├── README.md
├── index.js
├── package.json
└── test
    └── test.sum.js


/README.md:
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 1 | Kahan summation algorithm for Node.js
 2 | =====================================
 3 | 
 4 | Since JavaScript's only numerical data type is Number, internally represented 
 5 | as floating-point number, calculating a sum of numbers can introduce significant 
 6 | numerical error. This library aims to reduce the error by employing
 7 | [Kahan's summation algorithm](http://en.wikipedia.org/wiki/Kahan_summation_algorithm).
 8 | 
 9 | Example
10 | -------
11 | 
12 | ```js
13 | // Naive summation
14 | [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7].reduce(function(a, b) { return a + b })
15 | // -> 15.299999999999999
16 | 
17 | // Kahan summation
18 | require('kahan').sum([0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7])
19 | // -> 15.3
20 | ```
21 | 


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/index.js:
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 1 | // Implements http://en.wikipedia.org/wiki/Kahan_summation_algorithm
 2 | 
 3 | function kahanSum(inputArray) {
 4 |   var sum = 0.0
 5 |     , c = 0.0 // A running compensation for lost low-order bits.
 6 |   for(var i = 0; i < inputArray.length; i++) {
 7 |     y = inputArray[i] - c // So far, so good: c is zero.
 8 |     t = sum + y           // Alas, sum is big, y small, so low-order digits of y are lost.
 9 |     c = (t - sum) - y     // (t - sum) recovers the high-order part of y; subtracting y recovers -(low part of y)
10 |     sum = t               // Algebraically, c should always be zero. Beware eagerly optimising compilers!
11 |   }                       // Next time around, the lost low part will be added to y in a fresh attempt.
12 |   return sum
13 | }
14 | 
15 | module.exports = {
16 |   sum: kahanSum
17 | }
18 | 


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/package.json:
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 1 | {
 2 |   "name": "kahan",
 3 |   "version": "0.0.3",
 4 |   "description": "Kahan summation algorithm for node.js",
 5 |   "repository": {
 6 |     "type": "git",
 7 |     "url": "git://github.com/viktors/node-kahan.git"
 8 |   },
 9 |   "author": "Viktors Rotanovs <viktors.rotanovs@gmail.com> (http://rotanovs.com)",
10 |   "dependencies": {
11 |     "expresso": ">=0.7.7"
12 |   },
13 |   "main": "index",
14 |   "engines": {
15 |     "node": ">= 0.4.0 < 0.5.0"
16 |   },
17 |   "files": [
18 |     ""
19 |   ],
20 |   "devDependencies": {}
21 | }


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/test/test.sum.js:
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 1 | // Run $ expresso
 2 | 
 3 | var assert = require('assert'),
 4 |     kahan = require('../index.js')
 5 | 
 6 | var testArray = [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7]
 7 | 
 8 | module.exports = 
 9 |   { 'test naive summation': function() { assert.equal(15.299999999999999, testArray.reduce(function(a, b) { return a + b })) }
10 |   , 'test Kahan summation': function() { assert.equal(15.3, kahan.sum(testArray)) }
11 |   }
12 | 


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