Add support for links in markdown

ios/MullvadVPN/Classes/Markdown/AttributedStringSupport/AttributedMarkdown.swift

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+//
+//  AttributedMarkdownProtocol.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+/// Type of markdown element.
+enum MarkdownElement {
+    /// Bold text.
+    case bold
+    /// URL link.
+    case link
+}
+
+/// Callback type used to override the attributed string attributes during parsing.
+typealias MarkdownEffectCallback = (MarkdownElement, String) -> [NSAttributedString.Key: Any]
+
+/// Type implementing conversion from markdown to attributed string.
+protocol AttributedMarkdown {
+    /// Convert the type to attributed string.
+    ///
+    /// - Parameters:
+    ///   - options: markdown styling options.
+    ///   - applyEffect: the callback used to override the string attributes during parsing.
+    /// - Returns: the attributed string.
+    func attributedString(options: MarkdownStylingOptions, applyEffect: MarkdownEffectCallback?) -> NSAttributedString
+}
+
+extension NSAttributedString.Key {
+    /// The attributed string key used in place of `.link` whos text color is not customizable in UILabels.
+    /// The value associated with this key can be a `String` or an `URL`.
+    static let hyperlink = NSAttributedString.Key("HyperLink")
+}
+
+extension AttributedMarkdown {
+    /// Convert the type to attributed string.
+    ///
+    /// - Parameter options: markdown styling options.
+    /// - Returns: the attributed string.
+    func attributedString(options: MarkdownStylingOptions) -> NSAttributedString {
+        attributedString(options: options, applyEffect: nil)
+    }
+}

ios/MullvadVPN/Classes/Markdown/AttributedStringSupport/MarkdownBoldNode+AttributedString.swift

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+//
+//  MarkdownBoldNode+AttributedString.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+extension MarkdownBoldNode: AttributedMarkdown {
+    func attributedString(options: MarkdownStylingOptions, applyEffect: MarkdownEffectCallback?) -> NSAttributedString {
+        let string = text?.withUnicodeLineSeparators() ?? ""
+        var attributes: [NSAttributedString.Key: Any] = [.font: options.boldFont]
+
+        if let textColor = options.textColor {
+            attributes[.foregroundColor] = textColor
+        }
+
+        attributes.merge(applyEffect?(.bold, string) ?? [:], uniquingKeysWith: { $1 })
+
+        return NSAttributedString(string: string, attributes: attributes)
+    }
+}

ios/MullvadVPN/Classes/Markdown/AttributedStringSupport/MarkdownDocumentNode+AttributedString.swift

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+//
+//  MarkdownDocumentNode+AttributedString.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+extension MarkdownDocumentNode: AttributedMarkdown {
+    func attributedString(options: MarkdownStylingOptions, applyEffect: MarkdownEffectCallback?) -> NSAttributedString {
+        var isPrecededByParagraph = false
+
+        return children.reduce(into: NSMutableAttributedString()) { partialResult, node in
+            guard let transformableNode = node as? AttributedMarkdown else { return }
+
+            defer { isPrecededByParagraph = node.isParagraph }
+
+            // Add newline between paragraphs.
+            if node.isParagraph, isPrecededByParagraph {
+                partialResult.append(NSAttributedString(
+                    string: "\n",
+                    attributes: [.font: options.font, .paragraphStyle: options.paragraphStyle]
+                ))
+            }
+
+            let attributedString = transformableNode.attributedString(options: options, applyEffect: applyEffect)
+            partialResult.append(attributedString)
+        }
+    }
+}
+
+private extension MarkdownNode {
+    var isParagraph: Bool {
+        type == .paragraph
+    }
+}

ios/MullvadVPN/Classes/Markdown/AttributedStringSupport/MarkdownLinkNode+AttributedString.swift

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+//
+//  MarkdownLinkNode+AttributedString.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+extension MarkdownLinkNode: AttributedMarkdown {
+    func attributedString(options: MarkdownStylingOptions, applyEffect: MarkdownEffectCallback?) -> NSAttributedString {
+        var attributes: [NSAttributedString.Key: Any] = [.font: options.font, options.linkAttribute.attributeKey: url]
+
+        if let linkColor = options.linkColor {
+            attributes[.foregroundColor] = linkColor
+        }
+
+        attributes.merge(applyEffect?(.link, title) ?? [:], uniquingKeysWith: { $1 })
+
+        return NSAttributedString(string: title, attributes: attributes)
+    }
+}

ios/MullvadVPN/Classes/Markdown/AttributedStringSupport/MarkdownParagraphNode+AttributedString.swift

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+//
+//  MarkdownParagraphNode+AttributedString.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+extension MarkdownParagraphNode: AttributedMarkdown {
+    func attributedString(options: MarkdownStylingOptions, applyEffect: MarkdownEffectCallback?) -> NSAttributedString {
+        let mutableAttributedString = children.compactMap { $0 as? AttributedMarkdown }
+            .reduce(into: NSMutableAttributedString()) { partialResult, node in
+                let attributedString = node.attributedString(options: options, applyEffect: applyEffect)
+                partialResult.append(attributedString)
+            }
+
+        let range = NSRange(location: 0, length: mutableAttributedString.length)
+        mutableAttributedString.addAttribute(.paragraphStyle, value: options.paragraphStyle, range: range)
+
+        return mutableAttributedString
+    }
+}

ios/MullvadVPN/Classes/Markdown/AttributedStringSupport/MarkdownTextNode+AttributedString.swift

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+//
+//  MarkdownTextNode+AttributedString.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+extension MarkdownTextNode: AttributedMarkdown {
+    func attributedString(options: MarkdownStylingOptions, applyEffect: MarkdownEffectCallback?) -> NSAttributedString {
+        let string = text?.withUnicodeLineSeparators() ?? ""
+        var attributes: [NSAttributedString.Key: Any] = [.font: options.font]
+
+        if let textColor = options.textColor {
+            attributes[.foregroundColor] = textColor
+        }
+
+        return NSAttributedString(string: string, attributes: attributes)
+    }
+}

ios/MullvadVPN/Classes/Markdown/AttributedStringSupport/NSAttributedString+Markdown.swift

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+//
+//  NSAttributedString+Markdown.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 19/11/2021.
+//  Copyright © 2021 Mullvad VPN AB. All rights reserved.
+//
+
+import UIKit
+
+extension NSAttributedString {
+    /// Initialize the attributed string from markdown.
+    ///
+    /// - Parameters:
+    ///   - markdownString: the markdown string.
+    ///   - options: markdown styling options.
+    ///   - applyEffect: the callback used to override the string attributes during parsing.
+    convenience init(
+        markdownString: String,
+        options: MarkdownStylingOptions,
+        applyEffect: MarkdownEffectCallback? = nil
+    ) {
+        var parser = MarkdownParser(markdown: markdownString)
+        let document = parser.parse()
+
+        let attributedString = document.attributedString(options: options, applyEffect: applyEffect)
+
+        self.init(attributedString: attributedString)
+    }
+}

ios/MullvadVPN/Classes/Markdown/AttributedStringSupport/String+UnicodeLineSeparator.swift

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+//
+//  String+UnicodeLineSeparator.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+/// Unicode line separator.
+/// Declared on macOS as `NSLineSeparatorCharacter` but not on iOS.
+private let unicodeLineSeparator: Character = "\u{2028}"
+
+extension String {
+    /// Return a new string with all line seprators `\r\n` or `\n` replaced with unicode line separator.
+    ///
+    /// `NSAttributedString` treats `\n` as a paragraph separator.
+    /// - Returns: a new string with all line separators converted to unicode line separator.
+    func withUnicodeLineSeparators() -> String {
+        String(map { ch in
+            ch.isNewline ? unicodeLineSeparator : ch
+        })
+    }
+}

ios/MullvadVPN/Classes/Markdown/IteratorProtocol+String.swift

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+//
+//  IteratorProtocol+String.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+extension IteratorProtocol where Element == Character {
+    /// Collect characters into a string while the predicate evaluates to `true`. Rethrows errors thrown by the predicate.
+    ///
+    /// - Parameter predicate: The predicate to evaluate each character before appending it to the result string.
+    /// - Returns: The result string.
+    mutating func take(while predicate: (Character) throws -> Bool) rethrows -> String {
+        var accummulated = ""
+
+        while let char = next() {
+            guard try predicate(char) else { break }
+
+            accummulated.append(char)
+        }
+
+        return accummulated
+    }
+}

ios/MullvadVPN/Classes/Markdown/MarkdownNode.swift

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+//
+//  MarkdownNode.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+/// The base type defining markdown node.
+/// Do not instantiate this type directly. Use one of its subclasses instead.
+class MarkdownNode: CustomDebugStringConvertible {
+    /// The type of node.
+    let type: MarkdownNodeType
+
+    /// The child nodes.
+    private(set) var children: [MarkdownNode] = []
+
+    /// The parent node.
+    private(set) weak var parent: MarkdownNode?
+
+    init(type: MarkdownNodeType, children: [MarkdownNode] = []) {
+        self.type = type
+        children.forEach { addChild($0) }
+    }
+
+    /// Returns last child.
+    var lastChild: MarkdownNode? {
+        return children.last
+    }
+
+    /// Add child node.
+    func addChild(_ child: MarkdownNode) {
+        child.parent = self
+        children.append(child)
+    }
+
+    /// Remove child.
+    func removeChild(_ child: MarkdownNode) {
+        children.removeAll { childFromArray in
+            guard child === childFromArray else { return false }
+
+            child.parent = nil
+            return true
+        }
+    }
+
+    /// Detach this node from parent.
+    func removeFromParent() {
+        parent?.removeChild(self)
+    }
+
+    var debugDescription: String {
+        // Subclasses should override this method.
+        return "\(self)"
+    }
+
+    /// Returns a recursive description of a markdown subtree. Useful when debugging.
+    ///
+    /// - Parameter level: indentation level.
+    /// - Returns: recursive description of a subtree
+    func recursiveDescription(level: Int = 0) -> String {
+        let indent = String(repeating: "  ", count: level)
+        var str = ""
+
+        let descriptionLines = debugDescription.components(separatedBy: .newlines)
+        if let firstLine = descriptionLines.first {
+            str += "\(indent)+ \(firstLine)"
+        }
+        descriptionLines.dropFirst().forEach { line in
+            str += "\n\(indent)  \(line)"
+        }
+
+        for child in children {
+            str += "\n" + child.recursiveDescription(level: level + 1)
+        }
+
+        return str
+    }
+
+    /// Test equality.
+    ///
+    /// Default implementation only checks node types.
+    ///
+    /// - Parameter other: other node.
+    /// - Returns: `true` if objects are equal, otherwise `false`.
+    func isEqualTo(_ other: MarkdownNode) -> Bool {
+        guard type == other.type && children.count == other.children.count else { return false }
+
+        return zip(children, other.children).allSatisfy { $0.isEqualTo($1) }
+    }
+}
+
+extension MarkdownNode: Equatable {
+    static func == (lhs: MarkdownNode, rhs: MarkdownNode) -> Bool {
+        return lhs.isEqualTo(rhs)
+    }
+}

ios/MullvadVPN/Classes/Markdown/MarkdownNodeType.swift

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+//
+//  MarkdownNodeType.swift
+//  MullvadVPN
+//
+//  Created by pronebird on 03/11/2023.
+//  Copyright © 2023 Mullvad VPN AB. All rights reserved.
+//
+
+import Foundation
+
+/// The type of node used within the markdown tree.
+enum MarkdownNodeType {
+    /// The unstyled text fragment.
+    case text
+
+    /// The bold text node.
+    /// Syntax: `**Proceed carefully in unknown waters!**`
+    case bold
+
+    /// The URL link node.
+    /// Syntax: `[Mullvad VPN](https://mullvad.net)`
+    case link
+
+    /// The paragraph node.
+    /// Typically groups of elements separated by two newline characters form a paragraph.
+    case paragraph
+
+    /// The fragment of a markdown document.
+    case document
+}