Recipe Scaler

Project roadmap

1. 01

   Define the recipe format

   ~1h

   Write README.md with an example recipe file in YAML: a title, the servings, and ingredients each with an amount (such as 1 1/2, 3/4, 2 or 0.5), a unit (cup, tbsp, tsp, g, kg, ml, l, or none, as in 2 eggs) and a name. Show the command (scale RECIPE --servings 6) and the exact output expected. Create a Gemfile with minitest. Done when: README.md shows the format, the command and the expected output.

2. 02

   Parse amounts and ingredients

   ~1.5h

   Write an Amount parser that turns text like 1 1/2, 3/4, 2 and 0.5 into a Rational, using regular expressions, with a clear error for text it doesn't understand. Create an Ingredient class (amount, unit and name) and a Recipe class loaded from the YAML file, validating that servings is a positive whole number. Done when: unit tests show every amount format parses to the right Rational.

3. 03

   Scale the recipe

   ~1h

   Add Recipe#scale(new_servings), which multiplies every amount by new_servings / servings as exact fractions and returns a new recipe, leaving the original unchanged. Ingredients without an amount (such as salt to taste) stay as they are. Done when: scaling from 4 to 6 servings multiplies every amount by 3/2 exactly.

4. 04

   Format amounts nicely

   ~1.5h

   Print amounts as cooks write them: whole numbers, mixed fractions like 1 1/2, and fractions rounded to the nearest 1/8 for cups and spoons, while grams and millilitres are shown as whole numbers. Convert units when it reads better: 3 tsp or more becomes tablespoons, 16 tbsp becomes 1 cup, and 1,000 g becomes 1 kg. Done when: scaled recipes print friendly amounts like 2 1/4 cups instead of 2.25.

5. 05

   Build the command

   ~1h

   Use OptionParser for --servings (required, 1 to 100) and --help, read the recipe file given as an argument, and print the title, the new number of servings, and the ingredient list. Show clear messages for a missing file, invalid YAML, and bad amounts, and exit with code 1. Done when: the command in README.md prints exactly the expected output.

6. 06

   Test it

   ~1.5h

   Write Minitest tests: parsing every amount format, scaling up and down (including to 1 serving), rounding to eighths, unit conversions, ingredients without amounts, and error messages for invalid input. Done when: ruby -Itest test/*_test.rb passes (or rake test).

Resources

• DocsRuby documentation: Rational
• DocsRuby documentation: OptionParser
• DocsRuby documentation: YAML (Psych)
• DocsMinitest
• DocsRuby in Twenty Minutes