Birthday problem solver in Python

This project calculates the generalized birthday problem in the Python language using the Decimal class. It can be used from the command line as a standalone application or as a dependency from another project.

Synopsis

Treats the generalized birthday problem for arbitrary values.

Calculates the generalized birthday problem, the probability P that, when sampling uniformly at random N times (with replacement) from a set of D unique items, there is a non-unique item among the N samples. In the original birthday problem formulation, N is 23 and D is 366 (or 365) for a risk of P ≈ 0.5 = 50% of at least two people having the same birthday.

In mathematical terms, this can be expressed as

since the probability of picking all of N unique is equal to the number of ways to pick N unique samples divided by number of ways to pick any N samples. This, of course, given the assumption that all D items are equally probable.

The project supports calculating both the probability P from N and D (using exact method, exact method with Stirling's approximation in the calculation of faculties or Taylor approximation) and N from D and P (using exact method or Taylor approximation). Both approximations get asymptotically close to the exact result as D grows towards infinity. The exact method should not be used for larger numbers. For extremely small probabilities P, the exact method with Stirling's approximation used for faculties may become unstable as it involves many more different operations than the Taylor approximation which, each, results in small round-offs. Another source of error in this case arises from the use of Stirling's formula for two calculations of faculties (D! and (D - N)!). Since one of these ((D - N)!) diverges slightly more from the exact result than the other (D!), the difference between these (used for calculations in log space) might introduce small errors when P is extremely small. A good check to see whether the approximation in question is suffering or not is to compare it to the Taylor approximation and see whether they match well.

Parameter legend

Name	Type	Effect	CLI flag
D	integer	The size of the set to sample from	-
N	integer	The number of samples sampled from D	-n
P	floating point number	The probability of a non-unique sample in N	-p
binary	boolean	Whether to interpret D and N as base-2 logarithms	-b
combinations	boolean	Whether to interpret D as the size of a set from which we must yield the actual size, D!, of the set to sample from	-c
taylor	boolean	Whether to calculate P or N with Taylor approximation	-t
stirling	boolean	Whether to calculate P with exact method using Stirling's approximation in calculation of faculties	-s
exact	boolean	Whether to calculate P or N with exact method	-e
all	boolean	Whether to calculate P or N with all methods (implies -s -t -e for P and -t -e for N)	-a
json	boolean	Whether to output answer as a Json object or as text	-j
prec	integer	Decimals in the solution where applicable (in [0, 10] with default 10)	--prec

Versions

This project uses Python 3.7 or higher.

Usage

Command-line

Command-line usage can be summarized with following input parameters:

D [-n SAMPLES] [-p PROBABILITY] [-b] [-c] [-t] [-s] [-e] [-a] [-j] [--prec PREC]

The meaning of the flags can be found in the table further up. An additional --help flag is also available to provide information.

Examples

Example usage of standalone application on command line:

Example 1

Calculate the probability P of at least one non-unique birthday among N= 23 persons with all available methods:

> python BirthdayProblem.py 366 -n 23 -a

Example 2:

Calculate, approximatively, the number of times N a deck of cards has to be shuffled to have a P = 50% probability of seeing a repeated shuffle:

> python BirthdayProblem.py 52 -p 0.5 -c -t

Example 3:

Calculate, with approximative methods, the probability P of a collision in a 128-bit crypto when encrypting N = 2^64 = 18 446 744 073 709 551 616 blocks with the same key and output answer as a Json object with at most 5 decimals:

> python BirthdayProblem.py 128 -n 64 -b -s -t -j --prec 5

Help:

Use the following command on the standalone application to get information about usage:

> python BirthdayProblem.py --help

As a dependency

The following shows example usage of this project in another application:

from BirthdayProblem import BirthdayProblem
from decimal import *

# Program.py

Solver = BirthdayProblem.Solver
CalcPrecision = Solver.CalcPrecision

[p, pMethod] = Solver.solveForP(Decimal('366'), Decimal('23'), False, False, CalcPrecision.EXACT)
[n, nMethod] = Solver.solveForN(Decimal('52'), Decimal('0.5'), False, True, CalcPrecision.TAYLOR)

The functions to call have signatures

def solveForP(dOrDLog, nOrNLog, isBinary, isCombinations, method)
def solveForN(dOrDLog, p, isBinary, isCombinations, method)

and may throw exceptions.

Testing

To run tests, simply execute

> python RunTests.py

Notes

• This project is also available in Kotlin at https://github.com/fast-reflexes/BirthdayProblem-Kotlin
• This project is available as an online solver at https://www.bdayprob.com

Author

Elias Lousseief (2020)

Change log

• v. 1.0

• v. 1.1

  • Added rounding upwards (ceiling) instead of regular rounding (half up) on non-logarithmic solutions for N.
  • Removed output approximation character on non-logarithmic solutions for N.
  • Added flag --prec for command-line interface allowing the user to choose output precision, where applicable, in [0, 10] with default 10.

• v. 1.2

  • Corrected calculation of adjusted precision during calculations.
  • Added constants for repeatedly used values and replaced relevant uses.
  • Corrected exact / naive calculation of m take n.
  • Adjusted method using Stirling's formula so that it always returns probablity 0 or more (can otherwise return slightly less than 0 due to precision errors).
  • Changed preprocessing so that the application fails whenever the max precision is insufficient to represent the resulting log of set size D.
  • Small vocabular fixes in descriptions and comments.
  • Fixed tests.

• v. 1.3

  • Improved exception handling:
    • Error codes
    • Exception class
    • Improved checks on input sizes before starting calculations
    • Returning error codes when terminating program with an error
    • Returning more specific error messages when a method fails
  • Simplified conversion to log10 form in _BirthdayProblemNumberFormatter.toLog10ReprOrNone.
  • Corrected precision bug in _BirthdayProblemSolverChecked.birthdayProblemInv.
  • Corrected minor bug in _BirthdayProblemTextFormatter.methodToText.
  • Simplified _BirthdayProblemTextFormatter.methodToText and _BirthdayProblemTextFormatter.methodToDescription.

• v. 1.4

  • Added exact method for calculating N given D and P using a numerical approach, this means that from now on multiple solution strategies can be used for this calculation as well (earlier this calculation always used Taylor approximation).
  • Fixed bug in method facultyLog for when input is 0. Since 0! = 1 and the return value is in log space, the correct answer is 0 and not 1.
  • Added trivial use case for calculating N using D and P when D is 1 and P is neither 0 nor 1 (in this case the answer is always 2).

• v. 1.4.1

  • Documentation, text and smaller fixes forgotten in v. 1.4.
  • Added tests where the project is used as a library (previous tests only used the project's command line API).

• v. 1.4.2

  • Corrected wrong method flag returned when solving for N with overflow in D.
  • Added exact (naive) calculation of D parameter when -c flag is used. Earlier this relied on the Sterling approximation. It still relies on Sterling approximation when -b and -c are used in combination with an input D larger than 15.