// Copyright 2014 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 http2

import (
	"bytes"
	"fmt"
	"net/http"
	"sort"
	"time"

	"golang.org/x/net/http2/hpack"
)

// writeFramer is implemented by any type that is used to write frames.
type writeFramer interface {
	writeFrame(writeContext) error
}

// writeContext is the interface needed by the various frame writer
// types below. All the writeFrame methods below are scheduled via the
// frame writing scheduler (see writeScheduler in writesched.go).
//
// This interface is implemented by *serverConn.
//
// TODO: decide whether to a) use this in the client code (which didn't
// end up using this yet, because it has a simpler design, not
// currently implementing priorities), or b) delete this and
// make the server code a bit more concrete.
type writeContext interface {
	Framer() *Framer
	Flush() error
	CloseConn() error
	// HeaderEncoder returns an HPACK encoder that writes to the
	// returned buffer.
	HeaderEncoder() (*hpack.Encoder, *bytes.Buffer)
}

// endsStream reports whether the given frame writer w will locally
// close the stream.
func endsStream(w writeFramer) bool {
	switch v := w.(type) {
	case *writeData:
		return v.endStream
	case *writeResHeaders:
		return v.endStream
	case nil:
		// This can only happen if the caller reuses w after it's
		// been intentionally nil'ed out to prevent use. Keep this
		// here to catch future refactoring breaking it.
		panic("endsStream called on nil writeFramer")
	}
	return false
}

type flushFrameWriter struct{}

func (flushFrameWriter) writeFrame(ctx writeContext) error {
	return ctx.Flush()
}

type writeSettings []Setting

func (s writeSettings) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteSettings([]Setting(s)...)
}

type writeGoAway struct {
	maxStreamID uint32
	code        ErrCode
}

func (p *writeGoAway) writeFrame(ctx writeContext) error {
	err := ctx.Framer().WriteGoAway(p.maxStreamID, p.code, nil)
	if p.code != 0 {
		ctx.Flush() // ignore error: we're hanging up on them anyway
		time.Sleep(50 * time.Millisecond)
		ctx.CloseConn()
	}
	return err
}

type writeData struct {
	streamID  uint32
	p         []byte
	endStream bool
}

func (w *writeData) String() string {
	return fmt.Sprintf("writeData(stream=%d, p=%d, endStream=%v)", w.streamID, len(w.p), w.endStream)
}

func (w *writeData) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteData(w.streamID, w.endStream, w.p)
}

// handlerPanicRST is the message sent from handler goroutines when
// the handler panics.
type handlerPanicRST struct {
	StreamID uint32
}

func (hp handlerPanicRST) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteRSTStream(hp.StreamID, ErrCodeInternal)
}

func (se StreamError) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteRSTStream(se.StreamID, se.Code)
}

type writePingAck struct{ pf *PingFrame }

func (w writePingAck) writeFrame(ctx writeContext) error {
	return ctx.Framer().WritePing(true, w.pf.Data)
}

type writeSettingsAck struct{}

func (writeSettingsAck) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteSettingsAck()
}

// writeResHeaders is a request to write a HEADERS and 0+ CONTINUATION frames
// for HTTP response headers or trailers from a server handler.
type writeResHeaders struct {
	streamID    uint32
	httpResCode int         // 0 means no ":status" line
	h           http.Header // may be nil
	trailers    []string    // if non-nil, which keys of h to write. nil means all.
	endStream   bool

	date          string
	contentType   string
	contentLength string
}

func (w *writeResHeaders) writeFrame(ctx writeContext) error {
	enc, buf := ctx.HeaderEncoder()
	buf.Reset()

	if w.httpResCode != 0 {
		enc.WriteField(hpack.HeaderField{
			Name:  ":status",
			Value: httpCodeString(w.httpResCode),
		})
	}

	encodeHeaders(enc, w.h, w.trailers)

	if w.contentType != "" {
		enc.WriteField(hpack.HeaderField{Name: "content-type", Value: w.contentType})
	}
	if w.contentLength != "" {
		enc.WriteField(hpack.HeaderField{Name: "content-length", Value: w.contentLength})
	}
	if w.date != "" {
		enc.WriteField(hpack.HeaderField{Name: "date", Value: w.date})
	}

	headerBlock := buf.Bytes()
	if len(headerBlock) == 0 && w.trailers == nil {
		panic("unexpected empty hpack")
	}

	// For now we're lazy and just pick the minimum MAX_FRAME_SIZE
	// that all peers must support (16KB). Later we could care
	// more and send larger frames if the peer advertised it, but
	// there's little point. Most headers are small anyway (so we
	// generally won't have CONTINUATION frames), and extra frames
	// only waste 9 bytes anyway.
	const maxFrameSize = 16384

	first := true
	for len(headerBlock) > 0 {
		frag := headerBlock
		if len(frag) > maxFrameSize {
			frag = frag[:maxFrameSize]
		}
		headerBlock = headerBlock[len(frag):]
		endHeaders := len(headerBlock) == 0
		var err error
		if first {
			first = false
			err = ctx.Framer().WriteHeaders(HeadersFrameParam{
				StreamID:      w.streamID,
				BlockFragment: frag,
				EndStream:     w.endStream,
				EndHeaders:    endHeaders,
			})
		} else {
			err = ctx.Framer().WriteContinuation(w.streamID, endHeaders, frag)
		}
		if err != nil {
			return err
		}
	}
	return nil
}

type write100ContinueHeadersFrame struct {
	streamID uint32
}

func (w write100ContinueHeadersFrame) writeFrame(ctx writeContext) error {
	enc, buf := ctx.HeaderEncoder()
	buf.Reset()
	enc.WriteField(hpack.HeaderField{Name: ":status", Value: "100"})
	return ctx.Framer().WriteHeaders(HeadersFrameParam{
		StreamID:      w.streamID,
		BlockFragment: buf.Bytes(),
		EndStream:     false,
		EndHeaders:    true,
	})
}

type writeWindowUpdate struct {
	streamID uint32 // or 0 for conn-level
	n        uint32
}

func (wu writeWindowUpdate) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteWindowUpdate(wu.streamID, wu.n)
}

func encodeHeaders(enc *hpack.Encoder, h http.Header, keys []string) {
	// TODO: garbage. pool sorters like http1? hot path for 1 key?
	if keys == nil {
		keys = make([]string, 0, len(h))
		for k := range h {
			keys = append(keys, k)
		}
		sort.Strings(keys)
	}
	for _, k := range keys {
		vv := h[k]
		k = lowerHeader(k)
		isTE := k == "transfer-encoding"
		for _, v := range vv {
			// TODO: more of "8.1.2.2 Connection-Specific Header Fields"
			if isTE && v != "trailers" {
				continue
			}
			enc.WriteField(hpack.HeaderField{Name: k, Value: v})
		}
	}
}