CHGenerator/Sources/Generator/Files/FileUpdateChecker.swift

import Foundation
import CryptoKit

final class FileUpdateChecker {

    private static let hashesFileName = "hashes.json"

    private let input: URL

    private var hashesFile: URL {
        input.appendingPathComponent(FileUpdateChecker.hashesFileName)
    }
    
    /**
     The hashes of all accessed files from the previous run

     The key is the relative path to the file from the source
     */
    private var previousFiles: [String : Data] = [:]

    /**
     The paths of all files which were accessed, with their new hashes

     This list is used to check if a file was modified, and to write all accessed files back to disk
     */
    private var accessedFiles: [String : Data] = [:]

    private var source: String {
        "FileUpdateChecker"
    }

    init(input: URL) {
        self.input = input
        guard hashesFile.exists else {
            log.add(info: "No file hashes loaded, regarding all content as new", source: source)
            return
        }
        let data: Data
        do {
            data = try Data(contentsOf: hashesFile)
        } catch {
            log.add(
                warning: "File hashes could not be read, regarding all content as new",
                source: source,
                error: error)
            return
        }
        do {
            self.previousFiles = try JSONDecoder().decode(from: data)
        } catch {
            log.add(
                warning: "File hashes could not be decoded, regarding all content as new",
                source: source,
                error: error)
            return
        }
    }

    func fileHasChanged(at path: String) -> Bool {
        guard let oldHash = previousFiles[path] else {
            // Image wasn't used last time, so treat as new
            return true
        }
        guard let newHash = accessedFiles[path] else {
            // Each image should have been loaded once
            // before using this function
            fatalError()
        }
        return oldHash != newHash
    }

    func didLoad(_ data: Data, at path: String) {
        accessedFiles[path] = SHA256.hash(data: data).data
    }


    func writeDetectedFileChangesToDisk() {
        do {
            let encoder = JSONEncoder()
            encoder.outputFormatting = .prettyPrinted
            let data = try encoder.encode(accessedFiles)
            try data.write(to: hashesFile)
        } catch {
            log.add(warning: "Failed to save file hashes", source: source, error: error)
        }
    }

}