Fix GetGeneralCode handling of zh-CN and zh-TW (#1476)

Author: imnasnainaec

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 ### Fixed
+- [SIL.WritingSystems] Fix IetfLanguageTag.GetGeneralCode to handle cases when zh-CN or zh-TW is a prefix and not the whole string.
 - [SIL.Windows.Forms] Prevent BetterLabel from responding to OnTextChanged when it has been disposed.
 
 ### Changed

SIL.WritingSystems.Tests/IetfLanguageTagTests.cs

@@ -689,6 +689,7 @@ public void TryGetSubtags_ComplexPrivateLanguageCode_ReturnsValidResults()
 		[TestCase("qaa", ExpectedResult = "qaa")]
 		[TestCase("qed", ExpectedResult = "qed")]
 		[TestCase("qed-Lepc-x-rubbish", ExpectedResult = "qed")]
+		[TestCase("zh-CN-fonipa", ExpectedResult = "zh")]
 		public string GetLanguagePart_ValidIetfTag_ReturnsLanguage(string tag)
 		{
 			return IetfLanguageTag.GetLanguagePart(tag);
@@ -1109,5 +1110,20 @@ public void GetBestLanguageName_ForLotsOfVariants_FindsExpectedName(string codeV
 			Assert.That(name, Is.EqualTo(expectedResult));
 		}
 		#endregion
+
+		[TestCase("tpi-AR", ExpectedResult = "tpi")]
+		[TestCase("tpi-Lepc-BR-fonipa-x-blah", ExpectedResult = "tpi")]
+		[TestCase("qaa", ExpectedResult = "qaa")]
+		[TestCase("qed-Lepc-x-rubbish", ExpectedResult = "qed")]
+		[TestCase("zh-BN", ExpectedResult = "zh")]
+		[TestCase("zh-CN-fonipa", ExpectedResult = "zh-CN")]
+		[TestCase("zh-Hans", ExpectedResult = "zh")]
+		[TestCase("zh-TW", ExpectedResult = "zh-TW")]
+		[TestCase("a", ExpectedResult = "a")]
+		[TestCase("-", ExpectedResult = "")]
+		public string GetGeneralCode_ReturnsCode(string tag)
+		{
+			return IetfLanguageTag.GetGeneralCode(tag);
+		}
 	}
 }

SIL.WritingSystems/IetfLanguageTag.cs

@@ -1116,6 +1116,9 @@ public static IEnumerable<VariantSubtag> GetVariantSubtags(string langTag)
 		/// <summary>
 		/// Gets the language part of the specified language tag.
 		/// </summary>
+		/// <remarks>
+		/// Returns `zh` for both `zh-CN` and `zh-TW`.
+		/// </remarks>
 		[PublicAPI]
 		public static string GetLanguagePart(string langTag)
 		{
@@ -1523,10 +1526,12 @@ public static string GetGeneralCode(string code)
 			// Though you might be tempted to simplify this by using GetLanguagePart, don't: this
 			// methods works with three-letter codes even if there is a valid 2-letter code that
 			// should be used instead.
+			if (code.StartsWith(ChineseSimplifiedTag))
+				return ChineseSimplifiedTag;
+			if (code.StartsWith(ChineseTraditionalTag))
+				return ChineseTraditionalTag;
 			var idxCountry = code.IndexOf("-");
-			if (idxCountry == -1 || code == ChineseSimplifiedTag || code == ChineseTraditionalTag)
-				return code;
-			return code.Substring(0, idxCountry);
+			return idxCountry == -1 ? code : code.Substring(0, idxCountry);
 		}
 
 		/// <summary>