What Makes a Good Hash Function for Strings?

How can I create an effective hash function for strings in Java?

public int simpleStringHash(String input) {
    int sum = 0;
    for (int i = 0; i < Math.min(5, input.length()); i++) {
        sum += input.charAt(i);
    }
    return sum;
}

When designing a hash function for strings, efficiency and collision resistance are key factors to consider. A simple approach, like summing the Unicode values of characters, may lead to high collision rates, especially for similar strings, thus compromising the effectiveness of the hash function. Here we explore why this technique might be insufficient and present better alternatives.

public int improvedStringHash(String input) {
    int hash = 0;
    int prime = 31;
    for (int i = 0; i < input.length(); i++) {
        hash = prime * hash + input.charAt(i);
    }
    return hash;
}

Causes

• High collision rate due to the limited range of outputs.
• Ignoring the entire string can lead to loss of information.
• Sums can be easily manipulated by modifying a few characters.

Solutions

• Utilize a polynomial rolling hash function that considers character positions.
• Incorporate prime numbers in the hashing process for better distribution.
• Use established hashing algorithms like SHA-256 or MurmurHash for robustness and performance.

Mistake: Using a hash function that only processes the first few characters.

Solution: Ensure that the hash function considers all characters to minimize collision.

Mistake: Failing to test the hash function with different string lengths and patterns.

Solution: Conduct thorough testing with various input scenarios to observe collision rates.