StringView.kt

class StringView(
    val string: String,
    val offset: Int,
    val length: Int,
) : Iterable<Char> {
    constructor(string: String) : this(string, 0, string.length)

    init {
        val end = offset + length
        assert(offset >= 0 && offset < string.length)
        assert(end <= string.length)
    }

    override fun iterator() = StringViewIterator(this)

    operator fun get(idx: Int) = string[offset + idx]

    fun substring(offset: Int, length: Int): StringView {
        return StringView(string, this.offset + offset, length)
    }

    fun startsWith(pattern: StringView): Boolean {
        if (pattern.length > this.length) {
            return false
        }

        for (i in 0 until pattern.length) {
            if (this[i] != pattern[i]) {
                return false
            }
        }
        return true
    }

    fun endsWith(pattern: StringView): Boolean {
        if (pattern.length > this.length) {
            return false
        }

        val idxDelta = this.length - pattern.length
        for (i in (pattern.length - 1) downTo 0) {
            if (this[i + idxDelta] != pattern[i]) {
                return false
            }
        }
        return true
    }
}

class StringViewIterator(private val stringView: StringView) : Iterator<Char> {
    var offset: Int = 0

    override fun hasNext(): Boolean = offset < stringView.length

    override fun next(): Char {
        val result = stringView.string[stringView.offset + offset]
        offset++
        return result
    }
}

fun String.view() = StringView(this)
fun String.subView(offset: Int, length: Int) = StringView(this, offset, length)