package util

import (
	"encoding/json"
	"fmt"
	"hash/fnv"

	"regexp"
)

// ComputeName hashes the given object and prefixes it with prefix.
// The algorithm in use is Fowler–Noll–Vo hash function and is not
// cryptographically secure. Using a cryptographically secure hash is
// not necessary.
func ComputeName(prefix string, obj interface{}) (string, error) {
	objectBytes, err := json.Marshal(obj)
	if err != nil {
		return "", err
	}

	hashF := fnv.New32()
	_, err = hashF.Write(objectBytes)
	if err != nil {
		return "", err
	}

	// we're shortening to stay under 64 as we use this in services
	// and pods down the road for ACME resources.
	prefix = DNSSafeShortenTo52Characters(prefix)

	return fmt.Sprintf("%s-%d", prefix, hashF.Sum32()), nil
}

// DNSSafeShortenTo52Characters shortens the input string to 52 chars and ensures the last char is an alpha-numeric character.
func DNSSafeShortenTo52Characters(in string) string {
	if len(in) >= 52 {
		validCharIndexes := regexp.MustCompile(`[a-zA-Z\d]`).FindAllStringIndex(fmt.Sprintf("%.52s", in), -1)
		in = in[:validCharIndexes[len(validCharIndexes)-1][1]]
	}

	return in
}