// written in the D programming language
// author: Timon Gehr

import std.stdio;
import std.conv;
import core.memory;

class Lazy(T){
	T delegate() compute;
	this(T delegate() compute){this.compute=compute;}
	this(T value){this.value=value;}
	T value;
	final T opCall(){
		if(compute==null) return value;
		value=compute(), compute=null;
		return value;
	}
	auto opDispatch(string op,T...)(T args){return lz({return mixin("opCall()."~op)(args);});}
	override final string toString(){return to!string(opCall());}
}

Lazy!T lz(T)(T delegate() compute){return new Lazy!T(compute);} // lazy
Lazy!T st(T)(T value){return new Lazy!T(value);}                // strict

template Type(S){alias typeof({S a;return a();}()) Type;}

struct List(T){
	Lazy!T head;
	Lazy!(List!T) tail;

	string toString(){
		if(empty) return "[]";
		char[] r=['['];
		for(auto xs=this;!xs.empty;xs=xs.tail()) r~=to!string(xs.head())~",";
		r[$-1]=']';
		return cast(string)r;
	}
	T front(){return head();}
	bool empty(){return head is null;}
	static List!T Empty;
	static this(){Empty=List!T(null,lz({assert(0,"Tried to get tail of empty list!");return List.init;}));}
	this(Lazy!T x, Lazy!(List!T) xs){
		head=x;
		tail=xs;
	}
	Lazy!T opIndex(int i){return drop(i-1,st(this))().head;}
	Lazy!(List!T) opSlice(int a,int b){return take(b-a,drop(a-1,st(this)));}
}

auto cons(T)(Lazy!T x, Lazy!(List!T) xs){return lz({return List!T(x,xs);});}

//auto list(T)(Lazy!T[] args...){return lz({return List!T(args);});} // =O?
auto list(T)(Lazy!T[] args...){if(args==[]) return st(Empty!T); return cons(args[0],list(args[1..$]));}
auto list(T)(T[] args...) if(!is(T U==Lazy!U)){if(args==[]) return st(Empty!T); return cons(st(args[0]),list(args[1..$]));}

template Empty(T){alias List!T.Empty Empty;}
template LList(T){alias Lazy!(List!T) LList;}

LList!S map(T,S)(Lazy!S delegate(Lazy!T) f, Lazy!(List!T) xs){
	return lz({
			if(xs().empty) return Empty!S;
			return cons(f(xs.head), map(f,xs.tail))();
		});
}

LList!T take(T)(int n,Lazy!(List!T) xs){
	return lz({
			if(n<=0) return Empty!T;
			return cons(xs().head,lz({return take(n-1,xs().tail)();}))();
		});
}

LList!T drop(T)(int n,Lazy!(List!T) xs){
	return lz({
			auto r=xs();
			foreach(i;0..n) r=r.tail();
			return r;
		});
}

LList!T hamming(T)(){
	LList!T merge(LList!T xs, LList!T ys){
		return lz({
				auto x=xs.head, y=ys.head;
				if(x()<y()) return cons(x,merge(xs.tail,ys))();
				else if(x()>y()) return cons(y,merge(xs,ys.tail))();
				else return cons(x,merge(xs.tail,ys.tail))();
			});
	}
	return lz({
			LList!T r;
			r=cons(st(1),lz({return merge(merge(map((Lazy!T a){return lz({return 2*a();});},r),map((Lazy!T a){return lz({return 3*a();});},r)),map((Lazy!T a){return lz({return 5*a();});},r))();}));
			return r();
		});
}

void main(){
	auto h=hamming!int();
	writeln(take(20,h));
	writeln(h()[1690]);
	// next would require unwieldy std.bigint
}