rpick is a command line tool that helps you to pick items from a list, using configurable algorithms.

An example use case for this is picking a restaurant. You might want to generally go to restaurants you haven't visited in a while, but you also might not want to use a strict least recently used model and spice things up with some element of chance, with restaurants you've least recently visited getting a boost in their chances.

rpick keeps its state in a YAML file in your home config directory called rpick.yml. For now, users must create this file by hand, and rpick will manage it from there. To get started with some examples, create ~/.config/rpick.yml like this:

---
prs:
  model: even
  choices:
    - paper
    - rock
    - scissors
restaurant:
  model: gaussian
  choices:
    - Spirits
    - Lucky 32
    - Centro
    - Sitti
    - Cookout

Then you can ask rpick to pick a game of paper rock scissors for you:

$ rpick prs
Choice is scissors. Accept? (Y/n)

Note that it would be bad to use the Gaussian model for paper rock scissors, because you have a statistical advantage of guessing that model's results. Let's take a look at the Gaussian model:

$ rpick restaurant
Choice is Lucky 32. Accept? (Y/n)

If you say yes, it will rewrite the yaml file like this since we used the Gaussian model:

---
prs:
  model: even
  choices:
    - paper
    - rock
    - scissors
restaurant:
  model: gaussian
  stddev_scaling_factor: 3.0
  choices:
    - Spirits
    - Centro
    - Sitti
    - Cookout
    - Lucky 32

Note that we passed prs and then restaurant as arguments when we called rpick - this told rpick to look for those objects in rpick.yml to find out which models to use and which choices were available. This parameter is required, but its possible values are defined by you in your config file.

The model field in the config file defines which mathematical model to use to pick from the given choices. See the Models section below for more information about which models are available and how you can configure them.

It added one setting to your restaurant object that wasn't there originally: stddev_scaling_factor. You can read more about this setting in the Gaussian model documentation below.

This project is available on crates.io.

Models

rpick is capable of a few different algorithms for picking choices: even, gaussian, lottery, and weighted.

Even

The even distribution model is the simplest available choice model. It will give an even chance to each item in the list of strings to be chosen. It requires two keys:

• model: This must be set to the string "even", in order to select this model.
• choices: This is a list of strings that are the options for the model to choose from.

Example:

convertible_top:
  model: even
  choices:
    - up
    - down

You might want to consult the weather before using rpick for this use case…

Gaussian

The gaussian distribution model is more complex. It uses the Gaussian distribution to prefer choices that have been less recently chosen. Things near the top of the list of choices have the highest probability of being chosen, while things at the end of the list have the lowest chance. Once an item has been picked and the user has accepted the choice, the list is saved to disk with the picked item moved to the end of the list. This model accepts three keys:

• model: This must be set to the string "gaussian", in order to select this model.
• stddev_scaling_factor is used to derive the standard deviation; the standard deviation is the length of the list of choices, divided by this scaling factor. Thus, a larger scaling factor will result in a stronger preference for items near the top of the list, and a smaller scaling factor will result in a more even distribution among the choices. Note that the smaller the scaling factor is, the longer rpick will take to make a decision, on average. The default is 3.0, which is chosen because it places the last item on the list at three standard deviations, giving it a 0.03% chance of being chosen. This key is optional, and defaults to 3.0.
• choices: This is a list of strings that are the options for the model to choose from.

Example:

album:
  model: gaussian
  stddev_scaling_factor: 5.0
  choices:
    - Fountains of Wayne/Fountains Of Wayne
    - Beck/Odelay
    - "Townes Van Zandt/High, Low and In Between"
    - Tori Amos/From The Choirgirl Hotel
    - Zao/Parade Of Chaos

Lottery

The lottery distribution model is a dynamic version of the weighted model. Each of the choices has a certain number of lottery tickets that influence how likely they are to be picked that round. Once an item is picked, it loses all of its lottery tickets and every choice that wasn't picked gains more lottery tickets. It accepts three keys:

• model: This must be set to the string "lottery", in order to select this model.
• choices: This must be a list of objects. Each object accepts three keys:
  • name: This is required, and is the name of the choice.
  • tickets: The current number of lottery tickets that this choice has. This is optional, an integer, and defaults to 1.
  • weight: This is an integer expressing how many lottery tickets are given to this choice when it is not chosen. You can use this to influence how often this item gets favored relative to the other choices. It is optional, and defaults to 1.

Example:

activity:
  model: lottery
  choices:
    - name: exercise
    - name: read documentation
    - name: watch tv
      weight: 1000

Weighted

The weighted distribution model is a more general version of the even model that allows you to express different weights for each of the choices. It accepts two keys:

• model: This must be set to the string "weighted", in order to select this model.
• choices: This must be a list of objects. Each object accepts two keys:
  • name: This is required, and is the name of the choice.
  • weight: This is an integer expressing the weight for the choice. It is optional, and defaults to 1.

Example:

cereal:
  model: weighted
  choices:
    - name: generic bran flakes
    - name: cracklin oat bran
      weight: 1000

Changelog

0.2.0

• #3: Added a new even distribution model, which does a nice flat random pick.
• #4: Added a new weighted distribution model, which does a weighted random pick.
• 95b32b1e: Added a new lottery distribution model, which gives lottery tickets to unpicked items and resets the picked item's lottery tickets to 0.

0.1.0

• #1: Added a new stddev_scaling_factor setting, which is optional and defaults to 3.0.
• #2: The model now defaults to "gaussian", so users don't have to define it by hand.