package chatcmpl

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

// Stream reads Server-Sent Events carrying Chat Completions chunks.
type Stream struct {
	scanner *bufio.Scanner
}

// NewStream wraps a reader of upstream SSE data.
func NewStream(r io.Reader) *Stream {
	sc := bufio.NewScanner(r)
	sc.Buffer(make([]byte, 0, 64*1024), 16*1024*1024)
	return &Stream{scanner: sc}
}

// Next returns the next chunk. It reports io.EOF once the upstream
// stream is finished (either by "[DONE]" or by closing).
func (s *Stream) Next() (*StreamChunk, error) {
	for s.scanner.Scan() {
		line := strings.TrimRight(s.scanner.Text(), "\r")
		if line == "" || strings.HasPrefix(line, ":") {
			continue
		}
		if !strings.HasPrefix(line, "data:") {
			continue
		}
		data := strings.TrimSpace(strings.TrimPrefix(line, "data:"))
		if data == "" {
			continue
		}
		if data == "[DONE]" {
			return nil, io.EOF
		}
		var chunk StreamChunk
		if err := json.Unmarshal([]byte(data), &chunk); err != nil {
			return nil, fmt.Errorf("decode upstream chunk: %w", err)
		}
		return &chunk, nil
	}
	if err := s.scanner.Err(); err != nil {
		return nil, err
	}
	return nil, io.EOF
}