platekit.go

// Package platekit provides functions to generate random license plate strings.
//
// The license plate format is: `XX-000-000-XX`.
package platekit

import (
	"fmt"
	"math"
	"math/rand/v2"
)

const (
	alphabet       = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	alphabetLength = len(alphabet)

	numDigitsInGroup  = 3
	numLettersInGroup = 2

	seedMultiplier = 31
	maxDigit       = 10
)

// Generate returns a random license plate string.
func Generate() string {
	return GenerateFromSeed(rand.Int())
}

// GenerateFromSeed returns a license plate string based on a given seed.
func GenerateFromSeed(seed int) string {
	return plateFromSeed(seed)
}

// GenerateFromString returns a license plate string based on a string value.
func GenerateFromString(value string) string {
	seed := seedFromString(value)
	return plateFromSeed(seed)
}

// splitMix32 returns a deterministic pseudo-random float64 generator based on the seed.
func splitMix32(seed int) func() float64 {
	return func() float64 {
		seed += 0x9e3779b9
		hash := seed ^ (seed >> 16)
		hash = hash * 0x21f0aaad
		hash ^= hash >> 15
		hash *= 0x735a2d97
		hash ^= hash >> 15
		return float64(hash) / float64(math.MaxInt)
	}
}

// seedFromString creates a deterministic seed from a string.
func seedFromString(value string) int {
	var seed int
	for _, char := range value {
		seed = seed*seedMultiplier + int(char)
	}
	return seed
}

// plateFromSeed generates a license plate string from a seed.
func plateFromSeed(seed int) string {
	random := splitMix32(seed)
	return fmt.Sprintf("%s-%s-%s-%s",
		generateLetters(random),
		generateDigits(random),
		generateDigits(random),
		generateLetters(random),
	)
}

// generateLetters returns a string of random uppercase letters of length numLettersInGroup.
func generateLetters(random func() float64) string {
	letters := make([]byte, numLettersInGroup)
	for i := range numLettersInGroup {
		letters[i] = alphabet[int(math.Floor(random()*float64(alphabetLength)))]
	}
	return string(letters)
}

// generateDigits returns a string of random digits of length numDigitsInGroup.
func generateDigits(random func() float64) string {
	digits := make([]byte, numDigitsInGroup)
	for i := range numDigitsInGroup {
		digits[i] = byte('0' + int(math.Floor(random()*maxDigit)))
	}
	return string(digits)
}