class CreditCardVisualTransformation: VisualTransformation {
    override fun filter(text: AnnotatedString): TransformedText {
        val trimmed = if (text.text.length >= 16) text.text.substring(0..15) else text.text
        var out = ""
        for (i in trimmed.indices) {
            out += trimmed[i]
            if (i % 4 == 3 && i != 15) out += "-"
        }

        /**
         * The offset translator should ignore the hyphen characters, so conversion from
         *  original offset to transformed text works like
         *  - The 4th char of the original text is 5th char in the transformed text.
         *  - The 13th char of the original text is 15th char in the transformed text.
         *  Similarly, the reverse conversion works like
         *  - The 5th char of the transformed text is 4th char in the original text.
         *  - The 12th char of the transformed text is 10th char in the original text.
         */

        return TransformedText(AnnotatedString(out),
            object : OffsetMap {
                override fun originalToTransformed(offset: Int): Int {
                    if (offset <= 3) return offset
                    if (offset <= 7) return offset + 1
                    if (offset <= 11) return offset + 2
                    if (offset <= 16) return offset + 3
                    return 19
                }

                override fun transformedToOriginal(offset: Int): Int {
                    if (offset <= 4) return offset
                    if (offset <= 9) return offset - 1
                    if (offset <= 14) return offset - 2
                    if (offset <= 19) return offset - 3
                    return 16
                }
            })
    }
}