add indentationStyle to MarkupFormatter.Options

Sources/Markdown/Walker/Walkers/MarkupFormatter.swift

@@ -238,6 +238,21 @@ public struct MarkupFormatter: MarkupWalker {
             /// Precede Doxygen commands with an at-sign (`@`).
             case at = "@"
         }
+
+        public enum IndentationStyle: String, CaseIterable {
+            case spaces = "spaces" // use spaces for indentation
+            case tabs = "tabs"   // Use tab characters for indentation
+
+            /// Generates an indentation string based on the current indentation style.
+            func indentation() -> String {
+                switch self {
+                case .spaces:
+                    return String("    ")
+                case .tabs:
+                    return String("\t")
+                }
+            }
+        }
 
         // MARK: Option Properties
 
@@ -253,6 +268,7 @@ public struct MarkupFormatter: MarkupWalker {
         var preferredLineLimit: PreferredLineLimit?
         var customLinePrefix: String
         var doxygenCommandPrefix: DoxygenCommandPrefix
+        var indentationStyle: IndentationStyle
 
         /**
          Create a set of formatting options to use when printing an element.
@@ -269,6 +285,8 @@ public struct MarkupFormatter: MarkupWalker {
             - preferredHeadingStyle: The preferred heading style.
             - lineLimit: The preferred maximum line length and method for splitting ``Text`` elements in an attempt to maintain that line length.
             - customLinePrefix: An addition prefix to print at the start of each line, useful for adding documentation comment markers.
+            - indentationStyle: Whether to use spaces or tabs when indenting.
+
          */
         public init(unorderedListMarker: UnorderedListMarker = .dash,
                     orderedListNumerals: OrderedListNumerals = .allSame(1),
@@ -281,7 +299,8 @@ public struct MarkupFormatter: MarkupWalker {
                     preferredHeadingStyle: PreferredHeadingStyle = .atx,
                     preferredLineLimit: PreferredLineLimit? = nil,
                     customLinePrefix: String = "",
-                    doxygenCommandPrefix: DoxygenCommandPrefix = .backslash) {
+                    doxygenCommandPrefix: DoxygenCommandPrefix = .backslash,
+                    indentationStyle: IndentationStyle = .spaces) {
             self.unorderedListMarker = unorderedListMarker
             self.orderedListNumerals = orderedListNumerals
             self.useCodeFence = useCodeFence
@@ -296,6 +315,7 @@ public struct MarkupFormatter: MarkupWalker {
             self.thematicBreakLength = max(3, thematicBreakLength)
             self.customLinePrefix = customLinePrefix
             self.doxygenCommandPrefix = doxygenCommandPrefix
+            self.indentationStyle = indentationStyle
         }
 
         /// The default set of formatting options.
@@ -431,12 +451,12 @@ public struct MarkupFormatter: MarkupWalker {
                 prefix += "> "
             } else if element is UnorderedList {
                 if unorderedListCount > 0 {
-                    prefix += "  "
+                    prefix += formattingOptions.indentationStyle.indentation()
                 }
                 unorderedListCount += 1
             } else if element is OrderedList {
                 if orderedListCount > 0 {
-                    prefix += "   "
+                    prefix += formattingOptions.indentationStyle.indentation()
                 }
                 orderedListCount += 1
             } else if !(element is ListItem),
@@ -461,11 +481,12 @@ public struct MarkupFormatter: MarkupWalker {
                     // Unordered list markers are of fixed length.
                     prefix += "  "
                 } else if let numeralPrefix = numeralPrefix(for: parentListItem) {
+                    // I don't know exactly what will happen when this produces an indent of 4 or more spaces.
                     prefix += String(repeating: " ", count: numeralPrefix.count)
                 }
             }
             if element.parent is BlockDirective {
-                prefix += "    "
+                prefix += formattingOptions.indentationStyle.indentation()
             }
         }
         return prefix

Tools/markdown-tool/Commands/FormatCommand.swift

@@ -52,6 +52,7 @@ extension MarkupFormatter.Options.ThematicBreakCharacter: ExpressibleByArgument
 extension MarkupFormatter.Options.EmphasisMarker: ExpressibleByArgument {}
 extension MarkupFormatter.Options.PreferredHeadingStyle: ExpressibleByArgument {}
 extension MarkupFormatter.Options.PreferredLineLimit.SplittingElement:  ExpressibleByArgument {}
+extension MarkupFormatter.Options.IndentionStyle: ExpressibleByArgument {}
 
 extension MarkdownCommand {
     /**
@@ -108,6 +109,9 @@ extension MarkdownCommand {
         @Option(help: "The kind of element to use to split text elements while enforcing --preferred-maximum-line-length; choices: \(MarkupFormatter.Options.PreferredLineLimit.SplittingElement.allCases.map { $0.rawValue }.joined(separator: ", "))")
         var lineSplittingElement: MarkupFormatter.Options.PreferredLineLimit.SplittingElement = .softBreak
 
+        @Option(help: "The indentation style to use when emitting indented blocks; choices:\(MarkupFormatter.Options.IndentationStyle.allCases.map { $0.rawValue }.joined(separator: ", "))")
+        var indentationStyle: MarkupFormatter.Options.IndentationStyle = .spaces
+
         @Option(help: "Prepend this prefix to every line")
         var customLinePrefix: String = ""
 
@@ -232,7 +236,8 @@ extension MarkdownCommand {
                                                   condenseAutolinks: condenseAutolinks,
                                                   preferredHeadingStyle: preferredHeadingStyle,
                                                   preferredLineLimit: preferredLineLimit,
-                                                  customLinePrefix: customLinePrefix)
+                                                  customLinePrefix: customLinePrefix,
+                                                  indentationStyle: indentationStyle)
             let formatted = document.format(options: formatOptions)
             print(formatted)
             if checkStructuralEquivalence {