Add a JPEG comment marker to file in pure Swift.

AddJPEGComment.swift

//
// Inspired by wrjpgcom of libjpeg
// https://github.com/libjpeg-turbo/libjpeg-turbo/blob/master/wrjpgcom.c
//
// https://stackoverflow.com/a/46045524/5536516
//
// Note: To add an EXIF UserComment go here:
// https://gist.github.com/nyg/c90f36abbd30f72c8b6681ef23db886b

import Foundation

/**
 Adds a comment to the given JPEG image.

 The comment marker (COM) will be insert just before the first SOF
 marker found.

 1. It causes the new comment to appear after, rather than before,
 existing comments,
 2. and ensures that comments come after any JFIF or JFXX markers,
 as required by the JFIF specification.


 - parameters:
 - jpegData: The JPEG image as a Data instance. Will be modified
 by the function.
 - comment: The comment that will be added to the image.
 */
func addJPEGComment(to jpegData: inout Data, _ comment: String) {

    // Define JPEG markers.
    let markerStart: UInt8 = 0xFF
    let comMarker: UInt8 = 0xFE

    let sofMarkers: [UInt8] = [
        0xC0, 0xC1, 0xC2, 0xC3, 0xC5, 0xC6, 0xC7, 0xC9, 0xCA, 0xCB, 0xCD, 0xCE, 0xCF,
        0xD9 // End of Image
    ]

    // Find range of first SOF marker, or EOI.
    var firstSOFRange: Range<Data.Index>?
    for marker in sofMarkers {
        if let range = jpegData.range(of: Data(bytes: [ markerStart, marker ])) {
            firstSOFRange = range
            break
        }
    }

    guard let firstSOFIndex = firstSOFRange?.lowerBound
        else { fatalError("No SOF or EOI marker found.") }

    // Create the comment byte array.
    let length = comment.lengthOfBytes(using: .utf8) + 2
    let l1 = UInt8(length >> 8) & markerStart
    let l2 = UInt8(length) & markerStart
    let commentArray = [ markerStart, comMarker, l1, l2 ] + [UInt8](comment.utf8)

    // Insert the comment array into image data object.
    jpegData.insert(contentsOf: commentArray, at: firstSOFIndex)
}

/* Usage example */

guard let jpegURL = Bundle.main.url(forResource: "image", withExtension: "jpg")
    else { fatalError("File not found.") }

guard var jpegData = try? Data(contentsOf: jpegURL)
    else { fatalError("File could not be read.") }

addJPEGComment(to: &jpegData, "This is a JPEG comment.")

guard let jpegOutputURL = try? FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent("image_com.jpg")
    else { fatalError("Destination URL not created") }

try jpegData.write(to: jpegOutputURL)

print("rdjpgcom \(jpegOutputURL.path)")