Keep the first body line's indentation when rewriting a document [patch]

Frontmatter.Test/BodyIndentationTests.cs

@@ -0,0 +1,63 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Tests that rewriting a document keeps the indentation of the body's first content line, so a body that
+/// opens with an indented code block is not turned into a prose paragraph.
+/// </summary>
+[TestClass]
+public class BodyIndentationTests
+{
+	private static readonly string Nl = Environment.NewLine;
+
+	private static readonly string IndentedBody = $"    int x = 1;{Nl}    int y = 2;{Nl}{Nl}Text";
+
+	private static string Document() => $"---{Nl}title: T{Nl}---{Nl}{IndentedBody}{Nl}";
+
+	[TestMethod]
+	public void ExtractBody_IndentedCodeBlockFirst_KeepsIndentation() =>
+		Assert.AreEqual(IndentedBody, Frontmatter.ExtractBody(Document()));
+
+	[TestMethod]
+	public void RemoveFrontmatter_IndentedCodeBlockFirst_KeepsIndentation() =>
+		Assert.AreEqual($"{IndentedBody}{Nl}", Frontmatter.RemoveFrontmatter(Document()));
+
+	[TestMethod]
+	public void ReplaceFrontmatter_IndentedCodeBlockFirst_KeepsIndentation()
+	{
+		string result = Frontmatter.ReplaceFrontmatter(Document(), new Dictionary<string, object> { ["title"] = "U" });
+
+		Assert.AreEqual(IndentedBody, Frontmatter.ExtractBody(result));
+	}
+
+	[TestMethod]
+	public void AddFrontmatter_ExistingFrontmatterAndIndentedCodeBlockFirst_KeepsIndentation()
+	{
+		string result = Frontmatter.AddFrontmatter(Document(), new Dictionary<string, object> { ["date"] = 2024 });
+
+		Assert.AreEqual(IndentedBody, Frontmatter.ExtractBody(result));
+	}
+
+	[TestMethod]
+	public void AddFrontmatter_NoFrontmatterAndIndentedCodeBlockFirst_KeepsIndentation()
+	{
+		string result = Frontmatter.AddFrontmatter($"    code{Nl}{Nl}Text{Nl}", new Dictionary<string, object> { ["title"] = "U" });
+
+		Assert.AreEqual($"    code{Nl}{Nl}Text", Frontmatter.ExtractBody(result));
+	}
+
+	[TestMethod]
+	public void ExtractBody_TabIndentedFirstLineAfterBlankLines_DropsOnlyTheBlankLines()
+	{
+		string input = $"---{Nl}title: T{Nl}---{Nl}{Nl}  {Nl}\tindented{Nl}{Nl}";
+
+		Assert.AreEqual("\tindented", Frontmatter.ExtractBody(input));
+	}
+
+	[TestMethod]
+	public void ExtractBody_WhitespaceOnlyBody_ReturnsEmpty() =>
+		Assert.AreEqual(string.Empty, Frontmatter.ExtractBody($"---{Nl}title: T{Nl}---{Nl}  {Nl}\t{Nl}"));
+}

Frontmatter/Frontmatter.cs

@@ -136,7 +136,7 @@
 		}
 
 		List<Dictionary<string, object>> frontmatterObjects = ExtractFrontmatterObjects(input, out _);
 		return frontmatterObjects.Count > 0 ? frontmatterObjects.First() : null;
 	}
 
 	/// <summary>
@@ -198,7 +198,7 @@
 
 		string yamlFrontmatter = YamlSerializer.SerializeYamlObject(frontmatter).Trim();
 		string nl = Environment.NewLine;
-		return $"{FrontmatterDelimiter}{nl}{yamlFrontmatter}{nl}{FrontmatterDelimiter}{nl}{input.Trim()}{nl}";
+		return $"{FrontmatterDelimiter}{nl}{yamlFrontmatter}{nl}{FrontmatterDelimiter}{nl}{TrimBody(input)}{nl}";
 	}
 
 	/// <summary>
@@ -220,7 +220,7 @@
 		ExtractFrontmatterObjects(input, out string body);
 		string yamlFrontmatter = YamlSerializer.SerializeYamlObject(frontmatter).Trim();
 		string nl = Environment.NewLine;
-		return $"{FrontmatterDelimiter}{nl}{yamlFrontmatter}{nl}{FrontmatterDelimiter}{nl}{body.Trim()}{nl}";
+		return $"{FrontmatterDelimiter}{nl}{yamlFrontmatter}{nl}{FrontmatterDelimiter}{nl}{TrimBody(body)}{nl}";
 	}
 
 	/// <summary>
@@ -239,7 +239,7 @@
 		}
 
 		ExtractFrontmatterObjects(input, out string body);
-		return body.Trim() + Environment.NewLine;
+		return TrimBody(body) + Environment.NewLine;
 	}
 
 	/// <summary>
@@ -253,7 +253,7 @@
 		Ensure.NotNull(input);
 
 		ExtractFrontmatterObjects(input, out string body);
-		return body.Trim();
+		return TrimBody(body);
 	}
 
 	/// <summary>
@@ -283,7 +283,7 @@
 		}
 
 		// Then add any remaining properties that weren't in the standard order
 		foreach (KeyValuePair<string, object> property in frontmatter)
 		{
 			if (!sortedFrontmatter.ContainsKey(property.Key))
 			{
@@ -333,7 +333,7 @@
 				continue;
 			}
 
 			if (YamlSerializer.TryParseYamlObject(section, out Dictionary<string, object>? frontmatterObject) && frontmatterObject != null)
 			{
 				frontmatterObjects.Add(frontmatterObject);
 			}
@@ -478,7 +478,7 @@
 			lines.Add((start, i));
 			if (c == '\r' && i + 1 < input.Length && input[i + 1] == '\n')
 			{
 				i++;
 			}
 
 			start = i + 1;
@@ -488,6 +488,32 @@
 		return lines;
 	}
 
+	/// <summary>
+	/// Trims a document body for output, dropping the blank lines before its first content line and the
+	/// whitespace after its last. Unlike <see cref="string.Trim()"/>, it keeps the first content line's
+	/// indentation, so a body that opens with an indented code block or nested list content survives.
+	/// </summary>
+	/// <param name="body">The body to trim.</param>
+	/// <returns>The body without leading blank lines or trailing whitespace.</returns>
+	private static string TrimBody(string body)
+	{
+		int start = 0;
+		for (int i = 0; i < body.Length; i++)
+		{
+			char c = body[i];
+			if (c == '\n')
+			{
+				start = i + 1;
+			}
+			else if (!char.IsWhiteSpace(c))
+			{
+				break;
+			}
+		}
+
+		return body[start..].TrimEnd();
+	}
+
 	/// <summary>
 	/// Checks whether a line is a frontmatter delimiter, allowing trailing whitespace.
 	/// </summary>
@@ -513,7 +539,7 @@
 		}
 
 		// Then, add properties from dictionary b that don't exist in a
 		foreach (KeyValuePair<string, object> kvp in b)
 		{
 			if (!combinedFrontmatterObject.ContainsKey(kvp.Key))
 			{