Obfuscation / Hiding text with TransformationMethod

obfuscation-transformer.kt

import android.graphics.Rect
import android.text.method.TransformationMethod
import android.view.View

class ObfuscationTransformation(val type: Type) : TransformationMethod {
    enum class Type {
        Email, Phone
    }

    override fun getTransformation(source: CharSequence, view: View?): CharSequence {
        return PassCharSequence(source, type)
    }

    override fun onFocusChanged(
        view: View?,
        sourceText: CharSequence?,
        focused: Boolean,
        direction: Int,
        previouslyFocusedRect: Rect?
    ) {
        /* noop */
    }

    private class PassCharSequence(private val charSequence: CharSequence, private val type: Type) :
        CharSequence {
        override val length: Int
            get() = charSequence.length

        override fun get(index: Int): Char {
            return getChar(index)
        }

        override fun subSequence(startIndex: Int, endIndex: Int): CharSequence {
            return PassCharSequence(charSequence.substring(startIndex, endIndex), type)
        }

        private fun getChar(index: Int): Char {
            return when (type) {
                Type.Email -> {
                    val atIdx = charSequence.indexOf("@")
                    when {
                        index < atIdx -> {
                            when (index != 0 && index < atIdx - 1) {
                                true -> "*".single()
                                false -> charSequence[index]
                            }
                        }
                        index > atIdx -> {
                            val dotIdx = charSequence.indexOfLast { char -> char == ".".single() }
                            when (index > atIdx + 1 && index < dotIdx - 1) {
                                true -> "*".single()
                                false -> charSequence[index]
                            }
                        }
                        else -> charSequence[index]
                    }
                }
                Type.Phone -> {
                    when(index > charSequence.length - (charSequence.length - 1) && index < charSequence.length - 2) {
                        true -> "*".single()
                        false -> charSequence[index]
                    }
                }
            }
        }
    }
}