capid

Stability: 3 - Stable

Capability Unique Identifier generator.

Contributors

@tristanls

Usage

var capid = require('capid');
var id = capid(); // returns Buffer

Overview

Given N number of possible identifiers and n random choices of identifiers, the expected number of choosing the same identifier at random is given by the formula n * (1 - (1 - 1/N)^(n-1)).

Let N = 2^336 (42 bytes).

Let n = 1e48 (1e48 ~= 2^159.45) id's created. (for comparison: 4e20 is the number of milliseconds since the beginning of the Universe (13.8 billion years); 1e41 (~= 2^136) is the number of events you'd get if every millisecond since the beginning of the Universe generated 4e20 events).

The expected number of randomly assigning the same identifier for n = 1e48 and N = 2^336 is 0.00000714. The expected number of randomly assigning the same identifier is 1 when n = 2^168, or more generally, when N = n^2.

This gives us guidance that we want N number of possible identifiers to be greater than n^2 where n is the number of identifiers chosen. Each additional bit of address space beyond the n^2 requirement halves the expected number of randomly assigning the same identifier.

Sources

• http://math.stackexchange.com/a/35798
• N = 2^336, n = 1e48