Closes #369

PluralRule for DualFromZeroToTwo: Does not cover value of 2

The `Dictionary<string, PluralRuleDelegate> IsoLangToDelegate` is backed by a default dictionary. It can be restored with `PluralRules.RestoreDefault()` if one of the delegates was changed. Both has global effect.

Extract class `CustomPluralRuleProvider` to its own file

`PluralRuleDelegate DualFromZeroToTwo`:
* with 3 words, the index is for counts of 0, > 0 and < 2, more than 2
* with 4 words, the index is for counts of 0, > 0 and < 2,  >= 2 and < 3, more than 3

Add unit tests

src/SmartFormat.Tests/Extensions/PluralLocalizationFormatterTests.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
 using NUnit.Framework;
@@ -23,12 +24,12 @@ private static SmartFormatter GetFormatter(SmartSettings? smartSettings = null)
 
     private static void TestAllResults(CultureInfo cultureInfo, string format, ExpectedResults expectedValuesAndResults)
     {
-        foreach (var test in expectedValuesAndResults)
+        foreach (var testResult in expectedValuesAndResults)
         {
             var smart = GetFormatter();
-            var value = test.Key;
-            var expected = test.Value;
-            var actual = smart.Format(cultureInfo, format, value);
+            var count = testResult.Key;
+            var expected = testResult.Value;
+            var actual = smart.Format(cultureInfo, format, count);
 
             Assert.That(actual, Is.EqualTo(expected));
             Debug.WriteLine(actual);
@@ -153,6 +154,45 @@ public void Test_English_Unsigned()
         }
     }
 
+    [TestCase(0, "{0} personne")] // 0 is singular
+    [TestCase(1, "{0} personne")] // 1 is singular
+    [TestCase(2, "{0} personnes")] // 2 or more is plural
+    [TestCase(50, "{0} personnes")]
+    public void Test_French_2words(int count, string expected)
+    {
+        var smart = GetFormatter();
+        var ci = CultureInfo.GetCultureInfo("fr");
+        var actual = smart.Format(ci, "{0:plural:{0} personne|{0} personnes}", count);
+
+        Assert.That(actual, Is.EqualTo(string.Format(ci, expected, count)));
+    }
+
+    [TestCase(0, "pas de personne")] // 0 is singular
+    [TestCase(1, "une personne")] // 1 is singular
+    [TestCase(2, "{0} personnes")] // 2 or more is plural
+    [TestCase(50, "{0} personnes")]
+    public void Test_French_3words(int count, string expected)
+    {
+        var smart = GetFormatter();
+        var ci = CultureInfo.GetCultureInfo("fr");
+        var actual = smart.Format(ci, "{0:plural:pas de personne|une personne|{0} personnes}", count);
+
+        Assert.That(actual, Is.EqualTo(string.Format(ci, expected, count)));
+    }
+
+    [TestCase(0, "pas de personne")] // 0 is singular
+    [TestCase(1, "une personne")] // 1 is singular
+    [TestCase(2, "deux personnes")] // 2 is plural
+    [TestCase(50, "{0} personnes")] // more than 2
+    public void Test_French_4words(int count, string expected)
+    {
+        var smart = GetFormatter();
+        var ci = CultureInfo.GetCultureInfo("fr");
+        var actual = smart.Format(ci, "{0:plural:pas de personne|une personne|deux personnes|{0} personnes}", count);
+
+        Assert.That(actual, Is.EqualTo(string.Format(ci, expected, count)));
+    }
+
     [Test]
     public void Test_Turkish()
     {
@@ -273,7 +313,7 @@ public void NamedFormatter_should_use_specific_language(string format, object ar
     [TestCase("{0:plural:zero|one|many}", new string[0], "zero")]
     [TestCase("{0:plural:zero|one|many}", new[] { "alice" }, "one")]
     [TestCase("{0:plural:zero|one|many}", new[] { "alice", "bob" }, "many")]
-    public void Test_should_allow_ienumerable_parameter(string format, object arg0, string expectedResult)
+    public void Should_Allow_IEnumerable_Parameter(string format, object arg0, string expectedResult)
     {
         var smart = GetFormatter();
         var culture = new CultureInfo("en-US");
@@ -282,23 +322,54 @@ public void Test_should_allow_ienumerable_parameter(string format, object arg0,
     }
 
     [Test]
-    public void Test_With_CustomPluralRuleProvider()
+    public void Use_CustomPluralRuleProvider()
     {
         var smart = GetFormatter();
-        var actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("de")), "{0:plural:Frau|Frauen}", new string[2], "more");
-        Assert.That(actualResult, Is.EqualTo("Frauen"));
 
-        actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("en")), "{0:plural:person|people}", new string[2], "more");
-        Assert.That(actualResult, Is.EqualTo("people"));
+        Assert.Multiple(() =>
+        {
+            // ** German **
+            var actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("de")),
+                "{0:plural:Frau|Frauen}", new string[2], "more");
+            Assert.That(actualResult, Is.EqualTo("Frauen"));
+
+            actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("de")),
+                "{0:plural:Frau|Frauen|einige Frauen|viele Frauen}", new string[4], "more");
+            Assert.That(actualResult, Is.EqualTo("viele Frauen"));
 
-        actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("en")), "{0:plural:person|people}", new string[1], "one");
-        Assert.That(actualResult, Is.EqualTo("person"));
+            // ** English **
 
-        actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("fr")), "{0:plural:une personne|deux personnes|plusieurs personnes}", new string[3], "several");
-        Assert.That(actualResult, Is.EqualTo("plusieurs personnes"));
+            actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("en")),
+                "{0:plural:person|people}", new string[2], "more");
+            Assert.That(actualResult, Is.EqualTo("people"));
 
-        actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("fr")), "{0:plural:une personne|deux personnes|plusieurs personnes|beaucoup de personnes}", new string[3], "several");
-        Assert.That(actualResult, Is.EqualTo("beaucoup de personnes"));
+            actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("en")),
+                "{0:plural:person|people}", new string[1], "one");
+            Assert.That(actualResult, Is.EqualTo("person"));
+
+            actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("fr")),
+                "{0:plural:pas de personne|une personne|plusieurs personnes}", Array.Empty<string>(), "none");
+            Assert.That(actualResult, Is.EqualTo("pas de personne"));
+
+            // ** French **
+
+            actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("fr")),
+                "{0:plural:pas de personne|une personne|plusieurs personnes}", new string[1], "one");
+            Assert.That(actualResult, Is.EqualTo("une personne"));
+
+            actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("fr")),
+                "{0:plural:pas de personne|une personne|deux personnes}", new string[2], "two");
+            Assert.That(actualResult, Is.EqualTo("deux personnes"));
+
+            actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("fr")),
+                "{0:plural:pas de personne|une personne|deux personnes|plusieurs personnes}", new string[3], "several");
+            Assert.That(actualResult, Is.EqualTo("plusieurs personnes"));
+
+            actualResult = smart.Format(new CustomPluralRuleProvider(PluralRules.GetPluralRule("fr")),
+                "{0:plural:une personne|deux personnes|plusieurs personnes|beaucoup de personnes}", new string[3],
+                "many");
+            Assert.That(actualResult, Is.EqualTo("beaucoup de personnes"));
+        });
     }
 
     [TestCase("{0:plural:one|many} {1:plural:one|many} {2:plural:one|many}", "many one many")]
@@ -318,7 +389,10 @@ public void Should_Process_Signed_And_Unsigned_Numbers()
     {
         var smart = GetFormatter();
         foreach (var number in new object[]
-                     { (long)123, (ulong)123, (short)123, (ushort)123, (int)123, (uint)123 })
+                 {
+                     (long)123, (ulong)123, (short)123, (ushort)123, (int)123, (uint)123,
+                     (long)-123, (short) -123, (int) -123
+                 })
         {
             Assert.That(smart.Format("{0:plural(en):zero|one|many}", number), Is.EqualTo("many"));
         }
@@ -363,4 +437,48 @@ public void Pluralization_With_Changed_SplitChar(int numOfPeople, string format,
         var result = smart.Format(format, data);
         Assert.That(result, Is.EqualTo(expected));
     }
+
+    [TestCase(0, "nobody", "pas de personne")] // 0 is singular
+    [TestCase(1, "{0} person", "{0} personne")] // 1 is singular
+    [TestCase(2, "{0} people", "{0} personnes")] // 2 or more is plural
+    [TestCase(5, "a couple of people", "quelques personnes")]
+    [TestCase(15, "many people", "beaucoup de gens")]
+    [TestCase(50, "a lot of people", "une foule de personnes")]
+    public void Pluralization_With_Changed_Default_Rule_Delegate(int count, string rawExpectedEnglish, string rawExpectedFrench)
+    {
+        // Note: This test changes a default rule delegate *globally*.
+        //       It is not recommended, but possible.
+        PluralRules.IsoLangToDelegate["en"] = (value, wordsCount) =>
+        {
+            if (wordsCount != 6) return -1;
+
+            return Math.Abs(value) switch
+            {
+                <= 0 => 0,
+                > 0 and < 2 => 1,
+                >= 2 and < 3 => 2,
+                > 2 and < 10 => 3,
+                >= 10 and < 20 => 4,
+                >= 20 => 5
+            };
+        };
+        // Use the same rule delegate for English and French:
+        PluralRules.IsoLangToDelegate["fr"] = PluralRules.IsoLangToDelegate["en"];
+
+        var smart = GetFormatter();
+        var ciEnglish = CultureInfo.GetCultureInfo("en");
+        var ciFrench = CultureInfo.GetCultureInfo("fr");
+
+        var actualEnglish = smart.Format(ciEnglish, "{0:plural:nobody|{} person|{} people|a couple of people|many people|a lot of people}", count);
+        var actualFrench = smart.Format(ciFrench, "{0:plural:pas de personne|{} personne|{} personnes|quelques personnes|beaucoup de gens|une foule de personnes}", count);
+
+        // Restore default rule delegates:
+        PluralRules.RestoreDefault();
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(actualEnglish, Is.EqualTo(string.Format(ciEnglish, rawExpectedEnglish, count)));
+            Assert.That(actualFrench, Is.EqualTo(string.Format(ciFrench, rawExpectedFrench, count)));
+        });
+    }
 }

src/SmartFormat/Extensions/CustomPluralRuleProvider.cs

@@ -0,0 +1,44 @@
+//
+// Copyright SmartFormat Project maintainers and contributors.
+// Licensed under the MIT license.
+//
+
+using System;
+using SmartFormat.Utilities;
+
+namespace SmartFormat.Extensions;
+/// <summary>
+/// Use this class to provide custom plural rules to Smart.Format
+/// </summary>
+public class CustomPluralRuleProvider : IFormatProvider
+{
+    private readonly PluralRules.PluralRuleDelegate _pluralRule;
+
+    /// <summary>
+    /// Creates a new instance of a <see cref="CustomPluralRuleProvider"/>.
+    /// </summary>
+    /// <param name="pluralRule">The delegate for plural rules.</param>
+    public CustomPluralRuleProvider(PluralRules.PluralRuleDelegate pluralRule)
+    {
+        _pluralRule = pluralRule;
+    }
+
+    /// <summary>
+    /// Gets the format <see cref="object"/> for a <see cref="CustomPluralRuleProvider"/>.
+    /// </summary>
+    /// <param name="formatType"></param>
+    /// <returns>The format <see cref="object"/> for a <see cref="CustomPluralRuleProvider"/> or <see langword="null"/>.</returns>
+    public object? GetFormat(Type? formatType)
+    {
+        return formatType == typeof(CustomPluralRuleProvider) ? this : default;
+    }
+
+    /// <summary>
+    /// Gets the <see cref="PluralRules.PluralRuleDelegate"/> of the current <see cref="CustomPluralRuleProvider"/> instance.
+    /// </summary>
+    /// <returns></returns>
+    public PluralRules.PluralRuleDelegate GetPluralRule()
+    {
+        return _pluralRule;
+    }
+}

src/SmartFormat/Extensions/PluralLocalizationFormatter.cs

@@ -222,38 +222,3 @@ private static CultureInfo GetCultureInfo(IFormattingInfo formattingInfo)
     }
 }
 
-/// <summary>
-/// Use this class to provide custom plural rules to Smart.Format
-/// </summary>
-public class CustomPluralRuleProvider : IFormatProvider
-{
-    private readonly PluralRules.PluralRuleDelegate _pluralRule;
-
-    /// <summary>
-    /// Creates a new instance of a <see cref="CustomPluralRuleProvider"/>.
-    /// </summary>
-    /// <param name="pluralRule">The delegate for plural rules.</param>
-    public CustomPluralRuleProvider(PluralRules.PluralRuleDelegate pluralRule)
-    {
-        _pluralRule = pluralRule;
-    }
-
-    /// <summary>
-    /// Gets the format <see cref="object"/> for a <see cref="CustomPluralRuleProvider"/>.
-    /// </summary>
-    /// <param name="formatType"></param>
-    /// <returns>The format <see cref="object"/> for a <see cref="CustomPluralRuleProvider"/> or <see langword="null"/>.</returns>
-    public object? GetFormat(Type? formatType)
-    {
-        return formatType == typeof(CustomPluralRuleProvider) ? this : default;
-    }
-
-    /// <summary>
-    /// Gets the <see cref="PluralRules.PluralRuleDelegate"/> of the current <see cref="CustomPluralRuleProvider"/> instance.
-    /// </summary>
-    /// <returns></returns>
-    public PluralRules.PluralRuleDelegate GetPluralRule()
-    {
-        return _pluralRule;
-    }
-}