Explain the concept of a hash function and its main purpose in computer systems.

A hash function is an algorithm that takes input data of arbitrary size and transforms it into a fixed size, called a hash value or hash code. The main purpose of hash functions in computer systems is fast indexing and data retrieval, integrity verification, and ensuring data uniqueness.

For example, in hash tables, a hash function helps quickly find the needed element by converting the key into an array index. In cryptography, hash functions are used to create digital signatures and verify data authenticity.

Here's an example in Go on how to obtain the SHA-256 hash of a string:

package main

import (
    "crypto/sha256"
    "fmt"
)

func main() {
    data := "example data"
    hash := sha256.Sum256([]byte(data))
    fmt.Printf("SHA-256: %x\n", hash)
}