// Package lumberjack provides a rolling logger.
//
// Lumberjack is intended to be one part of a logging infrastructure.
// It is not an all-in-one solution, but instead is a pluggable
// component at the bottom of the logging stack that simply controls the files
// to which logs are written.
//
// Lumberjack plays well with any logger that can write to an io.Writer,
// including the standard library's log package.
//
// For example, to use lumberjack with the std lib's log package, just pass it
// into the SetOutput function when your application starts:
//
//   log.SetOutput(&lumberjack.Logger{
//       Dir: "/var/log/myapp/"
//       NameFormat: time.RFC822+".log",
//       MaxSize: lumberjack.Gigabyte,
//       MaxBackups: 3,
//       MaxAge: lumberjack.Week * 4,
//   ))
//
// Note that lumberjack assumes whatever is writing to it will use locks to
// prevent concurrent writes. Lumberjack does not implement its own lock.
//
// Lumberjack also assumes that only one process is writing to the output files.
// Using the same lumberjack configuration from multiple processes on the same
// machine will result in improper behavior.
package lumberjack

import (
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
	"sort"
	"time"
)

const (
	// Some helper constants to make your declarations easier to read.
	Megabyte = 1024 * 1024
	Gigabyte = 1024 * Megabyte

	// note that lumberjack days and weeks may not exactly conform to calendar
	// days and weeks due to daylight savings, leap seconds, etc.
	Day  = 24 * time.Hour
	Week = 7 * Day

	defaultNameFormat = "2006-01-02T15-04-05.000.log"
	defaultMaxSize    = 100 * Megabyte
)

// ensure we always implement io.WriteCloser
var _ io.WriteCloser = &Logger{}

// Logger is an io.WriteCloser that writes to a log file in the given directory
// with the given NameFormat.  NameFormat should include a time formatting
// layout in it that produces a valid unique filename for the OS.  For more
// about time formatting layouts, read http://golang.org/pkg/time/#pkg-
// constants.
//
// The date encoded in the filename by NameFormat is used to determine which log
// files are most recent in several situations.
//
// Logger opens or creates a logfile on first Write.  It looks for files in the
// directory that match its name format, and if the one with the most recent
// NameFormat date is less than MaxSize, it will open and append to that file.
// If no such file exists, or the file is >= MaxSize, a new file is created
// using the current time with NameFormat to generate the filename.
//
// Whenever a write would cause the current log file exceed MaxSize, a new file
// is created using the current time.
//
// Cleaning Up Old Log Files
//
// Whenever a new file gets created, old log files may be deleted.  The log file
// directory is scanned for files that match NameFormat.  The most recent files
// according to their NameFormat date will be retained, up to a number equal to
// MaxBackups (or all of them if MaxBackups is 0).  Any files with a last
// modified time (based on FileInfo.ModTime) older than MaxAge are deleted,
// regardless of MaxBackups.
//
// If MaxBackups and MaxAge are both 0, no old log files will be deleted.
type Logger struct {
	// Dir determines the directory in which to store log files.
	// It defaults to os.TempDir() if empty.
	Dir string

	// NameFormat is the time formatting layout used to generate filenames.
	// It defaults to "2006-01-02T15-04-05.000.log".
	NameFormat string

	// MaxSize is the maximum size in bytes of the log file before it gets
	// rolled. It defaults to 100 megabytes.
	MaxSize int64

	// MaxAge is the maximum time to retain old log files based on
	// FileInfo.ModTime.  The default is not to remove old log files based on
	// age.
	MaxAge time.Duration

	// MaxBackups is the maximum number of old log files to retain.  The default
	// is to retain all old log files (though MaxAge may still cause them to get
	// deleted.)
	MaxBackups int

	// LocalTime determines if the time used for formatting the filename is the
	// computer's local time.  The default is to use UTC time.
	LocalTime bool

	size int64
	file *os.File
}

// currentTime is only used for testing. Normally it's the time.Now() function.
var currentTime = time.Now

// Write implements io.Writer.  If a write would cause the log file to be larger
// than MaxSize, a new log file is created using the current time formatted with
// PathFormat.  If the length of the write is greater than MaxSize, an error is
// returned that satisfies IsWriteTooLong.
func (l *Logger) Write(p []byte) (n int, err error) {
	writeLen := int64(len(p))
	if writeLen > l.max() {
		return 0, writeTooLongError{fmt.Errorf(
			"write length %d exceeds maximum file size %d", writeLen, l.max(),
		)}
	}
	f := l.file
	rotate := l.size+writeLen > l.max()
	if f == nil {
		if f, err = l.openExistingOrNew(len(p)); err != nil {
			return 0, err
		}
	} else if rotate {
		if f, err = l.openNew(); err != nil {
			return 0, err
		}
	}

	n, err = f.Write(p)
	l.size += int64(n)

	if l.file != nil && rotate {
		l.file.Close()
	}
	l.file = f

	if rotate {
		if err := l.cleanup(); err != nil {
			return 0, err
		}
	}

	return n, err
}

// openNew opens a new log file for writing.
func (l *Logger) openNew() (*os.File, error) {
	err := os.MkdirAll(l.dir(), 0744)
	if err != nil {
		return nil, fmt.Errorf("can't make directories for new logfile: %s", err)
	}
	filename := l.genFilename()
	f, err := os.OpenFile(filename, os.O_CREATE|os.O_WRONLY, 0644)
	if err != nil {
		return nil, fmt.Errorf("can't open new logfile: %s", err)
	}
	info, err := f.Stat()
	if err != nil {
		// can't really do anything if close fails here
		_ = f.Close()
		return nil, fmt.Errorf("can't get size of new logfile: %s", err)
	}
	l.size = info.Size()
	return f, nil
}

// openExistingOrNew opens the most recently modified logfile in the log
// directory, if the current write would not put it over MaxSize.  If there is
// no such file or the write would put it over the MaxSize, a new file is
// created.
func (l *Logger) openExistingOrNew(writeLen int) (*os.File, error) {
	files, err := ioutil.ReadDir(l.dir())
	if os.IsNotExist(err) {
		return l.openNew()
	}
	if err != nil {
		return nil, fmt.Errorf("can't read files in log file directory: %s", err)
	}
	sort.Sort(byFormatTime{files, l.format()})
	for _, f := range files {
		if f.IsDir() {
			continue
		}

		if !l.isLogFile(f) {
			continue
		}

		// the first file we find that matches our pattern will be the most
		// recently modified log file.
		if f.Size()+int64(writeLen) < l.max() {
			filename := filepath.Join(l.dir(), f.Name())
			file, err := os.OpenFile(filename, os.O_APPEND|os.O_WRONLY, 0644)
			if err == nil {
				return file, nil
			}
			// if we fail to open the old log file for some reason, just ignore
			// it and open a new log file.
		}
		break
	}
	return l.openNew()
}

// genFilename generates the name of the logfile from the current time.
func (l *Logger) genFilename() string {
	t := currentTime()
	if !l.LocalTime {
		t = t.UTC()
	}
	return filepath.Join(l.dir(), t.Format(l.format()))
}

// cleanup deletes old log files, keeping at most l.MaxBackups files, as long as
// none of them are older than MaxAge.
func (l *Logger) cleanup() error {
	if l.MaxBackups == 0 && l.MaxAge == 0 {
		return nil
	}

	files, err := l.oldLogFiles()
	if err != nil {
		return err
	}

	var deletes []os.FileInfo

	if l.MaxBackups > 0 {
		deletes = files[l.MaxBackups:]
		files = files[:l.MaxBackups]
	}
	if l.MaxAge > 0 {
		cutoff := currentTime().Add(-1 * l.MaxAge)

		for _, f := range files {
			if f.ModTime().Before(cutoff) {
				deletes = append(deletes, f)
			}
		}
	}

	if len(deletes) == 0 {
		return nil
	}

	go deleteAll(l.dir(), deletes)

	return nil
}

func deleteAll(dir string, files []os.FileInfo) {
	// remove files on a separate goroutine
	for _, f := range files {
		// what am I going to do, log this?
		_ = os.Remove(filepath.Join(dir, f.Name()))
	}
}

// oldLogFiles returns the list of backup log files stored in the same
// directory as the current log file, sorted by ModTime
func (l *Logger) oldLogFiles() ([]os.FileInfo, error) {
	files, err := ioutil.ReadDir(l.dir())
	if err != nil {
		return nil, fmt.Errorf("can't read log file directory: %s", err)
	}
	logFiles := []os.FileInfo{}

	for _, f := range files {
		if f.IsDir() {
			continue
		}
		if filepath.Base(f.Name()) == filepath.Base(l.file.Name()) {
			continue
		}

		if l.isLogFile(f) {
			logFiles = append(logFiles, f)
		}
	}

	sort.Sort(byFormatTime{logFiles, l.format()})

	return logFiles, nil
}

func (l *Logger) isLogFile(f os.FileInfo) bool {
	_, err := time.Parse(l.format(), filepath.Base(f.Name()))
	return err == nil
}

// Close implements io.Closer, and closes the current logfile.
func (l *Logger) Close() error {
	if l.file != nil {
		err := l.file.Close()
		l.file = nil
		return err
	}
	return nil
}

func (l *Logger) max() int64 {
	if l.MaxSize == 0 {
		return defaultMaxSize
	}
	return l.MaxSize
}

func (l *Logger) dir() string {
	if l.Dir != "" {
		return l.Dir
	}
	return os.TempDir()
}

func (l *Logger) format() string {
	if l.NameFormat != "" {
		return l.NameFormat
	}
	return defaultNameFormat
}

// byFormatTime sorts by newest time formatted in the name.
type byFormatTime struct {
	files  []os.FileInfo
	format string
}

func (b byFormatTime) Less(i, j int) bool {
	return b.time(i).After(b.time(j))
}

func (b byFormatTime) Swap(i, j int) {
	b.files[i], b.files[j] = b.files[j], b.files[i]
}

func (b byFormatTime) Len() int {
	return len(b.files)
}

func (b byFormatTime) time(i int) time.Time {
	t, err := time.Parse(b.format, filepath.Base(b.files[i].Name()))
	if err != nil {
		return time.Time{}
	}
	return t
}

// IsWriteTooLong returns whether the given error reports a write to Logger that
// exceeds the Logger's MaxSize.
func IsWriteTooLong(err error) bool {
	if err == nil {
		return false
	}
	_, ok := err.(writeTooLongError)
	return ok
}

type writeTooLongError struct {
	error
}