package yamux

import (
	"fmt"
	"io"
	"net"
	"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

	// PingBacklog is used to limit how many ping acks we can queue.
	PingBacklog 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
	LogOutput io.Writer

	// ReadBufSize controls the size of the read buffer.
	//
	// Set to 0 to disable it.
	ReadBufSize int

	// WriteCoalesceDelay is the maximum amount of time we'll delay
	// coalescing a packet before sending it. This should be on the order of
	// micro-milliseconds.
	WriteCoalesceDelay time.Duration

	// MaxMessageSize is the maximum size of a message that we'll send on a
	// stream. This ensures that a single stream doesn't hog a connection.
	MaxMessageSize uint32
}

// DefaultConfig is used to return a default configuration
func DefaultConfig() *Config {
	return &Config{
		AcceptBacklog:          256,
		PingBacklog:            32,
		EnableKeepAlive:        true,
		KeepAliveInterval:      30 * time.Second,
		ConnectionWriteTimeout: 10 * time.Second,
		MaxStreamWindowSize:    initialStreamWindow,
		LogOutput:              os.Stderr,
		ReadBufSize:            4096,
		MaxMessageSize:         64 * 1024, // Means 64KiB/10s = 52kbps minimum speed.
		WriteCoalesceDelay:     100 * time.Microsecond,
	}
}

// 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.MaxMessageSize < 1024 {
		return fmt.Errorf("MaxMessageSize must be greater than a kilobyte")
	}
	if config.WriteCoalesceDelay < 0 {
		return fmt.Errorf("WriteCoalesceDelay must be >= 0")
	}
	if config.PingBacklog < 1 {
		return fmt.Errorf("PingBacklog must be > 0")
	}
	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 net.Conn, config *Config) (*Session, error) {
	if config == nil {
		config = DefaultConfig()
	}
	if err := VerifyConfig(config); err != nil {
		return nil, err
	}
	return newSession(config, conn, false, config.ReadBufSize), nil
}

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

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