76 lines
2.1 KiB
Go
76 lines
2.1 KiB
Go
// Package uniuri generates random strings good for use in URIs to identify
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// unique objects.
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//
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// Example usage:
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//
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// s := uniuri.New() // s is now "apHCJBl7L1OmC57n"
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//
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// A standard string created by New() is 16 bytes in length and consists of
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// Latin upper and lowercase letters, and numbers (from the set of 62 allowed
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// characters), which means that it has ~95 bits of entropy. To get more
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// entropy, you can use NewLen(UUIDLen), which returns 20-byte string, giving
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// ~119 bits of entropy, or any other desired length.
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//
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// Functions read from crypto/rand random source, and panic if they fail to
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// read from it.
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package uniuri
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import (
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"crypto/rand"
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"io"
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)
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const (
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// Standard length of uniuri string to achive ~95 bits of entropy.
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StdLen = 16
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// Length of uniurl string to achive ~119 bits of entropy, closest
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// to what can be losslessly converted to UUIDv4 (122 bits).
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UUIDLen = 20
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)
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// Standard characters allowed in uniuri string.
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var StdChars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")
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// New returns a new random string of the standard length, consisting of
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// standard characters.
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func New() string {
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return NewLenChars(StdLen, StdChars)
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}
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// NewLen returns a new random string of the provided length, consisting of
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// standard characters.
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func NewLen(length int) string {
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return NewLenChars(length, StdChars)
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}
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// NewLenChars returns a new random string of the provided length, consisting
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// of the provided byte slice of allowed characters (maximum 256).
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func NewLenChars(length int, chars []byte) string {
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b := make([]byte, length)
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r := make([]byte, length+(length/4)) // storage for random bytes.
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clen := len(chars)
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if clen > 256 {
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panic("uniuri: maximum length of charset for NewLenChars is 256")
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}
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maxrb := 256 - (256 % len(chars))
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i := 0
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for {
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if _, err := io.ReadFull(rand.Reader, r); err != nil {
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panic("error reading from random source: " + err.Error())
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}
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for _, byte := range r {
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c := int(byte)
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if c > maxrb {
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// Skip this number to avoid modulo bias.
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continue
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}
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b[i] = chars[c%clen]
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i++
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if i == length {
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return string(b)
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}
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}
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}
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panic("unreachable")
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}
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