// Package duration contains routines to parse standard units of time.
package duration

import (
	"errors"
	"time"
)

// Duration is a standard unit of time.
type Duration time.Duration

// String returns a string representing the duration in the form "3d1h3m".
// Leading zero units are omitted. As a special case, durations less than one
// second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure
// that the leading digit is non-zero. Duration more than a day or more than a
// week lose granularity and are truncated to resp. days-hours-minutes and
// weeks-days-hours. The zero duration formats as 0s.
func (d Duration) String() string {
	// Largest time is 2540400h10m10.000000000s
	var buf [32]byte
	w := len(buf)

	u := uint64(d)
	neg := d < 0
	if neg {
		u = -u
	}

	if u < uint64(Second) {
		// Special case: if duration is smaller than a second,
		// use smaller units, like 1.2ms
		var prec int
		w--
		buf[w] = 's'
		w--
		switch {
		case u == 0:
			return "0s"
		case u < uint64(Microsecond):
			// print nanoseconds
			prec = 0
			buf[w] = 'n'
		case u < uint64(Millisecond):
			// print microseconds
			prec = 3
			// U+00B5 'µ' micro sign == 0xC2 0xB5
			w-- // Need room for two bytes.
			copy(buf[w:], "µ")
		default:
			// print milliseconds
			prec = 6
			buf[w] = 'm'
		}
		w, u = fmtFrac(buf[:w], u, prec)
		w = fmtInt(buf[:w], u)

	} else if u > uint64(Week) {
		// Special case: if duration is larger than a week,
		// use bigger units like 4w3d2h
		w--
		buf[w] = 'h'

		u /= uint64(Hour)

		// u is now integer hours
		w = fmtInt(buf[:w], u%24)
		u /= 24

		// u is now integer days
		if u > 0 {
			w--
			buf[w] = 'd'
			w = fmtInt(buf[:w], u%7)
			u /= 7

			// u is now integer weeks
			// Stop at hours because days can be different lengths.
			if u > 0 {
				w--
				buf[w] = 'w'
				w = fmtInt(buf[:w], u)
			}
		}

	} else if u > uint64(Day) {
		// Special case: if duration is larger than a day,
		// use bigger units like 3d2h6m
		w--
		buf[w] = 'm'

		u /= uint64(Minute)

		// u is now integer minutes
		w = fmtInt(buf[:w], u%60)
		u /= 60

		// u is now integer hours
		if u > 0 {
			w--
			buf[w] = 'h'
			w = fmtInt(buf[:w], u%24)
			u /= 24

			// u is now integer weeks
			if u > 0 {
				w--
				buf[w] = 'd'
				w = fmtInt(buf[:w], u)
			}
		}

	} else {
		w--
		buf[w] = 's'

		w, u = fmtFrac(buf[:w], u, 9)

		// u is now integer seconds
		w = fmtInt(buf[:w], u%60)
		u /= 60

		// u is now integer minutes
		if u > 0 {
			w--
			buf[w] = 'm'
			w = fmtInt(buf[:w], u%60)
			u /= 60

			// u is now integer hours
			// Stop at hours because days can be different lengths.
			if u > 0 {
				w--
				buf[w] = 'h'
				w = fmtInt(buf[:w], u)
			}
		}
	}

	if neg {
		w--
		buf[w] = '-'
	}

	return string(buf[w:])
}

// fmtFrac formats the fraction of v/10**prec (e.g., ".12345") into the
// tail of buf, omitting trailing zeros.  it omits the decimal
// point too when the fraction is 0.  It returns the index where the
// output bytes begin and the value v/10**prec.
func fmtFrac(buf []byte, v uint64, prec int) (nw int, nv uint64) {
	// Omit trailing zeros up to and including decimal point.
	w := len(buf)
	print := false
	for i := 0; i < prec; i++ {
		digit := v % 10
		print = print || digit != 0
		if print {
			w--
			buf[w] = byte(digit) + '0'
		}
		v /= 10
	}
	if print {
		w--
		buf[w] = '.'
	}
	return w, v
}

// fmtInt formats v into the tail of buf.
// It returns the index where the output begins.
func fmtInt(buf []byte, v uint64) int {
	w := len(buf)
	if v == 0 {
		w--
		buf[w] = '0'
	} else {
		for v > 0 {
			w--
			buf[w] = byte(v%10) + '0'
			v /= 10
		}
	}
	return w
}

// Nanoseconds returns the duration as an integer nanosecond count.
func (d Duration) Nanoseconds() int64 { return int64(d) }

// Seconds returns the duration as a floating point number of seconds.
func (d Duration) Seconds() float64 {
	sec := d / Second
	nsec := d % Second
	return float64(sec) + float64(nsec)*1e-9
}

// Hours returns the duration as a floating point number of hours.
func (d Duration) Hours() float64 {
	hour := d / Hour
	nsec := d % Hour
	return float64(hour) + float64(nsec)*(1e-9/60/60)
}

// Days returns the duration as a floating point number of days.
func (d Duration) Days() float64 {
	hour := d / Hour
	nsec := d % Hour
	return float64(hour) + float64(nsec)*(1e-9/60/60/24)
}

// Weeks returns the duration as a floating point number of days.
func (d Duration) Weeks() float64 {
	hour := d / Hour
	nsec := d % Hour
	return float64(hour) + float64(nsec)*(1e-9/60/60/24/7)
}

// Minutes returns the duration as a floating point number of minutes.
func (d Duration) Minutes() float64 {
	min := d / Minute
	nsec := d % Minute
	return float64(min) + float64(nsec)*(1e-9/60)
}

// Standard unit of time.
var (
	Nanosecond  = Duration(time.Nanosecond)
	Microsecond = Duration(time.Microsecond)
	Millisecond = Duration(time.Millisecond)
	Second      = Duration(time.Second)
	Minute      = Duration(time.Minute)
	Hour        = Duration(time.Hour)
	Day         = Hour * 24
	Week        = Day * 7
	Fortnight   = Week * 2
	Month       = Day * 30    // Approximation
	Year        = Day * 365   // Approximation
	Decade      = Year * 10   // Approximation
	Century     = Year * 100  // Approximation
	Millennium  = Year * 1000 // Approximation
)

var errLeadingInt = errors.New("duration: bad [0-9]*") // never printed

// leadingInt consumes the leading [0-9]* from s.
func leadingInt(s string) (x int64, rem string, err error) {
	i := 0
	for ; i < len(s); i++ {
		c := s[i]
		if c < '0' || c > '9' {
			break
		}
		if x > (1<<63-1)/10 {
			// overflow
			return 0, "", errLeadingInt
		}
		x = x*10 + int64(c) - '0'
		if x < 0 {
			// overflow
			return 0, "", errLeadingInt
		}
	}
	return x, s[i:], nil
}

var unitMap = map[string]int64{
	"ns": int64(Nanosecond),
	"us": int64(Microsecond),
	"µs": int64(Microsecond), // U+00B5 = micro symbol
	"μs": int64(Microsecond), // U+03BC = Greek letter mu
	"ms": int64(Millisecond),
	"s":  int64(Second),
	"m":  int64(Minute),
	"h":  int64(Hour),
	"d":  int64(Day),
	"w":  int64(Week),
	"y":  int64(Year), // Approximation
}

// ParseDuration parses a duration string.
// A duration string is a possibly signed sequence of
// decimal numbers, each with optional fraction and a unit suffix,
// such as "300ms", "-1.5h" or "2h45m".
// Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h", "d", "w", "y".
func ParseDuration(s string) (Duration, error) {
	// [-+]?([0-9]*(\.[0-9]*)?[a-z]+)+
	orig := s
	var d int64
	neg := false

	// Consume [-+]?
	if s != "" {
		c := s[0]
		if c == '-' || c == '+' {
			neg = c == '-'
			s = s[1:]
		}
	}
	// Special case: if all that is left is "0", this is zero.
	if s == "0" {
		return 0, nil
	}
	if s == "" {
		return 0, errors.New("time: invalid duration " + orig)
	}
	for s != "" {
		var (
			v, f  int64       // integers before, after decimal point
			scale float64 = 1 // value = v + f/scale
		)

		var err error

		// The next character must be [0-9.]
		if !(s[0] == '.' || '0' <= s[0] && s[0] <= '9') {
			return 0, errors.New("time: invalid duration " + orig)
		}
		// Consume [0-9]*
		pl := len(s)
		v, s, err = leadingInt(s)
		if err != nil {
			return 0, errors.New("time: invalid duration " + orig)
		}
		pre := pl != len(s) // whether we consumed anything before a period

		// Consume (\.[0-9]*)?
		post := false
		if s != "" && s[0] == '.' {
			s = s[1:]
			pl := len(s)
			f, s, err = leadingInt(s)
			if err != nil {
				return 0, errors.New("time: invalid duration " + orig)
			}
			for n := pl - len(s); n > 0; n-- {
				scale *= 10
			}
			post = pl != len(s)
		}
		if !pre && !post {
			// no digits (e.g. ".s" or "-.s")
			return 0, errors.New("time: invalid duration " + orig)
		}

		// Consume unit.
		i := 0
		for ; i < len(s); i++ {
			c := s[i]
			if c == '.' || '0' <= c && c <= '9' {
				break
			}
		}
		if i == 0 {
			return 0, errors.New("time: missing unit in duration " + orig)
		}
		u := s[:i]
		s = s[i:]
		unit, ok := unitMap[u]
		if !ok {
			return 0, errors.New("time: unknown unit " + u + " in duration " + orig)
		}
		if v > (1<<63-1)/unit {
			// overflow
			return 0, errors.New("time: invalid duration " + orig)
		}
		v *= unit
		if f > 0 {
			// float64 is needed to be nanosecond accurate for fractions of hours.
			// v >= 0 && (f*unit/scale) <= 3.6e+12 (ns/h, h is the largest unit)
			v += int64(float64(f) * (float64(unit) / scale))
			if v < 0 {
				// overflow
				return 0, errors.New("time: invalid duration " + orig)
			}
		}
		d += v
		if d < 0 {
			// overflow
			return 0, errors.New("time: invalid duration " + orig)
		}
	}

	if neg {
		d = -d
	}
	return Duration(d), nil
}