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# Adler

Arduino Library for Adler-32 and experimental Adler-16 checksum.


## Description

This library provides a Adler32 checksum of a data array.
Furthermore since 0.2.0 an experimental Adler-16 implementation is added.
This one is often faster as it uses a smaller checksum than the Adler32.
The price is that Adler16 is less sensitive than the Adler32.
Still it will have its niches where it will be useful.

Relates to https://github.com/RobTillaart/CRC

Relates to https://github.com/RobTillaart/Fletcher

Tested on Arduino UNO only.

0.2.0 is a breaking change, file names have been changed to be more
in line with the CRC library.
- Adler.h for the static functions
- Adler32.h for the Adler32 class
- Adler16.h for the Adler16 class.


## Interface


### Adler class

Use **\#include "Adler32.h"** or **\#include "Adler16.h"**

The interface for the Adler16 is very similar. 

- **Adler32()** Constructor, initializes internals.
- **void begin(uint8_t s1 = 1, uint8_t s2 = 0)** resets the internals.
optional setting start values for s1 and s2. Note this is not part of the standard.
This allows a restart from a specific index in a buffer.
- **void add(uint8_t value)** add a single value to the checksum.
- **uint32_t add(const uint8_t \* array, uint8_t length)** add an array of values to the checksum.
Returns the current checksum.
- **uint32_t addFast(const uint8_t \* array, uint8_t length)** add an array of values to the checksum. 
Is faster by trading PROGMEM for performance.
Returns the current checksum.
- **uint32_t getAdler()** get the current checksum.
- **uint32_t count()** get the number of items added. Merely a debugging feature, 
can overflow without affecting checksum.

The class is typically used for streaming very large blocks of data,
optional with intermediate checksum tests.


## Performance Adler32

Only tested on UNO and ESP32 yet.
If you have data of other platforms, please let me know.

Numbers measured with **Adler32_performance.ino**.


### add(value)

The **add(value)** adds one byte and does a subtraction
instead of a modulo.

| Version | Function | UNO 16 MHz | ESP32 240 MHz |
|:-------:|:---------|:----------:|:-------------:|
| 0.1.0   |   add    |   5.6 us   |               |
| 0.1.2   |   add    |   6.6 us   |               |
| 0.2.0   |   add    |   5.9 us   |    1.7 us     |


### add(lorem) 868 chars

The **add(array, length)** is a straightforward loop
over the array and has a small footprint.

| Version | Function | UNO 16 MHz | ESP32 240 MHz |
|:-------:|:---------|:----------:|:-------------:|
| 0.1.0   |   add    |            |               |
| 0.1.2   |   add    |  6392 us   |               |
| 0.2.0   |   add    |  5748 us   |     145 us    |


Note: **add()** is about 6.6 us per byte.


### addFast(lorem) 868 chars

The **addFast(array, length)** is faster than the 
reference **add(array, length)** and uses 108 bytes more (UNO).
So the function has a larger footprint. 
Depending on your needs, choose performance or footprint. 

See **Adler32_performance_addFast.ino**


| Version | Function | UNO 16 MHz | ESP32 240 MHz |
|:-------:|:---------|:----------:|:-------------:|
| 0.1.0   | addFast  |            |               |
| 0.1.2   | addFast  |  1348 us   |               |
| 0.2.0   | addFast  |  1348 us   |      66 us    |

Note: **addFast()** is less than 2 us per byte.


## Performance Adler16

Only tested on UNO and ESP32 yet.
If you have data of other platforms, please let me know.

Numbers measured with **Adler16_performance.ino**.


### add(value)

The **add(value)** adds one byte and does a subtraction
instead of a modulo.

| Version | Function | UNO 16 MHz | ESP32 240 MHz |
|:-------:|:---------|:----------:|:-------------:|
| 0.2.0   | add      |   4.0 us   |     1.8 us    |

The per byte performance of the Adler16 (on UNO) is faster 
than the Adler32 **add(value)**. The reason is that a 16 bit 
subtraction on an UNO is faster than a 32 bit subtraction.


### add(lorem) 868 chars

The **add(array, length)** is a straightforward loop
over the array and has a small footprint.

| Version | Function | UNO 16 MHz | ESP32 240 MHz |
|:-------:|:---------|:----------:|:-------------:|
| 0.2.0   | add      |  4040 us   |    160 us     |

Note: **add()** is about 6.6 us per byte.


### addFast(lorem) 868 chars

The **addFast(array, length)** is faster than the 
reference **add(array, length)**.

| Version | Function | UNO 16 MHz | ESP32 240 MHz |
|:-------:|:---------|:----------:|:-------------:|
| 0.2.0   | addFast  |  1968 us   |     79 us     |

The gain of the faster 16 bit modulo meets the frequency of
doing the modulo more often.


## Interface static functions

The functions are straightforward.

Use **\#include "Adler.h"**

- **uint32_t adler32(uint8_t \*data, uint16_t length)** length in units of 1 byte = 8 bits.
- **uint16_t adler16(uint8_t \*data, uint16_t length)** length in units of 1 byte = 8 bits.

The functions are typically used for an in memory buffer to calculate the checksum once. 
Think of packets in a network, records in a database, or a checksum for an configuration in EEPROM.


### Performance

Only tested on UNO and ESP32 yet.
If you have data of other platforms, please let me know.

Numbers measured with **Adler_performance.ino**.

Lorem Ipsum text = 868 bytes.

| Version | Function | UNO 16 MHz | ESP32 240 MHz |
|:-------:|:---------|:----------:|:-------------:|
| 0.1.0   | Adler32  |  1116 us   |               |
| 0.1.2   | Adler32  |  1116 us   |               |
| 0.2.0   | Adler32  |  1116 us   |     60 us     |
| 0.2.0   | Adler16  |  1736 us   |     75 us     |


#### UNO

Adler32 average 1116 / 868 = 1.29 us per byte.
Adler16 average 1736 / 868 = 2.00 us per byte. (~1.5x slower !)

Adler16 does more often the modulo math as it reaches halfway uint16_t 
faster than Adler32 reaches halfway uint32_t.

As the Adler16 is less performant as the Adler32 (on all tested platforms), 
it is often the best to use the 32 bit version.


## Operation

See examples.


## Future

- Adler64 ?
  - would need a large prime (which) => 4294967291
  - max uint32_t       = 4.294.967.296
  - max uint32_t prime = 4.294.967.291
  - need printHelpers library for printing.
  - only on request.


#### Wont

- do the string wrappers need strlen() ? parameter.
  - yes, as string can be processed partially.
- no return value for **add(value)** 
  - would create too much overhead for repeated calls.