问题描述

我可以对 64 位整数进行乘除运算的最准确方法是在 32 位和 64 位程序中(在 Visual C++ 中)吗?(在溢出的情况下，我需要结果 mod 2⁶⁴.)

What is the most accurate way I can do a multiply-and-divide operation for 64-bit integers that works in both 32-bit and 64-bit programs (in Visual C++)? (In case of overflow, I need the result mod 2⁶⁴.)

(我正在寻找类似 MulDiv64 的东西，除了这个使用内联汇编，仅适用于 32 位程序.)

(I'm looking for something like MulDiv64, except that this one uses inline assembly, which only works in 32-bit programs.)

显然，转换为 double 并返回是可能的，但我想知道是否有更准确的方法而不是太复杂.(即，我不是在这里寻找任意精度的算术库！)

Obviously, casting to double and back is possible, but I'm wondering if there's a more accurate way that isn't too complicated. (i.e. I'm not looking for arbitrary-precision arithmetic libraries here!)

推荐答案

由于这被标记为 Visual C++，我将提供一个滥用 MSVC 特定内在函数的解决方案.

这个例子相当复杂.它是 GMP 和 java.math.BigInteger 用于大除法的相同算法的高度简化版本.

This example is fairly complicated. It's a highly simplified version of the same algorithm that is used by GMP and java.math.BigInteger for large division.

虽然我有一个更简单的算法，但它可能慢了大约 30 倍.

Although I have a simpler algorithm in mind, it's probably about 30x slower.

此解决方案具有以下约束/行为:

• 它需要 x64.它不会在 x86 上编译.
• 商不为零.
• 商在 64 位溢出时饱和.

请注意，这是针对无符号整数的情况.围绕它构建一个包装器以使其也适用于已签名的案例是微不足道的.此示例还应生成正确截断的结果.

Note that this is for the unsigned integer case. It's trivial to build a wrapper around this to make it work for signed cases as well. This example should also produce correctly truncated results.

这段代码没有经过全面测试.但是，它已经通过了我抛出的所有测试用例.
(即使是我故意构建以试图破坏的用例算法.)

This code is not fully tested. However, it has passed all the tests cases that I've thrown at it.
(Even cases that I've intentionally constructed to try to break the algorithm.)

#include <intrin.h>

uint64_t muldiv2(uint64_t a, uint64_t b, uint64_t c){
    //  Normalize divisor
    unsigned long shift;
    _BitScanReverse64(&shift,c);
    shift = 63 - shift;

    c <<= shift;

    //  Multiply
    a = _umul128(a,b,&b);
    if (((b << shift) >> shift) != b){
        cout << "Overflow" << endl;
        return 0xffffffffffffffff;
    }
    b = __shiftleft128(a,b,shift);
    a <<= shift;


    uint32_t div;
    uint32_t q0,q1;
    uint64_t t0,t1;

    //  1st Reduction
    div = (uint32_t)(c >> 32);
    t0 = b / div;
    if (t0 > 0xffffffff)
        t0 = 0xffffffff;
    q1 = (uint32_t)t0;
    while (1){
        t0 = _umul128(c,(uint64_t)q1 << 32,&t1);
        if (t1 < b || (t1 == b && t0 <= a))
            break;
        q1--;
//        cout << "correction 0" << endl;
    }
    b -= t1;
    if (t0 > a) b--;
    a -= t0;

    if (b > 0xffffffff){
        cout << "Overflow" << endl;
        return 0xffffffffffffffff;
    }

    //  2nd reduction
    t0 = ((b << 32) | (a >> 32)) / div;
    if (t0 > 0xffffffff)
        t0 = 0xffffffff;
    q0 = (uint32_t)t0;

    while (1){
        t0 = _umul128(c,q0,&t1);
        if (t1 < b || (t1 == b && t0 <= a))
            break;
        q0--;
//        cout << "correction 1" << endl;
    }

//    //  (a - t0) gives the modulus.
//    a -= t0;

    return ((uint64_t)q1 << 32) | q0;
}

请注意，如果您不需要完美截断的结果，则可以完全删除最后一个循环.如果这样做，答案将不会比正确的商大 2.

Note that if you don't need a perfectly truncated result, you can remove the last loop completely. If you do this, the answer will be no more than 2 larger than the correct quotient.

测试用例:

cout << muldiv2(4984198405165151231,6132198419878046132,9156498145135109843) << endl;
cout << muldiv2(11540173641653250113, 10150593219136339683, 13592284235543989460) << endl;
cout << muldiv2(449033535071450778, 3155170653582908051, 4945421831474875872) << endl;
cout << muldiv2(303601908757, 829267376026, 659820219978) << endl;
cout << muldiv2(449033535071450778, 829267376026, 659820219978) << endl;
cout << muldiv2(1234568, 829267376026, 1) << endl;
cout << muldiv2(6991754535226557229, 7798003721120799096, 4923601287520449332) << endl;
cout << muldiv2(9223372036854775808, 2147483648, 18446744073709551615) << endl;
cout << muldiv2(9223372032559808512, 9223372036854775807, 9223372036854775807) << endl;
cout << muldiv2(9223372032559808512, 9223372036854775807, 12) << endl;
cout << muldiv2(18446744073709551615, 18446744073709551615, 9223372036854775808) << endl;

输出:

3337967539561099935
8618095846487663363
286482625873293138
381569328444
564348969767547451
1023786965885666768
11073546515850664288
1073741824
9223372032559808512
Overflow
18446744073709551615
Overflow
18446744073709551615

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