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78 lines
2.6 KiB
78 lines
2.6 KiB
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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