MatrixText.kt - A Composeable that displays text Matrix style down the screen

MatrixText.kt

private const val MAX_SPEED = 25
private const val MIN_SPEED = 7
private const val MAX_FONT_SIZE = 64f
private const val MIN_FONT_SIZE = 24f
private val characters = listOf("ジ", "ェ", "ッ", "ト", "パ", "ッ", "ク", "構", "成")
private val colors = listOf(0xffcefbe4, 0xff81ec72, 0xff5cd646, 0xff54d13c, 0xff4ccc32, 0xff43c728)
@Composable
fun MatrixText(
  stripCount: Int = 25,
  lettersPerStrip: Int = 20,
  modifier: Modifier = Modifier
) {
  // Where each strip is in Y
  // Drive composition with this state
  val stripY = remember { mutableStateListOf<Int>() }
  // Where each strip is in X
  val stripX = remember { IntArray(stripCount).toMutableList() }
  // The speed of each strip
  val dY = remember { IntArray(stripCount).toMutableList() }
  // The font size of each strip
  val stripFontSize = remember { IntArray(stripCount).toMutableList() }

  val paint = Paint().asFrameworkPaint().apply {
    isAntiAlias = true
    typeface = Typeface.SANS_SERIF
  }

  Canvas(
    modifier
      .fillMaxSize()
      .background(Color.Black)) {
    val width = size.width
    val height = size.height

    for (i in 0 until stripCount) {
      var y = stripY.getOrNull(i)

      paint.textSize = stripFontSize[i].toFloat()
      if (y == null || (y > height + (lettersPerStrip * stripFontSize[i]))) {
        if (y == null) {
          stripY.add(0)
        }
        // Initialise a strip in a random location
        stripX[i] = (Random.nextFloat() * width).toInt()
        stripY[i] = -99 // Start off screen
        dY[i] = ((Random.nextFloat() * MAX_SPEED.dp.value) + MIN_SPEED.dp.value).toInt()
        stripFontSize[i] = ((Random.nextFloat() * MAX_FONT_SIZE) + MIN_FONT_SIZE).toInt()
      } else {
        val x = stripX[i]

        (0 until lettersPerStrip).forEach { _ ->
          val randChar = characters.random()
          paint.color = colors.random().toInt()

          drawIntoCanvas {
            it.nativeCanvas.drawText(randChar, x.toFloat(), y.toFloat(), paint)
          }

          y -= stripFontSize[i]
        }
      }
    }
  }

  LaunchedEffect(key1 = Unit) {
    while(true) {
      delay(60)
      for (i in 0 until stripCount) {
        // Increase the start position of each strip which will trigger a recomposition
        stripY[i] = stripY[i] + dY[i]
      }
    }
  }
}

private fun<T> SnapshotStateList<T>.getOrNull(index: Int): T? {
  if (index < 0 || index >= size) return null
  return get(index)
}