Swift 4: Inbuilt String.hashValue generates same result for different strings

Question

While generating hash values using Swift's built in string hashValue, we found a case where two different strings generate the same hash value... but the cause of the collision is very perplexing..

Take these two strings:

var str1 = """
00000000000000
00000000000000
00000000000000
00000000000000
00000000000000
00000000100000
00000000000000
00000000000
""" //Contains a single 1 character
var str2 = """
00000000000000
00000000000000
00000000000000
00000000000000
00000000000000
00000000000000
00000000000000
00000000000
""" //Contains only zeroes

str1 == str2 // false
str1.hashValue == str2.hashValue // true ..WAT?

• If we trim both strings by a single character, the hashValues are different.
• If we append different characters, the hashValues are different.
• If we append identical characters, the hashValues are the same.

Would love to understand what's going on here...

Thanks

Answer 1

Take a look at official documentation from Apple on Hashable protocol. It says:

A hash value, provided by a type’s hashValue property, is an integer that is the same for any two instances that compare equally. That is, for two instances a and b of the same type, if a == b, then a.hashValue == b.hashValue. The reverse is not true: Two instances with equal hash values are not necessarily equal to each other.

You can also get more by reading this post

Answer 2

Don’t ever assume that two items with the same hash value are equal. This happens because you XOR the individual hash values but XOR’ing a value with itself gives zero (A ^ A = 0).

Maybe this article would give you the answer

https://useyourloaf.com/blog/swift-hashable/

Cheers,