func (l *Log) header(s Severity, depth int) (*buffer, string, int) {
	// Lmmdd hh:mm:ss.uuuuuu threadid file:line]
	now := timeNow()
	_, file, line, ok := runtime.Caller(l.skip + depth)
	if !ok {
		file = "???"
		line = 1
	} else {
		slash := strings.LastIndex(file, "/")
		if slash >= 0 {
			file = file[slash+1:]
		}
	}
	if line < 0 {
		line = 0 // not a real line number, but acceptable to someDigits
	}
	if s > FatalLog {
		s = InfoLog // for safety.
	}
	buf := l.getBuffer()

	// Avoid Fprintf, for speed. The format is so simple that we can do it quickly by hand.
	// It's worth about 3X. Fprintf is hard.
	_, month, day := now.Date()
	hour, minute, second := now.Clock()
	buf.tmp[0] = severityChar[s]
	buf.twoDigits(1, int(month))
	buf.twoDigits(3, day)
	buf.tmp[5] = ' '
	buf.twoDigits(6, hour)
	buf.tmp[8] = ':'
	buf.twoDigits(9, minute)
	buf.tmp[11] = ':'
	buf.twoDigits(12, second)
	buf.tmp[14] = '.'
	buf.nDigits(6, 15, now.Nanosecond()/1000, '0')
	buf.tmp[21] = ' '
	buf.nDigits(7, 22, pid, ' ') // TODO: should be TID
	buf.tmp[29] = ' '
	buf.Write(buf.tmp[:30])
	buf.WriteString(file)
	buf.tmp[0] = ':'
	n := buf.someDigits(1, line)
	buf.tmp[n+1] = ']'
	buf.tmp[n+2] = ' '
	buf.Write(buf.tmp[:n+3])
	return buf, file, line
}
func (l *Log) header(s Severity, depth int) (*buffer, string, int) {
	// Lmmdd hh:mm:ss.uuuuuu threadid file:line]
	now := timeNow()
	_, file, line, ok := runtime.Caller(l.skip + depth)
	if !ok {
		file = "???"
		line = 1
	} else {
		slash := strings.LastIndex(file, "/")
		if slash >= 0 {
			file = file[slash+1:]
		}
	}
	if line < 0 {
		line = 0 // not a real line number, but acceptable to someDigits
	}
	if s > FatalLog {
		s = InfoLog // for safety.
	}
	buf := l.getBuffer()

	// Avoid Fprintf, for speed. The format is so simple that we can do it quickly by hand.
	// It's worth about 3X. Fprintf is hard.
	_, month, day := now.Date()
	hour, minute, second := now.Clock()
	buf.tmp[0] = severityChar[s]
	buf.twoDigits(1, int(month))
	buf.twoDigits(3, day)
	buf.tmp[5] = ' '
	buf.twoDigits(6, hour)
	buf.tmp[8] = ':'
	buf.twoDigits(9, minute)
	buf.tmp[11] = ':'
	buf.twoDigits(12, second)
	buf.tmp[14] = '.'
	buf.nDigits(6, 15, now.Nanosecond()/1000, '0')
	buf.tmp[21] = ' '
	buf.nDigits(7, 22, pid, ' ') // TODO: should be TID
	buf.tmp[29] = ' '
	buf.Write(buf.tmp[:30])
	buf.WriteString(file)
	buf.tmp[0] = ':'
	n := buf.someDigits(1, line)
	buf.tmp[n+1] = ']'
	buf.tmp[n+2] = ' '
	buf.Write(buf.tmp[:n+3])
	return buf, file, line
}
/*
header formats a log header as defined by the C++ implementation.
It returns a buffer containing the formatted header.
The depth specifies how many stack frames above lives the source line to be identified in the log message.

Log lines have this form:
	Lmmdd hh:mm:ss.uuuuuu threadid file:line] msg...
where the fields are defined as follows:
	L                A single character, representing the log level (eg 'I' for INFO)
	mm               The month (zero padded; ie May is '05')
	dd               The day (zero padded)
	hh:mm:ss.uuuuuu  Time in hours, minutes and fractional seconds
	threadid         The space-padded thread ID as returned by GetTID()
	file             The file name
	line             The line number
	msg              The user-supplied message
*/