Tusker/Tusker/HTMLConverter.swift

//
//  HTMLConverter.swift
//  Tusker
//
//  Created by Shadowfacts on 12/3/22.
//  Copyright © 2022 Shadowfacts. All rights reserved.
//

import UIKit
import SwiftSoup
import WebURL
import WebURLFoundationExtras

struct HTMLConverter {
    
    static let defaultFont = UIFont.systemFont(ofSize: 17)
    static let defaultColor = UIColor.label
    static let defaultParagraphStyle: NSParagraphStyle = {
        let style = NSMutableParagraphStyle()
        // 2 points is enough that it doesn't make things look weirdly spaced out, but leaves a slight gap when there are multiple lines of emojis
        style.lineSpacing = 2
        return style
    }()
    
    var font: UIFont = defaultFont
    var color: UIColor = defaultColor
    var paragraphStyle: NSParagraphStyle = defaultParagraphStyle
    
    func convert(_ html: String) -> NSAttributedString {
        let doc = try! SwiftSoup.parseBodyFragment(html)
        let body = doc.body()!
        
        let attributedText = attributedTextForHTMLNode(body)
        let mutAttrString = NSMutableAttributedString(attributedString: attributedText)
        mutAttrString.trimTrailingCharactersInSet(.whitespacesAndNewlines)
        mutAttrString.collapseWhitespace()
        
        mutAttrString.addAttribute(.paragraphStyle, value: paragraphStyle, range: mutAttrString.fullRange)
        
        return mutAttrString
    }
    
    private func attributedTextForHTMLNode(_ node: Node, usePreformattedText: Bool = false) -> NSAttributedString {
        switch node {
        case let node as TextNode:
            let text: String
            if usePreformattedText {
                text = node.getWholeText()
            } else {
                text = node.text()
            }
            return NSAttributedString(string: text, attributes: [.font: font, .foregroundColor: color])
        case let node as Element:
            let attributed = NSMutableAttributedString(string: "", attributes: [.font: font, .foregroundColor: color])
            for child in node.getChildNodes() {
                attributed.append(attributedTextForHTMLNode(child, usePreformattedText: usePreformattedText || node.tagName() == "pre"))
            }
            
            switch node.tagName() {
            case "br":
                // need to specify defaultFont here b/c otherwise it uses the default 12pt Helvetica which
                // screws up its determination of the line height making multiple lines of emojis squash together
                attributed.append(NSAttributedString(string: "\n", attributes: [.font: font]))
            case "a":
                let href = try! node.attr("href")
                if let webURL = WebURL(href),
                   let url = URL(webURL) {
                    attributed.addAttribute(.link, value: url, range: attributed.fullRange)
                } else if let url = URL(string: href) {
                    attributed.addAttribute(.link, value: url, range: attributed.fullRange)
                }
            case "p":
                attributed.append(NSAttributedString(string: "\n\n", attributes: [.font: font]))
            case "em", "i":
                let currentFont: UIFont
                if attributed.length == 0 {
                    currentFont = font
                } else {
                    currentFont = attributed.attribute(.font, at: 0, effectiveRange: nil) as? UIFont ?? font
                }
                attributed.addAttribute(.font, value: currentFont.withTraits(.traitItalic)!, range: attributed.fullRange)
            case "strong", "b":
                let currentFont: UIFont
                if attributed.length == 0 {
                    currentFont = font
                } else {
                    currentFont = attributed.attribute(.font, at: 0, effectiveRange: nil) as? UIFont ?? font
                }
                attributed.addAttribute(.font, value: currentFont.withTraits(.traitBold)!, range: attributed.fullRange)
            case "del":
                attributed.addAttribute(.strikethroughStyle, value: NSUnderlineStyle.single.rawValue, range: attributed.fullRange)
            case "code":
                // TODO: this probably breaks with dynamic type
                attributed.addAttribute(.font, value: UIFont.monospacedSystemFont(ofSize: font.pointSize, weight: .regular), range: attributed.fullRange)
            case "pre":
                attributed.append(NSAttributedString(string: "\n\n"))
                attributed.addAttribute(.font, value: UIFont.monospacedSystemFont(ofSize: font.pointSize, weight: .regular), range: attributed.fullRange)
            case "ol", "ul":
                attributed.append(NSAttributedString(string: "\n\n"))
                attributed.trimLeadingCharactersInSet(.whitespacesAndNewlines)
            case "li":
                let parentEl = node.parent()!
                let parentTag = parentEl.tagName()
                let bullet: NSAttributedString
                if parentTag == "ol" {
                    let index = (try? node.elementSiblingIndex()) ?? 0
                    // we use the monospace digit font so that the periods of all the list items line up
                    // TODO: this probably breaks with dynamic type
                    bullet = NSAttributedString(string: "\(index + 1).\t", attributes: [.font: UIFont.monospacedDigitSystemFont(ofSize: font.pointSize, weight: .regular), .foregroundColor: color])
                } else if parentTag == "ul" {
                    bullet = NSAttributedString(string: "\u{2022}\t", attributes: [.font: font, .foregroundColor: color])
                } else {
                    bullet = NSAttributedString()
                }
                attributed.insert(bullet, at: 0)
                attributed.append(NSAttributedString(string: "\n", attributes: [.font: font]))
            default:
                break
            }
            
            return attributed
        default:
            fatalError("Unexpected node type \(type(of: node))")
        }
    }
    
    
}