MongoDB的NumberLong只有54位签名？

发布于 2024-12-09 17:57:35 字数 1111 浏览 4 评论 0原文

MongoDB 的 NumberLong 真的是 64 位有符号整数吗？真的是 64 位吗？

MongoDB 的 NumberLong 据说是一个 64 位有符号整数，这意味着我们可以使用 -2^63 <= x <= 2^63-1，其中 x 是一个 NumberLong。但是，对 NumberLong(x) 加 1 或减 1 不会返回 x <= -2^54 或 x >= 2^54 的预期值，但会返回 -2^53 <= 的正确值x <= 2^53。
因此，可靠的 NumberLong 数字似乎是 54 位有符号整数。

这是为什么？
我做错了什么吗？

来自 mongo shell 的示例：

> NumberLong( Math.pow(2,54) )
NumberLong("18014398509481984")    // Expected
> NumberLong( Math.pow(2,54)-1 )
NumberLong("18014398509481984")    // **NOT** Expected
> NumberLong( -Math.pow(2,54) )
NumberLong("-18014398509481984")   // Expected
> NumberLong( -Math.pow(2,54)+1 )
NumberLong("-18014398509481984")   // **NOT** Expected

> NumberLong( Math.pow(2,53) )
NumberLong("9007199254740992")     // Expected
> NumberLong( Math.pow(2,53)-1 )
NumberLong("9007199254740991")     // Expected
> NumberLong( -Math.pow(2,53) )
NumberLong("-9007199254740992")    // Expected
> NumberLong( -Math.pow(2,53)+1 )
NumberLong("-9007199254740991")    // Expected

使用 MongoDB 2.0.0

原文

Is a MongoDB's NumberLong really 64 bit signed integer really 64 bit?

MongoDB's NumberLong is said to be a 64 bit signed integer, which should mean we can play with -2^63 <= x <= 2^63-1, where x is a NumberLong.
However, adding 1 or subtracting 1 from a NumberLong(x) does not return the expected value for x <= -2^54 or x >= 2^54, but correct values are returned for -2^53 <= x <= 2^53.
The reliable NumberLong numbers therefor seem to be 54 bit signed integers.

Why is this?
Am I doing someting wrong?

Sample from the mongo shell:

> NumberLong( Math.pow(2,54) )
NumberLong("18014398509481984")    // Expected
> NumberLong( Math.pow(2,54)-1 )
NumberLong("18014398509481984")    // **NOT** Expected
> NumberLong( -Math.pow(2,54) )
NumberLong("-18014398509481984")   // Expected
> NumberLong( -Math.pow(2,54)+1 )
NumberLong("-18014398509481984")   // **NOT** Expected

> NumberLong( Math.pow(2,53) )
NumberLong("9007199254740992")     // Expected
> NumberLong( Math.pow(2,53)-1 )
NumberLong("9007199254740991")     // Expected
> NumberLong( -Math.pow(2,53) )
NumberLong("-9007199254740992")    // Expected
> NumberLong( -Math.pow(2,53)+1 )
NumberLong("-9007199254740991")    // Expected

Using MongoDB 2.0.0

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溺深海 2024-12-16 17:57:35

哇，这真是令人费解。显然这里发生了某种舍入错误。

> NumberLong("18014398509481984")-NumberLong("1");
18014398509481984

> NumberLong("18014398509481984")-NumberLong("2");
18014398509481982

> NumberLong("18014398509481984")+NumberLong("1");
18014398509481984

> NumberLong("18014398509481984")+NumberLong("2");
18014398509481984

> NumberLong("18014398509481984")+NumberLong("3");
18014398509481988

这可能是 shell 运行的 JavaScript 引擎出了问题，而不是 MongoDB 本身。例如，检查一下 - $inc 工作正常：

> db.test.insert({twoTo54:NumberLong("18014398509481984")});
> db.test.update({},{$inc:{twoTo54:NumberLong("1")}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : NumberLong("18014398509481985") }
> db.test.update({},{$inc:{twoTo54:NumberLong("1")}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : NumberLong("18014398509481986") }
> db.test.update({},{$inc:{twoTo54:NumberLong("1")}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : NumberLong("18014398509481987") }

不过，您必须小心。如果您仅使用普通文字 1，它会将类型转换为数字，然后破坏 $inc：

> db.test.update({},{$inc:{twoTo54:1}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : 18014398509481988 }
> db.test.update({},{$inc:{twoTo54:1}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : 18014398509481988 }

即使您返回 $ inc 与 NumberLong("1")，它仍然损坏：

> db.test.update({},{$inc:{twoTo54:NumberLong("1")}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : 18014398509481988 }

绝对要记住。

Wow, this is puzzling. Clearly there is some sort of rounding error happening here.

> NumberLong("18014398509481984")-NumberLong("1");
18014398509481984

> NumberLong("18014398509481984")-NumberLong("2");
18014398509481982

> NumberLong("18014398509481984")+NumberLong("1");
18014398509481984

> NumberLong("18014398509481984")+NumberLong("2");
18014398509481984

> NumberLong("18014398509481984")+NumberLong("3");
18014398509481988

This is probably something wrong with the JavaScript engine that the shell runs in, rather than MongoDB itself. Check this out, for example--$inc works fine:

> db.test.insert({twoTo54:NumberLong("18014398509481984")});
> db.test.update({},{$inc:{twoTo54:NumberLong("1")}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : NumberLong("18014398509481985") }
> db.test.update({},{$inc:{twoTo54:NumberLong("1")}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : NumberLong("18014398509481986") }
> db.test.update({},{$inc:{twoTo54:NumberLong("1")}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : NumberLong("18014398509481987") }

You have to be careful, though. If you use just a normal literal 1, it will convert the type to a Number, which then breaks the $inc:

> db.test.update({},{$inc:{twoTo54:1}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : 18014398509481988 }
> db.test.update({},{$inc:{twoTo54:1}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : 18014398509481988 }

And even if you go back to $inc with NumberLong("1"), it's still broken:

> db.test.update({},{$inc:{twoTo54:NumberLong("1")}});
> db.test.find();
{ "_id" : ObjectId("4f204847756aa806028abce1"), "twoTo54" : 18014398509481988 }

Definitely good to keep in mind.

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