package client

import (
	"bufio"
	"encoding/json"
	"io"
	"strings"
)

// sseDecoder reads the subset of the Server-Sent Events format the server
// emits: "event:" and "data:" lines, one JSON payload per event.
type sseDecoder struct {
	sc *bufio.Scanner
}

func newSSEDecoder(r io.Reader) *sseDecoder {
	sc := bufio.NewScanner(r)
	sc.Buffer(make([]byte, 0, 8<<10), 1<<20)
	return &sseDecoder{sc: sc}
}

// next returns the next event, or nil for a comment or keepalive line.
func (d *sseDecoder) next() (*Event, error) {
	var (
		name string
		data string
	)
	for d.sc.Scan() {
		line := d.sc.Text()
		switch {
		case line == "":
			if data == "" {
				name = ""
				continue
			}
			var ev Event
			if err := json.Unmarshal([]byte(data), &ev); err != nil {
				return nil, nil // skip a payload we cannot read
			}
			if ev.Type == "" {
				ev.Type = name
			}
			return &ev, nil
		case strings.HasPrefix(line, ":"):
			continue
		case strings.HasPrefix(line, "event:"):
			name = strings.TrimSpace(strings.TrimPrefix(line, "event:"))
		case strings.HasPrefix(line, "data:"):
			data += strings.TrimSpace(strings.TrimPrefix(line, "data:"))
		}
	}
	if err := d.sc.Err(); err != nil {
		return nil, err
	}
	return nil, io.EOF
}