package main

import (
	"bufio"
	"flag"
	"fmt"
	"io"
	"os"
	"time"
)

var (
	format                   = "2006-01-02 15:04" // Format that will be used for times.
	outFormat                = format + " MST"    // Output format.
	tz                       = "Local"            // String descriptor for timezone.
	fromLoc   *time.Location = time.Local         // Go time.Location for the named timezone.
	toLoc     *time.Location = time.UTC           // Go time.Location for output timezone.
)

func usage(w io.Writer) {
	fmt.Fprintf(w, `Usage: utc [-f format] [-h] [-o format] [-q] [-u] [-z zone] [time(s)...]

utc converts times to UTC. If no arguments are provided, prints the
current time in UTC. If the only argument provided is "-", utc reads
newline-separated timestamps from standard input. If the argument is
"help", it will print an extended help message with examples.  If both
the input and output timezones are the same (e.g., the local time zone
is UTC), a warning message will be printed on standard error. This can
be suppressed with the -q option.

Flags:

	-f format	Go timestamp format for input times. See the Go docs
			(e.g. https://golang.org/pkg/time/#pkg-constants)
			for an explanation of this format.

			Default value: %s

	-h		Print this help message.

	-o format	Go timestamp format for outputting times. It uses
			the same format as the '-f' argument.

			Default value: %s

	-q		Suppress the timezone check warning message.

	-u		Timestamps are in UTC format and should be converted
			to the timezone specified by the -z argument (which
			defaults to '%s'). Note that this isn't particularly
			useful with no arguments.

	-z zone		Text form of the time zone; this can be in short
			time zone abbreviation (e.g. MST) or a location
			(e.g. America/Los_Angeles). This has no effect when
			printing the current time.

			Default value: %s
`, format, outFormat, tz, tz)
}

func usageExamples() {
	usage(os.Stdout)
	fmt.Println(`
Examples (note that the examples are done in the America/Los_Angeles /
PST8PDT time zone):

	+ Getting the current time in UTC:
	  $ utc
	  2016-06-14 14:30 PDT = 2016-06-14 21:30 UTC
	+ Converting a local timestamp to UTC:
	  $ utc '2016-06-14 21:30'
	  2016-06-14 21:30 PDT = 2016-06-15 04:30 UTC
	+ Converting a local EST timestamp to UTC (on a machine set to
  	  PST8PDT):
	  $ utc -z EST '2016-06-14 21:30'  
	  2016-06-14 21:30 EST = 2016-06-15 02:30 UTC
	+ Converting timestamps in the form '14-06-2016 3:04PM':
	  $ utc -f '02-01-2006 3:04PM' '14-06-2016 9:30PM'
	  2016-06-14 21:30 PDT = 2016-06-15 04:30 UTC
	+ Converting timestamps from standard input:
	  $ printf "2016-06-14 14:42\n2016-06-13 11:01" | utc -
	  2016-06-14 14:42 PDT = 2016-06-14 21:42 UTC
	  2016-06-13 11:01 PDT = 2016-06-13 18:01 UTC
	+ Converting a UTC timestamp to the local time zone:
	  $ utc -u '2016-06-14 21:30'
	  2016-06-14 21:30 UTC = 2016-06-14 14:30 PDT
	+ Converting a UTC timestamp to EST (on a machine set to
	  PST8PDT):
	  $ utc -u -z EST '2016-06-14 21:30'
	  2016-06-14 21:30 UTC = 2016-06-14 16:30 EST
	+ Using a different output format:
	  $ utc -o '2006-01-02T15:03:04-0700' '2016-06-14 21:30' 
	  2016-06-14T21:09:30-0700 = 2016-06-15T04:04:30+0000
	+ Example of the warning message when running utc on a machine
	  where the local time zone is UTC:
	  $ utc
	   
	  ==================================================================
	  Note: both input and output timezone offsets are the same --- this
	  program may not do what you expect it to.
	   
	  (Converting from UTC (offset +0000) to UTC (offset +0000).)
	  ==================================================================
	  2016-06-14 23:44 = 2016-06-14 23:44
	+ Example of the warning message when running utc on a machine
	  where the local time zone is GMT:
	  $ utc
	   
	  ==================================================================
	  Note: both input and output timezone offsets are the same --- this
	  program may not do what you expect it to.
	   
	  (Converting from GMT (offset +0000) to UTC (offset +0000).)
	  ==================================================================
	  2016-06-14 23:46 = 2016-06-14 23:46
`)
}

func getZone(loc *time.Location) (string, int) {
	return time.Now().In(loc).Zone()
}

func checkZones(quiet bool) {
	if quiet {
		return
	}

	toZone, toOff := getZone(toLoc)
	fromZone, fromOff := getZone(fromLoc)

	if toOff == fromOff {
		fmt.Fprintf(os.Stderr, `
==================================================================
Note: both input and output timezone offsets are the same --- this
program may not do what you expect it to.

(Converting from %s (offset %+05d) to %s (offset %+05d).)
==================================================================
`, fromZone, fromOff, toZone, toOff)
	}
}

func init() {
	var help, quiet, utc bool

	flag.Usage = func() { usage(os.Stderr) }
	flag.StringVar(&format, "f", format, "time format")
	flag.BoolVar(&help, "h", false, "print usage information")
	flag.StringVar(&outFormat, "o", outFormat, "output time format")
	flag.BoolVar(&quiet, "q", false, "suppress zone check warning")
	flag.BoolVar(&utc, "u", false, "timestamps are in UTC format")
	flag.StringVar(&tz, "z", tz, "time zone to convert from; if blank, the local timezone is used")

	flag.Parse()

	if help {
		if !utc {
			checkZones(quiet)
		}
		usage(os.Stdout)
		os.Exit(0)
	}

	if utc {
		var err error
		toLoc, err = time.LoadLocation(tz)
		if err != nil {
			fmt.Fprintf(os.Stderr, "Malformed timezone %s: %s\n", tz, err)
			os.Exit(1)
		}

		fromLoc = time.UTC
	} else {
		var err error
		fromLoc, err = time.LoadLocation(tz)
		if err != nil {
			fmt.Fprintf(os.Stderr, "Malformed timezone %s: %s\n", tz, err)
			os.Exit(1)
		}

		if fromLoc == time.UTC {

		}

		toLoc = time.UTC
	}

	checkZones(quiet)
}

func showTime(t time.Time) {
	fmt.Printf("%s = %s\n", t.Format(outFormat),
		t.In(toLoc).Format(outFormat))
}

func dumpTimes(times []string) bool {
	var errored bool

	for _, t := range times {
		u, err := time.ParseInLocation(format, t, fromLoc)
		if err != nil {
			errored = true
			fmt.Fprintf(os.Stderr, "Malformed time %s: %s\n", t, err)
			continue
		}

		showTime(u)
	}

	return errored
}

func main() {
	var times []string
	n := flag.NArg()

	switch n {
	case 0:
		showTime(time.Now())
		os.Exit(0)
	case 1:
		if flag.Arg(0) == "-" {
			s := bufio.NewScanner(os.Stdin)

			for s.Scan() {
				times = append(times, s.Text())
			}
		} else if flag.Arg(0) == "help" {
			usageExamples()
		} else {
			times = flag.Args()
		}
	default:
		times = flag.Args()
	}

	if dumpTimes(times) {
		os.Exit(1)
	}
}