Calculating PI to x DP

[Xotic750]

Hi!

Would you be interested in a function like this?

/*jslint maxerr: 50, indent: 4, browser: true, white: true, devel: true */
/*global BigNumber */

(function() {
    "use strict";

    function pi(digits) {
        digits = Math.floor(digits);
        var config = BigNumber.config(),
            previousConfig = {},
            k = 0,
            prop,
            sum,
            ta,
            tb,
            divisor,
            a,
            b;

        for (prop in config) {
            if (config.hasOwnProperty(prop)) {
                previousConfig[prop] = config[prop];
            }
        }

        BigNumber.config({
            DECIMAL_PLACES : digits,
            ROUNDING_MODE : 4,
            EXPONENTIAL_AT : [-7, 20],
            RANGE : [-1000000000, 1000000000],
            ERRORS : true
        });

        sum = new BigNumber(0);
        a = ta = new BigNumber(16).div(5);
        b = tb = new BigNumber(-4).div(239);
        while (!a.equals(b)) {
            divisor = 2 * k + 1;
            a = ta.div(divisor);
            b = tb.div(divisor);
            sum = sum.plus(a).plus(b);
            ta = ta.neg().div(25);
            tb = tb.neg().div(57121);
            k += 1;
        }

        for (prop in previousConfig) {
            if (previousConfig.hasOwnProperty(prop)) {
                config[prop] = previousConfig[prop];
            }
        }

        return sum;
    }

    console.log(pi(20).toString());
}());

jsfiddle

[MikeMcl]

Hello
I'm not looking to add a pi calculation function to the library at this point, but some visitors may find it useful.
I tried it out and it seems to work okay apart from the fact that the last digit is sometimes incorrect, e.g. with pi(1) and pi(100).
To avoid this the DECIMAL_PLACES value could be increased to say digits + 2 for the calculation and then sum could be round to digits before its return.
I am not clear why you are storing and setting all the config properties, surely it's only the DECIMAL_PLACES value that needs to be stored, set and then restored.
Also, it looks like you are using the Machin formula and there are others that are considerably faster, for example see http://beej.us/blog/data/pi-chudnovsky-gmp/.

[Xotic750]

I was storing and restoring because in some of the testing I was changing
these and found that if you change the rounding scheme then it causes the
PI calculation to go into an infinite loop. I haven't fully checked as to
why yet, just wanted to see if it would of any interest wrt the library.
The last digit is being rounded and perhaps it should be truncated instead,
something I will need to think about a little. I will take a look at the
link, thanks.

On 16 September 2013 21:04, Michael Mclaughlin notifications@github.comwrote:

Hello
I'm not looking to add a pi calculation function to the library at this
point, but some visitors may find it useful.
I tried it out and it seems to work okay apart from the fact that the last
digit is sometimes incorrect, e.g. with pi(1) and pi(100).
To avoid this the DECIMAL_PLACES value could be increased to say digits +
2 for the calculation and then sum could be round to digits before its
return.
I am not clear why you are storing and setting all the config properties,
surely it's only the DECIMAL_PLACES value that needs to be stored, set
and then restored.
It looks like you are using the Machin formula and there are others that
are considerably faster, for example see
http://beej.us/blog/data/pi-chudnovsky-gmp/.

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