modulo/quadratic-equation.hpp

• View this file on GitHub
• Last update: 2023-05-29 20:16:02+09:00
• Include: #include "modulo/quadratic-equation.hpp"

Depends on

• modint/arbitrary-montgomery-modint.hpp
• mod sqrt(Tonelli-Shanks algorithm) (modulo/mod-sqrt.hpp)

Code

#pragma once

#include "mod-sqrt.hpp"

template <typename mint>
vector<mint> QuadraticEquation(mint a, mint b, mint c) {
  assert(mint::get_mod() % 2 != 0);
  if (a == mint()) {
    if(b == mint()) {
      assert(c != mint());
      return {};
    }
    return vector<mint>{-c * b.inverse()};
  }
  mint ia = a.inverse(), inv2 = mint(2).inverse();
  b *= ia, c *= ia;
  auto D = mod_sqrt(((b * inv2).pow(2) - c).get(), mint::get_mod());
  if(D == -1) return {};
  if(D <= 1) return vector<mint>{-b * inv2 + D};
  return vector<mint>{-b * inv2 + D, -b * inv2 - D};
}

#line 2 "modulo/quadratic-equation.hpp"

#line 2 "modint/arbitrary-montgomery-modint.hpp"

#include <iostream>
using namespace std;

template <typename Int, typename UInt, typename Long, typename ULong, int id>
struct ArbitraryLazyMontgomeryModIntBase {
  using mint = ArbitraryLazyMontgomeryModIntBase;

  inline static UInt mod;
  inline static UInt r;
  inline static UInt n2;
  static constexpr int bit_length = sizeof(UInt) * 8;

  static UInt get_r() {
    UInt ret = mod;
    while (mod * ret != 1) ret *= UInt(2) - mod * ret;
    return ret;
  }
  static void set_mod(UInt m) {
    assert(m < (UInt(1u) << (bit_length - 2)));
    assert((m & 1) == 1);
    mod = m, n2 = -ULong(m) % m, r = get_r();
  }
  UInt a;

  ArbitraryLazyMontgomeryModIntBase() : a(0) {}
  ArbitraryLazyMontgomeryModIntBase(const Long &b)
      : a(reduce(ULong(b % mod + mod) * n2)){};

  static UInt reduce(const ULong &b) {
    return (b + ULong(UInt(b) * UInt(-r)) * mod) >> bit_length;
  }

  mint &operator+=(const mint &b) {
    if (Int(a += b.a - 2 * mod) < 0) a += 2 * mod;
    return *this;
  }
  mint &operator-=(const mint &b) {
    if (Int(a -= b.a) < 0) a += 2 * mod;
    return *this;
  }
  mint &operator*=(const mint &b) {
    a = reduce(ULong(a) * b.a);
    return *this;
  }
  mint &operator/=(const mint &b) {
    *this *= b.inverse();
    return *this;
  }

  mint operator+(const mint &b) const { return mint(*this) += b; }
  mint operator-(const mint &b) const { return mint(*this) -= b; }
  mint operator*(const mint &b) const { return mint(*this) *= b; }
  mint operator/(const mint &b) const { return mint(*this) /= b; }

  bool operator==(const mint &b) const {
    return (a >= mod ? a - mod : a) == (b.a >= mod ? b.a - mod : b.a);
  }
  bool operator!=(const mint &b) const {
    return (a >= mod ? a - mod : a) != (b.a >= mod ? b.a - mod : b.a);
  }
  mint operator-() const { return mint(0) - mint(*this); }
  mint operator+() const { return mint(*this); }

  mint pow(ULong n) const {
    mint ret(1), mul(*this);
    while (n > 0) {
      if (n & 1) ret *= mul;
      mul *= mul, n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const mint &b) {
    return os << b.get();
  }

  friend istream &operator>>(istream &is, mint &b) {
    Long t;
    is >> t;
    b = ArbitraryLazyMontgomeryModIntBase(t);
    return (is);
  }

  mint inverse() const {
    Int x = get(), y = get_mod(), u = 1, v = 0;
    while (y > 0) {
      Int t = x / y;
      swap(x -= t * y, y);
      swap(u -= t * v, v);
    }
    return mint{u};
  }

  UInt get() const {
    UInt ret = reduce(a);
    return ret >= mod ? ret - mod : ret;
  }

  static UInt get_mod() { return mod; }
};

// id に適当な乱数を割り当てて使う
template <int id>
using ArbitraryLazyMontgomeryModInt =
    ArbitraryLazyMontgomeryModIntBase<int, unsigned int, long long,
                                      unsigned long long, id>;
template <int id>
using ArbitraryLazyMontgomeryModInt64bit =
    ArbitraryLazyMontgomeryModIntBase<long long, unsigned long long, __int128_t,
                                      __uint128_t, id>;
#line 3 "modulo/mod-sqrt.hpp"

int64_t mod_sqrt(const int64_t &a, const int64_t &p) {
  assert(0 <= a && a < p);
  if (a < 2) return a;
  using Mint = ArbitraryLazyMontgomeryModInt<409075245>;
  Mint::set_mod(p);
  if (Mint(a).pow((p - 1) >> 1) != 1) return -1;
  Mint b = 1, one = 1;
  while (b.pow((p - 1) >> 1) == 1) b += one;
  int64_t m = p - 1, e = 0;
  while (m % 2 == 0) m >>= 1, e += 1;
  Mint x = Mint(a).pow((m - 1) >> 1);
  Mint y = Mint(a) * x * x;
  x *= a;
  Mint z = Mint(b).pow(m);
  while (y != 1) {
    int64_t j = 0;
    Mint t = y;
    while (t != one) {
      j += 1;
      t *= t;
    }
    z = z.pow(int64_t(1) << (e - j - 1));
    x *= z;
    z *= z;
    y *= z;
    e = j;
  }
  return x.get();
}

/**
 * @brief mod sqrt(Tonelli-Shanks algorithm)
 * @docs docs/modulo/mod-sqrt.md
 */
#line 4 "modulo/quadratic-equation.hpp"

template <typename mint>
vector<mint> QuadraticEquation(mint a, mint b, mint c) {
  assert(mint::get_mod() % 2 != 0);
  if (a == mint()) {
    if(b == mint()) {
      assert(c != mint());
      return {};
    }
    return vector<mint>{-c * b.inverse()};
  }
  mint ia = a.inverse(), inv2 = mint(2).inverse();
  b *= ia, c *= ia;
  auto D = mod_sqrt(((b * inv2).pow(2) - c).get(), mint::get_mod());
  if(D == -1) return {};
  if(D <= 1) return vector<mint>{-b * inv2 + D};
  return vector<mint>{-b * inv2 + D, -b * inv2 - D};
}

Back to top page