frenzy-ios/Reader/ExcerptGenerator.swift

//
//  ExcerptGenerator.swift
//  Reader
//
//  Created by Shadowfacts on 1/13/22.
//

import Foundation
import OSLog
import CoreData

// public so that it can be imported in ReaderTests even when Reader is compiled in release mode (w/ testing disabled)
public struct ExcerptGenerator {
    private init() {}
    
    private static let logger = Logger(subsystem: Bundle.main.bundleIdentifier!, category: "ExcerptGenerator")
    
    static func generateAll(_ fervorController: FervorController) async {
        let req = Item.fetchRequest()
        req.predicate = NSPredicate(format: "generatedExcerpt = NO")
        req.sortDescriptors = [NSSortDescriptor(key: "published", ascending: false)]
        req.fetchBatchSize = 50
        let ctx = fervorController.persistentContainer.newBackgroundContext()
        await ctx.perform {
            guard let items = try? ctx.fetch(req) else { return }
            var count = 0
            for item in items {
                if let excerpt = excerpt(for: item) {
                    item.excerpt = excerpt
                    count += 1
                    if count % 50 == 0 {
                        logger.debug("Generated \(count, privacy: .public) excerpts")
                    }
                }
                item.generatedExcerpt = true
            }
            logger.log("Generated excerpts for \(count, privacy: .public) items")
            if ctx.hasChanges {
                do {
                    // get the updated objects now, because this set is empty after .save is called
                    let updated = ctx.updatedObjects
                    try ctx.save()
                    
                    // make sure the view context has the newly added excerpts
                    NSManagedObjectContext.mergeChanges(fromRemoteContextSave: [
                        NSUpdatedObjectsKey: Array(updated)
                    ], into: [fervorController.persistentContainer.viewContext])
                } catch {
                    logger.error("Unable to save context: \(String(describing: error), privacy: .public)")
                }
            }
        }
    }
    
    public static func excerpt(for item: Item) -> String? {
        guard let content = item.content else {
            return nil
        }
        return excerpt(from: content)
    }
    
    public static func excerpt(from html: String) -> String? {
        var html = html
        
        let builder = lol_html_rewriter_builder_new()!
        let pSelector = lol_html_selector_parse("p", 1)!
        var userData = UserData()
        withUnsafeMutablePointer(to: &userData) { userDataPtr in
            let rawPtr = UnsafeMutableRawPointer(userDataPtr)
            let res = lol_html_rewriter_builder_add_element_content_handlers(builder, pSelector, elementHandler, rawPtr, nil, nil, textHandler, rawPtr)
            guard res == 0 else {
                lolHtmlError()
            }
            let memSettings = lol_html_memory_settings_t(preallocated_parsing_buffer_size: 1024, max_allowed_memory_usage: .max)
            let rewriter = lol_html_rewriter_build(builder, "utf-8", 5, memSettings, outputSink, nil, true)
            lol_html_rewriter_builder_free(builder)
            lol_html_selector_free(pSelector)
            
            guard let rewriter = rewriter else {
                lolHtmlError()
            }
            
            _ = html.withUTF8 { buffer in
                buffer.withMemoryRebound(to: CChar.self) { buffer in
                    lol_html_rewriter_write(rewriter, buffer.baseAddress!, buffer.count)
                }
            }
        }
        if userData.isInParagraph {
            return userData.paragraphText.htmlUnescape().trimmingCharacters(in: .whitespacesAndNewlines)
            // todo: steal css whitespace collapsing from tusker
        } else {
            return nil
        }
    }
    
    private static func lolHtmlError() -> Never {
        let lastError = lol_html_take_last_error()
        let message = String(bytesNoCopy: UnsafeMutableRawPointer(mutating: lastError.data!), length: lastError.len, encoding: .utf8, freeWhenDone: false)
        fatalError(message ?? "Unknown lol-html error")
    }
}

private struct UserData {
    var isInParagraph = false
    var paragraphText = ""
}

private func elementHandler(element: OpaquePointer!, userData: UnsafeMutableRawPointer!) -> lol_html_rewriter_directive_t {
    let userDataPtr = userData.assumingMemoryBound(to: UserData.self)
    if userDataPtr.pointee.isInParagraph {
        return LOL_HTML_STOP
    } else {
        let s = lol_html_element_tag_name_get(element)
        let tagName = String(bytesNoCopy: UnsafeMutableRawPointer(mutating: s.data), length: s.len, encoding: .utf8, freeWhenDone: false)!
        userDataPtr.pointee.isInParagraph = tagName == "p" || tagName == "P"
        lol_html_str_free(s)
        return LOL_HTML_CONTINUE
    }
}

private func textHandler(chunk: OpaquePointer!, userData: UnsafeMutableRawPointer!) -> lol_html_rewriter_directive_t {
    let userDataPtr = userData.assumingMemoryBound(to: UserData.self)
    if userDataPtr.pointee.isInParagraph {
        let s = lol_html_text_chunk_content_get(chunk)
        let content = String(bytesNoCopy: UnsafeMutableRawPointer(mutating: s.data), length: s.len, encoding: .utf8, freeWhenDone: false)!
        userDataPtr.pointee.paragraphText += content
        if userDataPtr.pointee.paragraphText.underestimatedCount >= 1024 {
            // lol-html seems to get confused by img tags with hundreds of kilobytes of data in their src attributes
            // and returns that data as text even though it's a tag
            // if the text is over 1024 characters so far, we assume that's what's happened
            // and abandon this attempt and try again at the next paragraph
            userDataPtr.pointee.paragraphText = ""
            userDataPtr.pointee.isInParagraph = false
        }
    }
    return LOL_HTML_CONTINUE
}

private func outputSink(chunk: UnsafePointer<CChar>!, chunkLen: Int, userData: UnsafeMutableRawPointer!) {
    // no-op
}