chore: add all problems as READMEs

day7/README.md

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+## \--- Day 7: No Space Left On Device ---
+
+You can hear birds chirping and raindrops hitting leaves as the expedition proceeds. Occasionally, you can even hear much louder sounds in the distance; how big do the animals get out here, anyway?
+
+The device the Elves gave you has problems with more than just its communication system. You try to run a system update:
+
+    $ system-update --please --pretty-please-with-sugar-on-top
+    Error: No space left on device
+
+Perhaps you can delete some files to make space for the update?
+
+You browse around the filesystem to assess the situation and save the resulting terminal output (your puzzle input). For example:
+
+    $ cd /
+    $ ls
+    dir a
+    14848514 b.txt
+    8504156 c.dat
+    dir d
+    $ cd a
+    $ ls
+    dir e
+    29116 f
+    2557 g
+    62596 h.lst
+    $ cd e
+    $ ls
+    584 i
+    $ cd ..
+    $ cd ..
+    $ cd d
+    $ ls
+    4060174 j
+    8033020 d.log
+    5626152 d.ext
+    7214296 k
+
+The filesystem consists of a tree of files (plain data) and directories (which can contain other directories or files). The outermost directory is called `/`. You can navigate around the filesystem, moving into or out of directories and listing the contents of the directory you're currently in.
+
+Within the terminal output, lines that begin with `$` are _commands you executed_, very much like some modern computers:
+
+- `cd` means _change directory_. This changes which directory is the current directory, but the specific result depends on the argument:
+  - `cd x` moves _in_ one level: it looks in the current directory for the directory named `x` and makes it the current directory.
+  - `cd ..` moves _out_ one level: it finds the directory that contains the current directory, then makes that directory the current directory.
+  - `cd /` switches the current directory to the outermost directory, `/`.
+- `ls` means _list_. It prints out all of the files and directories immediately contained by the current directory:
+  - `123 abc` means that the current directory contains a file named `abc` with size `123`.
+  - `dir xyz` means that the current directory contains a directory named `xyz`.
+
+Given the commands and output in the example above, you can determine that the filesystem looks visually like this:
+
+    - / (dir)
+      - a (dir)
+        - e (dir)
+          - i (file, size=584)
+        - f (file, size=29116)
+        - g (file, size=2557)
+        - h.lst (file, size=62596)
+      - b.txt (file, size=14848514)
+      - c.dat (file, size=8504156)
+      - d (dir)
+        - j (file, size=4060174)
+        - d.log (file, size=8033020)
+        - d.ext (file, size=5626152)
+        - k (file, size=7214296)
+
+Here, there are four directories: `/` (the outermost directory), `a` and `d` (which are in `/`), and `e` (which is in `a`). These directories also contain files of various sizes.
+
+Since the disk is full, your first step should probably be to find directories that are good candidates for deletion. To do this, you need to determine the _total size_ of each directory. The total size of a directory is the sum of the sizes of the files it contains, directly or indirectly. (Directories themselves do not count as having any intrinsic size.)
+
+The total sizes of the directories above can be found as follows:
+
+- The total size of directory `e` is _584_ because it contains a single file `i` of size 584 and no other directories.
+- The directory `a` has total size _94853_ because it contains files `f` (size 29116), `g` (size 2557), and `h.lst` (size 62596), plus file `i` indirectly (`a` contains `e` which contains `i`).
+- Directory `d` has total size _24933642_.
+- As the outermost directory, `/` contains every file. Its total size is _48381165_, the sum of the size of every file.
+
+To begin, find all of the directories with a total size of _at most 100000_, then calculate the sum of their total sizes. In the example above, these directories are `a` and `e`; the sum of their total sizes is `95437` (94853 + 584). (As in this example, this process can count files more than once!)
+
+Find all of the directories with a total size of at most 100000. _What is the sum of the total sizes of those directories?_
+
+Your puzzle answer was `1427048`.
+
+## \--- Part Two ---
+
+Now, you're ready to choose a directory to delete.
+
+The total disk space available to the filesystem is `70000000`. To run the update, you need unused space of at least `30000000`. You need to find a directory you can delete that will _free up enough space_ to run the update.
+
+In the example above, the total size of the outermost directory (and thus the total amount of used space) is `48381165`; this means that the size of the _unused_ space must currently be `21618835`, which isn't quite the `30000000` required by the update. Therefore, the update still requires a directory with total size of at least `8381165` to be deleted before it can run.
+
+To achieve this, you have the following options:
+
+- Delete directory `e`, which would increase unused space by `584`.
+- Delete directory `a`, which would increase unused space by `94853`.
+- Delete directory `d`, which would increase unused space by `24933642`.
+- Delete directory `/`, which would increase unused space by `48381165`.
+
+Directories `e` and `a` are both too small; deleting them would not free up enough space. However, directories `d` and `/` are both big enough! Between these, choose the _smallest_: `d`, increasing unused space by `24933642`.
+
+Find the smallest directory that, if deleted, would free up enough space on the filesystem to run the update. _What is the total size of that directory?_
+
+Your puzzle answer was `2940614`.
+
+Both parts of this puzzle are complete! They provide two gold stars: \*\*