New: i18n pluralization and variables support and more...

fixes: #1649, #1648, #1641, #1650

relative to: #1597

i18n/internal/message.go

@@ -0,0 +1,81 @@
+package internal
+
+import (
+	"fmt"
+	"sort"
+)
+
+// Renderer is responsible to render a translation based
+// on the given "args".
+type Renderer interface {
+	Render(args ...interface{}) (string, error)
+}
+
+// Message is the default Renderer for translation messages.
+// Holds the variables and the plurals of this key.
+// Each Locale has its own list of messages.
+type Message struct {
+	Locale *Locale
+
+	Key   string
+	Value string
+
+	Plural  bool
+	Plurals []*PluralMessage // plural forms by order.
+
+	Vars []Var
+}
+
+// AddPlural adds a plural message to the Plurals list.
+func (m *Message) AddPlural(form PluralForm, r Renderer) {
+	msg := &PluralMessage{
+		Form:     form,
+		Renderer: r,
+	}
+
+	if len(m.Plurals) == 0 {
+		m.Plural = true
+		m.Plurals = append(m.Plurals, msg)
+		return
+	}
+
+	for i, p := range m.Plurals {
+		if p.Form.String() == form.String() {
+			// replace
+			m.Plurals[i] = msg
+			return
+		}
+	}
+
+	m.Plurals = append(m.Plurals, msg)
+	sort.SliceStable(m.Plurals, func(i, j int) bool {
+		return m.Plurals[i].Form.Less(m.Plurals[j].Form)
+	})
+}
+
+// Render completes the Renderer interface.
+// It accepts arguments, which can resolve the pluralization type of the message
+// and its variables. If the Message is wrapped by a Template then the
+// first argument should be a map. The map key resolves to the pluralization
+// of the message is the "PluralCount". And for variables the user
+// should set a message key which looks like: %VAR_NAME%Count, e.g. "DogsCount"
+// to set plural count for the "Dogs" variable, case-sensitive.
+func (m *Message) Render(args ...interface{}) (string, error) {
+	if m.Plural {
+		if len(args) > 0 {
+			if pluralCount, ok := findPluralCount(args[0]); ok {
+				for _, plural := range m.Plurals {
+					if plural.Form.MatchPlural(pluralCount) {
+						return plural.Renderer.Render(args...)
+					}
+				}
+
+				return "", fmt.Errorf("key: %q: no registered plurals for <%d>", m.Key, pluralCount)
+			}
+		}
+
+		return "", fmt.Errorf("key: %q: missing plural count argument", m.Key)
+	}
+
+	return m.Locale.Printer.Sprintf(m.Key, args...), nil
+}