Add footnotes.

Sources/Markdown/Base/Markup.swift

@@ -71,6 +71,10 @@ func makeMarkup(_ data: _MarkupData) -> Markup {
         return SymbolLink(data)
     case .inlineAttributes:
         return InlineAttributes(data)
+    case .footnoteReference:
+        return FootnoteReference(data)
+    case .footnoteDefinition:
+        return FootnoteDefinition(data)
     case .doxygenDiscussion:
         return DoxygenDiscussion(data)
     case .doxygenNote:

Sources/Markdown/Base/RawMarkup.swift

@@ -52,6 +52,9 @@ enum RawMarkupData: Equatable {
     case tableRow
     case tableCell(colspan: UInt, rowspan: UInt)
 
+    case footnoteReference(footnoteID: String)
+    case footnoteDefinition(footnoteID: String)
+
     case doxygenDiscussion
     case doxygenNote
     case doxygenParam(name: String)
@@ -344,6 +347,14 @@ final class RawMarkup: ManagedBuffer<RawMarkupHeader, RawMarkup> {
         return .create(data: .tableCell(colspan: colspan, rowspan: rowspan), parsedRange: parsedRange, children: children)
     }
 
+    static func footnoteDefinition(footnoteID: String, parsedRange: SourceRange?, _ children: [RawMarkup]) -> RawMarkup {
+        return .create(data: .footnoteDefinition(footnoteID: footnoteID), parsedRange: parsedRange, children: children)
+    }
+
+    static func footnoteReference(footnoteID: String, parsedRange: SourceRange?, _ children: [RawMarkup]) -> RawMarkup {
+        return .create(data: .footnoteReference(footnoteID: footnoteID), parsedRange: parsedRange, children: children)
+    }
+
     static func doxygenDiscussion(parsedRange: SourceRange?, _ children: [RawMarkup]) -> RawMarkup {
         return .create(data: .doxygenDiscussion, parsedRange: parsedRange, children: children)
     }

Sources/Markdown/Block Nodes/Block Container Blocks/FootnoteDefinition.swift

@@ -0,0 +1,56 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2021 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+public struct FootnoteDefinition: BlockContainer {
+    public var _data: _MarkupData
+    init(_ raw: RawMarkup) throws {
+        guard case .footnoteDefinition = raw.data else {
+            throw RawMarkup.Error.concreteConversionError(from: raw, to: FootnoteDefinition.self)
+        }
+        let absoluteRaw = AbsoluteRawMarkup(markup: raw, metadata: MarkupMetadata(id: .newRoot(), indexInParent: 0))
+        self.init(_MarkupData(absoluteRaw))
+    }
+
+    init(_ data: _MarkupData) {
+        self._data = data
+    }
+}
+
+// MARK: - Public API
+
+public extension FootnoteDefinition {
+    // MARK: BasicBlockContainer
+
+    init<Children: Sequence>(footnoteID: String, _ children: Children) where Children.Element == BlockMarkup {
+        try! self.init(.footnoteDefinition(footnoteID: footnoteID, parsedRange: nil, children.map { $0.raw.markup }))
+    }
+
+    init(footnoteID: String, _ children: BlockMarkup...) {
+        self.init(footnoteID: footnoteID, children)
+    }
+
+    var footnoteID: String {
+        get {
+            guard case let .footnoteDefinition(footnoteID: footnoteID) = _data.raw.markup.data else {
+                fatalError("\(self) markup wrapped unexpected \(_data.raw)")
+            }
+            return footnoteID
+        }
+        set {
+            _data = _data.replacingSelf(.footnoteDefinition(footnoteID: newValue, parsedRange: nil, _data.raw.markup.copyChildren()))
+        }
+    }
+
+    // MARK: Visitation
+
+    func accept<V: MarkupVisitor>(_ visitor: inout V) -> V.Result {
+        return visitor.visitFootnoteDefinition(self)
+    }
+}

Sources/Markdown/Inline Nodes/Inline Containers/FootnoteReference.swift

@@ -0,0 +1,57 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2021 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+/// A reference to a footnote
+public struct FootnoteReference: InlineMarkup, InlineContainer {
+    public var _data: _MarkupData
+
+    init(_ raw: RawMarkup) throws {
+        guard case .footnoteReference = raw.data else {
+            throw RawMarkup.Error.concreteConversionError(from: raw, to: FootnoteReference.self)
+        }
+        let absoluteRaw = AbsoluteRawMarkup(markup: raw, metadata: MarkupMetadata(id: .newRoot(), indexInParent: 0))
+        self.init(_MarkupData(absoluteRaw))
+    }
+
+    init(_ data: _MarkupData) {
+        self._data = data
+    }
+}
+
+// MARK: - Public API
+
+public extension FootnoteReference {
+    init<Children: Sequence>(footnoteID: String, _ children: Children) where Children.Element == RecurringInlineMarkup {
+        try! self.init(.footnoteReference(footnoteID: footnoteID, parsedRange: nil, children.map { $0.raw.markup }))
+    }
+
+    init(footnoteID: String, _ children: RecurringInlineMarkup...) {
+        self.init(footnoteID: footnoteID, children)
+    }
+
+    /// The specified attributes in JSON5 format.
+    var footnoteID: String {
+        get {
+            guard case let .footnoteReference(footnoteID: footnoteID) = _data.raw.markup.data else {
+                fatalError("\(self) markup wrapped unexpected \(_data.raw)")
+            }
+            return footnoteID
+        }
+        set {
+            _data = _data.replacingSelf(.footnoteReference(footnoteID: newValue, parsedRange: nil, _data.raw.markup.copyChildren()))
+        }
+    }
+
+    // MARK: Visitation
+
+    func accept<V: MarkupVisitor>(_ visitor: inout V) -> V.Result {
+        return visitor.visitFootnoteReference(self)
+    }
+}

Sources/Markdown/Parser/CommonMarkConverter.swift

@@ -68,6 +68,9 @@ fileprivate enum CommonMarkNodeType: String {
     case tableCell = "table_cell"
 
     case taskListItem = "tasklist"
+
+    case footnoteReference = "footnote_reference"
+    case footnoteDefinition = "footnote_definition"
 }
 
 /// Represents the result of a cmark conversion: the current `MarkupConverterState` and the resulting converted node.
@@ -139,6 +142,16 @@ fileprivate struct MarkupConverterState {
         guard let type = CommonMarkNodeType(rawValue: typeString) else {
             fatalError("Unknown cmark node type '\(typeString)' encountered during conversion")
         }
+        if type == .unknown {
+            // NOTE: cmark does not expose strings for the footnote types, but
+            // does correctly identify their type.
+            switch cmark_node_get_type(node) {
+                case CMARK_NODE_FOOTNOTE_DEFINITION: return .footnoteDefinition
+                case CMARK_NODE_FOOTNOTE_REFERENCE: return .footnoteReference
+                default:
+                    fatalError("Unknown cmark node type '\(typeString)' encountered during conversion")
+            }
+        }
         return type
     }
 
@@ -232,6 +245,10 @@ struct MarkupParser {
             return convertTableCell(state)
         case .inlineAttributes:
             return convertInlineAttributes(state)
+        case .footnoteReference:
+            return convertFootnoteReference(state)
+        case .footnoteDefinition:
+            return convertFootnoteDefinition(state)
         default:
             fatalError("Unknown cmark node type '\(state.nodeType.rawValue)' encountered during conversion")
         }
@@ -608,6 +625,28 @@ struct MarkupParser {
         return MarkupConversion(state: childConversion.state.next(), result: .inlineAttributes(attributes: attributes, parsedRange: parsedRange, childConversion.result))
      }
 
+    private static func convertFootnoteReference(_ state: MarkupConverterState) -> MarkupConversion<RawMarkup> {
+        precondition(state.event == CMARK_EVENT_ENTER)
+        precondition(state.nodeType == .footnoteReference)
+        let parsedRange = state.range(state.node)
+        let childConversion = convertChildren(state)
+        let footnoteID = String(cString: cmark_node_get_literal(state.node))
+        precondition(childConversion.state.node == state.node)
+        precondition(childConversion.state.event == CMARK_EVENT_EXIT)
+        return MarkupConversion(state: childConversion.state.next(), result: .footnoteReference(footnoteID: footnoteID, parsedRange: parsedRange, childConversion.result))
+     }
+
+    private static func convertFootnoteDefinition(_ state: MarkupConverterState) -> MarkupConversion<RawMarkup> {
+        precondition(state.event == CMARK_EVENT_ENTER)
+        precondition(state.nodeType == .footnoteDefinition)
+        let parsedRange = state.range(state.node)
+        let childConversion = convertChildren(state)
+        let footnoteID = String(cString: cmark_node_get_literal(state.node))
+        precondition(childConversion.state.node == state.node)
+        precondition(childConversion.state.event == CMARK_EVENT_EXIT)
+        return MarkupConversion(state: childConversion.state.next(), result: .footnoteDefinition(footnoteID: footnoteID, parsedRange: parsedRange, childConversion.result))
+    }
+
     static func parseString(_ string: String, source: URL?, options: ParseOptions) -> Document {
         cmark_gfm_core_extensions_ensure_registered()
 
@@ -618,6 +657,7 @@ struct MarkupParser {
         if !options.contains(.disableSourcePosOpts) {
             cmarkOptions |= CMARK_OPT_SOURCEPOS
         }
+        cmarkOptions |= CMARK_OPT_FOOTNOTES
 
         let parser = cmark_parser_new(cmarkOptions)
 

Sources/Markdown/Rewriter/MarkupRewriter.swift

@@ -87,4 +87,11 @@ extension MarkupRewriter {
     public mutating func visitText(_ text: Text) -> Result {
         return defaultVisit(text)
     }
+    public mutating func visitFootnoteReference(_ footnoteReference: FootnoteReference) -> Result {
+        return defaultVisit(footnoteReference)
+    }
+
+    public mutating func visitFootnoteDefinition(_ footnoteDefinition: FootnoteDefinition) -> Result {
+        return defaultVisit(footnoteDefinition)
+    }
 }

Sources/Markdown/Visitor/MarkupVisitor.swift

@@ -275,6 +275,22 @@ public protocol MarkupVisitor<Result> {
      */
      mutating func visitInlineAttributes(_ attributes: InlineAttributes) -> Result
 
+    /**
+    Visit an `FootnoteReference` element and return the result.
+
+    - parameter attributes: An `FootnoteReference` element.
+    - returns: The result of the visit.
+     */
+    mutating func visitFootnoteReference(_ footnoteReference: FootnoteReference) -> Result
+
+    /**
+    Visit an `FootnoteDefinition` element and return the result.
+
+    - parameter attributes: An `FootnoteDefinition` element.
+    - returns: The result of the visit.
+     */
+    mutating func visitFootnoteDefinition(_ footnoteDefinition: FootnoteDefinition) -> Result
+
     /**
      Visit a `DoxygenDiscussion` element and return the result.
 
@@ -405,6 +421,12 @@ extension MarkupVisitor {
     public mutating func visitInlineAttributes(_ attributes: InlineAttributes) -> Result {
         return defaultVisit(attributes)
     }
+    public mutating func visitFootnoteReference(_ footnoteReference: FootnoteReference) -> Result {
+        return defaultVisit(footnoteReference)
+    }
+    public mutating func visitFootnoteDefinition(_ footnoteDefinition: FootnoteDefinition) -> Result {
+        return defaultVisit(footnoteDefinition)
+    }
     public mutating func visitDoxygenDiscussion(_ doxygenDiscussion: DoxygenDiscussion) -> Result {
         return defaultVisit(doxygenDiscussion)
     }

Sources/Markdown/Walker/Walkers/MarkupTreeDumper.swift

@@ -287,6 +287,14 @@ struct MarkupTreeDumper: MarkupWalker {
         dump(attributes, customDescription: "attributes: `\(attributes.attributes)`")
     }
 
+    mutating func visitFootnoteReference(_ footnoteReference: FootnoteReference) -> () {
+        dump(footnoteReference, customDescription: "footnoteID: `\(footnoteReference.footnoteID)`")
+    }
+
+    mutating func visitFootnoteDefinition(_ footnoteDefinition: FootnoteDefinition) -> () {
+        dump(footnoteDefinition, customDescription: "footnoteID: `\(footnoteDefinition.footnoteID)`")
+    }
+
     mutating func visitDoxygenParameter(_ doxygenParam: DoxygenParameter) -> () {
         dump(doxygenParam, customDescription: "parameter: \(doxygenParam.name)")
     }

Tests/MarkdownTests/Block Nodes/FootnoteTests.swift

@@ -0,0 +1,70 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2021 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+@testable import Markdown
+import XCTest
+
+class FootnoteTests: XCTestCase {
+    func testSimpleFootnote() {
+        let source = """
+        text with a footnote [^1].
+        [^1]: footnote definition.
+        """
+
+        let document = Document(parsing: source)
+
+        let expectedDump = """
+        Document @1:1-2:27
+        ├─ Paragraph @1:1-1:27
+        │  ├─ Text @1:1-1:22 "text with a footnote "
+        │  ├─ FootnoteReference @1:22-1:26 footnoteID: `1`
+        │  └─ Text @1:26-1:27 "."
+        └─ FootnoteDefinition @2:7-2:27 footnoteID: `1`
+           └─ Paragraph @2:7-2:27
+              └─ Text @2:7-2:27 "footnote definition."
+        """
+
+        XCTAssertEqual(expectedDump, document.debugDescription(options: .printSourceLocations))
+    }
+
+    func testBlockFootnote() {
+        let source = """
+        text with a block footnote [^1].
+        [^1]: This is a long footnote, including a quote:
+
+            > This is a multi-line quote, spanning
+            > multiple lines.
+
+            And then some more text.
+        """
+
+        let document = Document(parsing: source)
+
+        let expectedDump = """
+        Document @1:1-7:29
+        ├─ Paragraph @1:1-1:33
+        │  ├─ Text @1:1-1:28 "text with a block footnote "
+        │  ├─ FootnoteReference @1:28-1:32 footnoteID: `1`
+        │  └─ Text @1:32-1:33 "."
+        └─ FootnoteDefinition @2:7-7:29 footnoteID: `1`
+           ├─ Paragraph @2:7-2:50
+           │  └─ Text @2:7-2:50 "This is a long footnote, including a quote:"
+           ├─ BlockQuote @4:5-5:22
+           │  └─ Paragraph @4:7-5:22
+           │     ├─ Text @4:7-4:43 "This is a multi-line quote, spanning"
+           │     ├─ SoftBreak
+           │     └─ Text @5:7-5:22 "multiple lines."
+           └─ Paragraph @7:5-7:29
+              └─ Text @7:5-7:29 "And then some more text."
+        """
+
+        XCTAssertEqual(expectedDump, document.debugDescription(options: .printSourceLocations))
+    }
+  }