element-ios/Riot/Modules/MatrixKit/Utils/EventFormatter/MarkdownToHTMLRenderer.swift

/*
Copyright 2024 New Vector Ltd.
Copyright 2020 The Matrix.org Foundation C.I.C

SPDX-License-Identifier: AGPL-3.0-only
Please see LICENSE in the repository root for full details.
*/

import Foundation
import Down
import libcmark

@objc public protocol MarkdownToHTMLRendererProtocol: NSObjectProtocol {
    func renderToHTML(markdown: String) -> String?
}

@objcMembers
public class MarkdownToHTMLRenderer: NSObject {
    
    fileprivate var options: DownOptions = []
    
    //  Do not expose an initializer with options, because `DownOptions` is not ObjC compatible.
    public override init() {
        super.init()
    }
}

extension MarkdownToHTMLRenderer: MarkdownToHTMLRendererProtocol {
    
    public func renderToHTML(markdown: String) -> String? {
        do {
            let ast = try DownASTRenderer.stringToAST(markdown, options: options)
            defer {
                cmark_node_free(ast)
            }
            ast.repairLinks()
            return try DownHTMLRenderer.astToHTML(ast, options: options)
        } catch {
            MXLog.error("[MarkdownToHTMLRenderer] renderToHTML failed")
            return nil
        }
    }
    
}

@objcMembers
public class MarkdownToHTMLRendererHardBreaks: MarkdownToHTMLRenderer {
    
    public override init() {
        super.init()
        options = .hardBreaks
    }
    
}

// MARK: - AST-handling private extensions
private extension CMarkNode {
    /// Formatting symbol associated with given note type
    /// Note: this is only defined for node types that are handled in repairLinks
    var formattingSymbol: String {
        switch self.type {
        case CMARK_NODE_EMPH:
            return "_"
        case CMARK_NODE_STRONG:
            return "__"
        default:
            return ""
        }
    }

    /// Repairs links that were broken down by markdown formatting.
    /// Should be used on the first node of libcmark's AST
    /// (e.g. the object returned by DownASTRenderer.stringToAST).
    func repairLinks() {
        let iterator = cmark_iter_new(self)
        var text = ""
        var isInParagraph = false
        var previousNode: CMarkNode?
        var orphanNodes: [CMarkNode] = []
        var shouldUnlinkFormattingMode = false
        var event: cmark_event_type?
        while event != CMARK_EVENT_DONE {
            event = cmark_iter_next(iterator)

            guard let node = cmark_iter_get_node(iterator) else { return }

            if node.type == CMARK_NODE_PARAGRAPH {
                if event == CMARK_EVENT_ENTER {
                    isInParagraph = true
                } else {
                    isInParagraph = false
                    text = ""
                }
            }

            if isInParagraph {
                switch node.type {
                case CMARK_NODE_SOFTBREAK,
                     CMARK_NODE_LINEBREAK:
                    text = ""
                case CMARK_NODE_TEXT:
                    if let literal = node.literal {
                        text += literal
                        // Reset text if it ends up with a whitespace.
                        if text.last?.isWhitespace == true {
                            text = ""
                        }
                        // Only the last part could be a link conflicting with next node.
                        text = String(text.split(separator: " ").last ?? "")
                    }
                case CMARK_NODE_EMPH where previousNode?.type == CMARK_NODE_TEXT,
                     CMARK_NODE_STRONG where previousNode?.type == CMARK_NODE_TEXT:
                    if event == CMARK_EVENT_ENTER {
                        if !text.containedUrls.isEmpty,
                           let childLiteral = node.pointee.first_child.literal {
                            // If current text is a link, the formatted text is reverted back to a
                            // plain text as a part of the link.
                            let symbol = node.formattingSymbol
                            let nonFormattedText = "\(symbol)\(childLiteral)\(symbol)"
                            let replacementTextNode = cmark_node_new(CMARK_NODE_TEXT)
                            cmark_node_set_literal(replacementTextNode, nonFormattedText)
                            cmark_node_insert_after(previousNode, replacementTextNode)
                            // Set child literal to empty string so we dont read it.
                            // This avoids having to re-create the main
                            // iterator in the middle of the process.
                            cmark_node_set_literal(node.pointee.first_child, "")
                            let newIterator = cmark_iter_new(node)
                            _ = cmark_iter_next(newIterator)
                            cmark_node_unlink(node)
                            orphanNodes.append(node)
                            let nextNode = cmark_iter_get_node(newIterator)
                            cmark_node_insert_after(previousNode, nextNode)
                            shouldUnlinkFormattingMode = true
                        }
                    } else {
                        if shouldUnlinkFormattingMode {
                            cmark_node_unlink(node)
                            orphanNodes.append(node)
                            shouldUnlinkFormattingMode = false
                        }
                    }
                default:
                    break
                }
            }
            previousNode = node
        }

        // Free all nodes removed from the AST.
        // This is done as a last step to avoid messing
        // up with the main itertor.
        for orphanNode in orphanNodes {
            cmark_node_free(orphanNode)
        }
    }
}

private extension String {
    /// Returns array of URLs detected inside the String.
    var containedUrls: [NSTextCheckingResult] {
        guard let detector = try? NSDataDetector(types: NSTextCheckingResult.CheckingType.link.rawValue),
              let percentEncoded = self.addingPercentEncoding(withAllowedCharacters: .urlHostAllowed) else {
            return []
        }

        return detector.matches(in: percentEncoded, options: [], range: NSRange(location: 0, length: percentEncoded.utf16.count))
    }
}