You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: format.go
    path of file: ./repos/cointop/vendor/golang.org/x/text/internal/number
    
    the code of the file until where you have to start completion: // Copyright 2017 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.

package number

import (
	"strconv"
	"unicode/utf8"

	"golang.org/x/text/language"
)

// TODO:
// - grouping of fractions
// - allow user-defined superscript notation (such as <sup>4</sup>)
// - same for non-breaking spaces, like &nbsp;

// A VisibleDigits computes digits, comma placement and trailing zeros as they
// will be shown to the user.
type VisibleDigits interface {
	Digits(buf []by
    
func (f *Formatter) Render(dst []byte, d Digits) []byte {
	var result []byte
	var postPrefix, preSuffix int
	if d.IsScientific {
		result, postPrefix, preSuffix = appendScientific(dst, f, &d)
	} else {
		result, postPrefix, preSuffix = appendDecimal(dst, f, &d)
	}
	if f.PadRune == 0 {
		return result
	}
	width := int(f.FormatWidth)
	if count := utf8.RuneCount(result); count < width {
		insertPos := 0
		switch f.Flags & PadMask {
		case PadAfterPrefix:
			insertPos = postPrefix
		case PadBeforeSuffix:
			insertPos = preSuffix
		case PadAfterSuffix:
			insertPos = len(result)
		}
		num := width - count
		pad := [utf8.UTFMax]byte{' '}
		sz := 1
		if r := f.PadRune; r != 0 {
			sz = utf8.EncodeRune(pad[:], r)
		}
		extra := sz * num
		if n := len(result) + extra; n < cap(result) {
			result = result[:n]
			copy(result[insertPos+extra:], result[insertPos:])
		} else {
			buf := make([]byte, n)
			copy(buf, result[:insertPos])
			copy(buf[insertPos+extra:], result[insertPos:])
			result = buf
		}
		for ; num > 0; num-- {
			insertPos += copy(result[insertPos:], pad[:sz])
		}
	}
	return result
}