Implement atbash-cipher in jq

jq/atbash-cipher/HELP.md

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+# Help
+
+## Running the tests
+
+Each exercise contains a test file.
+Run the tests using the `bats` program.
+
+```bash
+bats test-hello-world.bats
+```
+
+`bats` will need to be installed.
+See the [Testing on the Bash track][bash] page for instructions to install `bats` for your system.
+
+### bats is implemented in bash
+
+The bats file is a bash script, with some special functions recognized by the `bats` command.
+You'll see some tests that look like
+
+```sh
+jq -f some-exercise.jq <<< "{some,json,here}"
+```
+
+That `<<<` syntax is a bash [Here String][here-string].
+It sends the string on the right-hand side into the standard input of the program on the left-hand side.
+It is ([approximately][so]) the same as
+
+```sh
+echo "{some,json,here}" | jq -f some-exercise.jq
+```
+
+## Help for assert functions
+
+The tests use functions from the [bats-assert][bats-assert] library.
+Help for the various `assert*` functions can be found there.
+
+## Skipped tests
+
+Solving an exercise means making all its tests pass.
+By default, only one test (the first one) is executed when you run the tests.
+This is intentional, as it allows you to focus on just making that one test pass.
+Once it passes, you can enable the next test by commenting out or removing the
+
+    [[ $BATS_RUN_SKIPPED == true ]] || skip
+
+annotations prepending other tests.
+
+## Overriding skips
+
+To run all tests, including the ones with `skip` annotations, you can run:
+
+```bash
+BATS_RUN_SKIPPED=true bats test-some-exercise.bats
+```
+
+It can be convenient to use a wrapper function to save on typing: in `bash` you can do:
+
+```bash
+bats() {
+    BATS_RUN_SKIPPED=true command bats *.bats
+}
+```
+
+Then run tests with just:
+
+```bash
+bats
+```
+
+[bash]: https://exercism.org/docs/tracks/bash/tests
+[bats-assert]: https://github.com/bats-core/bats-assert
+[here-string]: https://www.gnu.org/software/bash/manual/bash.html#Here-Strings
+[so]: https://unix.stackexchange.com/a/80372/4667
+
+## Submitting your solution
+
+You can submit your solution using the `exercism submit atbash-cipher.jq` command.
+This command will upload your solution to the Exercism website and print the solution page's URL.
+
+It's possible to submit an incomplete solution which allows you to:
+
+- See how others have completed the exercise
+- Request help from a mentor
+
+## Need to get help?
+
+If you'd like help solving the exercise, check the following pages:
+
+- The [jq track's documentation](https://exercism.org/docs/tracks/jq)
+- The [jq track's programming category on the forum](https://forum.exercism.org/c/programming/jq)
+- [Exercism's programming category on the forum](https://forum.exercism.org/c/programming/5)
+- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
+
+Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
+
+Need help?
+
+- Go to the [Exercism Community forum](https://forum.exercism.org) to get support and ask questions (or just chat!)
+  - Use the [Exercism Support](https://forum.exercism.org/c/support/8) category if you face any issues with working in the web editor, or downloading or submitting your exercises locally.
+  - Use the [Programming:jq](https://forum.exercism.org/c/programming/jq/133) category for jq-specific topics.
+- [StackOverflow](https://stackoverflow.com/questions/tagged/jq) can be used to search for your problem and see if it has been answered already.
+  You can also ask and answer questions.
+- [Github issue tracker](https://github.com/exercism/jq/issues) is where we track our development and maintainance of `jq` exercises in exercism.
+  If none of the above links help you, feel free to post an issue here.

jq/atbash-cipher/README.md

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+# Atbash Cipher
+
+Welcome to Atbash Cipher on Exercism's jq Track.
+If you need help running the tests or submitting your code, check out `HELP.md`.
+
+## Instructions
+
+Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
+
+The Atbash cipher is a simple substitution cipher that relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards.
+The first letter is replaced with the last letter, the second with the second-last, and so on.
+
+An Atbash cipher for the Latin alphabet would be as follows:
+
+```text
+Plain:  abcdefghijklmnopqrstuvwxyz
+Cipher: zyxwvutsrqponmlkjihgfedcba
+```
+
+It is a very weak cipher because it only has one possible key, and it is a simple mono-alphabetic substitution cipher.
+However, this may not have been an issue in the cipher's time.
+
+Ciphertext is written out in groups of fixed length, the traditional group size being 5 letters, leaving numbers unchanged, and punctuation is excluded.
+This is to make it harder to guess things based on word boundaries.
+All text will be encoded as lowercase letters.
+
+## Examples
+
+- Encoding `test` gives `gvhg`
+- Encoding `x123 yes` gives `c123b vh`
+- Decoding `gvhg` gives `test`
+- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`
+
+## Source
+
+### Created by
+
+- @glennj
+
+### Based on
+
+Wikipedia - https://en.wikipedia.org/wiki/Atbash

jq/atbash-cipher/atbash-cipher.jq

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+def chunk:
+  if length <= 5 then
+    .
+  else
+    "\(.[:5]) \(.[5:] | chunk)"
+  end
+;
+
+if .property == "encode" then
+  .input.phrase | 
+  ascii_downcase | 
+  explode | 
+  map(
+    select(
+      (. >= 48 and . <= 57) or (. >= 97 and . <= 122)
+    )
+  ) | 
+  map(
+    if . >= 97 then
+      122-(.-97)
+    else
+      .
+    end
+  ) | 
+  implode |
+  chunk
+else
+  .input.phrase |
+  explode | 
+  map(
+    select(
+      (. >= 48 and . <= 57) or (. >= 97 and . <= 122)
+    )
+  ) | 
+  map(
+    if . >= 97 then
+      122-(.-97)
+    else
+      .
+    end
+  ) | 
+  implode
+end