This code defines a Go package called `randid` for generating...

This code defines a Go package called randid for generating and working with 128-bit random IDs, along with a base64-like encoding mechanism for their string representation.

Detailed Breakdown:

1. ID Definition (type ID [2]uint64):

   • ID is defined as a 128-bit random value, represented using an array of two uint64 values ([2]uint64).

2. Constants:

   • Size: Represents the length in bytes of an ID (128 bits = 16 bytes).
   • StringLen: Represents the length of the base64-encoded string representation of an ID (22 characters without padding).

3. Converting ID to Bytes (func (id ID) Bytes() [Size]byte):

   • Converts the 128-bit ID into an array of 16 bytes.
   • The function extracts each byte from the two uint64 values in ID by applying bit-shifting and masking.

4. Converting ID to Base64-like String (func (id ID) String() string):

   • Converts the ID into a string representation using a custom unpadded base64-like encoding.
   • The function uses the encodeUnrolled method to encode the ID into 22 characters using a manually optimized approach.
   • It uses the unsafe package to efficiently convert the encoded byte array into a string.

5. Generating New IDs (func New() ID):

   • Generates a new 128-bit random ID using two calls to rand.Uint64() to populate the two uint64 values in the ID.

6. Custom Base64-like Encoding (func encodeUnrolled(dst *[StringLen]byte, src ID)):

   • Encodes the ID into a fixed-length custom base64-like string without padding characters (=).
   • Utilizes a map of 64 characters (encodeMap) to represent 6-bit chunks of the input ID (base64 encoding works with 6-bit blocks).
   • The implementation is manually unrolled for performance optimization, processing chunks of 3 bytes from the ID at a time and converting them into 4 encoded characters.

7. Custom Encoding Map (var encodeMap):

   • Defines the 64-character map used for encoding. It includes uppercase letters (A-Z), lowercase letters (a-z), digits (0-9), and two special characters (- and _)—a "URL-safe" base64-like encoding.

Key Points:

• The main functionality of this package is to generate random 128-bit identifiers (ID), convert them to a byte representation (Bytes()), and encode them into a compact and URL-safe string representation (String()).
• The encoding implemented here is a custom take on base64 encoding, optimized for fixed-size input (16 bytes) and omitting the need for padding.
• The code relies on the Go math/rand package (specifically rand.Uint64()) for generating the random numbers used in the ID.

Example Usage:

To generate and work with an ID:

package main

import (
	"fmt"
	"randid"
)

func main() {
	// Generate a new random ID
	id := randid.New()
	
	// Get the byte representation
	bytes := id.Bytes()
	fmt.Printf("Bytes: %x\n", bytes)

	// Get the string representation
	str := id.String()
	fmt.Printf("String: %s\n", str)
}

This program would:

1. Generate a random 128-bit ID.
2. Print the ID as a 16-byte hexadecimal array.
3. Print the URL-safe string representation (22 characters).

Summary:

This package provides a streamlined and compact way to generate, encode, and represent 128-bit random identifiers, useful for purposes like unique session tokens, object IDs, or other cases requiring collision-resistant random identifiers. The encoding used is a tailored and efficient variation of base64.