package main

import (
	"crypto/sha256"
	"strings"
)

const crockfordAlphabet = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"

func buildDeviceID(publicKey []byte) string {
	material := append([]byte("dcauth.device.v1"), publicKey...)
	sum := sha256.Sum256(material)
	return formatCode("D1", sum[:])
}

func formatCode(prefix string, data []byte) string {
	encoded := base32Crockford(data)
	if len(encoded) > 20 {
		encoded = encoded[:20]
	}
	groups := make([]string, 0, 5)
	for len(encoded) > 0 {
		size := 4
		if len(encoded) < size {
			size = len(encoded)
		}
		groups = append(groups, encoded[:size])
		encoded = encoded[size:]
	}
	return prefix + "-" + strings.Join(groups, "-")
}

func base32Crockford(data []byte) string {
	var builder strings.Builder
	var buffer uint32
	var bits uint
	for _, value := range data {
		buffer = (buffer << 8) | uint32(value)
		bits += 8
		for bits >= 5 {
			bits -= 5
			builder.WriteByte(crockfordAlphabet[(buffer>>bits)&31])
		}
	}
	if bits > 0 {
		builder.WriteByte(crockfordAlphabet[(buffer<<(5-bits))&31])
	}
	return builder.String()
}