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Finding the smallest good base in JavaScript
Good Base
For an integer num, we call k (k >= 2) a good base of num, if all digits of num base k are 1.
For instance: 13 base 3 is 111, hence 3 is a good base for num = 13
Problem
We are required to write a JavaScript function that takes in string str that represents a number as the only argument. The function should return the string representation of the smallest possible number which is a good base for str.
For example, if the input to the function is −
const str = "4681";
Then the output should be −
const output = "8";
Output Explanation:
Because 4681 base 8 is 11111
Example
The code for this will be −
const str = "4681";
const smallestGoodBase = (n = '1') => {
const N = BigInt(n), bigint2 = BigInt(2), bigint1 = BigInt(1), bigint0 = BigInt(0)
let maxLen = countLength(N, bigint2) // result at most maxLen 1s
const findInHalf = (length, smaller = bigint2, bigger = N) => {
if (smaller > bigger){
return [false];
};
if (smaller == bigger) {
return [valueOf1s(smaller, length) == N, smaller]
};
let mid = (smaller + bigger) / bigint2;
let val = valueOf1s(mid, length);
if(val == N){
return [true, mid];
};
if (val > N){
return findInHalf(length, smaller, mid - bigint1);
};
return findInHalf(length, mid + bigint1, bigger);
};
for (let length = maxLen; length > 0; length--) {
let [found, base] = findInHalf(length);
if(found){
return '' + base;
}
};
return '' + (N - 1);
function valueOf1s(base, lengthOf1s) {
let t = bigint1
for (let i = 1; i < lengthOf1s; i++) {
t *= base
t += bigint1
}
return t
}
function countLength(N, base) {
let t = N, len = 0
while (t > bigint0) {
t /= base
len++
}
return len
}
};
console.log(smallestGoodBase(str));Output
And the output in the console will be −
8
Updated on: 04-Mar-2021
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