Decimal Amount Visual Transformation - Jetpack compose Visual Transformation for decimal input. Cursor works well! There might be room for optimization

DecimalAmountTransformation.kt

package io.untec.utils.masks

import android.util.Log
import androidx.compose.ui.text.AnnotatedString
import androidx.compose.ui.text.TextRange
import androidx.compose.ui.text.input.OffsetMapping
import androidx.compose.ui.text.input.TextFieldValue
import androidx.compose.ui.text.input.TransformedText
import androidx.compose.ui.text.input.VisualTransformation
import java.math.BigDecimal
import java.text.DecimalFormat
import java.text.DecimalFormatSymbols
import java.util.*

private val groupingSymbol = ' '
private val decimalSymbol = '.'

private val numberFormatter: DecimalFormat = DecimalFormat("#,###").apply {
    decimalFormatSymbols = DecimalFormatSymbols(Locale.getDefault()).apply {
        groupingSeparator = groupingSymbol
        decimalSeparator = decimalSymbol
    }
}

class DecimalAmountTransformation : VisualTransformation {

    override fun filter(text: AnnotatedString): TransformedText {
        val transformation = reformat(text.text)

        return TransformedText(
            AnnotatedString(transformation.formatted ?: ""),
            object : OffsetMapping {
                override fun originalToTransformed(offset: Int): Int {
                    return transformation.originalToTransformed[offset]
                }

                override fun transformedToOriginal(offset: Int): Int {
                    return transformation.transformedToOriginal[offset]
                }
            },
        )
    }

    private fun reformat(original: String): Transformation {
        val parts = original.split(decimalSymbol)
        check(parts.size < 3) { "original text must have only one dot (use filteredDecimalText)" }

        val hasEndDot = original.endsWith('.')
        var formatted = original

        Log.d("original_tag", original)

        if (original.isNotEmpty() && parts.size == 1) {
            formatted = numberFormatter.format(BigDecimal(parts[0]))

            if (hasEndDot) {
                formatted += decimalSymbol
            }
        } else if (parts.size == 2) {
            val numberPart = numberFormatter.format(BigDecimal(parts[0]))
            val decimalPart = parts[1]

            formatted = "$numberPart.$decimalPart"
        }

        val originalToTransformed = mutableListOf<Int>()
        val transformedToOriginal = mutableListOf<Int>()
        var specialCharsCount = 0

        formatted.forEachIndexed { index, char ->
            if (groupingSymbol == char) {
                specialCharsCount++
            } else {
                originalToTransformed.add(index)
            }
            transformedToOriginal.add(index - specialCharsCount)
        }
        originalToTransformed.add(originalToTransformed.maxOrNull()?.plus(1) ?: 0)
        transformedToOriginal.add(transformedToOriginal.maxOrNull()?.plus(1) ?: 0)

        return Transformation(formatted, originalToTransformed, transformedToOriginal)
    }
}

data class Transformation(
    val formatted: String?,
    val originalToTransformed: List<Int>,
    val transformedToOriginal: List<Int>,
)

DecimalnputFilter.kt

private val decimalSymbol = '.'

object InputFilterRegex {
    val DecimalInput by lazy { Regex("^(\\d*\\.?)+\$") }
}


fun filteredDecimalText(input: TextFieldValue): TextFieldValue {
    var inputText = input.text.replaceFirst(regex = Regex("^0+(?!$)"), "")
    var startsWithDot = input.text.startsWith(decimalSymbol)

    var selectionStart = input.selection.start
    var selectionEnd = input.selection.end

    if (startsWithDot) {
        inputText = "0$inputText"

        if (selectionStart == selectionEnd) {
            selectionStart++
            selectionEnd++
        } else {
            selectionEnd++
        }
    }

    val parts = inputText.split(decimalSymbol)
    var text = if (parts.size > 1) {
        parts[0] + decimalSymbol + parts.subList(1, parts.size).joinToString("")
    } else {
        parts.joinToString("")
    }

    if (text.startsWith(decimalSymbol)) {
        text = "0$text"
    }

    return input.copy(text = text, selection = TextRange(selectionStart, selectionEnd))
}

UsageScreen.kt

    BasicTextField(
        value = value,
        onValueChange = {
            if (!it.text.contains(InputFilterRegex.DecimalInput)) {
                return@BasicTextField
            }
            onValueChange(filteredDecimalText(it))
        },
        visualTransformation = DecimalAmountTransformation(),
    )