Coding Challenge #40 Top Slide Shuffle - Card Shuffling

 // top slide shuffle
// This is how i shuffle cards when playing a card game.
// I take a random selection from the top (10-50% of the deck) and slide it into the deck so that different parts of the top selection go into the deck at different places. In other words, I insert into the deck randomly a selection of the deck from the top.
// i named it that because i 'slide off' a portion of the top cards

// ary: the input array that will be shuffled
// howMany: the number of cards to take from the top (highest index is the top, 0 is the bottom)
function topSlideShuffle(ary, howMany){
    if(howMany >= ary.length) return ary; // trying to skim too many cards, grabbed whole deck
    
    var output = [];
    var ctop = ary.splice(ary.length - howMany); // get last howMany cards in ary, also removing them from ary
    
    // so... in real life, while you're holding the top part of the deck, the cards would remain in order. The bottom-most card of the top part wouldn't end up on top of the top-most card of the top part during the same shuffle. We'll replicate this retained order... We'll do this by grabbing howMany random indices, then incrementing them so that no 2 share the same index, then sorting them.
    var rands = [];
    for(var xint = 0;xint < howMany;xint++){ // get random indices
        rands.push(Math.round(Math.random() * (ary.length-1)));
    }
    
    // make sure no 2 indices are the same, increment (keep it sorted)
    rands.sort((a,b)=>{return a>b;}); // sort least to greatest
    for(var xint = 0, len = rands.length;xint < len;xint++){
        for(var yint = xint; yint < len;yint++){
            if(xint == yint) continue;
            if(rands[xint] == rands[yint]){
                rands[yint]++;
                rands.sort((a,b)=>{return a>b;});
                xint--;
                break;
            }
        }
    }
    
    // now we insert them at the randomized indices
    for(var xint = 0, len = ctop.length;xint < len;xint++){
        // insert the card via concat()
        var deck_top = ary.splice(rands[xint]+xint); // cards above insertion index
        ary = ary.concat(ctop[xint]);
        ary = ary.concat(deck_top);
    }
    
    return ary;
}

function test(ary, howMany){
    console.log("Top slide shuffling " + howMany + " top cards into the deck randomly. " + ary.toString() + " => " + topSlideShuffle(ary,howMany).toString());
}

test([1,2,3,4,5,6,7,8], 3);
test([1,2,3,4,5,6,7,8], 3);
test([1,2,3,4,5,6,7,8], 5);
test([1,2,3,4,5,6,7,8], 8);