Scala: Most concise conversion of a CSS color string to RGB integers

Question

I am trying to get the RGB values of a CSS color string and wonder how good my code is:

object Color {
  def stringToInts(colorString: String): Option[(Int, Int, Int)] = {
    val trimmedColorString: String = colorString.trim.replaceAll("#", "")
    val longColorString: Option[String] = trimmedColorString.length match {
      // allow only strings with either 3 or 6 letters
      case 3 => Some(trimmedColorString.flatMap(character => s"$character$character"))
      case 6 => Some(trimmedColorString)
      case _ => None
    }
    val values: Option[Seq[Int]] = longColorString.map(_
      .foldLeft(Seq[String]())((accu, character) => accu.lastOption.map(_.toSeq) match {
        case Some(Seq(_, _)) => accu :+ s"$character" // previous value is complete => start with succeeding
        case Some(Seq(c)) => accu.dropRight(1) :+ s"$c$character" // complete the previous value
        case _ => Seq(s"$character") // start with an incomplete first value
      })
      .flatMap(hexString => scala.util.Try(Integer.parseInt(hexString, 16)).toOption)
      // .flatMap(hexString => try {
      //  Some(Integer.parseInt(hexString, 16))
      // } catch {
      //   case _: Exception => None
      // })
    )
    values.flatMap(values => values.size match {
      case 3 => Some((values.head, values(1), values(2)))
      case _ => None
    })
  }
}

// example:

println(Color.stringToInts("#abc")) // prints Some((170,187,204))

You may run that example on https://scastie.scala-lang.org

The parts of that code I am most unsure about are

• the match in the foldLeft (is it a good idea to use string interpolation or can the code be written shorter without string interpolation?)
• Integer.parseInt in conjunction with try (can I use a prettier alternative in Scala?) (solved thanks to excellent comment by Xavier Guihot)

But I expect most parts of my code to be improvable. I do not want to introduce new libraries in addition to com.itextpdf to shorten my code, but using com.itextpdf functions is an option. (The result of stringToInts is going to be converted into a new com.itextpdf.kernel.colors.DeviceRgb(...), thus I have installed com.itextpdf anyway.)

Tests defining the expected function:

import org.scalatest.{BeforeAndAfterEach, FunSuite}

class ColorTest extends FunSuite with BeforeAndAfterEach {

  test("shorthand mixed case color") {
    val actual: Option[(Int, Int, Int)] = Color.stringToInts("#Fa#F")
    val expected = (255, 170, 255)
    assert(actual === Some(expected))
  }

  test("mixed case color") {
    val actual: Option[(Int, Int, Int)] = Color.stringToInts("#1D9a06")
    val expected = (29, 154, 6)
    assert(actual === Some(expected))
  }

  test("too short long color") {
    val actual: Option[(Int, Int, Int)] = Color.stringToInts("#1D9a6")
    assert(actual === None)
  }

  test("too long shorthand color") {
    val actual: Option[(Int, Int, Int)] = Color.stringToInts("#1D9a")
    assert(actual === None)
  }

  test("invalid color") {
    val actual: Option[(Int, Int, Int)] = Color.stringToInts("#1D9g06")
    assert(actual === None)
  }

}

Answer 1

At the moment of writing this answer the other answers don't properly handle rgb(), rgba() and named colors cases. Color strings that start with hashes (#) are only a part of the deal.

As you have iText7 as a dependency and iText7 has a pdfHTML add-on which means the logic for parsing CSS colors obviously must be somewhere in iText7 and, more importantly, it must handle various range of CSS color cases. The question is only about finding the right place. Fortunately, this API is public and easy to use.

The method you are interested in is WebColors.getRGBAColor() from package com.itextpdf.kernel.colors which accepts a CSS color string a returns a 4-element array with R, G, B, A values (last one stands for alpha, i.e. transparency).

You can use those values to create a color right away (code in Java):

float[] rgbaColor = WebColors.getRGBAColor("#ababab");
Color color = new DeviceRgb(rgbaColor[0], rgbaColor[1], rgbaColor[2]);

In Scala it must be something like

val rgbaColor = WebColors.getRGBAColor("#ababab");
val color = new DeviceRgb(rgbaColor(0), rgbaColor(1), rgbaColor(2));