pb071(pexam): add ‹garbage_collect›

Signed-off-by: Matej Focko <xfocko@fi.muni.cz>

pb071/pexam/b-garbage_collect.md

@@ -0,0 +1,167 @@
+---
+slug: garbage_collect
+title: Practice exam B
+description: |
+  Garbage everywhere…
+---
+
+# Garbage Collection
+
+:::caution Exam environment
+
+* During the exam you will be provided with a barebone _exam session_ on the
+  _faculty computers_.
+* In browser you are only allowed to have the following tabs open:
+  * [C documentation](https://en.cppreference.com)
+  * page containing the assignment
+* You **are not** allowed to use your own source code, e.g. prepared beforehand
+  or from the seminars.
+* You have **5 minutes** to read through the assignment and ask any follow-up
+  questions should be there something unclear.
+* You have **60 minutes** to work on the assignment, afterward your work will be
+  discussed with your seminar tutor.
+
+:::
+
+You have gotten into a trouble during your regular upgrade of your archLinux[^1]
+installation… You've been carelessly running the upgrades for months and forgot
+about clearing up the caches.
+
+Your task is to write a program `garbage_collect` that will evaluate the shell
+history provided as a file and will try to find files or directories that are
+suspiciously big and decide which of them should be deleted to free some space.
+
+## Format of the shell history
+
+You are provided one file consisting of the captured buffer of the terminal. You
+can see only two commands being used:
+
+1. `cd ‹somewhere›` that changes the current working directory.
+
+   At the beginning you start in the root of the filesystem (i.e. `/`).
+   
+   You are **guaranteed** that `‹somewhere›` is:
+   * `.` that is a current working directory (i.e. does nothing),
+   * `..` that moves you up one level (in case you are in `/`, does nothing), or
+   * is a valid directory in the current working directory.
+
+   :::caution
+
+   There are no guarantees or restrictions on the names of the files or
+   directories!
+
+   :::
+
+1. `ls` that will list files in the current working directory and their
+   respective sizes. If there is a directory in the current working it has `dir`
+   instead of the size.
+
+```
+$ ls
+dir a
+14848514 b.txt
+8504156 c.dat
+dir d
+$ cd a
+$ cd .
+$ cd .
+$ cd .
+$ 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
+```
+
+For this input, you will get following file system:
+```
+- / (dir, size=48381165)
+  - a (dir, size=94853)
+    - e (dir, size=584)
+      - 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, size=24933642)
+    - j (file, size=4060174)
+    - d.log (file, size=8033020)
+    - d.ext (file, size=5626152)
+    - k (file, size=7214296)
+```
+
+## Format of the output
+
+Your program should support 2 switches:
+
+* `-gt ‹min_size›` that will print out suspiciously big files.
+* `-f ‹total_size› ‹min_unused›` that will print out a file to be deleted.
+
+### `-gt ‹min_size›`
+
+With this switch you are provided one additional argument:
+
+* `min_size` that is the lower bound (inclusive) for size of any file or
+  directory that is supposed to be listed.
+
+When your program is being run with this switch, it is is supposed to print out
+all files **and** directories that are bigger than the provided `min_size`.
+
+### `-f ‹total_size› ‹min_unused›`
+
+With this switch you are provided two additional arguments:
+
+* `total_size` that is a total size of the filesystem[^2].
+* `min_unused` that is a minimum of free space required for an upgrade.
+
+Your program should find **exactly one** file or a directory that is of the
+smallest size, but big enough to free enough space for the upgrade to proceed.
+
+In other words, if that file or directory is deleted, following should hold:
+$$
+\mathtt{total\_size} - \mathtt{used} \geq \mathtt{min\_unused}
+$$
+
+## Example usage
+
+You can have a look at the example usage of your program. We can run your
+program from the shell like
+
+    $ ./garbage_collect shell_history.txt -gt 10000000
+    24933642 /d
+    14848514 /b.txt
+    48381165 /
+
+    $ ./garbage_collect shell_history.txt -f 70000000 30000000
+    24933642 /d
+
+## Requirements and notes
+
+* Define **structures** (and **enumerations**, if applicable) for the parsed
+  information from the files.
+* For keeping the “records”, use some **dynamic** data structure.
+  * Don't forget to consider pros and cons of using _specific_ data structures
+    before going through implementing.
+* You **are not required** to produce 1:1 output to the provided examples, they
+  are just a hint to not waste your time tinkering with a user experience.
+* If any of the operations on the input files should fail,
+  **you are expected to** handle the situation _accordingly_.
+* Failures of any other common functions (e.g. functions used for memory
+  management) should be handled in **the same way** as they were in the
+  homeworks and seminars.
+* Your program **must free** all the resources before exiting.
+
+[^1]: Also applies to Fedora, but… we use arch btw :wink:
+[^2]: duh!