace one line error message with all lines (full message)
		err = &Error{code, message}
	}
	return
}

// DotReader returns a new [Reader] that satisfies Reads using the
// decoded text of a dot-encoded block read from r.
// The returned Reader is only valid until the next call
// to a method on r.
//
// Dot encoding is a common framing used for data blocks
// in text protocols such as SMTP.  The data consists of a sequence
// of lines, each of which ends in "\r\n".  The sequence itself
// ends at a line containing just a dot: ".\r\n".  Lines beginning
// with a dot are escaped with an additional dot to avoid
// looking like the end of the sequence.
//
// The decoded form returned by the Reader's Read method
// rewrites the "\r\n" line endings into the simpler "\n",
// removes leading dot escapes if present, and stops with error [io.EOF]
// after consuming (and discarding) the end-of-sequence line.
func (r *Reader) DotReader() io.Reader {
	r.closeDot()
	r.dot = &dotReader{r: r}
	return r.dot
}

type dotReader struct {
	r     *Reader
	state int
}

// Read satisfies reads by decoding dot-encoded data read from d.r.
func (d *dotReader) Read(b []byte) (n int, err error) {
	// Run data through a simple state machine to
	// elide leading dots, rewrite trailing \r\n into \n,
	// and detect ending .\r\n line.
	const (
		stateBeginLine = iota // beginning of line; initial state; must be zero
		stateDot              // read . at beginning of line
		stateDotCR            // read .\r at beginning of line
		stateCR               // read \r (possibly at end of line)
		stateData             // reading data in middle of line
		stateEOF              // reached .\r\n end marker line
	)
	br := d.r.R
	for n < len(b) && d.state != stateEOF {
		var c byte
		c, err = br.ReadByte()
		if err != nil {
			if err == io.EOF {
				err = io.ErrUnexpectedEOF
			}
			break
		}
		switch d.state {
		case stateBeginLine:
			if c == '.' {
				d.state = stateDot
				continue
			}
			if c == '\r' {
				d.state = stateCR
				continue
			}
			d.state = stateData

		case stateDot:
			if c == '\r' {
				d.state = stateDotCR
				continue
			}
			if c == '\n' {
				d.state = stateEOF
				continue
			}
			d.state = stateData

		case stateDotCR:
			if c == '\n' {
				d.state = stateEOF
				continue
			}
			// Not part of .\r\n.
			// Consume leading dot and emit saved \r.
			br.UnreadByte()
			c = '\r'
			d.state = stateData

		case stateCR:
			if c == '\n' {
				d.state = stateBeginLine
				break
			}
			// Not part of \r\n. Emit saved \r
			br.UnreadByte()
			c = '\r'
			d.state = stateData

		case stateData:
			if c == '\r' {
				d.state = stateCR
				continue
			}
			if c == '\n' {
				d.state = stateBeginLine
			}
		}
		b[n] = c
		n++
	}
	if err == nil && d.state == stateEOF {
		err = io.EOF
	}
	if err != nil && d.r.dot == d {
		d.r.dot = nil
	}
	return
}

// closeDot drains the current DotReader if any,
// making sure that it reads until the ending dot line.
func (r *Reader) closeDot() {
	if r.dot == nil {
		return
	}
	buf := make([]byte, 128)
	for r.dot != nil {
		// When Read reaches EOF or an error,
		// it will set r.dot == nil.
		r.dot.Read(buf)
	}
}

// ReadDotBytes reads a dot-encoding and returns the decoded data.
//
// See the documentation for the [Reader.DotReader] method for details about dot-encoding.
func (r *Reader) ReadDotBytes() ([]byte, error) {
	return io.ReadAll(r.DotReader())
}

// ReadDotLines reads a dot-encoding and returns a slice
// containing the decoded lines, with the final \r\n or \n elided from each.
//
// See the documentation for the [Reader.DotReader] method for details about dot-encoding.
func (r *Reader) ReadDotLines() ([]string, error) {
	// We could use ReadDotBytes and then Split it,
	// but reading a line at a time avoids needing a
	// large contiguous block of memory and is simpler.
	var v []string
	var err error
	for {
		var line string
		line, err = r.ReadLine()
		if err != nil {
			if err == io.EOF {
				err = io.ErrUnexpectedEOF
			}
			break
		}

		// Dot by itself marks end; otherwise cut one dot.
		if len(line) > 0 && line[0] == '.' {
			if len(line) == 1 {
				break
			}
			line = line[1:]
		}
		v = append(v, line)
	}
	return v, err
}

var colon = []byte(":")

// ReadMIMEHeader reads a MIME-style header from r.
// The header is a sequence of possibly continued Key: Value lines
// ending in a blank line.
// The returned map m maps [CanonicalMIMEHeaderKey](key) to a
// sequence of values in the same order encountered in the input.
//
// For example, consider this input:
//
//	My-Key: Value 1
//	Long-Key: Even
//	       Longer Value
//	My-Key: Value 2
//
// Given that input, ReadMIMEHeader returns the map:
//
//	map[string][]string{
//		"My-Key": {"Value 1", "Value 2"},
//		"Long-Key": {"Even Longer Value"},
//	}
func (r *Reader) ReadMIMEHeader() (MIMEHeader, error) {
	return readMIMEHeader(r, math.MaxInt64, math.MaxInt64)
}

// readMIMEHeader is accessed from mime/multipart.
//go:linkname readMIMEHeader

// readMIMEHeader is a version of ReadMIMEHeader which takes a limit on the header size.
// It is called by the mime/multipart package.
func readMIMEHeader(r *Reader, maxMemory, maxHeaders int64) (MIMEHeader, error) {
	// Avoid lots of small slice allocations later by allocating one
	// large one ahead of time which we'll cut up into smaller
	// slices. If this isn't big enough later, we allocate small ones.
	var strs []string
	hint := r.upcomingHeaderKeys()
	if hint > 0 {
		if hint > 1000 {
			hint = 1000 // set a cap to avoid overallocation
		}
		strs = make([]string, hint)
	}

	m := make(MIMEHeader, hint)

	// Account for 400 bytes of overhead for the MIMEHeader, plus 200 bytes per entry.
	// Benchmarking map creation as of go1.20, a one-entry MIMEHeader is 416 bytes and large
	// MIMEHeaders average about 200 bytes per entry.
	maxMemory -= 400
	const mapEntryOverhead = 200

	// The first line cannot start with a leading space.
	if buf, err := r.R.Peek(1); err == nil && (buf[0] == ' ' || buf[0] == '\t') {
		const errorLimit = 80 // arbitrary limit on how much of the line we'll quote
		line, err := r.readLineSlice(errorLimit)
		if err != nil {
			return m, err
		}
		return m, ProtocolError("malformed MIME header initial line: " + string(line))
	}

	for {
		kv, err := r.readContinuedLineSlice(maxMemory, mustHaveFieldNameColon)
		if len(kv) == 0 {
			return m, err
		}

		// Key ends at first colon.
		k, v, ok := bytes.Cut(kv, colon)
		if !ok {
			return m, ProtocolError("malformed MIME header line: " + string(kv))
		}
		key, ok := canonicalMIMEHeaderKey(k)
		if !ok {
			return m, ProtocolError("malformed MIME header line: " + string(kv))
		}
		for _, c := range v {
			if !validHeaderValueByte(c) {
				return m, ProtocolError("malformed MIME header line: " + string(kv))
			}
		}

		maxHeaders--
		if maxHeaders < 0 {
			return nil, errMessageTooLarge
		}

		// Skip initial spaces in value.
		value := string(bytes.TrimLeft(v, " \t"))

		vv := m[key]
		if vv == nil {
			maxMemory -= int64(len(key))
			maxMemory -= mapEntryOverhead
		}
		maxMemory -= int64(len(value))
		if maxMemory < 0 {
			return m, errMessageTooLarge
		}
		if vv == nil && len(strs) > 0 {
			// More than likely this will be a single-element key.
			// Most headers aren't multi-valued.
			// Set the capacity on strs[0] to 1, so any future append
			// won't extend the slice into the other strings.
			vv, strs = strs[:1:1], strs[1:]
			vv[0] = value
			m[key] = vv
		} else {
			m[key] = append(vv, value)
		}

		if err != nil {
			return m, err
		}
	}
}

// noValidation is a no-op validation func for readContinuedLineSlice
// that permits any lines.
func noValidation(_ []byte) error { return nil }

// mustHaveFieldNameColon ensures that, per RFC 7230, the
// field-name is on a single line, so the first line must
// contain a colon.
func mustHaveFieldNameColon(line []byte) error {
	if bytes.IndexByte(line, ':') < 0 {
		return ProtocolError(fmt.Sprintf("malformed MIME header: missing colon: %q", line))
	}
	return nil
}

var nl = []byte("\n")

// upcomingHeaderKeys returns an approximation of the number of keys
// that will be in this header. If it gets confused, it returns 0.
func (r *Reader) upcomingHeaderKeys() (n int) {
	// Try to determine the 'hint' size.
	r.R.Peek(1) // force a buffer load if empty
	s := r.R.Buffered()
	if s == 0 {
		return
	}
	peek, _ := r.R.Peek(s)
	for len(peek) > 0 && n < 1000 {
		var line []byte
		line, peek, _ = bytes.Cut(peek, nl)
		if len(line) == 0 || (len(line) == 1 && line[0] == '\r') {
			// Blank line separating headers from the body.
			break
		}
		if line[0] == ' ' || line[0] == '\t' {
			// Folded continuation of the previous line.
			continue
		}
		n++
	}
	return n
}

// CanonicalMIMEHeaderKey returns the canonical format of the
// MIME header key s. The canonicalization converts the first
// letter and any letter following a hyphen to upper case;
// the rest are converted to lowercase. For example, the
// canonical key for "accept-encoding" is "Accept-Encoding".
// MIME header keys are assumed to be ASCII only.
// If s contains a space or invalid header field bytes as
// defined by RFC 9112, it is returned without modifications.
func CanonicalMIMEHeaderKey(s string) string {
	// Quick check for canonical encoding.
	upper := true
	for i := 0; i < len(s); i++ {
		c := s[i]
		if !validHeaderFieldByte(c) {
			return s
		}
		if upper && 'a' <= c && c <= 'z' {
			s, _ = canonicalMIMEHeaderKey([]byte(s))
			return s
		}
		if !upper && 'A' <= c && c <= 'Z' {
			s, _ = canonicalMIMEHeaderKey([]byte(s))
			return s
		}
		upper = c == '-'
	}
	return s
}

const toLower = 'a' - 'A'

// validHeaderFieldByte reports whether c is a valid byte in a header
// field name. RFC 7230 says:
//
//	header-field   = field-name ":" OWS field-value OWS
//	field-name     = token
//	tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
//	        "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
//	token = 1*tchar
func validHeaderFieldByte(c byte) bool {
	// mask is a 128-bit bitmap with 1s for allowed bytes,
	// so that the byte c can be tested with a shift and an and.
	// If c >= 128, then 1<<c and 1<<(c-64) will both be zero,
	// and this function will return false.
	const mask = 0 |
		(1<<(10)-1)<<'0' |
		(1<<(26)-1)<<'a' |
		(1<<(26)-1)<<'A' |
		1<<'!' |
		1<<'#' |
		1<<'$' |
		1<<'%' |
		1<<'&' |
		1<<'\'' |
		1<<'*' |
		1<<'+' |
		1<<'-' |
		1<<'.' |
		1<<'^' |
		1<<'_' |
		1<<'`' |
		1<<'|' |
		1<<'~'
	return ((uint64(1)<<c)&(mask&(1<<64-1)) |
		(uint64(1)<<(c-64))&(mask>>64)) != 0
}

// validHeaderValueByte reports whether c is a valid byte in a header
// field value. RFC 7230 says:
//
//	field-content  = field-vchar [ 1*( SP / HTAB ) field-vchar ]
//	field-vchar    = VCHAR / obs-text
//	obs-text       = %x80-FF
//
// RFC 5234 says:
//
//	HTAB           =  %x09
//	SP             =  %x20
//	VCHAR          =  %x21-7E
func validHeaderValueByte(c byte) bool {
	// mask is a 128-bit bitmap with 1s for allowed bytes,
	// so that the byte c can be tested with a shift and an and.
	// If c >= 128, then 1<<c and 1<<(c-64) will both be zero.
	// Since this is the obs-text range, we invert the mask to
	// create a bitmap with 1s for disallowed bytes.
	const mask = 0 |
		(1<<(0x7f-0x21)-1)<<0x21 | // VCHAR: %x21-7E
		1<<0x20 | // SP: %x20
		1<<0x09 // HTAB: %x09
	return ((uint64(1)<<c)&^(mask&(1<<64-1)) |
		(uint64(1)<<(c-64))&^(mask>>64)) == 0
}

// canonicalMIMEHeaderKey is like CanonicalMIMEHeaderKey but is
// allowed to mutate the provided byte slice before returning the
// string.
//
// For invalid inputs (if a contains spaces or non-token bytes), a
// is unchanged and a string copy is returned.
//
// ok is true if the header key contains only valid characters and spaces.
// ReadMIMEHeader accepts header keys containing spaces, but does not
// canonicalize them.
func canonicalMIMEHeaderKey(a []byte) (_ string, ok bool) {
	if len(a) == 0 {
		return "", false
	}

	// See if a looks like a header key. If not, return it unchanged.
	noCanon := false
	for _, c := range a {
		if validHeaderFieldByte(c) {
			continue
		}
		// Don't canonicalize.
		if c == ' ' {
			// We accept invalid headers with a space before the
			// colon, but must not canonicalize them.
			// See https://go.dev/issue/34540.
			noCanon = true
			continue
		}
		return string(a), false
	}
	if noCanon {
		return string(a), true
	}

	upper := true
	for i, c := range a {
		// Canonicalize: first letter upper case
		// and upper case after each dash.
		// (Host, User-Agent, If-Modified-Since).
		// MIME headers are ASCII only, so no Unicode issues.
		if upper && 'a' <= c && c <= 'z' {
			c -= toLower
		} else if !upper && 'A' <= c && c <= 'Z' {
			c += toLower
		}
		a[i] = c
		upper = c == '-' // for next time
	}
	commonHeaderOnce.Do(initCommonHeader)
	// The compiler recognizes m[string(byteSlice)] as a special
	// case, so a copy of a's bytes into a new string does not
	// happen in this map lookup:
	if v := commonHeader[string(a)]; v != "" {
		return v, true
	}
	return string(a), true
}

// commonHeader interns common header strings.
var commonHeader map[string]string

var commonHeaderOnce sync.Once

func initCommonHeader() {
	commonHeader = make(map[string]string)
	for _, v := range []string{
		"Accept",
		"Accept-Charset",
		"Accept-Encoding",
		"Accept-Language",
		"Accept-Ranges",
		"Cache-Control",
		"Cc",
		"Connection",
		"Content-Id",
		"Content-Language",
		"Content-Length",
		"Content-Transfer-Encoding",
		"Content-Type",
		"Cookie",
		"Date",
		"Dkim-Signature",
		"Etag",
		"Expires",
		"From",
		"Host",
		"If-Modified-Since",
		"If-None-Match",
		"In-Reply-To",
		"Last-Modified",
		"Location",
		"Message-Id",
		"Mime-Version",
		"Pragma",
		"Received",
		"Return-Path",
		"Server",
		"Set-Cookie",
		"Subject",
		"To",
		"User-Agent",
		"Via",
		"X-Forwarded-For",
		"X-Imforwards",
		"X-Powered-By",
	} {
		commonHeader[v] = v
	}
}
