png mixed content

MakePNG.kt

import io.javalin.Javalin
import java.util.zip.CRC32
import java.util.zip.Deflater

/*
    main requires:
        io.javalin:javalin:2.7.0
        org.slf4j:slf4j-simple:1.7.25

    test at:
        https://skhmt.github.io/https.html
 */
fun main(args: Array<String>) {
    val port = 80

    val app = Javalin.create()
    app.enableCorsForAllOrigins()
    app.start(port)
    println("running on port $port")

    app.get("/get.png") { ctx ->
        val msg = ctx.queryParam("msg")
        println("${ctx.ip()}: msg=$msg")

        val png = makePNG(msg!!)
        ctx.result(png.inputStream())
    }
}

fun makePNG(data: String): ByteArray {
    val dataAsByteArray = data.toByteArray(Charsets.UTF_8) // string (utf8) as a byte array

    return (pngSignature() +
            pngIHDR(dataAsByteArray.size,1) +
            pngIDAT(dataAsByteArray) +
            pngIEND())
}

// PNG Signature - https://www.w3.org/TR/PNG/#5PNG-file-signature
fun pngSignature(): ByteArray {
    return byteArrayOf(-119,80,78,71,13,10,26,10)
}

// PNG IHDR chunk - https://www.w3.org/TR/PNG/#11IHDR
fun pngIHDR(width: Int, height: Int): ByteArray {
    val ihdrLength = byteArrayOf(0,0,0,13)
    val ihdrType = byteArrayOf(73,72,68,82)
    val ihdrData = byteArrayOf(
        *intToBA(width), // width
        *intToBA(height), // height
        8, // bitdepth - 8 so each pixel is a byte
        0, // color type - 0 is greyscale
        0,0,0 // compression, filter, and interlace methods - must be 0
    )
    val ihdrCRC = getCRC(ihdrType, ihdrData)

    return (ihdrLength +
            ihdrType +
            ihdrData +
            ihdrCRC)
}

// PNG IDAT chunk - https://www.w3.org/TR/PNG/#11IDAT
fun pngIDAT(data: ByteArray): ByteArray {
    val idatType = byteArrayOf(73,68,65,84)

    val idatData = deflate(byteArrayOf(0, *data)) // filter type 0 (no filter)

    val idatCRC = getCRC(idatType, idatData)

    val idatLength = intToBA(idatData.size) // compressed data length

    return (idatLength +
            idatType +
            idatData +
            idatCRC)
}

// PNG IEND chunk - https://www.w3.org/TR/PNG/#11IEND
fun pngIEND(): ByteArray {
    return byteArrayOf(0,0,0,0,73,69,78,68,-82,66,96,-126)
}

// convenience function to use Java's built-in CRC32 library
private fun getCRC(ba1: ByteArray, ba2: ByteArray): ByteArray {
    val ba = ba1 + ba2
    val crc = CRC32()
    crc.update(ba)
    return intToBA(crc.value.toInt())
}

// this is faster than using a for-loop and faster than ByteBuffer by 15-20x
private fun intToBA(num: Int): ByteArray {
    val ba = ByteArray(4)
    ba[3] = (num and 0xff).toByte()
    ba[2] = ((num shr 8) and 0xff).toByte()
    ba[1] = ((num shr 16) and 0xff).toByte()
    ba[0] = ((num shr 24) and 0xff).toByte()
    return ba
}

// convenience function to use Java's built-in Deflater library
// the buffer can be modified if necessary; it should probably be 32767
private fun deflate(ba: ByteArray): ByteArray {
    val buffer = ByteArray(1048576) // 2^20 = 1024*1024 = 1MB max
    val deflater = Deflater()
    deflater.setInput(ba)
    deflater.finish()
    val length = deflater.deflate(buffer)

    return buffer.sliceArray(0 until length)
}