package fetch

import (
	"errors"
	"fmt"
	"io"
	"sync/atomic"
	"time"
)

const (
	bufferSize   = 1 << 20
	stallTimeout = 2 * time.Minute
)

func copyBody(destination io.Writer, body io.ReadCloser, remaining int64, record func(int64) error) error {
	var stalled atomic.Bool
	stallTimer := time.AfterFunc(stallTimeout, func() {
		stalled.Store(true)
		body.Close()
	})
	defer stallTimer.Stop()

	buffer := make([]byte, bufferSize)
	for remaining != 0 {
		readSize := len(buffer)
		if remaining > 0 && remaining < int64(readSize) {
			readSize = int(remaining)
		}

		count, readErr := body.Read(buffer[:readSize])
		if count > 0 {
			stallTimer.Reset(stallTimeout)
			if _, err := destination.Write(buffer[:count]); err != nil {
				return err
			}
			if err := record(int64(count)); err != nil {
				return err
			}
			if remaining > 0 {
				remaining -= int64(count)
			}
		}
		if errors.Is(readErr, io.EOF) {
			return nil
		}
		if readErr != nil {
			if stalled.Load() {
				return ErrStalled
			}
			return fmt.Errorf("%w: %w", errConnectionFailed, readErr)
		}
	}
	return nil
}