NumberToWordsExtension.ToWords add ability to convert billions

src/Humanizer.Tests/NumberToWordsTests.cs

@@ -1,26 +1,41 @@
 using Xunit;
+using Xunit.Extensions;
 
 namespace Humanizer.Tests
 {
     public class NumberToWordsTests
     {
-        [Fact]
-        public void ToWords()
+        [InlineData(1, "one")]
+        [InlineData(10, "ten")]
+        [InlineData(11, "eleven")]
+        [InlineData(122, "one hundred and twenty-two")]
+        [InlineData(3501, "three thousand five hundred and one")]
+        [InlineData(100, "one hundred")]
+        [InlineData(1000, "one thousand")]
+        [InlineData(100000, "one hundred thousand")]
+        [InlineData(1000000, "one million")]
+        [InlineData(10000000, "ten million")]
+        [InlineData(100000000, "one hundred million")]
+        [InlineData(1000000000, "one billion")]
+        [InlineData(111, "one hundred and eleven")]
+        [InlineData(1111, "one thousand one hundred and eleven")]
+        [InlineData(111111, "one hundred and eleven thousand one hundred and eleven")]
+        [InlineData(1111111, "one million one hundred and eleven thousand one hundred and eleven")]
+        [InlineData(11111111, "eleven million one hundred and eleven thousand one hundred and eleven")]
+        [InlineData(111111111, "one hundred and eleven million one hundred and eleven thousand one hundred and eleven")]
+        [InlineData(1111111111, "one billion one hundred and eleven million one hundred and eleven thousand one hundred and eleven")]
+        [InlineData(123, "one hundred and twenty-three")]
+        [InlineData(1234, "one thousand two hundred and thirty-four")]
+        [InlineData(12345, "twelve thousand three hundred and forty-five")]
+        [InlineData(123456, "one hundred and twenty-three thousand four hundred and fifty-six")]
+        [InlineData(1234567, "one million two hundred and thirty-four thousand five hundred and sixty-seven")]
+        [InlineData(12345678, "twelve million three hundred and forty-five thousand six hundred and seventy-eight")]
+        [InlineData(123456789, "one hundred and twenty-three million four hundred and fifty-six thousand seven hundred and eighty-nine")]
+        [InlineData(1234567890, "one billion two hundred and thirty-four million five hundred and sixty-seven thousand eight hundred and ninety")]
+        [Theory]
+        public void Test(int number, string expected)
         {
-            Assert.Equal("one", 1.ToWords());
-            Assert.Equal("ten", 10.ToWords());
-            Assert.Equal("eleven", 11.ToWords());
-            Assert.Equal("one hundred and twenty-two", 122.ToWords());
-            Assert.Equal("three thousand five hundred and one", 3501.ToWords());
-        }
-
-        [Fact]
-        public void RoundNumbersHaveNoSpaceAtTheEnd()
-        {
-            Assert.Equal("one hundred", 100.ToWords());
-            Assert.Equal("one thousand", 1000.ToWords());
-            Assert.Equal("one hundred thousand", 100000.ToWords());
-            Assert.Equal("one million", 1000000.ToWords());
+            Assert.Equal(expected, number.ToWords());
         }
     }
 }

src/Humanizer/NumberToWordsExtension.cs

@@ -35,6 +35,12 @@ private static string ToEnglishWords(int number)
 
             var parts = new List<string>();
 
+            if ((number / 1000000000) > 0)
+            {
+                parts.Add(string.Format("{0} billion", ToWords(number / 1000000000)));
+                number %= 1000000000;
+            }
+
             if ((number / 1000000) > 0)
             {
                 parts.Add(string.Format("{0} million", ToWords(number / 1000000)));