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79 lines
2.6 KiB
Markdown
79 lines
2.6 KiB
Markdown
12 months ago
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JavaScript Numbers are represented as [IEEE 754 double-precision floats](http://steve.hollasch.net/cgindex/coding/ieeefloat.html). Unfortunately, this means they lose integer precision for values beyond +/- 2^^53. For projects that need to accurately handle 64-bit ints, such as [node-thrift](https://github.com/wadey/node-thrift), a performant, Number-like class is needed. Int64 is that class.
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Int64 instances look and feel much like JS-native Numbers. By way of example ...
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```js
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// First, let's illustrate the problem ...
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> (0x123456789).toString(16)
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'123456789' // <- what we expect.
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> (0x123456789abcdef0).toString(16)
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'123456789abcdf00' // <- Ugh! JS doesn't do big ints. :(
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// So let's create a couple Int64s using the above values ...
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// Require, of course
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> Int64 = require('node-int64')
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// x's value is what we expect (the decimal value of 0x123456789)
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> x = new Int64(0x123456789)
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[Int64 value:4886718345 octets:00 00 00 01 23 45 67 89]
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// y's value is Infinity because it's outside the range of integer
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// precision. But that's okay - it's still useful because it's internal
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// representation (octets) is what we passed in
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> y = new Int64('123456789abcdef0')
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[Int64 value:Infinity octets:12 34 56 78 9a bc de f0]
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// Let's do some math. Int64's behave like Numbers. (Sorry, Int64 isn't
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// for doing 64-bit integer arithmetic (yet) - it's just for carrying
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// around int64 values
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> x + 1
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4886718346
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> y + 1
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Infinity
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// Int64 string operations ...
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> 'value: ' + x
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'value: 4886718345'
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> 'value: ' + y
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'value: Infinity'
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> x.toString(2)
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'100100011010001010110011110001001'
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> y.toString(2)
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'Infinity'
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// Use JS's isFinite() method to see if the Int64 value is in the
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// integer-precise range of JS values
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> isFinite(x)
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true
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> isFinite(y)
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false
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// Get an octet string representation. (Yay, y is what we put in!)
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> x.toOctetString()
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'0000000123456789'
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> y.toOctetString()
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'123456789abcdef0'
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// Finally, some other ways to create Int64s ...
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// Pass hi/lo words
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> new Int64(0x12345678, 0x9abcdef0)
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[Int64 value:Infinity octets:12 34 56 78 9a bc de f0]
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// Pass a Buffer
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> new Int64(new Buffer([0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf0]))
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[Int64 value:Infinity octets:12 34 56 78 9a bc de f0]
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// Pass a Buffer and offset
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> new Int64(new Buffer([0,0,0,0,0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf0]), 4)
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[Int64 value:Infinity octets:12 34 56 78 9a bc de f0]
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// Pull out into a buffer
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> new Int64(new Buffer([0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf0])).toBuffer()
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<Buffer 12 34 56 78 9a bc de f0>
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// Or copy into an existing one (at an offset)
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> var buf = new Buffer(1024);
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> new Int64(new Buffer([0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf0])).copy(buf, 512);
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```
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