Instagram ID Parsing

shortcode.go

package api

import (
	"encoding/base64"
	"fmt"
	"strings"
	"time"
)

// See https://gist.github.com/sclark39/9daf13eea9c0b381667b61e3d2e7bc11
// and https://instagram-engineering.com/sharding-ids-at-instagram-1cf5a71e5a5c?gi=8583b512a3ca

const (
	InstagramEpoch = 1314220021721
)

var (
	encoder = base64.NewEncoding(`ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_`)
)

type ID int64

func NewID(unixMillis, shardID, sequenceID int64) ID {
	result := (unixMillis - InstagramEpoch) << (64 - 41)
	result = result | (shardID << (64 - 41 - 13))
	result = result | (sequenceID % 1024)
	return ID(result)
}

func ShortcodeID(shortcode string) (ID, error) {
	var decoded [16]byte
	n, err := encoder.Decode(decoded[:], []byte(strings.Repeat("A", 12-len(shortcode))+shortcode))
	if err != nil {
		return ID(0), err
	}

	var result int64
	for i := 0; i < n; i++ {
		result <<= 8
		result |= int64(decoded[i])
	}
	return ID(result), nil
}

func (id ID) Shortcode() string {
	var idBytes [9]byte
	for i := 8; i >= 0; i-- {
		idBytes[i] = byte(id & 0xFF)
		id >>= 8
	}

	// Encode the ID bytes using base64
	shortcode := encoder.EncodeToString(idBytes[:])
	// Replace 'A' with a space
	shortcode = strings.ReplaceAll(shortcode, "A", " ")
	// Strip leading spaces
	shortcode = strings.TrimLeft(shortcode, " ")
	// Replace remaining spaces with 'A'
	return strings.ReplaceAll(shortcode, " ", "A")
}

func (id ID) CreationTime() int64 {
	return (int64(id) >> (64 - 41)) + InstagramEpoch
}

func (id ID) ShardID() int64 {
	return (int64(id) >> (64 - 41 - 13)) & 0b1111111111111
}

func (id ID) SequenceID() int64 {
	return int64(id) & 0b1111111111
}

func (id ID) String() string {
	return fmt.Sprintf("%s - %d (Sequence ID: %d, Shard ID: %d, Created: %v)", id.Shortcode(), int64(id), id.SequenceID(), id.ShardID(), time.UnixMilli(id.CreationTime()))
}