package main

import (
	"crypto/sha1"
	"encoding/hex"
	"fmt"
	"strings"
)

func digest(secret, publicKey []byte) (string, error) {
	hash, err := func() (hash []byte, err error) {
		h := sha1.New()
		_, err = h.Write(secret)
		if err != nil {
			return nil, err
		}
		_, err = h.Write(publicKey)
		if err != nil {
			return nil, err
		}
		return h.Sum(nil), nil
	}()
	if err != nil {
		return "", fmt.Errorf("error writing sha1: %v", err)
	}

	var s strings.Builder
	if (hash[0] & 0x80) == 0x80 {
		hash = twosComplement(hash)
		s.WriteRune('-')
	}
	s.WriteString(strings.TrimLeft(hex.EncodeToString(hash), "0"))
	return s.String(), nil
}

func twosComplement(p []byte) []byte {
	carry := true
	for i := len(p) - 1; i >= 0; i-- {
		p[i] = ^p[i]
		if carry {
			carry = p[i] == 0xff
			p[i]++
		}
	}
	return p
}