// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Parse "zoneinfo" time zone file.
// This is a fairly standard file format used on OS X, Linux, BSD, Sun, and others.
// See tzfile(5), http://en.wikipedia.org/wiki/Zoneinfo,
// and ftp://munnari.oz.au/pub/oldtz/

package time

import (
	"io";
	"once";
	"os"
)

const (
	headerSize = 4+16+4*7;

	zoneDir = "/usr/share/zoneinfo/";
)

// Simple I/O interface to binary blob of data.
type data struct {
	p []byte;
	error bool;
}


func (d *data) read(n int) []byte {
	if len(d.p) < n {
		d.p = nil;
		d.error = true;
		return nil;
	}
	p := d.p[0:n];
	d.p = d.p[n:len(d.p)];
	return p
}

func (d *data) big4() (n uint32, ok bool) {
	p := d.read(4);
	if len(p) < 4 {
		d.error = true;
		return 0, false
	}
	return uint32(p[0]) << 24 | uint32(p[1]) << 16 | uint32(p[2]) << 8 | uint32(p[3]), true
}

func (d *data) byte() (n byte, ok bool) {
	p := d.read(1);
	if len(p) < 1 {
		d.error = true;
		return 0, false
	}
	return p[0], true
}


// Make a string by stopping at the first NUL
func byteString(p []byte) string {
	for i := 0; i < len(p); i++ {
		if p[i] == 0 {
			return string(p[0:i])
		}
	}
	return string(p)
}

// Parsed representation
type zone struct {
	utcoff int;
	isdst bool;
	name string;
}

type zonetime struct {
	time int32;		// transition time, in seconds since 1970 GMT
	zone *zone;		// the zone that goes into effect at that time
	isstd, isutc bool;	// ignored - no idea what these mean
}

func parseinfo(bytes []byte) (zt []zonetime, ok bool) {
	d := data{bytes, false};

	// 4-byte magic "TZif"
	if magic := d.read(4); string(magic) != "TZif" {
		return nil, false
	}

	// 1-byte version, then 15 bytes of padding
	var p []byte;
	if p = d.read(16); len(p) != 16 || p[0] != 0 && p[0] != '2' {
		return nil, false
	}
	vers := p[0];

	// six big-endian 32-bit integers:
	//	number of UTC/local indicators
	//	number of standard/wall indicators
	//	number of leap seconds
	//	number of transition times
	//	number of local time zones
	//	number of characters of time zone abbrev strings
	const (
		NUTCLocal = iota;
		NStdWall;
		NLeap;
		NTime;
		NZone;
		NChar
	)
	var n [6]int;
	for i := 0; i < 6; i++ {
		nn, ok := d.big4();
		if !ok {
			return nil, false
		}
		n[i] = int(nn);
	}

	// Transition times.
	txtimes := data{d.read(n[NTime]*4), false};

	// Time zone indices for transition times.
	txzones := d.read(n[NTime]);

	// Zone info structures
	zonedata := data{d.read(n[NZone]*6), false};

	// Time zone abbreviations.
	abbrev := d.read(n[NChar]);

	// Leap-second time pairs
	leapdata := data{d.read(n[NLeap]*8), false};

	// Whether tx times associated with local time types
	// are specified as standard time or wall time.
	isstd := d.read(n[NStdWall]);

	// Whether tx times associated with local time types
	// are specified as UTC or local time.
	isutc := d.read(n[NUTCLocal]);

	if d.error {	// ran out of data
		return nil, false
	}

	// If version == 2, the entire file repeats, this time using
	// 8-byte ints for txtimes and leap seconds.
	// We won't need those until 2106.

	// Now we can build up a useful data structure.
	// First the zone information.
	//	utcoff[4] isdst[1] nameindex[1]
	z := make([]zone, n[NZone]);
	for i := 0; i < len(z); i++ {
		var ok bool;
		var n uint32;
		if n, ok = zonedata.big4(); !ok {
			return nil, false
		}
		z[i].utcoff = int(n);
		var b byte;
		if b, ok = zonedata.byte(); !ok {
			return nil, false
		}
		z[i].isdst = b != 0;
		if b, ok = zonedata.byte(); !ok || int(b) >= len(abbrev) {
			return nil, false
		}
		z[i].name = byteString(abbrev[b:len(abbrev)])
	}

	// Now the transition time info.
	zt = make([]zonetime, n[NTime]);
	for i := 0; i < len(zt); i++ {
		var ok bool;
		var n uint32;
		if n, ok = txtimes.big4(); !ok {
			return nil, false
		}
		zt[i].time = int32(n);
		if int(txzones[i]) >= len(z) {
			return nil, false
		}
		zt[i].zone = &z[txzones[i]];
		if i < len(isstd) {
			zt[i].isstd = isstd[i] != 0
		}
		if i < len(isutc) {
			zt[i].isutc = isutc[i] != 0
		}
	}
	return zt, true
}

func readinfofile(name string) ([]zonetime, bool) {
	buf, err := io.ReadFile(name);
	if err != nil {
		return nil, false
	}
	return parseinfo(buf);
}

var zones []zonetime

func setupZone() {
	// consult $TZ to find the time zone to use.
	// no $TZ means use the system default /etc/localtime.
	// $TZ="" means use UTC.
	// $TZ="foo" means use /usr/share/zoneinfo/foo.

	tz, err := os.Getenverror("TZ");
	var ok bool;
	switch {
	case err == os.ENOENV:
		zones, ok = readinfofile("/etc/localtime");
	case len(tz) > 0:
		zones, ok = readinfofile(zoneDir + tz);
	case len(tz) == 0:
		// do nothing: use UTC
	}
}

func lookupTimezone(sec int64) (zone string, offset int) {
	once.Do(setupZone);
	if len(zones) == 0 {
		return "UTC", 0
	}

	// Binary search for entry with largest time <= sec
	tz := zones;
	for len(tz) > 1 {
		m := len(tz)/2;
		if sec < int64(tz[m].time) {
			tz = tz[0:m]
		} else {
			tz = tz[m:len(tz)]
		}
	}
	z := tz[0].zone;
	return z.name, z.utcoff
}