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// 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.

// +build darwin freebsd linux netbsd openbsd

// 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 (
        "errors"
        "syscall"
)

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

// 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:]
        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)
}

var badData = errors.New("malformed time zone information")

func loadZoneData(bytes []byte) (l *Location, err error) {
        d := data{bytes, false}

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

        // 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, badData
        }

        // 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, badData
                }
                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
        d.read(n[NLeap] * 8)

        // 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, badData
        }

        // 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]
        zone := make([]zone, n[NZone])
        for i := range zone {
                var ok bool
                var n uint32
                if n, ok = zonedata.big4(); !ok {
                        return nil, badData
                }
                zone[i].offset = int(n)
                var b byte
                if b, ok = zonedata.byte(); !ok {
                        return nil, badData
                }
                zone[i].isDST = b != 0
                if b, ok = zonedata.byte(); !ok || int(b) >= len(abbrev) {
                        return nil, badData
                }
                zone[i].name = byteString(abbrev[b:])
        }

        // Now the transition time info.
        tx := make([]zoneTrans, n[NTime])
        for i := range tx {
                var ok bool
                var n uint32
                if n, ok = txtimes.big4(); !ok {
                        return nil, badData
                }
                tx[i].when = int64(int32(n))
                if int(txzones[i]) >= len(zone) {
                        return nil, badData
                }
                tx[i].index = txzones[i]
                if i < len(isstd) {
                        tx[i].isstd = isstd[i] != 0
                }
                if i < len(isutc) {
                        tx[i].isutc = isutc[i] != 0
                }
        }

        // Commited to succeed.
        l = &Location{zone: zone, tx: tx}

        // Fill in the cache with information about right now,
        // since that will be the most common lookup.
        sec, _ := now()
        for i := range tx {
                if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
                        l.cacheStart = tx[i].when
                        l.cacheEnd = 1<<63 - 1
                        if i+1 < len(tx) {
                                l.cacheEnd = tx[i+1].when
                        }
                        l.cacheZone = &l.zone[tx[i].index]
                }
        }

        return l, nil
}

func loadZoneFile(name string) (l *Location, err error) {
        buf, err := readFile(name)
        if err != nil {
                return
        }
        return loadZoneData(buf)
}

func initTestingZone() {
        syscall.Setenv("TZ", "America/Los_Angeles")
        initLocal()
}

// Many systems use /usr/share/zoneinfo, Solaris 2 has
// /usr/share/lib/zoneinfo, IRIX 6 has /usr/lib/locale/TZ.
var zoneDirs = []string{
        "/usr/share/zoneinfo/",
        "/usr/share/lib/zoneinfo/",
        "/usr/lib/locale/TZ/",
}

func initLocal() {
        // 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, ok := syscall.Getenv("TZ")
        switch {
        case !ok:
                z, err := loadZoneFile("/etc/localtime")
                if err == nil {
                        localLoc = *z
                        localLoc.name = "Local"
                        return
                }
        case tz != "" && tz != "UTC":
                if z, err := loadLocation(tz); err == nil {
                        localLoc = *z
                        return
                }
        }

        // Fall back to UTC.
        localLoc.name = "UTC"
}

func loadLocation(name string) (*Location, error) {
        for _, zoneDir := range zoneDirs {
                if z, err := loadZoneFile(zoneDir + name); err == nil {
                        z.name = name
                        return z, nil
                }
        }
        return nil, errors.New("unknown time zone " + name)
}