PatchAttributedString.swift

//: Playground - noun: a place where people can play

import UIKit
import PlaygroundSupport
import Differ

//let viewController = GraphViewController(string1: "kitten", string2: "sitting")
//viewController.view.frame = CGRect(x: 0, y: 0, width: 500, height: 500)
//PlaygroundPage.current.liveView = viewController.view

//print("Done")

extension NSAttributedString {
    struct PatchAttributedString {
        let before: NSAttributedString
        let after: NSAttributedString
    }

    func patchAttributedString(with string: String) -> PatchAttributedString {
        let baseAttributes = attributes(at: 0, effectiveRange: nil)
        let before = mutableCopy() as! NSMutableAttributedString
        let after = NSMutableAttributedString(string: string, attributes: baseAttributes)
        let (beforeTags, beforeRanges) = self.tags
        let (afterTags, afterRanges) = after.tags
        //
        let deleteAttribute: [NSAttributedStringKey: Any] = baseAttributes.merging([
            NSAttributedStringKey.foregroundColor: UIColor.red
            ], uniquingKeysWith: { $1 })
        let insertAttribute: [NSAttributedStringKey: Any] = baseAttributes.merging([
            NSAttributedStringKey.foregroundColor: UIColor.green
            ], uniquingKeysWith: { $1 })

        let pathResult = patch(from: beforeTags, to: afterTags)
        var indexOffset = 0
        pathResult.forEach {
            switch $0 {
            case .deletion(let index):
                before.addAttributes(deleteAttribute, range: beforeRanges[index + indexOffset])
            case .insertion(let index, _):
                after.addAttributes(insertAttribute, range: afterRanges[index])
                indexOffset += 1
            }
        }
        return PatchAttributedString(
            before: before.copy() as! NSAttributedString,
            after: after.copy() as! NSAttributedString
        )
    }

    var tags: ([String], [NSRange]) {
        let options = NSLinguisticTagger.Options.omitWhitespace
        let tagger = NSLinguisticTagger(tagSchemes: NSLinguisticTagger.availableTagSchemes(forLanguage: "en"), options: Int(options.rawValue))
        tagger.string = self.string
        let range = NSRange(location: 0, length: string.utf16.count)
        var tags = [String]()
        var ranges = [NSRange]()
        tagger.enumerateTags(in: range, scheme: .nameTypeOrLexicalClass, options: options) { (tag, tokenRange, _, _) in
            let token = (self.string as NSString).substring(with: tokenRange)
            tags.append(token)
            ranges.append(tokenRange)
        }
        return (tags, ranges)
    }
}

let beforeString = "The quick brown fox jumps over the lazy dog"
let afterString  = "quick brown fax jumps over the dog hoge"

let p = NSAttributedString(string: beforeString, attributes: [
    NSAttributedStringKey.foregroundColor: UIColor.white])
    .patchAttributedString(with: afterString)
p.before
p.after