An Android InputFilter for a decimal range

RangeInputFilter.kt

import android.text.InputFilter
import android.text.Spanned

class RangeInputFilter(private val min: Int, private val max: Int) : InputFilter {
  override fun filter(
    source: CharSequence,
    start: Int,
    end: Int,
    dest: Spanned,
    dstart: Int,
    dend: Int
  ): CharSequence? {
    if (start == end) {
      // Always allow the empty replacement.
      return null
    }
    var realStart = start
    if (dstart == 0) {
      while (source[realStart] == '0') {
        realStart++
        if (realStart == end) {
          // Minor optimization when the replacement text is all preceding 0s.
          return ""
        }
      }
    }
    val resultLength = dstart + end - realStart + dest.length - dend
    val result = StringBuilder(resultLength)
      .append(dest, 0, dstart)
      .append(source, realStart, end)
      .append(dest, dend, dest.length)
      .toString()
    return try {
      val input = result.toInt()
      if (input in min..max) {
        if (realStart == start) {
          // Accept the new input.
          null
        } else {
          // Accept the new input without the preceding 0s.
          source.subSequence(realStart, end)
        }
      } else {
        // Reject the new input and return to the original text.
        dest.subSequence(dstart, dend)
      }
    } catch (e: NumberFormatException) {
      // Reject the new input and return to the original text.
      dest.subSequence(dstart, dend)
    }
  }
}