/*
 * 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 psiphon

import (
	"crypto/rand"
	"crypto/x509"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"math/big"
	"net"
	"net/url"
	"os"
	"runtime"
	"strings"
	"sync"
	"syscall"
	"time"
)

// Contains is a helper function that returns true
// if the target string is in the list.
func Contains(list []string, target string) bool {
	for _, listItem := range list {
		if listItem == target {
			return true
		}
	}
	return false
}

// FlipCoin is a helper function that randomly
// returns true or false. If the underlying random
// number generator fails, FlipCoin still returns
// a result.
func FlipCoin() bool {
	randomInt, _ := MakeSecureRandomInt(2)
	return randomInt == 1
}

// MakeSecureRandomInt is a helper function that wraps
// MakeSecureRandomInt64.
func MakeSecureRandomInt(max int) (int, error) {
	randomInt, err := MakeSecureRandomInt64(int64(max))
	return int(randomInt), err
}

// MakeSecureRandomInt64 is a helper function that wraps
// crypto/rand.Int, which returns a uniform random value in [0, max).
func MakeSecureRandomInt64(max int64) (int64, error) {
	randomInt, err := rand.Int(rand.Reader, big.NewInt(max))
	if err != nil {
		return 0, ContextError(err)
	}
	return randomInt.Int64(), nil
}

// MakeSecureRandomBytes is a helper function that wraps
// crypto/rand.Read.
func MakeSecureRandomBytes(length int) ([]byte, error) {
	randomBytes := make([]byte, length)
	n, err := rand.Read(randomBytes)
	if err != nil {
		return nil, ContextError(err)
	}
	if n != length {
		return nil, ContextError(errors.New("insufficient random bytes"))
	}
	return randomBytes, nil
}

// MakeSecureRandomPadding selects a random padding length in the indicated
// range and returns a random byte array of the selected length.
// In the unlikely case where an underlying MakeRandom functions fails,
// the padding is length 0.
func MakeSecureRandomPadding(minLength, maxLength int) []byte {
	var padding []byte
	paddingSize, err := MakeSecureRandomInt(maxLength - minLength)
	if err != nil {
		NoticeAlert("MakeSecureRandomPadding: MakeSecureRandomInt failed")
		return make([]byte, 0)
	}
	paddingSize += minLength
	padding, err = MakeSecureRandomBytes(paddingSize)
	if err != nil {
		NoticeAlert("MakeSecureRandomPadding: MakeSecureRandomBytes failed")
		return make([]byte, 0)
	}
	return padding
}

// MakeRandomPeriod returns a random duration, within a given range.
// In the unlikely case where an  underlying MakeRandom functions fails,
// the period is the minimum.
func MakeRandomPeriod(min, max time.Duration) (duration time.Duration) {
	period, err := MakeSecureRandomInt64(max.Nanoseconds() - min.Nanoseconds())
	if err != nil {
		NoticeAlert("NextRandomRangePeriod: MakeSecureRandomInt64 failed")
	}
	duration = min + time.Duration(period)
	return
}

// MakeRandomStringHex returns a hex encoded random string.
// byteLength specifies the pre-encoded data length.
func MakeRandomStringHex(byteLength int) (string, error) {
	bytes, err := MakeSecureRandomBytes(byteLength)
	if err != nil {
		return "", ContextError(err)
	}
	return hex.EncodeToString(bytes), nil
}

// MakeRandomStringBase64 returns a base64 encoded random string.
// byteLength specifies the pre-encoded data length.
func MakeRandomStringBase64(byteLength int) (string, error) {
	bytes, err := MakeSecureRandomBytes(byteLength)
	if err != nil {
		return "", ContextError(err)
	}
	return base64.RawURLEncoding.EncodeToString(bytes), nil
}

func DecodeCertificate(encodedCertificate string) (certificate *x509.Certificate, err error) {
	derEncodedCertificate, err := base64.StdEncoding.DecodeString(encodedCertificate)
	if err != nil {
		return nil, ContextError(err)
	}
	certificate, err = x509.ParseCertificate(derEncodedCertificate)
	if err != nil {
		return nil, ContextError(err)
	}
	return certificate, nil
}

// FilterUrlError transforms an error, when it is a url.Error, removing
// the URL value. This is to avoid logging private user data in cases
// where the URL may be a user input value.
// This function is used with errors returned by net/http and net/url,
// which are (currently) of type url.Error. In particular, the round trip
// function used by our HttpProxy, http.Client.Do, returns errors of type
// url.Error, with the URL being the url sent from the user's tunneled
// applications:
// https://github.com/golang/go/blob/release-branch.go1.4/src/net/http/client.go#L394
func FilterUrlError(err error) error {
	if urlErr, ok := err.(*url.Error); ok {
		err = &url.Error{
			Op:  urlErr.Op,
			URL: "",
			Err: urlErr.Err,
		}
	}
	return err
}

// TrimError removes the middle of over-long error message strings
func TrimError(err error) error {
	const MAX_LEN = 100
	message := fmt.Sprintf("%s", err)
	if len(message) > MAX_LEN {
		return errors.New(message[:MAX_LEN/2] + "..." + message[len(message)-MAX_LEN/2:])
	}
	return err
}

// getFunctionName is a helper that extracts a simple function name from
// full name returned byruntime.Func.Name(). This is used to declutter
// log messages containing function names.
func getFunctionName(pc uintptr) string {
	funcName := runtime.FuncForPC(pc).Name()
	index := strings.LastIndex(funcName, "/")
	if index != -1 {
		funcName = funcName[index+1:]
	}
	return funcName
}

// GetParentContext returns the parent function name and source file
// line number.
func GetParentContext() string {
	pc, _, line, _ := runtime.Caller(2)
	return fmt.Sprintf("%s#%d", getFunctionName(pc), line)
}

// ContextError prefixes an error message with the current function
// name and source file line number.
func ContextError(err error) error {
	if err == nil {
		return nil
	}
	pc, _, line, _ := runtime.Caller(1)
	return fmt.Errorf("%s#%d: %s", getFunctionName(pc), line, err)
}

// IsAddressInUseError returns true when the err is due to EADDRINUSE/WSAEADDRINUSE.
func IsAddressInUseError(err error) bool {
	if err, ok := err.(*net.OpError); ok {
		if err, ok := err.Err.(*os.SyscallError); ok {
			if err.Err == syscall.EADDRINUSE {
				return true
			}
			// Special case for Windows (WSAEADDRINUSE = 10048)
			if errno, ok := err.Err.(syscall.Errno); ok {
				if 10048 == int(errno) {
					return true
				}
			}
		}
	}
	return false
}

// SyncFileWriter wraps a file and exposes an io.Writer. At predefined
// steps, the file is synced (flushed to disk) while writing.
type SyncFileWriter struct {
	file  *os.File
	step  int
	count int
}

// NewSyncFileWriter creates a SyncFileWriter.
func NewSyncFileWriter(file *os.File) *SyncFileWriter {
	return &SyncFileWriter{
		file:  file,
		step:  2 << 16,
		count: 0}
}

// Write implements io.Writer with periodic file syncing.
func (writer *SyncFileWriter) Write(p []byte) (n int, err error) {
	n, err = writer.file.Write(p)
	if err != nil {
		return
	}
	writer.count += n
	if writer.count >= writer.step {
		err = writer.file.Sync()
		writer.count = 0
	}
	return
}

// GetCurrentTimestamp returns the current time in UTC as
// an RFC 3339 formatted string.
func GetCurrentTimestamp() string {
	return time.Now().UTC().Format(time.RFC3339)
}

// TruncateTimestampToHour truncates an RFC 3339 formatted string
// to hour granularity. If the input is not a valid format, the
// result is "".
func TruncateTimestampToHour(timestamp string) string {
	t, err := time.Parse(time.RFC3339, timestamp)
	if err != nil {
		NoticeAlert("failed to truncate timestamp: %s", err)
		return ""
	}
	return t.Truncate(1 * time.Hour).Format(time.RFC3339)
}

// IsFileChanged uses os.Stat to check if the name, size, or last mod time of the
// file has changed (which is a heuristic, but sufficiently robust for users of this
// function). Returns nil if file has not changed; otherwise, returns a changed
// os.FileInfo which may be used to check for subsequent changes.
func IsFileChanged(path string, previousFileInfo os.FileInfo) (os.FileInfo, error) {

	fileInfo, err := os.Stat(path)
	if err != nil {
		return nil, ContextError(err)
	}

	changed := previousFileInfo == nil ||
		fileInfo.Name() != previousFileInfo.Name() ||
		fileInfo.Size() != previousFileInfo.Size() ||
		fileInfo.ModTime() != previousFileInfo.ModTime()

	if !changed {
		return nil, nil
	}

	return fileInfo, nil
}

// Reloader represents a read-only, in-memory reloadable data object. For example,
// a JSON data file that is loaded into memory and accessed for read-only lookups;
// and from time to time may be reloaded from the same file, updating the memory
// copy.
type Reloader interface {

	// Reload reloads the data object. Reload returns a flag indicating if the
	// reloadable target has changed and reloaded or remains unchanged. By
	// convention, when reloading fails the Reloader should revert to its previous
	// in-memory state.
	Reload() (bool, error)

	// WillReload indicates if the data object is capable of reloading.
	WillReload() bool

	// LogDescription returns a description to be used for logging
	// events related to the Reloader.
	LogDescription() string
}

// ReloadableFile is a file-backed Reloader. This type is intended to be embedded
// in other types that add the actual reloadable data structures.
//
// ReloadableFile has a multi-reader mutex for synchronization. Its Reload() function
// will obtain a write lock before reloading the data structures. Actually reloading
// action is to be provided via the reloadAction callback (for example, read the contents
// of the file and unmarshall the contents into data structures). All read access to
// the data structures should be guarded by RLocks on the ReloadableFile mutex.
//
// reloadAction must ensure that data structures revert to their previous state when
// a reload fails.
//
type ReloadableFile struct {
	sync.RWMutex
	fileName     string
	fileInfo     os.FileInfo
	reloadAction func(string) error
}

// NewReloadableFile initializes a new ReloadableFile
func NewReloadableFile(
	fileName string,
	reloadAction func(string) error) ReloadableFile {

	return ReloadableFile{
		fileName:     fileName,
		reloadAction: reloadAction,
	}
}

// WillReload indicates whether the ReloadableFile is capable
// of reloading.
func (reloadable *ReloadableFile) WillReload() bool {
	return reloadable.fileName != ""
}

// Reload checks if the underlying file has changed (using IsFileChanged semantics, which
// are heuristics) and, when changed, invokes the reloadAction callback which should
// reload, from the file, the in-memory data structures.
// All data structure readers should be blocked by the ReloadableFile mutex.
func (reloadable *ReloadableFile) Reload() (bool, error) {

	if !reloadable.WillReload() {
		return false, nil
	}

	// Check whether the file has changed _before_ blocking readers

	reloadable.RLock()
	changedFileInfo, err := IsFileChanged(reloadable.fileName, reloadable.fileInfo)
	reloadable.RUnlock()
	if err != nil {
		return false, ContextError(err)
	}

	if changedFileInfo == nil {
		return false, nil
	}

	// ...now block readers

	reloadable.Lock()
	defer reloadable.Unlock()

	err = reloadable.reloadAction(reloadable.fileName)
	if err != nil {
		return false, ContextError(err)
	}

	reloadable.fileInfo = changedFileInfo

	return true, nil
}

func (reloadable *ReloadableFile) LogDescription() string {
	return reloadable.fileName
}