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59 lines
1.8 KiB
C#
59 lines
1.8 KiB
C#
using System;
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namespace Obfuz.Utils
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{
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internal static class MathUtil
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{
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//public static int ModInverseOdd32(int sa)
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//{
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// uint a = (uint)sa;
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// if (a % 2 == 0)
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// throw new ArgumentException("Input must be an odd number.", nameof(a));
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// uint x = 1; // 初始解:x₀ = 1 (mod 2)
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// for (int i = 0; i < 5; i++) // 迭代5次(2^1 → 2^32)
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// {
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// int shift = 2 << i; // 当前模数为 2^(2^(i+1))
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// ulong mod = 1UL << shift; // 使用 ulong 避免溢出
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// ulong ax = (ulong)a * x; // 计算 a*x(64位避免截断)
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// ulong term = (2 - ax) % mod;
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// x = (uint)((x * term) % mod); // 更新 x,结果截断为 uint
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// }
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// return (int)x; // 最终解为 x₅ mod 2^32
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//}
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public static int ModInverse32(int sa)
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{
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uint x = (uint)sa;
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if ((x & 1) == 0)
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throw new ArgumentException("x must be odd (coprime with 2^32)");
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uint inv = x;
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inv = inv * (2 - x * inv); // 1
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inv = inv * (2 - x * inv); // 2
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inv = inv * (2 - x * inv); // 3
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inv = inv * (2 - x * inv); // 4
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inv = inv * (2 - x * inv); // 5
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return (int)inv;
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}
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public static long ModInverse64(long sx)
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{
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ulong x = (ulong)sx;
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if ((x & 1) == 0)
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throw new ArgumentException("x must be odd (coprime with 2^64)");
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ulong inv = x;
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inv *= 2 - x * inv; // 1
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inv *= 2 - x * inv; // 2
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inv *= 2 - x * inv; // 3
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inv *= 2 - x * inv; // 4
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inv *= 2 - x * inv; // 5
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inv *= 2 - x * inv; // 6
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return (long)inv;
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}
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}
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}
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