2020 day 21: init for C version

2020/day21/README.org

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+** --- Day 21: Allergen Assessment ---
+You reach the train's last stop and the closest you can get to your
+vacation island without getting wet. There aren't even any boats here,
+but nothing can stop you now: you build a raft. You just need a few
+days' worth of food for your journey.
+
+You don't speak the local language, so you can't read any ingredients
+lists. However, sometimes, allergens are listed in a language you /do/
+understand. You should be able to use this information to determine
+which ingredient contains which allergen and work out which foods are
+safe to take with you on your trip.
+
+You start by compiling a list of foods (your puzzle input), one food per
+line. Each line includes that food's /ingredients list/ followed by some
+or all of the allergens the food contains.
+
+Each allergen is found in exactly one ingredient. Each ingredient
+contains zero or one allergen. /Allergens aren't always marked/; when
+they're listed (as in =(contains nuts, shellfish)= after an ingredients
+list), the ingredient that contains each listed allergen will be
+/somewhere in the corresponding ingredients list/. However, even if an
+allergen isn't listed, the ingredient that contains that allergen could
+still be present: maybe they forgot to label it, or maybe it was labeled
+in a language you don't know.
+
+For example, consider the following list of foods:
+
+#+begin_example
+mxmxvkd kfcds sqjhc nhms (contains dairy, fish)
+trh fvjkl sbzzf mxmxvkd (contains dairy)
+sqjhc fvjkl (contains soy)
+sqjhc mxmxvkd sbzzf (contains fish)
+#+end_example
+
+The first food in the list has four ingredients (written in a language
+you don't understand): =mxmxvkd=, =kfcds=, =sqjhc=, and =nhms=. While
+the food might contain other allergens, a few allergens the food
+definitely contains are listed afterward: =dairy= and =fish=.
+
+The first step is to determine which ingredients /can't possibly/
+contain any of the allergens in any food in your list. In the above
+example, none of the ingredients =kfcds=, =nhms=, =sbzzf=, or =trh= can
+contain an allergen. Counting the number of times any of these
+ingredients appear in any ingredients list produces /=5=/: they all
+appear once each except =sbzzf=, which appears twice.
+
+Determine which ingredients cannot possibly contain any of the allergens
+in your list. /How many times do any of those ingredients appear?/
+
+Your puzzle answer was =2211=.
+
+** --- Part Two ---
+Now that you've isolated the inert ingredients, you should have enough
+information to figure out which ingredient contains which allergen.
+
+In the above example:
+
+- =mxmxvkd= contains =dairy=.
+- =sqjhc= contains =fish=.
+- =fvjkl= contains =soy=.
+
+Arrange the ingredients /alphabetically by their allergen/ and separate
+them by commas to produce your /canonical dangerous ingredient list/.
+(There should /not be any spaces/ in your canonical dangerous ingredient
+list.) In the above example, this would be /=mxmxvkd,sqjhc,fvjkl=/.
+
+Time to stock your raft with supplies. /What is your canonical dangerous
+ingredient list?/
+
+Your puzzle answer was
+=vv,nlxsmb,rnbhjk,bvnkk,ttxvphb,qmkz,trmzkcfg,jpvz=.
+
+Both parts of this puzzle are complete! They provide two gold stars: **
+
+At this point, all that is left is for you to [[/2020][admire your
+Advent calendar]].