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69 lines
2.4 KiB
69 lines
2.4 KiB
5 years ago
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# buffer-more-ints: Add support for more integer widths to Buffer
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Node's Buffer only supports reading and writing integers of a limited
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range of widths. This module provides support for more widths, so
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that integers from 1 to 8 bytes (64 bits) can be accessed. The
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support takes two forms. Firstly, as stand-alone functions similar to
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the integer reading/writing methods of Buffer:
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$ node
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> var moreints = require('buffer-more-ints')
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undefined
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> moreints.readInt64BE(new Buffer("0000deadbeef0000", "hex"), 0).toString(16)
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'deadbeef0000'
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Read and write functions for the regular widths (8, 16, 32) are also
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present in this module, for consistency.
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The second form is methods patched into `Buffer.prototype`, installed
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by requiring `'buffer-more-ints/polyfill'`:
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$ node
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> require('buffer-more-ints/polyfill')
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{}
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> new Buffer("0000deadbeef0000", "hex").readInt64BE(0).toString(16)
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'deadbeef0000'
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buffer-more-ints/polyfill also adds methods `readIntBE`, `writeIntBE`,
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and their LE and UInt counterparts, which take an initial argument
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giving the width of the integer in bytes:
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> var b = new Buffer(3);
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> b.writeIntLE(3, -123456, 0);
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> b.toString('hex')
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'c01dfe'
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> b.readIntLE(3, 0);
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-123456
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The functions added by buffer-more-ints are all implemented in terms
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of the core Buffer functions. Part way through writing the code, I
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discovered that node.js currently implements those in JavaScript, so
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this doesn't lead to performance benefits. But should node ever
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switch to implementing its Buffer operations natively, this
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module should get a speed boost.
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## Limitations
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As JavaScript uses IEEE754 doubles for numbers, the contiguous range
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of integers it can represent is [-2^53 - 1, 2^53 - 1]. So only
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integer widths up to 6 bytes or 48 bits can be read exactly. Reads of
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7 or 8 bytes (56 or 64 bits) will return the closest value that can be
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represented as a JavaScript number.
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In certain situations it might be important to check that a JavaScript
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number was read exactly. The `isContiguousInt` or
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`Buffer.isContiguousInt` (polyfill) function will determine this:
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> Buffer.isContiguousInt(0x1fffffffffffff);
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true
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> Buffer.isContiguousInt(0x20000000000000);
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false
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And `assertContiguousInt` asserts that a number is so:
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> Buffer.assertContiguousInt(0x1fffffffffffff);
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undefined
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> Buffer.assertContiguousInt(0x20000000000000);
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AssertionError: number cannot be represented as a contiguous integer
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