Transforming EditText value into currency

CurrencyOffsetMapping.kt

import androidx.compose.ui.text.input.OffsetMapping

class CurrencyOffsetMapping(originalText: String, formattedText: String) : OffsetMapping {
    private val originalLength: Int = originalText.length
    private val indexes = findDigitIndexes(originalText, formattedText)

    private fun findDigitIndexes(firstString: String, secondString: String): List<Int> {
        val digitIndexes = mutableListOf<Int>()
        var currentIndex = 0
        for (digit in firstString) {
            // Find the index of the digit in the second string
            val index = secondString.indexOf(digit, currentIndex)
            if (index != -1) {
                digitIndexes.add(index)
                currentIndex = index + 1
            } else {
                // If the digit is not found, return an empty list
                return emptyList()
            }
        }
        return digitIndexes
    }

    override fun originalToTransformed(offset: Int): Int {
        if (offset >= originalLength) {
            return indexes.last() + 1
        }
        return indexes[offset]
    }

    override fun transformedToOriginal(offset: Int): Int {
        return indexes.indexOfFirst { it >= offset }.takeIf { it != -1 } ?: originalLength
    }
}

CurrencyVisualTransformation.kt

import android.util.Log
import androidx.compose.runtime.Composable
import androidx.compose.runtime.remember
import androidx.compose.ui.platform.LocalInspectionMode
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
import androidx.core.text.isDigitsOnly
import java.text.NumberFormat
import java.util.Currency

private class CurrencyVisualTransformation(
    currencyCode: String
) : VisualTransformation {

    private val numberFormatter = NumberFormat.getCurrencyInstance().apply {
        currency = Currency.getInstance(currencyCode)
        maximumFractionDigits = 0
    }

    override fun filter(text: AnnotatedString): TransformedText {
        val originalText = text.text.trim()
        if (originalText.isEmpty()) {
            return TransformedText(text, OffsetMapping.Identity)
        }
        if (originalText.isDigitsOnly().not()) {
            Log.w("TAG", "Prize visual transformation require using digits only but found [$originalText]")
            return TransformedText(text, OffsetMapping.Identity)
        }

        val formattedText = numberFormatter.format(originalText.toInt())
        return TransformedText(
            AnnotatedString(formattedText),
            CurrencyOffsetMapping(originalText, formattedText)
        )
    }
}

@Composable
fun rememberCurrencyVisualTransformation(currency: String): VisualTransformation {
    val inspectionMode = LocalInspectionMode.current
    return remember(currency) {
        if (inspectionMode) {
            VisualTransformation.None
        } else {
            CurrencyVisualTransformation(currency)
        }
    }
}

SampleUse.kt

import androidx.compose.foundation.text.KeyboardOptions
import androidx.compose.material.TextField
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.ui.text.input.KeyboardType

private const val MAX_VALUE = 10000

@Composable
fun SampleUse() {
    var value by remember { mutableStateOf("") }
    val currencyVisualTransformation = rememberCurrencyVisualTransformation(currency = "USD")
    TextField(
        value = value,
        onValueChange = { newValue ->
            val trimmed = newValue.trimStart('0').trim { it.isDigit().not() }
            if (trimmed.isEmpty() || trimmed.toInt() <= MAX_VALUE) {
                value = trimmed
            }
        },
        keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Number),
        visualTransformation = currencyVisualTransformation
    )
}