Javascript Hash Unusual Behavior with Short Strings

Question

I tried to create a hash function in Javascript, and I noticed that one version of it had strange behavior with two-letter strings.

    function hash(seed) {
            var genseed = 0;
            for(i=0;i<seed.length;++i) {
                    genseed += seed.charCodeAt(i)**((seed.length-1)-i);
            }
            console.log(genseed)
    }

When you call hash on two-letter strings like hash("AB") and hash("AX"), you get the exact same result. Yes I know about hash collision, but here it seems that so long as the first letter is the same, the second letter changes nothing, so is it just being ignored? Why exactly does this happen?

Why don't you just use **i instead of the more complicated formula? — Barmar
– Barmar, Commented May 24, 2017 at 20:36
@OliverCharlesworth seed.length is the number of characters in the input string. i is the number of times the for loop has already run (in this scenario anyway) — Benjamin Aguilar
– Benjamin Aguilar, Commented May 25, 2017 at 14:17

Hodrobond · Accepted Answer · 2017-05-24 20:32:50Z

1

((seed.length-1)-i) is 0 when the length is 1 and i=1, or rather: every time the last character is reached. I'd advise removing the -1, otherwise you're just ignoring the last character.

function hash(seed) {
  var genseed = 0;
  for (i = 0; i < seed.length; ++i) {
    genseed += seed.charCodeAt(i) ** ((seed.length) - i);
  }
  console.log(genseed)
}

hash("AB");
hash("AX");

answered May 24, 2017 at 20:32

Hodrobond

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Benjamin Aguilar Over a year ago

That's what I ended up doing, I just wanted to know why it was ignoring the last letter. Thanks

Joseph Marikle · Accepted Answer · 2017-05-24 20:32:55Z

0

The problem is this part: ** ((seed.length-1)-i). For the last character, that evaluates to ** 0. Any number to the power of 0 equals 1. Therefor, the last character is always evaluated as 1. Change it to ** (seed.length-i) and it shouldn't have that issue.

answered May 24, 2017 at 20:32

Joseph Marikle

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Comments

Umashankar Das · Accepted Answer · 2017-05-24 20:34:37Z

0

In case of 2 letter strings. since, ur total iteration is 2 seed.length becomes equal to 2.

so,in the second iteration (seed.length-1)-i = 2 - 1 -1 = 0. Therefore what you're getting is basically the first genseed which is equal to 66 from the 1st iteration.

answered May 24, 2017 at 20:34

Umashankar Das

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