// Package anubis implements a native Go client for getting past the Anubis
// (TecharoHQ) proof-of-work anti-scraper proxy without a headless browser.
//
// The challenge is embedded in the interstitial HTML as a JSON document inside
// a <script id="anubis_challenge"> element. The client must find an integer
// nonce such that SHA-256(randomData + nonce) has `difficulty` leading hex-zero
// digits, then submit it to the pass-challenge endpoint to obtain a signed-JWT
// auth cookie.
package anubis

import (
	"crypto/sha256"
	"encoding/hex"
	"strconv"
)

// sha256Hex returns the hex-encoded SHA-256 of s. This is the answer to a
// "preact" Anubis challenge: result = SHA256(randomData).
func sha256Hex(s string) string {
	sum := sha256.Sum256([]byte(s))
	return hex.EncodeToString(sum[:])
}

// Solve performs the Anubis proof-of-work: it searches for the smallest nonce
// (starting at 0) such that SHA-256(randomData + nonce) has `difficulty`
// leading hex-zero digits. It returns the hex-encoded hash and the nonce.
//
// The zero check mirrors the Anubis worker: the first floor(difficulty/2)
// bytes must be zero and, when difficulty is odd, the high nibble of the next
// byte must also be zero.
func Solve(randomData string, difficulty int) (hash string, nonce uint64) {
	prefix := []byte(randomData)
	zeroBytes := difficulty / 2
	oddNibble := difficulty%2 != 0

	// Reusable buffer: challenge prefix + decimal nonce, refilled each round.
	buf := make([]byte, 0, len(prefix)+20)

	for n := uint64(0); ; n++ {
		buf = append(buf[:0], prefix...)
		buf = strconv.AppendUint(buf, n, 10)
		sum := sha256.Sum256(buf)

		if leadingZeros(sum[:], zeroBytes, oddNibble) {
			return hex.EncodeToString(sum[:]), n
		}
	}
}

// leadingZeros reports whether sum has the required number of leading hex-zero
// digits: zeroBytes fully-zero bytes, plus (when oddNibble) a zero high nibble
// in the following byte.
func leadingZeros(sum []byte, zeroBytes int, oddNibble bool) bool {
	for i := range zeroBytes {
		if sum[i] != 0 {
			return false
		}
	}
	if oddNibble {
		return sum[zeroBytes]>>4 == 0
	}
	return true
}