duration.go

package main

import (
	"errors"
	"time"
)

var errLeadingInt = errors.New("time: 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<<63-10)/10 {
			// overflow
			return 0, "", errLeadingInt
		}
		x = x*10 + int64(c) - '0'
	}
	return x, s[i:], nil
}

var unitMap = map[string]float64{
	"ns": float64(time.Nanosecond),
	"us": float64(time.Microsecond),
	"µs": float64(time.Microsecond), // U+00B5 = micro symbol
	"μs": float64(time.Microsecond), // U+03BC = Greek letter mu
	"ms": float64(time.Millisecond),
	"s":  float64(time.Second),
	"m":  float64(time.Minute),
	"h":  float64(time.Hour),
	"d":  float64(time.Hour * 24),
	"w":  float64(time.Hour * 24 * 7),
}

// 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".
func ParseDuration(s string) (time.Duration, error) {
	// [-+]?([0-9]*(\.[0-9]*)?[a-z]+)+
	orig := s
	f := float64(0)
	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 != "" {
		g := float64(0) // this element of the sequence

		var x int64
		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)
		x, s, err = leadingInt(s)
		if err != nil {
			return 0, errors.New("time: invalid duration " + orig)
		}
		g = float64(x)
		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)
			x, s, err = leadingInt(s)
			if err != nil {
				return 0, errors.New("time: invalid duration " + orig)
			}
			scale := 1
			for n := pl - len(s); n > 0; n-- {
				scale *= 10
			}
			g += float64(x) / float64(scale)
			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)
		}

		f += g * unit
	}

	if neg {
		f = -f
	}
	return time.Duration(f), nil
}