package com.example.random
import java.security.SecureRandom
// Thread-safe instance of SecureRandom backed by OS entropy
private val secureRandom = SecureRandom()
// Constant definitions with underscore separators for readability
private const val MIN_8_DIGIT_INT = 10_000_000
private const val MAX_8_DIGIT_INT = 99_999_999
private const val CODE_UPPER_EXCLUSIVE = 100_000_000
/**
* Generates a cryptographically secure 8-digit integer between 10,000,000 and 99,999,999 (inclusive).
* Guaranteed to start with a non-zero digit (1-9).
*
* @return An 8-digit positive integer.
*/
fun generate8DigitNumber(): Int {
// Kotlin's IntRange (min..max) handles inclusive bounds cleanly.
// SecureRandom.nextInt(min, maxExclusive) is available in Java 17+/Kotlin JVM.
// For universal compatibility across all JVM versions, we use range arithmetic:
val rangeSize = (MAX_8_DIGIT_INT - MIN_8_DIGIT_INT) + 1 // 90,000,000
return MIN_8_DIGIT_INT + secureRandom.nextInt(rangeSize)
}
/**
* Generates an 8-character numeric string code between "00000000" and "99999999".
* Preserves leading zeros using String.format, making it suitable for OTPs, PINs, or tokens.
*
* @return An 8-character numeric string padded with leading zeros if necessary.
*/
fun generate8DigitCode(): String {
// Generates a raw value in the range [0, 99_999_999]
val rawValue = secureRandom.nextInt(CODE_UPPER_EXCLUSIVE)
// String formatting with %08d guarantees a minimum width of 8 digits with leading zero padding
return String.format("%08d", rawValue)
}
// ==========================================
// Example Execution
// ==========================================
fun main() {
println("=== Generating 8-Digit Integers [10000000 - 99999999] ===")
repeat(3) { index ->
val randomNumber = generate8DigitNumber()
println("Generated integer #${index + 1}: $randomNumber")
}
println("\n=== Generating 8-Digit String Codes [00000000 - 99999999] ===")
repeat(3) { index ->
val randomCode = generate8DigitCode()
println("Generated OTP code #${index + 1}: $randomCode")
}
}
/*
run:
=== Generating 8-Digit Integers [10000000 - 99999999] ===
Generated integer #1: 55374651
Generated integer #2: 60501712
Generated integer #3: 42917903
=== Generating 8-Digit String Codes [00000000 - 99999999] ===
Generated OTP code #1: 13815866
Generated OTP code #2: 28291987
Generated OTP code #3: 12289359
*/
