CurrencyVisualTransformation.kt

import java.text.DecimalFormat

import androidx.core.text.isDigitsOnly
import androidx.compose.ui.text.AnnotatedString
import androidx.compose.ui.text.input.OffsetMapping
import androidx.compose.ui.text.input.TransformedText
import androidx.compose.ui.text.input.VisualTransformation

private val String.isValidForFormatting get(): Boolean =
    isNotBlank() && isDigitsOnly() && length <= 6

private fun String.formatToAmount(): String =
    if (isValidForFormatting) {
        DecimalFormat("#,##,###").format(this.toLong())
    } else {
        this
    }

class CurrencyVisualTransformation(private val separator: Char = ' ') : VisualTransformation {
    override fun filter(text: AnnotatedString): TransformedText {
        val originalText = text.text
        val formattedText = originalText.formatToAmount()
        val annotatedText = AnnotatedString(text = formattedText)
        val offsetMapping = object : OffsetMapping {
            override fun originalToTransformed(offset: Int): Int {
                if (originalText.isValidForFormatting) {
                    val separators = formattedText.count { character -> character == separator }
                    return when {
                        offset <= 1 -> offset
                        offset <= 3 -> if (separators >= 1) offset + 1 else offset
                        offset <= 5 -> if (separators == 2) offset + 2 else offset + 1
                        else -> 8
                    }
                }
                return offset
            }

            override fun transformedToOriginal(offset: Int): Int {
                if (originalText.isValidForFormatting) {
                    val separators = formattedText.count { character -> character == separator }
                    return when (offset) {
                        8, 7 -> offset - 2
                        6 -> if (separators == 1) 5 else 4
                        5 -> if (separators == 1) 4 else if (separators == 2) 3 else offset
                        4, 3 -> if (separators >= 1) offset - 1 else offset
                        2 -> if (separators == 2) 1 else offset
                        else -> offset
                    }
                }
                return offset
            }
        }

        return TransformedText(
            text = annotatedText,
            offsetMapping = offsetMapping
        )
    }
}