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separate puzzle and solving strategies

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embedded-workshop-book/src/SUMMARY.md
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- [Link Quality Indicator (LQI)](./link-quality.md)
- [Radio In ](./radio-in.md)
- [Radio Puzzle](./radio-puzzle.md)
- [Radio Puzzele Help](./radio-puzzle-help.md)
- [Starting a Project from Scratch](./from-scratch.md)
- [Next Steps](./beginner-next-steps.md)
- [Advanced Workbook](./advanced-workbook.md)

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embedded-workshop-book/src/radio-puzzle-help.md Normal file
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# Help
## Use a dictionary.
Our suggestion is to use a dictionary / map. `std::collections::HashMap` is not available in `no_std` code (without linking to a global allocator) but you can use one of the stack-allocated maps in the [`heapless`] crate. It supplies a stack-allocated, fixed-capacity version of the `std::Vec` type which will come in handy to store byte arrays. To store character mappings we recommend using a `heapless::IndexMap`.
`heapless` is already declared as a dependency in the Cargo.toml of the project so you can directly import it into the application code using a `use` statement.
[`heapless`]: https://docs.rs/heapless
[crates.io]: https://crates.io/crates/heapless
``` rust
use heapless::Vec; // like `std::Vec` but stack-allocated
use heapless::FnvIndexMap; // a dictionary / map
use heapless::consts::*; // defines U16, U32, U64... etc. to set the size of the IndexMap
fn main() {
// A hash map with a capacity of 16 key-value pairs allocated on the stack
let mut my_map = FnvIndexMap::<_, _, U16>::new();
my_map.insert(b'A', b'~').unwrap();
// A vector with a fixed capacity of 8 elements allocated on the stack
let mut my_vec = Vec::<_, U8>::new();
my_vec.push(b'A').unwrap();
}
```
If you haven't used a stack-allocated collection before note that you'll need to specify the capacity of the collection as a type parameter using one of the "type-level values" in the `heapless::consts` module. The [`heapless::IndexMap` documentation][indexMap] of the `heapless` crate has some usage examples, as does the [`heapless::Vec` documentation][vec].
[indexMap]: https://docs.rs/heapless/0.5.5/heapless/struct.IndexMap.html
[vec]: https://docs.rs/heapless/0.5.5/heapless/struct.Vec.html
## Note the difference between character literals and byte literals!
Something you will likely run into while solving this exercise are *character* literals (`'c'`) and *byte* literals (`b'c'`). The former has type [`char`] and represent a single Unicode "scalar value". The latter has type `u8` (1-byte integer) and it's mainly a convenience for getting the value of ASCII characters, for instance `b'A'` is the same as the `65u8` literal.
[`char`]: https://doc.rust-lang.org/std/primitive.char.html
*IMPORTANT* you do not need to use the `str` or `char` API to solve this problem, other than for printing purposes. Work directly with slices of bytes (`[u8]`) and bytes (`u8`); and only convert those to `str` or `char` when you are about to print them.
P.S. The plaintext string is *not* stored in `puzzle.hex` so running `strings` on it will not give you the answer.
## Recommended Steps:
Each step is demonstrated in a separate example so if for example you only need a quick reference of how to use the map API you can step / example number 2.
1. Send a one letter packet (e.g. `A`) to the radio to get a feel for how the mapping works. Then do a few more letters. Check out example `radio-puzzle-1`
2. Get familiar with the dictionary API. Do some insertions and look ups. What happens if the dictionary gets full? See `radio-puzzle-2`
3. Next, get mappings from the radio and insert them into the dictionary. See `radio-puzzle-3`
4. You'll probably want a buffer to place the plaintext in. We suggest using `heapless::Vec` for this. See `radio-puzzle-4` (NB It is also possible to decrypt the packet in place)
5. Simulate decryption: fetch the encrypted string and "process" each of its bytes. See `radio-puzzle-5`
6. Now merge steps 3 and 5: build a dictionary, retrieve the secret string and do the reverse mapping to decrypt the message. See `radio-puzzle-6`
7. As a final step, send the decrypted string to the Dongle and check if it was correct or not. See `radio-puzzle-7`
For your reference, we have provided a complete solution in the `src/bin/radio-puzzle-solution.rs` file. That solution is based on the seven steps outlined above. Did you solve the puzzle in a different way?
If you solved the puzzle using a `Vec` buffer you can try solving it without the buffer as a stretch goal. You may find the [slice methods][slice] that let you mutate its data useful. A solution that does not use the `Vec` buffer can be found in the `radio-puzzle-solution-2` file.
[slice]: https://doc.rust-lang.org/std/primitive.slice.html#methods

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embedded-workshop-book/src/radio-puzzle.md
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# Radio Puzzle
For this section you'll need to flash the `puzzle.hex` program on the Dongle. Follow the instructions from the "nRF52840 Dongle" section but flash the `puzzle.hex` program instead of the `loopback.hex` one -- don't forget to put the Dongle in bootloader mode before invoking `dongle-flash`.
Like in the previous sections the Dongle will listen for radio packets -- this time over *channel 25* -- while also logging messages over a USB/serial interface.
Open the `beginner/apps` folder in VS Code; then open the `src/bin/radio-puzzle.rs` file.
Your task in this section is to decrypt the [substitution cipher] encrypted *ASCII* string stored in the Dongle. The string has been encrypted using *simple substitution*.
[substitution cipher]: https://en.wikipedia.org/wiki/Substitution_cipher
✅ Flash the `puzzle.hex` program on the Dongle. Follow the instructions from the "nRF52840 Dongle" section but flash the `puzzle.hex` program instead of the `loopback.hex` one -- don't forget to put the Dongle in bootloader mode before invoking `dongle-flash`.
Like in the previous sections the Dongle will listen for radio packets -- this time over *channel 25* -- while also logging messages over a USB/serial interface.
✅ Open the `beginner/apps` folder in VS Code; then open the `src/bin/radio-puzzle.rs` file. Run the program.
## Dongle Responses
The Dongle will respond differently depending on the length of the incoming packet:
- On zero-sized packets it will respond with the encrypted string.
- On one-byte sized packets it will respond with the *direct* mapping from a *plaintext* letter (single `u8` value) -- the letter contained in the packet -- to the *ciphertext* letter (`u8` value).
- On packets of any other length the Dongle will respond with the string `correct` if it received the decrypted string, otherwise it will respond with the `incorrect` string.
The Dongle will always respond with packets that are valid UTF-8 so you can use `str::from_utf8` on the response packets.
Our suggestion is to use a dictionary / map. `std::collections::HashMap` is not available in `no_std` code (without linking to a global allocator) but you can use one of the stack-allocated maps in the [`heapless`] crate. It supplies a stack-allocated, fixed-capacity version of the `std::Vec` type which will come in handy to store byte arrays. To store character mappings we recommend using a `heapless::IndexMap`.
`heapless` is already declared as a dependency in the Cargo.toml of the project so you can directly import it into the application code using a `use` statement.
[`heapless`]: https://docs.rs/heapless
[crates.io]: https://crates.io/crates/heapless
``` rust
use heapless::Vec; // like `std::Vec` but stack-allocated
use heapless::FnvIndexMap; // a dictionary / map
use heapless::consts::*; // defines U16, U32, U64... etc. to set the size of the IndexMap
fn main() {
// A hash map with a capacity of 16 key-value pairs allocated on the stack
let mut my_map = FnvIndexMap::<_, _, U16>::new();
my_map.insert(b'A', b'~').unwrap();
// A vector with a fixed capacity of 8 elements allocated on the stack
let mut my_vec = Vec::<_, U8>::new();
my_vec.push(b'A').unwrap();
}
```
If you haven't used a stack-allocated collection before note that you'll need to specify the capacity of the collection as a type parameter using one of the "type-level values" in the `heapless::consts` module. The [`heapless::IndexMap` documentation][indexMap] of the `heapless` crate has some usage examples, as does the [`heapless::Vec` documentation][vec].
[indexMap]: https://docs.rs/heapless/0.5.5/heapless/struct.IndexMap.html
[vec]: https://docs.rs/heapless/0.5.5/heapless/struct.Vec.html
Something you will likely run into while solving this exercise are *character* literals (`'c'`) and *byte* literals (`b'c'`). The former has type [`char`] and represent a single Unicode "scalar value". The latter has type `u8` (1-byte integer) and it's mainly a convenience for getting the value of ASCII characters, for instance `b'A'` is the same as the `65u8` literal.
[`char`]: https://doc.rust-lang.org/std/primitive.char.html
*IMPORTANT* you do not need to use the `str` or `char` API to solve this problem, other than for printing purposes. Work directly with slices of bytes (`[u8]`) and bytes (`u8`); and only convert those to `str` or `char` when you are about to print them.
P.S. The plaintext string is *not* stored in `puzzle.hex` so running `strings` on it will not give you the answer.
These are our recommended steps to tackle the problem. Each step is demonstrated in a separate example so if for example you only need a quick reference of how to use the map API you can step / example number 2.
1. Send a one letter packet (e.g. `A`) to the radio to get a feel for how the mapping works. Then do a few more letters. Check out example `radio-puzzle-1`
2. Get familiar with the dictionary API. Do some insertions and look ups. What happens if the dictionary gets full? See `radio-puzzle-2`
3. Next, get mappings from the radio and insert them into the dictionary. See `radio-puzzle-3`
4. You'll probably want a buffer to place the plaintext in. We suggest using `heapless::Vec` for this. See `radio-puzzle-4` (NB It is also possible to decrypt the packet in place)
5. Simulate decryption: fetch the encrypted string and "process" each of its bytes. See `radio-puzzle-5`
6. Now merge steps 3 and 5: build a dictionary, retrieve the secret string and do the reverse mapping to decrypt the message. See `radio-puzzle-6`
7. As a final step, send the decrypted string to the Dongle and check if it was correct or not. See `radio-puzzle-7`
For your reference, we have provided a complete solution in the `src/bin/radio-puzzle-solution.rs` file. That solution is based on the seven steps outlined above. Did you solve the puzzle in a different way?
If you solved the puzzle using a `Vec` buffer you can try solving it without the buffer as a stretch goal. You may find the [slice methods][slice] that let you mutate its data useful. A solution that does not use the `Vec` buffer can be found in the `radio-puzzle-solution-2` file.
[slice]: https://doc.rust-lang.org/std/primitive.slice.html#methods
The Dongle will always respond with packets that are valid UTF-8 so you can use `str::from_utf8` on the response packets.