109 lines
2.4 KiB
Go
109 lines
2.4 KiB
Go
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package id
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import (
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"crypto/rand"
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"errors"
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"math"
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)
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// DefaultAlphabet is the alphabet used for ID characters by default
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var DefaultAlphabet = []rune("6789BCDFGHJKLMNPQRTWbcdfghjkmnpqrtwz")
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// DefaultSize is the size used for ID by default
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var DefaultSize = 16
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// getMask generates bit mask used to obtain bits from the random bytes that are used to get index of random character
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// from the alphabet. Example: if the alphabet has 6 = (110)_2 characters it is sufficient to use mask 7 = (111)_2
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func getMask(alphabetSize int) int {
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for i := 1; i <= 8; i++ {
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mask := (2 << uint(i)) - 1
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if mask >= alphabetSize-1 {
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return mask
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}
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}
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return 0
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}
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// New returns new id or error
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func New(opts ...Option) (string, error) {
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options := NewOptions(opts...)
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if len(options.Alphabet) == 0 || len(options.Alphabet) > 255 {
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return "", errors.New("alphabet must not be empty and contain no more than 255 chars")
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}
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if options.Size <= 0 {
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return "", errors.New("size must be positive integer")
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}
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chars := options.Alphabet
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mask := getMask(len(chars))
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// estimate how many random bytes we will need for the ID, we might actually need more but this is tradeoff
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// between average case and worst case
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ceilArg := 1.6 * float64(mask*options.Size) / float64(len(options.Alphabet))
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step := int(math.Ceil(ceilArg))
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id := make([]rune, options.Size)
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bytes := make([]byte, step)
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for j := 0; ; {
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_, err := rand.Read(bytes)
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if err != nil {
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return "", err
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}
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for i := 0; i < step; i++ {
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currByte := bytes[i] & byte(mask)
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if currByte < byte(len(chars)) {
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id[j] = chars[currByte]
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j++
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if j == options.Size {
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return string(id[:options.Size]), nil
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}
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}
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}
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}
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}
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// Must is the same as New but panics on error
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func Must(opts ...Option) string {
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id, err := New(opts...)
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if err != nil {
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panic(err)
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}
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return id
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}
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// Options contains id deneration options
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type Options struct {
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Alphabet []rune
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Size int
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}
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// Option func signature
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type Option func(*Options)
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// Alphabet specifies alphabet to use
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func Alphabet(alphabet string) Option {
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return func(o *Options) {
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o.Alphabet = []rune(alphabet)
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}
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}
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// Size specifies id size
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func Size(size int) Option {
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return func(o *Options) {
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o.Size = size
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}
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}
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// NewOptions returns new Options struct filled by opts
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func NewOptions(opts ...Option) Options {
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options := Options{
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Alphabet: DefaultAlphabet,
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Size: DefaultSize,
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}
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for _, o := range opts {
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o(&options)
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}
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return options
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}
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