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// 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 (
// A FormatFunc formats a number.
type FormatFunc func(x interface{}, opts ...Option) Formatter
// NewFormat creates a FormatFunc based on another FormatFunc and new options.
// Use NewFormat to cash the creation of formatters.
func NewFormat(format FormatFunc, opts ...Option) FormatFunc {
o := *format(nil).options
n := len(o.options)
o.options = append(o.options[:n:n], opts...)
return func(x interface{}, opts ...Option) Formatter {
return newFormatter(&o, opts, x)
type options struct {
verbs string
initFunc initFunc
options []Option
pluralFunc func(t language.Tag, scale int) (f plural.Form, n int)
type optionFlag uint16
const (
hasScale optionFlag = 1 << iota
type initFunc func(f *number.Formatter, t language.Tag)
func newFormatter(o *options, opts []Option, value interface{}) Formatter {
if len(opts) > 0 {
n := *o
n.options = opts
o = &n
return Formatter{o, value}
func newOptions(verbs string, f initFunc) *options {
return &options{verbs: verbs, initFunc: f}
type Formatter struct {
value interface{}
// Format implements format.Formatter. It is for internal use only for now.
func (f Formatter) Format(state format.State, verb rune) {
// TODO: consider implementing fmt.Formatter instead and using the following
// piece of code. This allows numbers to be rendered mostly as expected
// when using fmt. But it may get weird with the spellout options and we
// may need more of format.State over time.
// lang := language.Und
// if s, ok := state.(format.State); ok {
// lang = s.Language()
// }
lang := state.Language()
if !strings.Contains(f.verbs, string(verb)) {
fmt.Fprintf(state, "%%!%s(%T=%v)", string(verb), f.value, f.value)
var p number.Formatter
f.initFunc(&p, lang)
for _, o := range f.options.options {
o(lang, &p)
if w, ok := state.Width(); ok {
p.FormatWidth = uint16(w)
if prec, ok := state.Precision(); ok {
switch verb {
case 'd':
case 'f':
case 'e':
p.SetPrecision(prec + 1)
case 'g':
var d number.Decimal
d.Convert(p.RoundingContext, f.value)
state.Write(p.Format(nil, &d))
// Digits returns information about which logical digits will be presented to
// the user. This information is relevant, for instance, to determine plural
// forms.
func (f Formatter) Digits(buf []byte, tag language.Tag, scale int) number.Digits {
var p number.Formatter
f.initFunc(&p, tag)
if scale >= 0 {
// TODO: this only works well for decimal numbers, which is generally
// fine.
var d number.Decimal
d.Convert(p.RoundingContext, f.value)
return number.FormatDigits(&d, p.RoundingContext)