StringExtensions.kt

import android.graphics.Typeface
import android.text.SpannableString
import android.text.Spanned
import android.text.style.StrikethroughSpan
import android.text.style.StyleSpan
import android.text.style.UnderlineSpan

private val BOLD_REGEX = Regex("\\*([^*]+)\\*")
private val ITALIC_REGEX = Regex("_([^_]+)_")
private val STRIKE_REGEX = Regex("~([^~]+)~")
private val UNDERLINE_REGEX = Regex("#([^#]+)#")
private val EFFECT_REGEX_LIST = listOf(BOLD_REGEX, ITALIC_REGEX, STRIKE_REGEX, UNDERLINE_REGEX)

private data class EffectIndex(val start: Int, val end: Int, val type: Int)

private fun spanByType(type: Int): Any {
    return when (type) {
        0 -> StyleSpan(Typeface.BOLD)
        1 -> StyleSpan(Typeface.ITALIC)
        2 -> StrikethroughSpan()
        3 -> UnderlineSpan()
        else -> throw IllegalStateException("unknown type")
    }
}

fun String?.nullIfBlank(): String? {
    return if (isNullOrBlank()) null else this
}

fun String.createSpannable(): SpannableString {
    val values = EFFECT_REGEX_LIST.flatMapIndexed { type, regex ->
        val matches = regex.findAll(this)
        matches.map { result ->
            val range = result.groups[1]?.range ?: throw IllegalStateException("no match at regex?")
            EffectIndex(range.first - 1, range.last + 1, type)
        }
    }

    val indices = values.flatMap { listOf(it.start, it.end) }.distinct().sorted()

    fun indexedPosition(index: Int): Int {
        val position = indices.indexOf(index)
        return index - position
    }

    val finalString = this
        .replace(BOLD_REGEX, "$1")
        .replace(ITALIC_REGEX, "$1")
        .replace(STRIKE_REGEX, "$1")
        .replace(UNDERLINE_REGEX, "$1")

    val spannable = SpannableString(finalString).apply {
        values.forEach { effect ->
            val span = spanByType(effect.type)
            setSpan(span, indexedPosition(effect.start), indexedPosition(effect.end), Spanned.SPAN_INCLUSIVE_INCLUSIVE)
        }
    }

    return spannable
}