package gateway

import (
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/textproto"
	"strconv"
	"strings"
	"time"
)

// errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
// all of the byte-range-spec values is greater than the content size.
var errNoOverlap = errors.New("invalid range: failed to overlap")

func headerGetExact(h http.Header, key string) string {
	if v := h[key]; len(v) > 0 {
		return v[0]
	}
	return ""
}

// httpServeContent replies to the request using the content in the
// provided Reader.
//
// The main benefit of httpServeContent over io.Copy is that it handles Range requests properly,
// handles If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since, and If-Range requests.
//
// If modtime is not the zero time or Unix epoch, ServeContent
// includes it in a Last-Modified header in the response. If the
// request includes an If-Modified-Since header, ServeContent uses
// modtime to decide whether the content needs to be sent at all.
//
// If the caller has set w's ETag header formatted per RFC 7232, section 2.3,
// ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
//
// Notable differences from http.ServeContent
// 1. Takes an io.Reader instead of an io.ReaderSeeker
// 2. Requires the size to be passed in explicitly instead of discovered via Seeker behavior
// 3. Only handles a single HTTP Range, if multiple are requested it returns the first
// 4. The passed io.Reader must start at wherever the HTTP Range Request will start
// 4. Requires the Content-Type header to already be set
// 5. Does not require the name to be passed in for content sniffing
// 6. content may be nil for HEAD requests
func httpServeContent(w http.ResponseWriter, r *http.Request, modtime time.Time, size int64, content io.Reader) {
	if size < 0 {
		// Should never happen but just to be sure
		http.Error(w, "negative content size computed", http.StatusInternalServerError)
		return
	}

	setLastModified(w, modtime)
	done, rangeReq := checkPreconditions(w, r, modtime)
	if done {
		return
	}

	code := http.StatusOK

	// handle Content-Range header.
	sendSize := size
	ranges, err := parseRange(rangeReq, size)
	switch err {
	case nil:
	case errNoOverlap:
		if size == 0 {
			// Some clients add a Range header to all requests to
			// limit the size of the response. If the file is empty,
			// ignore the range header and respond with a 200 rather
			// than a 416.
			ranges = nil
			break
		}
		w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
		fallthrough
	default:
		http.Error(w, err.Error(), http.StatusRequestedRangeNotSatisfiable)
		return
	}
	if sumRangesSize(ranges) > size {
		// The total number of bytes in all the ranges
		// is larger than the size of the file by
		// itself, so this is probably an attack, or a
		// dumb client. Ignore the range request.
		ranges = nil
	}

	// We only support a single range request, if more than one is submitted we just send back the first
	if len(ranges) > 0 {
		ra := ranges[0]
		// RFC 7233, Section 4.1:
		// "If a single part is being transferred, the server
		// generating the 206 response MUST generate a
		// Content-Range header field, describing what range
		// of the selected representation is enclosed, and a
		// payload consisting of the range.
		// ...
		// A server MUST NOT generate a multipart response to
		// a request for a single range, since a client that
		// does not request multiple parts might not support
		// multipart responses."

		sendSize = ra.length
		code = http.StatusPartialContent
		w.Header().Set("Content-Range", ra.contentRange(size))
	}

	w.Header().Set("Accept-Ranges", "bytes")
	if w.Header().Get("Content-Encoding") == "" {
		w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
	}

	w.WriteHeader(code)

	if r.Method != http.MethodHead {
		io.CopyN(w, content, sendSize)
	}
}

// scanETag determines if a syntactically valid ETag is present at s. If so,
// the ETag and remaining text after consuming ETag is returned. Otherwise,
// it returns "", "".
func scanETag(s string) (etag string, remain string) {
	s = textproto.TrimString(s)
	start := 0
	if strings.HasPrefix(s, "W/") {
		start = 2
	}
	if len(s[start:]) < 2 || s[start] != '"' {
		return "", ""
	}
	// ETag is either W/"text" or "text".
	// See RFC 7232 2.3.
	for i := start + 1; i < len(s); i++ {
		c := s[i]
		switch {
		// Character values allowed in ETags.
		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
		case c == '"':
			return s[:i+1], s[i+1:]
		default:
			return "", ""
		}
	}
	return "", ""
}

// etagStrongMatch reports whether a and b match using strong ETag comparison.
// Assumes a and b are valid ETags.
func etagStrongMatch(a, b string) bool {
	return a == b && a != "" && a[0] == '"'
}

// etagWeakMatch reports whether a and b match using weak ETag comparison.
// Assumes a and b are valid ETags.
func etagWeakMatch(a, b string) bool {
	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
}

// condResult is the result of an HTTP request precondition check.
// See https://tools.ietf.org/html/rfc7232 section 3.
type condResult int

const (
	condNone condResult = iota
	condTrue
	condFalse
)

func checkIfMatch(w http.ResponseWriter, r *http.Request) condResult {
	im := r.Header.Get("If-Match")
	if im == "" {
		return condNone
	}
	for {
		im = textproto.TrimString(im)
		if len(im) == 0 {
			break
		}
		if im[0] == ',' {
			im = im[1:]
			continue
		}
		if im[0] == '*' {
			return condTrue
		}
		etag, remain := scanETag(im)
		if etag == "" {
			break
		}
		if etagStrongMatch(etag, headerGetExact(w.Header(), "Etag")) {
			return condTrue
		}
		im = remain
	}

	return condFalse
}

func checkIfUnmodifiedSince(r *http.Request, modtime time.Time) condResult {
	ius := r.Header.Get("If-Unmodified-Since")
	if ius == "" || isZeroTime(modtime) {
		return condNone
	}
	t, err := http.ParseTime(ius)
	if err != nil {
		return condNone
	}

	// The Last-Modified header truncates sub-second precision so
	// the modtime needs to be truncated too.
	modtime = modtime.Truncate(time.Second)
	if modtime.Before(t) || modtime.Equal(t) {
		return condTrue
	}
	return condFalse
}

func checkIfNoneMatch(w http.ResponseWriter, r *http.Request) condResult {
	inm := headerGetExact(r.Header, "If-None-Match")
	if inm == "" {
		return condNone
	}
	buf := inm
	for {
		buf = textproto.TrimString(buf)
		if len(buf) == 0 {
			break
		}
		if buf[0] == ',' {
			buf = buf[1:]
			continue
		}
		if buf[0] == '*' {
			return condFalse
		}
		etag, remain := scanETag(buf)
		if etag == "" {
			break
		}
		if etagWeakMatch(etag, headerGetExact(w.Header(), "Etag")) {
			return condFalse
		}
		buf = remain
	}
	return condTrue
}

func checkIfModifiedSince(r *http.Request, modtime time.Time) condResult {
	if r.Method != http.MethodGet && r.Method != http.MethodHead {
		return condNone
	}
	ims := r.Header.Get("If-Modified-Since")
	if ims == "" || isZeroTime(modtime) {
		return condNone
	}
	t, err := http.ParseTime(ims)
	if err != nil {
		return condNone
	}
	// The Last-Modified header truncates sub-second precision so
	// the modtime needs to be truncated too.
	modtime = modtime.Truncate(time.Second)
	if modtime.Before(t) || modtime.Equal(t) {
		return condFalse
	}
	return condTrue
}

func checkIfRange(w http.ResponseWriter, r *http.Request, modtime time.Time) condResult {
	if r.Method != http.MethodGet && r.Method != http.MethodHead {
		return condNone
	}
	ir := headerGetExact(r.Header, "If-Range")
	if ir == "" {
		return condNone
	}
	etag, _ := scanETag(ir)
	if etag != "" {
		if etagStrongMatch(etag, w.Header().Get("Etag")) {
			return condTrue
		} else {
			return condFalse
		}
	}
	// The If-Range value is typically the ETag value, but it may also be
	// the modtime date. See golang.org/issue/8367.
	if modtime.IsZero() {
		return condFalse
	}
	t, err := http.ParseTime(ir)
	if err != nil {
		return condFalse
	}
	if t.Unix() == modtime.Unix() {
		return condTrue
	}
	return condFalse
}

// isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
func isZeroTime(t time.Time) bool {
	return t.IsZero() || t.Equal(unixEpochTime)
}

func setLastModified(w http.ResponseWriter, modtime time.Time) {
	if !isZeroTime(modtime) {
		w.Header().Set("Last-Modified", modtime.UTC().Format(http.TimeFormat))
	}
}

func writeNotModified(w http.ResponseWriter) {
	// RFC 7232 section 4.1:
	// a sender SHOULD NOT generate representation metadata other than the
	// above listed fields unless said metadata exists for the purpose of
	// guiding cache updates (e.g., Last-Modified might be useful if the
	// response does not have an ETag field).
	h := w.Header()
	delete(h, "Content-Type")
	delete(h, "Content-Length")
	delete(h, "Content-Encoding")
	if h.Get("Etag") != "" {
		delete(h, "Last-Modified")
	}
	w.WriteHeader(http.StatusNotModified)
}

// checkPreconditions evaluates request preconditions and reports whether a precondition
// resulted in sending StatusNotModified or StatusPreconditionFailed.
func checkPreconditions(w http.ResponseWriter, r *http.Request, modtime time.Time) (done bool, rangeHeader string) {
	// This function carefully follows RFC 7232 section 6.
	ch := checkIfMatch(w, r)
	if ch == condNone {
		ch = checkIfUnmodifiedSince(r, modtime)
	}
	if ch == condFalse {
		w.WriteHeader(http.StatusPreconditionFailed)
		return true, ""
	}
	switch checkIfNoneMatch(w, r) {
	case condFalse:
		if r.Method == http.MethodGet || r.Method == http.MethodHead {
			writeNotModified(w)
			return true, ""
		} else {
			w.WriteHeader(http.StatusPreconditionFailed)
			return true, ""
		}
	case condNone:
		if checkIfModifiedSince(r, modtime) == condFalse {
			writeNotModified(w)
			return true, ""
		}
	}

	rangeHeader = headerGetExact(r.Header, "Range")
	if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
		rangeHeader = ""
	}
	return false, rangeHeader
}

// httpRange specifies the byte range to be sent to the client.
type httpRange struct {
	start, length int64
}

func (r httpRange) contentRange(size int64) string {
	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
}

// parseRange parses a Range header string as per RFC 7233.
// errNoOverlap is returned if none of the ranges overlap.
func parseRange(s string, size int64) ([]httpRange, error) {
	if s == "" {
		return nil, nil // header not present
	}
	const b = "bytes="
	if !strings.HasPrefix(s, b) {
		return nil, errors.New("invalid range")
	}
	var ranges []httpRange
	noOverlap := false
	for _, ra := range strings.Split(s[len(b):], ",") {
		ra = textproto.TrimString(ra)
		if ra == "" {
			continue
		}
		start, end, ok := strings.Cut(ra, "-")
		if !ok {
			return nil, errors.New("invalid range")
		}
		start, end = textproto.TrimString(start), textproto.TrimString(end)
		var r httpRange
		if start == "" {
			// If no start is specified, end specifies the
			// range start relative to the end of the file,
			// and we are dealing with <suffix-length>
			// which has to be a non-negative integer as per
			// RFC 7233 Section 2.1 "Byte-Ranges".
			if end == "" || end[0] == '-' {
				return nil, errors.New("invalid range")
			}
			i, err := strconv.ParseInt(end, 10, 64)
			if i < 0 || err != nil {
				return nil, errors.New("invalid range")
			}
			if i > size {
				i = size
			}
			r.start = size - i
			r.length = size - r.start
		} else {
			i, err := strconv.ParseInt(start, 10, 64)
			if err != nil || i < 0 {
				return nil, errors.New("invalid range")
			}
			if i >= size {
				// If the range begins after the size of the content,
				// then it does not overlap.
				noOverlap = true
				continue
			}
			r.start = i
			if end == "" {
				// If no end is specified, range extends to end of the file.
				r.length = size - r.start
			} else {
				i, err := strconv.ParseInt(end, 10, 64)
				if err != nil || r.start > i {
					return nil, errors.New("invalid range")
				}
				if i >= size {
					i = size - 1
				}
				r.length = i - r.start + 1
			}
		}
		ranges = append(ranges, r)
	}
	if noOverlap && len(ranges) == 0 {
		// The specified ranges did not overlap with the content.
		return nil, errNoOverlap
	}
	return ranges, nil
}

func sumRangesSize(ranges []httpRange) (size int64) {
	for _, ra := range ranges {
		size += ra.length
	}
	return
}

// seekToStartOfFirstRange seeks to the start of the first Range if the request is an HTTP Range Request
func (i *handler) seekToStartOfFirstRange(w http.ResponseWriter, r *http.Request, data io.Seeker) bool {
	rangeHeader := r.Header.Get("Range")
	if rangeHeader != "" {
		ranges, err := parseRangeWithoutLength(rangeHeader)
		if err != nil {
			i.webError(w, r, fmt.Errorf("invalid range request: %w", err), http.StatusBadRequest)
			return false
		}
		if len(ranges) > 0 {
			ra := &ranges[0]
			err = seekToRangeStart(data, ra)
			if err != nil {
				i.webError(w, r, fmt.Errorf("could not seek to location in range request: %w", err), http.StatusBadRequest)
				return false
			}
		}
	}
	return true
}

func seekToRangeStart(data io.Seeker, ra *ByteRange) error {
	if ra != nil && ra.From != 0 {
		if _, err := data.Seek(int64(ra.From), io.SeekStart); err != nil {
			return err
		}
	}
	return nil
}
