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# DEVRANDOM

Arduino library to wrap a random generator in a stream.


## Description

The library implements a stream class that mimics the /dev/random
device of a Linux system. It can be used for testing with streams.


## Interface

### Constructor

- **DEVRANDOM()** Constructor.
- **DEVRANDOM(char \* str)** Constructor, seeding with a string of characters.
- **DEVRANDOM(uint32_t value)** Constructor, seeding with a integer value.
- **DEVRANDOM(float value)** Constructor, seeding with a float value.


### Streeam interface

To read and to reseed the random generator.

- **int available()** There is always 1 next byte available.
- **int peek()** will give you next byte.
- **int read()** will give you next byte and generate a new one.
- **size_t write(uint8_t data)** data will be used for reseeding the random number generator (RNG), mode 0 and 3.
In digitalRead and analogRead mode the seed is used to XOR
the value, so it does have some effect.
- **size_t write(uint8_t \* buffer, size_t size)** speeds up reseeding 
for some print / println calls.
- **void flush()** to keep the CI happy.

As **write()** reseeds the RNG, printing to **DEVRANDOM** will also reseed the RNG. 
E.g. **dr.println("Hello world");** or **dr.println(3.45678);** will reseed DEVRANDOM too.
See examples.

If footprint is an issue one could remove the **size_t write(uint8_t \* buffer, size_t size)**
from the library and implement a stripped version in **write()**.
Reseeding will become slower but as reseeding is not used that often
this might be a good trade off.


### Random generator selection

- **void useRandom()** use the build in software random number generator. This is the default, but platform dependant.
- **void useDigitalRead(uint8_t pin)** use digitalRead to read 8 bits from a defined pin.
One can build a hardware RNG that flips between 0 and 1 very rapidly and unpredictably.
Connect this signal to the pin and it will be read and generate a random byte.
The seed value from the write is used as an XOR byte.
- **void useAnalogRead(uint8_t pin)** use the analogRead to read 8 bits
This can be fed with any analogue noise source.
The seed value from the write is used as a XOR byte.
- **void useMarsaglia()** uses the Marsaglia pseudo random generator.
This one is quite fast and good, and more important portable.
- **uint8_t getMode()** returns the source of randomness.

|  Mode   |  DEVRANDOM_MODE  | Type                   |
|:-------:|:----------------:|:-----------------------|
|    0    | build in random  | depends on platform    |
|    1    | digitalRead      | hardware external      |
|    2    | analogRead       | hardware external      |
|    3    | Marsaglia        | software portable PRNG |


There might be other RNG's in the future. 
If you have an interesting and fast PRNG to be included please let me know. 


### Obsolete

- **useSW()** replaced by **useRandom()**. 
- **useHW(uint8_t pin)** replaced by **useDigitalRead()**.
- **useAR(uint8_t pin)** replaced by **useAnalogRead()**.


## Operation

See example sketches.


### Example

As shown in the example one can use fscanf() to read larger data types, 

```cpp
  DEVRANDOM dr;
  uint32_t x;
  fscanf((FILE*) &dr, "%lu", &x);
  Serial.println(x);
```

However float is not supported standard in the fscanf() by UNO and strings (%s) generate garbage.
So a password generator is a bit more difficult (and a good exercise).


## Future

- improve documentation.
- add examples.
- add other (portable) PRNG.
- **available()** returns 1, 
  - should that be more e.g. INT_MAX
  - a random number > 0 ?
  - ?
- when changing mode should \_next == **peek()** be reset?
  - yes, think so ```_next = _rnd();```
  - when already in that mode? (=> complex)