package yamux

import (
	"fmt"
	"io"
	"os"
	"time"
)

// Config is used to tune the Yamux session
type Config struct {
	// AcceptBacklog is used to limit how many streams may be
	// waiting an accept.
	AcceptBacklog int

	// EnableKeepalive is used to do a period keep alive
	// messages using a ping.
	EnableKeepAlive bool

	// KeepAliveInterval is how often to perform the keep alive
	KeepAliveInterval time.Duration

	// MaxStreamWindowSize is used to control the maximum
	// window size that we allow for a stream.
	MaxStreamWindowSize uint32

	// LogOutput is used to control the log destination
	LogOutput io.Writer
}

// DefaultConfig is used to return a default configuration
func DefaultConfig() *Config {
	return &Config{
		AcceptBacklog:       256,
		EnableKeepAlive:     true,
		KeepAliveInterval:   30 * time.Second,
		MaxStreamWindowSize: initialStreamWindow,
		LogOutput:           os.Stderr,
	}
}

// VerifyConfig is used to verify the sanity of configuration
func VerifyConfig(config *Config) error {
	if config.AcceptBacklog <= 0 {
		return fmt.Errorf("backlog must be positive")
	}
	if config.KeepAliveInterval == 0 {
		return fmt.Errorf("keep-alive interval must be positive")
	}
	if config.MaxStreamWindowSize < initialStreamWindow {
		return fmt.Errorf("MaxStreamWindowSize must be larger than %d", initialStreamWindow)
	}
	return nil
}

// Server is used to initialize a new server-side connection.
// There must be at most one server-side connection. If a nil config is
// provided, the DefaultConfiguration will be used.
func Server(conn io.ReadWriteCloser, config *Config) (*Session, error) {
	if config == nil {
		config = DefaultConfig()
	}
	if err := VerifyConfig(config); err != nil {
		return nil, err
	}
	return newSession(config, conn, false), nil
}

// Client is used to initialize a new client-side connection.
// There must be at most one client-side connection.
func Client(conn io.ReadWriteCloser, config *Config) (*Session, error) {
	if config == nil {
		config = DefaultConfig()
	}

	if err := VerifyConfig(config); err != nil {
		return nil, err
	}
	return newSession(config, conn, true), nil
}