brackets.kt

private val openingBrackets = "([{<"
private val closingBrackets = ")]}>"

fun String.isBalancedComplex(): Boolean =
        fold(Stack<Char>()) { acc, c ->
            when (c) {
                // For kotlin beginners: `apply(..)` == `{acc.push(c); acc}`: just a shorthand to eliminate 1 line of code, nothing fancy here
                in openingBrackets -> acc.apply { push(c) } 
                in closingBrackets -> {
                    if (acc.isEmpty()) return false //  leading closing braket like ')))'
                    val last = acc.pop()
                    val doesntMatch = closingBrackets.indexOf(c) != openingBrackets.indexOf(last) // inefficient but brief
                    if (doesntMatch) return false
                    else acc
                }
                else -> acc // do nothing its an non-bracket symbol
            }
        }.isEmpty()