/*
 * Copyright (c) 2015, Psiphon Inc.
 * All rights reserved.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

package protocol

import (
	"bufio"
	"bytes"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"strings"

	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common"
)

// ServerEntry represents a Psiphon server. It contains information
// about how to establish a tunnel connection to the server through
// several protocols. Server entries are JSON records downloaded from
// various sources.
type ServerEntry struct {
	IpAddress                     string   `json:"ipAddress"`
	WebServerPort                 string   `json:"webServerPort"` // not an int
	WebServerSecret               string   `json:"webServerSecret"`
	WebServerCertificate          string   `json:"webServerCertificate"`
	SshPort                       int      `json:"sshPort"`
	SshUsername                   string   `json:"sshUsername"`
	SshPassword                   string   `json:"sshPassword"`
	SshHostKey                    string   `json:"sshHostKey"`
	SshObfuscatedPort             int      `json:"sshObfuscatedPort"`
	SshObfuscatedKey              string   `json:"sshObfuscatedKey"`
	Capabilities                  []string `json:"capabilities"`
	Region                        string   `json:"region"`
	MeekServerPort                int      `json:"meekServerPort"`
	MeekCookieEncryptionPublicKey string   `json:"meekCookieEncryptionPublicKey"`
	MeekObfuscatedKey             string   `json:"meekObfuscatedKey"`
	MeekFrontingHost              string   `json:"meekFrontingHost"`
	MeekFrontingHosts             []string `json:"meekFrontingHosts"`
	MeekFrontingDomain            string   `json:"meekFrontingDomain"`
	MeekFrontingAddresses         []string `json:"meekFrontingAddresses"`
	MeekFrontingAddressesRegex    string   `json:"meekFrontingAddressesRegex"`
	MeekFrontingDisableSNI        bool     `json:"meekFrontingDisableSNI"`

	// These local fields are not expected to be present in downloaded server
	// entries. They are added by the client to record and report stats about
	// how and when server entries are obtained.
	LocalSource    string `json:"localSource"`
	LocalTimestamp string `json:"localTimestamp"`
}

// GetCapability returns the server capability corresponding
// to the protocol.
func GetCapability(protocol string) string {
	return strings.TrimSuffix(protocol, "-OSSH")
}

// SupportsProtocol returns true if and only if the ServerEntry has
// the necessary capability to support the specified tunnel protocol.
func (serverEntry *ServerEntry) SupportsProtocol(protocol string) bool {
	requiredCapability := GetCapability(protocol)
	return common.Contains(serverEntry.Capabilities, requiredCapability)
}

// GetSupportedProtocols returns a list of tunnel protocols supported
// by the ServerEntry's capabilities.
func (serverEntry *ServerEntry) GetSupportedProtocols(excludeMeek bool) []string {
	supportedProtocols := make([]string, 0)
	for _, protocol := range SupportedTunnelProtocols {
		if excludeMeek && TunnelProtocolUsesMeek(protocol) {
			continue
		}
		if serverEntry.SupportsProtocol(protocol) {
			supportedProtocols = append(supportedProtocols, protocol)
		}
	}
	return supportedProtocols
}

// DisableImpairedProtocols modifies the ServerEntry to disable
// the specified protocols.
// Note: this assumes that protocol capabilities are 1-to-1.
func (serverEntry *ServerEntry) DisableImpairedProtocols(impairedProtocols []string) {
	capabilities := make([]string, 0)
	for _, capability := range serverEntry.Capabilities {
		omit := false
		for _, protocol := range impairedProtocols {
			requiredCapability := GetCapability(protocol)
			if capability == requiredCapability {
				omit = true
				break
			}
		}
		if !omit {
			capabilities = append(capabilities, capability)
		}
	}
	serverEntry.Capabilities = capabilities
}

// SupportsSSHAPIRequests returns true when the server supports
// SSH API requests.
func (serverEntry *ServerEntry) SupportsSSHAPIRequests() bool {
	return common.Contains(serverEntry.Capabilities, CAPABILITY_SSH_API_REQUESTS)
}

func (serverEntry *ServerEntry) GetUntunneledWebRequestPorts() []string {
	ports := make([]string, 0)
	if common.Contains(serverEntry.Capabilities, CAPABILITY_UNTUNNELED_WEB_API_REQUESTS) {
		// Server-side configuration quirk: there's a port forward from
		// port 443 to the web server, which we can try, except on servers
		// running FRONTED_MEEK, which listens on port 443.
		if !serverEntry.SupportsProtocol(TUNNEL_PROTOCOL_FRONTED_MEEK) {
			ports = append(ports, "443")
		}
		ports = append(ports, serverEntry.WebServerPort)
	}
	return ports
}

// EncodeServerEntry returns a string containing the encoding of
// a ServerEntry following Psiphon conventions.
func EncodeServerEntry(serverEntry *ServerEntry) (string, error) {
	serverEntryContents, err := json.Marshal(serverEntry)
	if err != nil {
		return "", common.ContextError(err)
	}

	return hex.EncodeToString([]byte(fmt.Sprintf(
		"%s %s %s %s %s",
		serverEntry.IpAddress,
		serverEntry.WebServerPort,
		serverEntry.WebServerSecret,
		serverEntry.WebServerCertificate,
		serverEntryContents))), nil
}

// DecodeServerEntry extracts server entries from the encoding
// used by remote server lists and Psiphon server handshake requests.
//
// The resulting ServerEntry.LocalSource is populated with serverEntrySource,
// which should be one of SERVER_ENTRY_SOURCE_EMBEDDED, SERVER_ENTRY_SOURCE_REMOTE,
// SERVER_ENTRY_SOURCE_DISCOVERY, SERVER_ENTRY_SOURCE_TARGET,
// SERVER_ENTRY_SOURCE_OBFUSCATED.
// ServerEntry.LocalTimestamp is populated with the provided timestamp, which
// should be a RFC 3339 formatted string. These local fields are stored with the
// server entry and reported to the server as stats (a coarse granularity timestamp
// is reported).
func DecodeServerEntry(
	encodedServerEntry, timestamp,
	serverEntrySource string) (serverEntry *ServerEntry, err error) {

	hexDecodedServerEntry, err := hex.DecodeString(encodedServerEntry)
	if err != nil {
		return nil, common.ContextError(err)
	}

	// Skip past legacy format (4 space delimited fields) and just parse the JSON config
	fields := bytes.SplitN(hexDecodedServerEntry, []byte(" "), 5)
	if len(fields) != 5 {
		return nil, common.ContextError(errors.New("invalid encoded server entry"))
	}

	serverEntry = new(ServerEntry)
	err = json.Unmarshal(fields[4], &serverEntry)
	if err != nil {
		return nil, common.ContextError(err)
	}

	// NOTE: if the source JSON happens to have values in these fields, they get clobbered.
	serverEntry.LocalSource = serverEntrySource
	serverEntry.LocalTimestamp = timestamp

	return serverEntry, nil
}

// ValidateServerEntry checks for malformed server entries.
// Currently, it checks for a valid ipAddress. This is important since
// the IP address is the key used to store/lookup the server entry.
// TODO: validate more fields?
func ValidateServerEntry(serverEntry *ServerEntry) error {
	ipAddr := net.ParseIP(serverEntry.IpAddress)
	if ipAddr == nil {
		errMsg := fmt.Sprintf("server entry has invalid ipAddress: '%s'", serverEntry.IpAddress)
		return common.ContextError(errors.New(errMsg))
	}
	return nil
}

// DecodeServerEntryList extracts server entries from the list encoding
// used by remote server lists and Psiphon server handshake requests.
// Each server entry is validated and invalid entries are skipped.
// See DecodeServerEntry for note on serverEntrySource/timestamp.
func DecodeServerEntryList(
	encodedServerEntryList, timestamp,
	serverEntrySource string) (serverEntries []*ServerEntry, err error) {

	serverEntries = make([]*ServerEntry, 0)
	for _, encodedServerEntry := range strings.Split(encodedServerEntryList, "\n") {
		if len(encodedServerEntry) == 0 {
			continue
		}

		// TODO: skip this entry and continue if can't decode?
		serverEntry, err := DecodeServerEntry(encodedServerEntry, timestamp, serverEntrySource)
		if err != nil {
			return nil, common.ContextError(err)
		}

		if ValidateServerEntry(serverEntry) != nil {
			// Skip this entry and continue with the next one
			// TODO: invoke a logging callback
			continue
		}

		serverEntries = append(serverEntries, serverEntry)
	}
	return serverEntries, nil
}

// StreamingServerEntryDecoder performs the DecodeServerEntryList
// operation, loading only one server entry into memory at a time.
type StreamingServerEntryDecoder struct {
	scanner           *bufio.Scanner
	timestamp         string
	serverEntrySource string
}

// NewStreamingServerEntryDecoder creates a new StreamingServerEntryDecoder.
func NewStreamingServerEntryDecoder(
	encodedServerEntryListReader io.Reader,
	timestamp, serverEntrySource string) *StreamingServerEntryDecoder {

	return &StreamingServerEntryDecoder{
		scanner:           bufio.NewScanner(encodedServerEntryListReader),
		timestamp:         timestamp,
		serverEntrySource: serverEntrySource,
	}
}

// Next reads and decodes, and validates the next server entry from the
// input stream, returning a nil server entry when the stream is complete.
//
// Limitations:
// - Each encoded server entry line cannot exceed bufio.MaxScanTokenSize,
//   the default buffer size which this decoder uses. This is 64K.
// - DecodeServerEntry is called on each encoded server entry line, which
//   will allocate memory to hex decode and JSON deserialze the server
//   entry. As this is not presently reusing a fixed buffer, each call
//   will allocate additional memory; garbage collection is necessary to
//   reclaim that memory for reuse for the next server entry.
//
func (decoder *StreamingServerEntryDecoder) Next() (*ServerEntry, error) {

	for {
		if !decoder.scanner.Scan() {
			return nil, common.ContextError(decoder.scanner.Err())
		}

		// TODO: use scanner.Bytes which doesn't allocate, instead of scanner.Text

		// TODO: skip this entry and continue if can't decode?
		serverEntry, err := DecodeServerEntry(
			decoder.scanner.Text(), decoder.timestamp, decoder.serverEntrySource)
		if err != nil {
			return nil, common.ContextError(err)
		}

		if ValidateServerEntry(serverEntry) != nil {
			// Skip this entry and continue with the next one
			// TODO: invoke a logging callback
			continue
		}

		return serverEntry, nil
	}
}