Bencode Encoder/Decoder in Kotlin (extended version) - https://en.wikipedia.org/wiki/Bencode

readme.md

Try it: http://try.kotlinlang.org/#/UserProjects/qqncr5tl9hc2a1sjt3qnh29n37/105sltrijnh63n5f7ago6fnosl

Ben.kt -> regular code, focus on readabilty

B.kt -> small code, but still readable

BFunKTional.kt -> small code, functional

Example.kt -> Parse benString and encode it again!

Results:

i42e -> 42
i0e -> 0
i-42e -> -42
4:spam -> spam
3:bar -> bar
3:foo -> foo
l4:spami42ee -> [spam, 42]
d3:bar4:spam3:fooi42ee -> {bar=spam, foo=42}

B.kt

class B(val s:String, var i:Int=0) {
    fun r(c:Int)=s.substring(i,i+c).apply{i+=length}
    fun u(c:Char)=s.substring(i,s.indexOf(c,i)).apply{i+=length+1}
    fun d():Any=when(r(1)[0]){
        'e'->Unit;'i'->u('e').toInt()
        'l'->ArrayList<Any>().apply{var o=d();while(o!=Unit){add(o);o=d()}}
        'd'->HashMap<String,Any>().apply{var o=d();while(o!=Unit){put(o as String,d());o=d()}}
        in('0'..'9')->r((s[i-1]+u(':')).toInt())
        else->throw IllegalStateException("Char:${s[i-1]}")
    }
}
fun Any.e():String=let{o->when(o){
    is Int->"i${o}e";is String->"${o.length}:$o"
    is List<*>->"l${o.joinToString(""){it!!.e()}}e"
    is Map<*,*>->"d${o.map{it.key!!.e()+it.value!!.e()}.joinToString("")}e"
    else->throw IllegalStateException()
}}

Ben.kt

class Ben(val str: String, var i: Int = 0) {
    fun read(count: Int) = str.substring(i, i + count).apply { i += length }
    fun readUntil(c: Char) = str.substring(i, str.indexOf(c, i)).apply { i += length + 1 }
    fun decode(): Any = when (read(1)[0]) {
        'i' -> readUntil('e').toInt()
        'l' -> ArrayList<Any>().apply {
            var obj = decode()
            while (obj != Unit) {
                add(obj)
                obj = decode()
            }
        }
        'd' -> HashMap<String, Any>().apply {
            var obj = decode()
            while (obj != Unit) {
                put(obj as String, decode())
                obj = decode()
            }
        }
        'e' -> Unit
        in ('0'..'9') -> read((str[i-1] + readUntil(':')).toInt())
        else -> throw IllegalStateException("Char: ${str[i-1]}")
    }

    companion object {
        fun encodeStr(obj: Any): String = when (obj) {
            is Int -> "i${obj}e"
            is String -> "${obj.length}:$obj"
            is List<*> -> "l${obj.joinToString("") { encodeStr(it!!) }}e"
            is Map<*, *> -> "d${obj.map { encodeStr(it.key!!) + encodeStr(it.value!!) }.joinToString("")}e"
            else -> throw IllegalStateException()
        }
    }
}

BFunKTional.kt

fun String.r(c:Int,i:Int)=substring(i,i+c).let{it to it.length+i}
fun String.u(c:Char,i:Int)=substring(i,indexOf(c,i)).let{it to it.length+i+1}
fun String.d(i:Int=0):Pair<Any,Int> =r(1,i).let{(r,i)->when(r[0]){
    'e'->Unit to i;'i'->u('e',i).let{(r,i)->r.toInt() to i}
    'l'->ArrayList<Any>().let{var(o,j)=d(i);while(o!=Unit){it.add(o);d(j).let{(o2,j2)->o=o2;j=j2}};it to j}
    'd'->HashMap<String,Any>().let{var(k,j)=d(i);while(k!=Unit){d(j).let{(v,z)->it.put(k as String,v);d(z).let{(k1,v1)->k=k1;j=v1}}};it to j}
    in('0'..'9')->u(':',i).let{(r,j)->(get(i-1)+r).toInt() to j}.let{(r,i)->r(r,i)}
    else->throw IllegalStateException("Char:${get(i-1)}")
}}
fun Any.e():String=let{o->when(o){
    is Int->"i${o}e";is String->"${o.length}:$o"
    is List<*>->"l${o.joinToString(""){it!!.e()}}e"
    is Map<*,*>->"d${o.map{(k,v)->k!!.e()+v!!.e()}.joinToString("")}e"
    else->throw IllegalStateException()
}}

Example.kt

fun main(args: Array<String>) {
    val n = listOf("i42e", "i0e", "i-42e")
    val s = listOf("4:spam", "3:bar", "3:foo")
    val l = "l${s[0]}${n[0]}e"
    val m = "d${s[1]}${s[0]}${s[2]}${n[0]}e"
    (n + s + l + m).forEach {
        val ben = Ben(it)
        val obj = ben.decode()
        val ben2 = Ben.encodeStr(obj)
        println("$it -> $obj")
        if (ben.str.length != ben.i) throw IllegalStateException("Malformed")
        if (it != ben2) throw IllegalStateException("Encoder fail: $it != $ben2")
    }
}

ExampleFunKTional.kt

fun main(args: Array<String>) {
    val n = listOf("i42e", "i0e", "i-42e")
    val s = listOf("4:spam", "3:bar", "3:foo")
    val l = "l${s[0]}${n[0]}e"
    val m = "d${s[1]}${s[0]}${s[2]}${n[0]}e"
    (n + s + l + m).forEach {
        val (obj,i) = it.d()
        val ben2 = obj.e()
        println("$it -> $obj")
        if (it.length != i) throw IllegalStateException("Malformed")
        if (it != ben2) throw IllegalStateException("Encoder fail: $it != $ben2")
    }
}