i18n: Add i18next plural resolver class

src/invidious/helpers/i18next.cr

@@ -80,4 +80,133 @@ module I18next::Plurals
     "sk"  => PluralForms::Special_Czech_Slovak,
     "sl"  => PluralForms::Special_Slovenian,
   }
+
+  # The array indices matches the PluralForms enum above
+  private NUMBERS = [
+    [1, 2],                # 1
+    [1, 2],                # 2
+    [1],                   # 3
+    [1, 2, 5],             # 4
+    [0, 1, 2, 3, 11, 100], # 5
+    [1, 2, 5],             # 6
+    [1, 2, 5],             # 7
+    [1, 2, 3, 8],          # 8
+    [1, 2],                # 9 (not used)
+    [1, 2, 3, 7, 11],      # 10
+    [1, 2, 3, 20],         # 11
+    [1, 2],                # 12
+    [0, 1],                # 13
+    [1, 2, 3, 4],          # 14
+    [1, 2, 10],            # 15
+    [1, 2, 0],             # 16
+    [1, 2],                # 17
+    [0, 1, 2],             # 18
+    [1, 2, 11, 20],        # 19
+    [1, 2, 20],            # 20
+    [5, 1, 2, 3],          # 21
+    [1, 2, 20, 21],        # 22
+  ]
+
+  # "or" ()
+  private NUMBERS_OR = [2, 1]
+
+  # -----------------------------------
+  #  I18next plural resolver class
+  # -----------------------------------
+
+  class Resolver
+    @@forms : Hash(String, PluralForms) = init_rules()
+    @@version : UInt8 = 3
+
+    # Options
+    property simplify_plural_suffix : Bool = true
+
+    # Suffixes
+    SUFFIXES_V1 = {
+      "",
+      "_plural_1",
+      "_plural_2",
+      "_plural_3",
+      "_plural_11",
+      "_plural_100",
+    }
+    SUFFIXES_V2 = {"_0", "_1", "_2", "_3", "_11", "_100"}
+    SUFFIXES_V3 = {"_0", "_1", "_2", "_3", "_4", "_5"}
+
+    def initialize(version : UInt8 = 3)
+      # Sanity checks
+      # V4 isn't supported, as it requires a full CLDR database.
+      if version > 4 || version == 0
+        raise "Invalid i18next version: v#{version}."
+      elsif version == 4
+        # Logger.error("Unsupported i18next version: v4. Falling back to v3")
+        @@version = 3
+      else
+        @@version = version
+      end
+    end
+
+    def self.init_rules
+      # Look into sets
+      PLURAL_SETS.each do |form, langs|
+        langs.each { |lang| @@forms[lang] = form }
+      end
+
+      # Add plurals from the "singles" set
+      @@forms.merge!(PLURAL_SINGLES)
+    end
+
+    def get_plural_form(locale : String) : PluralForms
+      # Extract the ISO 639-1 or 639-2 code from an RFC 5646
+      # language code, except for pt-BR which needs to be kept as-is.
+      if locale.starts_with?("pt-BR")
+        locale = "pt-BR"
+      else
+        locale = locale.split('-')[0]
+      end
+
+      return @@forms[locale] if @@forms[locale]?
+
+      # If nothing was found, then use the most common form, i.e
+      # one singular and one plural, as in english. Not perfect,
+      # but better than yielding an exception at the user.
+      return PluralForms::Single_not_one
+    end
+
+    def get_suffix(locale : String, count : Int) : String
+      # Checked count must be absolute. In i18next, `rule.noAbs` is used to
+      # determine if comparison should be done on a signed or unsigned integer,
+      # but this variable is never set, resulting in the comparison always
+      # being done on absolute numbers.
+      return get_suffix_retrocompat(locale, count.abs)
+    end
+
+    def get_suffix_retrocompat(locale : String, count : Int) : String
+      # Get plural form
+      plural_form = get_plural_form(locale)
+      rule_numbers = (locale == "or") ? NUMBERS_OR : NUMBERS[plural_form.to_i]
+
+      # Languages with no plural have no suffix
+      return "" if plural_form.none?
+
+      # Get the index and suffix for this number
+      # idx = Todo
+      suffix = rule_numbers[idx]
+
+      # Simple plurals are handled differently in all versions (but v4)
+      if @simplify_plural_suffix && rule_numbers.size == 2 && rule_numbers[0] == 1
+        return "_plural" if (suffix == 2)
+        return "" if (suffix == 1)
+      end
+
+      # More complex plurals
+      # TODO: support `options.prepend` for v2 and v3
+      # this.options.prepend && suffix.toString() ? this.options.prepend + suffix.toString() : suffix.toString()
+      case @version
+      when 1 then return SUFFIXES_V1[idx]
+      when 2 then return SUFFIXES_V2[idx]
+      else        return SUFFIXES_V3[idx]
+      end
+    end
+  end
 end