CardNumberVisualTransformation.kt

class CardNumberVisualTransformation : VisualTransformation {
    override fun filter(text: AnnotatedString): TransformedText {
        val trimmed = if (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 += " "
        }

        val creditCardOffsetTranslator = object : OffsetMapping {
            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
            }
        }

        return TransformedText(AnnotatedString(out), creditCardOffsetTranslator)
    }
}