Get indexes of substrings contained in a string in Kotlin-way

Question

I want to implement a function that will return the indexes of the substrings in the specified string. For now i did it in Java-style:

public fun String?.indexesOf(substr: String, ignoreCase: Boolean = true): List<Int> {
    var list = mutableListOf<Int>()
    if (substr.isNullOrBlank()) return list
    var count = 0;
    this?.split(substr, ignoreCase = ignoreCase)?.forEach {
        count += it.length
        list.add(count)
        count += substr.length
    }
    list.remove(list.get(list.size-1))
    return list
}

But I don't think this is a kotlin-way solution. Its most looks like typical java program but written in kotlin. How can this be implemented more elegantly using kotlin?

Answer 1

what would i do is the following:

fun ignoreCaseOpt(ignoreCase: Boolean) = 
    if (ignoreCase) setOf(RegexOption.IGNORE_CASE) else emptySet()

fun String?.indexesOf(pat: String, ignoreCase: Boolean = true): List<Int> =
    pat.toRegex(ignoreCaseOpt(ignoreCase))
        .findAll(this?: "")
        .map { it.range.first }
        .toList()

// check:
println("xabcaBd".indexesOf("ab", true))
println("xabcaBd".indexesOf("ab", false))
println("xabcaBd".indexesOf("abx", true))

val s: String? = null
println(s.indexesOf("aaa"))

// output:
[1, 4]
[1]
[]
[]

Answer 2

You could condense it down to something like this:

public fun String?.indexesOf(substr: String, ignoreCase: Boolean = true): List<Int> {
    return this?.let { 
        val regex = if (ignoreCase) Regex(substr, RegexOption.IGNORE_CASE) else Regex(substr)
        regex.findAll(this).map { it.range.start }.toList()
    } ?: emptyList()
}

Whether that's more efficient is a different matter. You'd have to test that.

---

If you wanted "aaa".indexesOf("aa") to return [0, 1] rather than just [0], you should be able to do that by modifying the regex to use positive lookahead, i.e.:

val regex = if (ignoreCase) Regex("(?=$substr)", RegexOption.IGNORE_CASE) else Regex("(?=$substr)")