package yamux

import (
	"fmt"
	"io"
	"log"
	"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

	// ConnectionWriteTimeout is meant to be a "safety valve" timeout after
	// we which will suspect a problem with the underlying connection and
	// close it. This is only applied to writes, where's there's generally
	// an expectation that things will move along quickly.
	ConnectionWriteTimeout 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. Either Logger or
	// LogOutput can be set, not both.
	LogOutput io.Writer

	// Logger is used to pass in the logger to be used. Either Logger or
	// LogOutput can be set, not both.
	Logger *log.Logger
}

// DefaultConfig is used to return a default configuration
func DefaultConfig() *Config {
	return &Config{
		AcceptBacklog:          256,
		EnableKeepAlive:        true,
		KeepAliveInterval:      30 * time.Second,
		ConnectionWriteTimeout: 10 * 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)
	}
	if config.LogOutput != nil && config.Logger != nil {
		return fmt.Errorf("both Logger and LogOutput may not be set, select one")
	} else if config.LogOutput == nil && config.Logger == nil {
		return fmt.Errorf("one of Logger or LogOutput must be set, select one")
	}
	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
}