Finding a repeated number using pigeon hole principle

find_repeat.js

'use strict'

// pigeon hole theory
// number range 1 - n
// array length n + 1

const num_list = [10, 2, 4, 3, 2, 5, 6, 9, 8, 7, 1];
const num_list2 = [10, 2, 4, 3, 5, 6, 9, 8, 7, 1, 8];

function findDuplicate(num_list) {
    var floor = 1;
    var ceiling = num_list.length - 1;

    while(floor < ceiling) {
        let actualItemCount = 0;

        let midPoint = Math.floor((ceiling - floor) / 2) + floor;
        let lowFloor = floor;
        let lowCeiling = midPoint;
        let highFloor = midPoint + 1;
        let highCeiling = ceiling;

        num_list.forEach(function(item) {
            if (item >= lowFloor && item <= lowCeiling) {
                actualItemCount++;
            }
        });

        let distinctPossibilities = lowCeiling - lowFloor + 1;

        if (actualItemCount > distinctPossibilities) {
            // update the bound
            floor = lowFloor;
            ceiling = lowCeiling;
        } else {
            // update the bound
            floor = highFloor;
            ceiling = highCeiling;
        }
    }

    return floor;
}

console.log(findDuplicate(num_list));
console.log(findDuplicate(num_list2));