What's Next

def whatsNext(arr):
    # set bits are at each even index
    last_idx = len(arr) -1
    
    # case: last_idx == 0
    if last_idx == 0:
        if arr[0] == 1:
            # input is [1] => 1, then the output is [1,1] => 10
            print(2)
            print(*[1, 1])
        else:
            # input is [x] => 1*x, then the output is [1,1, x-1] => 10(1*[x-1])
            print(3)
            print(*[1, 1, arr[0] - 1])
        return

    # find index for least significant set bit
    # last_idx & 1 can be 0 or 1, 1 if last_idx ends with 1
    i = last_idx-1 if last_idx & 1 else last_idx

    # case: last_idx == 1
    if last_idx == 1:
        # the input has 2 elements, 1s and 0s
        # if the input is [2, 1] => 110 then the output is [1, 2, 1] => 1001
        t = [1, arr[1] + 1, arr[0] - 1]
        
    # case: last bit group is off
    elif last_idx != i:
        t = arr[:i-1] + [arr[i-1] -1, 1, arr[-1] + 1, arr[i] -1]
    # case: last bit group is set
    else:
        t = arr[:i-1] + [arr[i-1] -1, 1, 1, arr[i] -1]

    # handle zeros
    if not t[-1]:
        t.pop(-1)

    if t.count(0):
        i_lst = []
        for i,a in enumerate(t):
            if not a:
                #  combine bits around zero
                t[i-1] += t[i+1]
                i_lst.append(i)
                
        # remove zero and extra bits
        for i in i_lst[-1:-1-len(i_lst):-1]:
            t.pop(i+1)
            t.pop(i)

    print(len(t))
    print(*t)

function whatsNext(arr) {
    // set bits are at each even index
    const last_idx = arr.length -1;
    
    // case: last_idx == 0
    if (last_idx == 0) {
        if (arr[0] == 1) {
            // input is [1] => 1, then the output is [1,1] => 10
            console.log(2);
            console.log('1 1');
        } else {
            // input is [x] => 1*x, then the output is [1,1, x-1] => 10(1*[x-1])
            console.log(3);
            console.log(`1 1 ${BigInt(arr[0]) - BigInt(1)}`);
        }
        return;
    }

    // find index for least significant set bit
    // last_idx & 1 can be 0 or 1, 1 if last_idx ends with 1
    const i = last_idx & 1 ? last_idx-1 : last_idx;
    let t;

    // case: last_idx == 1
    if (last_idx == 1) {
        // the input has 2 elements, 1s and 0s
        // if the input is [2, 1] => 110 then the output is [1, 2, 1] => 1001
        t = [1, arr[1] + 1, arr[0] - 1];
    } else if (last_idx != i) {
        // case: last bit group is off
        t = arr.slice(0, i-1).concat([arr[i-1] -1, 1, BigInt(arr[arr.length-1]) + BigInt(1), BigInt(arr[i]) - BigInt(1)]);
    } else {
        // case: last bit group is set
        t = arr.slice(0, i-1).concat([arr[i-1] -1, 1, 1, BigInt(arr[i]) - BigInt(1)]);
    }

    // handle zeros
    if (!t[t.length - 1]) t.pop();

    // check if we have 0s
    if (t.reduce((v, acc) => v === 0 ? acc+1 : acc, 0) > 0) {
        const i_lst = [];
        const tl = t.length;
        for (let i=0;i<tl;i++) {
            if (t[i] === 0) {
                //  combine bits around zero
                t[i-1] = BigInt(t[i-1]) + BigInt(t[i+1]);
                i_lst.push(i);
            }
        }

        // remove zero and extra bits
        for (let i=-1;i>-1-i_lst.length;i--) {
            t.splice(i_lst[i_lst.length+i], 2);
        }
    }

    console.log(t.length);
    console.log(t.reduce((v, acc) => `${v} ` + acc));
}