Allow separate strong emphasis marker

Sources/Markdown/Walker/Walkers/MarkupFormatter.swift

@@ -248,6 +248,7 @@ public struct MarkupFormatter: MarkupWalker {
         var thematicBreakCharacter: ThematicBreakCharacter
         var thematicBreakLength: UInt
         var emphasisMarker: EmphasisMarker
+        var strongEmphasisMarker: EmphasisMarker
         var condenseAutolinks: Bool
         var preferredHeadingStyle: PreferredHeadingStyle
         var preferredLineLimit: PreferredLineLimit?
@@ -264,7 +265,8 @@ public struct MarkupFormatter: MarkupWalker {
             - defaultCodeBlockLanguage: The default language string to use when code blocks don't have a language and will be printed as fenced code blocks.
             - thematicBreakCharacter: The character to use for thematic breaks.
             - thematicBreakLength: The length of printed thematic breaks.
-            - emphasisMarker: The character to use for emphasis and strong emphasis markers.
+            - emphasisMarker: The character to use for emphasis markers.
+            - strongEmphasisMarker: The character to use for strong emphasis markers. If no value is provided, uses `emphasisMarker`.
             - condenseAutolinks: Print links whose link text and destination match as autolinks, e.g. `<https://swift.org>`.
             - 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.
@@ -277,6 +279,7 @@ public struct MarkupFormatter: MarkupWalker {
                     thematicBreakCharacter: ThematicBreakCharacter = .dash,
                     thematicBreakLength: UInt = 5,
                     emphasisMarker: EmphasisMarker = .star,
+                    strongEmphasisMarker: EmphasisMarker? = nil,
                     condenseAutolinks: Bool = true,
                     preferredHeadingStyle: PreferredHeadingStyle = .atx,
                     preferredLineLimit: PreferredLineLimit? = nil,
@@ -288,6 +291,7 @@ public struct MarkupFormatter: MarkupWalker {
             self.defaultCodeBlockLanguage = defaultCodeBlockLanguage
             self.thematicBreakCharacter = thematicBreakCharacter
             self.emphasisMarker = emphasisMarker
+            self.strongEmphasisMarker = strongEmphasisMarker ?? emphasisMarker
             self.condenseAutolinks = condenseAutolinks
             self.preferredHeadingStyle = preferredHeadingStyle
             self.preferredLineLimit = preferredLineLimit
@@ -862,9 +866,9 @@ public struct MarkupFormatter: MarkupWalker {
     }
 
     public mutating func visitStrong(_ strong: Strong) {
-        print(String(repeating: formattingOptions.emphasisMarker.rawValue, count: 2), for: strong)
+        print(String(repeating: formattingOptions.strongEmphasisMarker.rawValue, count: 2), for: strong)
         descendInto(strong)
-        print(String(repeating: formattingOptions.emphasisMarker.rawValue, count: 2), for: strong)
+        print(String(repeating: formattingOptions.strongEmphasisMarker.rawValue, count: 2), for: strong)
     }
 
     public mutating func visitText(_ text: Text) {

Tests/MarkdownTests/Visitors/MarkupFormatterTests.swift

@@ -542,7 +542,40 @@ class MarkupFormatterOptionsTests: XCTestCase {
                 let printed = document.format(options: .init(emphasisMarker: .underline))
                 XCTAssertEqual(underline, printed)
             }
+        }
+
+        do {
+            let underline = "__strong__"
+            let star = "**strong**"
 
+            do {
+                let document = Document(parsing: underline)
+                let printed = document.format(options: .init(strongEmphasisMarker: .star))
+                XCTAssertEqual(star, printed)
+            }
+
+            do {
+                let document = Document(parsing: star)
+                let printed = document.format(options: .init(strongEmphasisMarker: .underline))
+                XCTAssertEqual(underline, printed)
+            }
+        }
+
+        do {
+            let underline = "__strong__"
+            let star = "**strong**"
+
+            do {
+                let document = Document(parsing: underline)
+                let printed = document.format(options: .init(emphasisMarker: .underline, strongEmphasisMarker: .star))
+                XCTAssertEqual(star, printed)
+            }
+
+            do {
+                let document = Document(parsing: star)
+                let printed = document.format(options: .init(emphasisMarker: .star, strongEmphasisMarker: .underline))
+                XCTAssertEqual(underline, printed)
+            }
         }
     }
 

Tools/markdown-tool/Commands/FormatCommand.swift

@@ -96,6 +96,9 @@ extension MarkdownCommand {
         @Option(help: "Emphasis marker; choices: \(MarkupFormatter.Options.EmphasisMarker.allCases.map { $0.rawValue }.joined(separator: ", "))")
         var emphasisMarker: String = "*"
 
+        @Option(help: "Strong emphasis marker; choices: \(MarkupFormatter.Options.EmphasisMarker.allCases.map { $0.rawValue }.joined(separator: ", "))")
+        var strongEmphasisMarker: String?
+
         @Flag(inversion: .prefixedNo, exclusivity: .chooseLast, help: "Condense links whose text matches their destination to 'autolinks' e.g. <https://swift.org>")
         var condenseAutolinks: Bool = true
 
@@ -206,6 +209,16 @@ extension MarkdownCommand {
                 throw ArgumentParser.ValidationError("The value '\(self.emphasisMarker)' is invalid for '--emphasis-marker'")
             }
 
+            let strongEmphasisMarker: MarkupFormatter.Options.EmphasisMarker?
+            if let marker = self.strongEmphasisMarker {
+                guard let marker = MarkupFormatter.Options.EmphasisMarker(argument: marker) else {
+                    throw ArgumentParser.ValidationError("The value '\(marker)' is invalid for '--strong-emphasis-marker'")
+                }
+                strongEmphasisMarker = marker
+            } else {
+                strongEmphasisMarker = nil
+            }
+
             guard let unorderedListMarker = MarkupFormatter.Options.UnorderedListMarker(argument: unorderedListMarker) else {
                 throw ArgumentParser.ValidationError("The value '\(self.unorderedListMarker)' is invalid for '--unordered-list-marker'")
             }
@@ -229,6 +242,7 @@ extension MarkdownCommand {
                                                   thematicBreakCharacter: thematicBreakCharacter,
                                                   thematicBreakLength: thematicBreakLength,
                                                   emphasisMarker: emphasisMarker,
+                                                  strongEmphasisMarker: strongEmphasisMarker,
                                                   condenseAutolinks: condenseAutolinks,
                                                   preferredHeadingStyle: preferredHeadingStyle,
                                                   preferredLineLimit: preferredLineLimit,