Better handling of the block chomping indicator

The Block chomping indicators affects how the final line break is
interpreted. So use "strip" when the value doesn't ends with a line
break, "clip" when the value ends with a line break, and "keep"
when the value ends with multiple line breaks.

Author: foriequal0

src/Microsoft.OpenApi/Writers/OpenApiYamlWriter.cs

@@ -182,12 +182,8 @@ public override void WriteValue(string value)
 
                 Writer.Write("|");
 
-                // Write chomping indicator when it ends with line break.
-                if (value.LastIndexOfAny(new []{'\n', '\r'}) == value.Length - 1)
-                {
-                    Writer.Write("+");
-                }
-                
+                WriteChompingIndicator(value);
+
                 // Write indentation indicator when it starts with spaces
                 if (value.StartsWith(" "))
                 {
@@ -222,6 +218,54 @@ public override void WriteValue(string value)
             }
         }
 
+        private void WriteChompingIndicator(string value)
+        {
+            var trailingNewlines = 0;
+            var end = value.Length - 1;
+            // We only need to know whether there are 0, 1, or more trailing newlines
+            while (end >= 0 && trailingNewlines < 2)
+            {
+                var found = value.LastIndexOfAny(new[] { '\n', '\r' }, end, 2);
+                if (found == -1 || found != end)
+                {
+                    // does not ends with newline
+                    break;
+                }
+
+                if (value[end] == '\r')
+                {
+                    // ends with \r
+                    end--;
+                }
+                else if (end > 0 && value[end - 1] == '\r')
+                {
+                    // ends with \r\n
+                    end -= 2;
+                }
+                else
+                {
+                    // ends with \n
+                    end -= 1;
+                }
+                trailingNewlines++;
+            }
+
+            switch (trailingNewlines)
+            {
+                case 0:
+                    // "strip" chomping indicator
+                    Writer.Write("-");
+                    break;
+                case 1:
+                    // "clip"
+                    break;
+                default:
+                    // "keep" chomping indicator
+                    Writer.Write("+");
+                    break;
+            }
+        }
+
         /// <summary>
         /// Write null value.
         /// </summary>

test/Microsoft.OpenApi.Tests/Writers/OpenApiWriterSpecialCharacterTests.cs

@@ -77,11 +77,13 @@ public void WriteStringWithSpecialCharactersAsYamlWorks(string input, string exp
         }
 
         [Theory]
-        [InlineData("multiline\r\nstring", "test: |\n  multiline\n  string")]
-        [InlineData("ends with\r\nline break\r\n", "test: |+\n  ends with\n  line break")]
-        [InlineData("  starts with\nspaces", "test: |2\n    starts with\n  spaces")]
-        [InlineData("  starts with\nspaces, and ends with line break\n", "test: |+2\n    starts with\n  spaces, and ends with line break")]
-        [InlineData("contains\n\n\nempty lines", "test: |\n  contains\n\n\n  empty lines")]
+        [InlineData("multiline\r\nstring", "test: |-\n  multiline\n  string")]
+        [InlineData("ends with\r\nline break\r\n", "test: |\n  ends with\n  line break")]
+        [InlineData("ends with\r\n2 line breaks\r\n\r\n", "test: |+\n  ends with\n  2 line breaks\n")]
+        [InlineData("ends with\r\n3 line breaks\r\n\r\n\r\n", "test: |+\n  ends with\n  3 line breaks\n\n")]
+        [InlineData("  starts with\nspaces", "test: |-2\n    starts with\n  spaces")]
+        [InlineData("  starts with\nspaces, and ends with line break\n", "test: |2\n    starts with\n  spaces, and ends with line break")]
+        [InlineData("contains\n\n\nempty lines", "test: |-\n  contains\n\n\n  empty lines")]
         [InlineData("no line breaks fallback ", "test: 'no line breaks fallback '")]
         public void WriteStringWithNewlineCharactersInObjectAsYamlWorks(string input, string expected)
         {
@@ -103,11 +105,13 @@ public void WriteStringWithNewlineCharactersInObjectAsYamlWorks(string input, st
         }
 
         [Theory]
-        [InlineData("multiline\r\nstring", "- |\n  multiline\n  string")]
-        [InlineData("ends with\r\nline break\r\n", "- |+\n  ends with\n  line break")]
-        [InlineData("  starts with\nspaces", "- |2\n    starts with\n  spaces")]
-        [InlineData("  starts with\nspaces, and ends with line break\n", "- |+2\n    starts with\n  spaces, and ends with line break")]
-        [InlineData("contains\n\n\nempty lines", "- |\n  contains\n\n\n  empty lines")]
+        [InlineData("multiline\r\nstring", "- |-\n  multiline\n  string")]
+        [InlineData("ends with\r\nline break\r\n", "- |\n  ends with\n  line break")]
+        [InlineData("ends with\r\n2 line breaks\r\n\r\n", "- |+\n  ends with\n  2 line breaks\n")]
+        [InlineData("ends with\r\n3 line breaks\r\n\r\n\r\n", "- |+\n  ends with\n  3 line breaks\n\n")]
+        [InlineData("  starts with\nspaces", "- |-2\n    starts with\n  spaces")]
+        [InlineData("  starts with\nspaces, and ends with line break\n", "- |2\n    starts with\n  spaces, and ends with line break")]
+        [InlineData("contains\n\n\nempty lines", "- |-\n  contains\n\n\n  empty lines")]
         [InlineData("no line breaks fallback ", "- 'no line breaks fallback '")]
         public void WriteStringWithNewlineCharactersInArrayAsYamlWorks(string input, string expected)
         {