DiceRoller service

app/models/stat.rb

@@ -11,11 +11,22 @@ class Stat < ApplicationRecord
                    uniqueness: true
   validates :value, presence: true,
                     numericality: true
+  validate :validate_roll_command
 
   before_validation :set_slug
 
+  def roll
+    DiceRoller.new(roll_command, stat: self).roll
+  end
+
   private
 
+    def validate_roll_command
+      return if roll_command.blank?
+
+      DiceRoller.new(roll_command).valid?
+    end
+
     def set_slug
       return if slug.present? || name.blank?
 

app/services/dice_roller.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+class DiceRoller
+  def initialize(roll_command, stat: nil)
+    @roll_command = roll_command
+    @stat = stat&.value
+  end
+
+  def roll
+    result = 0
+    operator = nil
+    roll_command_parts.each do |part|
+      case part
+      when /\A\d*d\d+\z/
+        operator.nil? ? (result += roll_dice(part)) : (result = result.send(operator, roll_dice(part)))
+      when /\A\d+\z/
+        operator.nil? ? (result += part.to_i) : (result = result.send(operator, part.to_i))
+      when "self"
+        operator.nil? ? (result += @stat) : (result = result.send(operator, @stat))
+      when /\A[+\-*\/]\z/
+        operator = part
+      end
+    end
+    result
+  end
+
+  def valid?
+    return if @roll_command.blank?
+
+    # No repeated math operators
+    return false if @roll_command.match?(/[+\-*\/]{2,}/)
+
+    # No leading or trailing math operators
+    return false if @roll_command.match?(/\A[+\-*\/]/) || @roll_command.match?(/[+\-*\/]\z/)
+
+    @roll_command.match?(
+      /
+      \A(
+        (\d*d\d*) |
+        ([+\-*\/]) |
+        (\d+) |
+        (self)
+      )*\z/xi,
+    )
+  end
+
+  private
+
+    def roll_command_parts
+      @roll_command.scan(/([+\-*\/])|(\d*d\d+)|(\d+)|(self)/xi)
+                   .flatten
+                   .compact_blank
+    end
+
+    def roll_dice(command)
+      parts = command.downcase.split("d").compact_blank
+      die_type = parts.last
+      dice_number = parts.length > 1 ? parts.first.to_i : 1
+
+      result = 0
+      dice_number.times do
+        result += rand(1..die_type.to_i)
+      end
+      result
+    end
+end

test/models/stat_test.rb

@@ -22,4 +22,10 @@ class StatTest < ActiveSupport::TestCase
     assert_not_equal existing_stat.slug, stat.slug
     assert_equal "#{existing_stat.slug}-2", stat.slug
   end
+
+  test "rolls with roll_command" do
+    stat = stats(:strength)
+    stat.roll_command = "1d6"
+    100.times { assert (1..6).cover?(stat.roll) }
+  end
 end

test/services/dice_roller_test.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class DiceRollerTest < ActiveSupport::TestCase
+  test "correctly validates strings" do
+    valid_strings = [
+      "2d6",
+      "12",
+      "self",
+      "d12",
+      "d20+self",
+      "8+D8-self",
+    ]
+
+    invalid_strings = [
+      "+",
+      "sel+10",
+      "d8++13",
+    ]
+
+    valid_strings.each do |roll_command|
+      assert DiceRoller.new(roll_command).valid?
+    end
+
+    invalid_strings.each do |roll_command|
+      assert_not DiceRoller.new(roll_command).valid?
+    end
+  end
+
+  test "rolls appropriate results" do
+    100.times do
+      assert (1..6).include? DiceRoller.new("d6").roll
+      assert (2..12).include? DiceRoller.new("2d6").roll
+      assert (11..30).include? DiceRoller.new("d20+self", stat: stats(:strength)).roll
+    end
+  end
+end

todo.md

@@ -1,5 +1,4 @@
 - fix inconsistencies in nested create: do we need to submit the id or use in url?
-- add stat stimulus controller to update stats
 - default avatars
 - add uuid/slug to characters and any other url-visible ids
 - shared/private notes