package core

import (
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"math"
	"math/big"
)

func RandomDecimal(bits int) (string, error) {
	max := new(big.Int).Lsh(big.NewInt(1), uint(bits))
	n, err := rand.Int(rand.Reader, max)
	if err != nil {
		return "", err
	}
	return n.String(), nil
}

func RandomSeed() (string, error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}

func ExpectedGuess(taskID, seed, clientID string, seq int64, bits int) string {
	h := sha256.Sum256([]byte(fmt.Sprintf("%s|%s|%s|%d", taskID, seed, clientID, seq)))
	n := new(big.Int).SetBytes(h[:])
	mod := new(big.Int).Lsh(big.NewInt(1), uint(bits))
	n.Mod(n, mod)
	return n.String()
}

func Distance(a, b string) (*big.Int, error) {
	x, ok := new(big.Int).SetString(a, 10)
	if !ok {
		return nil, fmt.Errorf("invalid integer")
	}
	y, ok := new(big.Int).SetString(b, 10)
	if !ok {
		return nil, fmt.Errorf("invalid integer")
	}
	d := new(big.Int).Sub(x, y)
	d.Abs(d)
	return d, nil
}

func log2Big(x *big.Int) float64 {
	if x.Sign() <= 0 {
		return 0
	}
	n := x.BitLen()
	if n <= 53 {
		return math.Log2(float64(x.Uint64()))
	}
	shift := n - 53
	top := new(big.Int).Rsh(new(big.Int).Set(x), uint(shift))
	return math.Log2(float64(top.Uint64())) + float64(shift)
}

func Score(distance *big.Int, bits int) float64 {
	if distance.Sign() == 0 {
		return 100
	}
	s := 100 * (1 - log2Big(new(big.Int).Add(distance, big.NewInt(1)))/float64(bits))
	if s < 0 {
		return 0
	}
	if s > 100 {
		return 100
	}
	return s
}

func Position(clientID string, score float64) (float64, float64, float64) {
	h := sha256.Sum256([]byte(clientID))
	f := func(i int) float64 { v := uint16(h[i])<<8 | uint16(h[i+1]); return float64(v)/65535*2 - 1 }
	x, y, z := f(0), f(2), f(4)
	norm := math.Sqrt(x*x + y*y + z*z)
	if norm < 0.001 {
		norm = 1
	}
	r := 0.25 + 13*(1-score/100)
	return r * x / norm, r * y / norm, r * z / norm
}