Prime Number Checker in Go

package main

import (
"fmt"
"math"
"net/http"
"strconv"
)

func isPrime(num int) bool {
if num <= 1 {
return false
}
if num <= 3 {
return true
}
if num%2 == 0 || num%3 == 0 {
return false
}
i := 5
for i*i <= num {
if num%i == 0 || num%(i+2) == 0 {
return false
}
i += 6
}
return true
}

func checkPrimeHandler(w http.ResponseWriter, r *http.Request) {
r.ParseForm()
numberStr := r.Form.Get("number")
number, err := strconv.Atoi(numberStr)
if err != nil {
http.Error(w, "Invalid input: "+err.Error(), http.StatusBadRequest)
return
}

isPrime := isPrime(number)
if isPrime {
fmt.Fprintf(w, "%d is a prime number.", number)
} else {
fmt.Fprintf(w, "%d is not a prime number.", number)
}
}

func main() {
http.HandleFunc("/check_prime", checkPrimeHandler)
fmt.Println("Server running on http://localhost:8080")
http.ListenAndServe(":8080", nil)
}

Explanation:

Function isPrime(n int) bool:

• Parameters: Takes an integer n as input.
• Returns: Returns true if n is a prime number, otherwise false.

Steps to Determine Primality:

1. Edge Cases:
   • Numbers less than or equal to 1 are not prime (false).
   • Handle small primes (2 and 3) directly (true).
   • Even numbers and multiples of 3 (except 2 and 3) are not prime (false).
2. Trial Division:
   • Loop through potential divisors starting from 5 up to sqrt(n), checking both i and i+2.
   • Increment i by 6 in each iteration to skip even numbers (since even numbers were already checked).
3. Efficiency:
   • This approach checks divisibility up to the square root of n, which reduces the number of iterations compared to checking all numbers up to n-1.

main Function:

• Test Cases: Tests the isPrime function with a list of prime numbers and non-prime numbers.
• Additional Test Cases: Includes edge cases and larger numbers to verify the correctness of the isPrime function.

Output:

The program will output whether each number in the test cases is prime or not.

How to Run:

To run the program, save it to a file (e.g., prime.go) and execute:

Summary:

This GoLang program efficiently determines whether a given number is prime using trial division up to the square root of the number, ensuring optimal performance for large inputs.