day23.m

currset = [7 8 9 4 6 5 1 2 3];%,10:1000000];
% currset = [3 8 9 1 2 5 4 6 7];%,10:1000000];
N = length(currset);
smartlist = zeros(N,0);
for i = 1:N
    smartlist(currset(i)) = currset(wrapN(i+1,N));
end
currcup = currset(1);
smartlist = iterate(smartlist,currcup,100);

part1 = zeros(8,1);
curr = 1;
for i = 1:8
    part1(i) = smartlist(curr);
    curr = smartlist(curr);
end
disp("Part1: " + strjoin(string(part1),''));
%part2 
currset = [7 8 9 4 6 5 1 2 3,10:1000000];
% currset = [3 8 9 1 2 5 4 6 7,10:1000000];

N = length(currset);
smartlist = zeros(N,0);
for i = 1:N
    smartlist(currset(i)) = currset(wrapN(i+1,N));
end
currcup = currset(1);
smartlist = iterate(smartlist,currcup,1e7);
part2 = uint64(smartlist(1)*smartlist(smartlist(1)))

function smartlist = iterate(smartlist,currcup,rounds)
N = length(smartlist);
for i = 1:rounds
    pickedup = [smartlist(currcup),smartlist(smartlist(currcup)),smartlist(smartlist(smartlist(currcup)))];
    smartlist(currcup) = smartlist(pickedup(end));
    dest = wrapN(currcup -1,N);
    while any(pickedup == dest)
        dest = wrapN(dest-1,N);
    end
    buff = smartlist(dest);
    smartlist(dest) = pickedup(1);
    smartlist(pickedup(end)) = buff;
    currcup = smartlist(currcup);
end
end

function r = wrapN(x,N)
    r = (1 + mod(x-1, N));
end
  
% for i = 1:100
%     if ~mod(i,10000)
%         disp(i);
%     end
%     startindex = wrapN(find(currset == currcup)+1,N);
%     range = wrapN(startindex:startindex+2,N);
%     pickedup = currset(range);
%     currset(range) = [];
%     dest = wrapN(currcup-1,N);
%     while any(pickedup == dest)
%         dest = wrapN(dest -1,N);
%     end
% %     disp(dest);
%     currset = [currset(1:find(currset==dest)),pickedup,currset(find(currset==dest)+1:end)];
%     disp("result: " + strjoin(string(currset)));
%     currcup = currset(wrapN(find(currset == currcup)+1,N));
% end