Awesome

AceSorting

Various sorting functions targeted for the Arduino environment, implemented using C++11 template functions. The library is intended to be compatible with most Arduino platforms, but some tradeoffs lean in favor of lower-end processors (e.g. AVR, lower-end STM32) as opposed to higher-end processors with larger amounts of RAM (e.g. ESP32, higher-end STM32).

Supports the following algorithms:

Bubble Sort
- bubbleSort() (not recommended)
Insertion Sort
- insertionSort() (recommended if N < ~100 or a stable sort is needed)
Selection Sort
- selectionSort() (not recommended)
Shell Sort
- shellSortClassic(): gap factor 2
- shellSortKnuth(): gap factor 3 (recommended)
- shellSortTokuda(): gap factor 2.25
Comb Sort
- combSort13(): gap factor 1.3 (13/10)
- combSort13m(): gap factor 1.3, modified for gaps 9 and 10 (recommended for 32-bit processors)
- combSort133(): gap factor 1.33 (4/3) (recommended for 8-bit processors)
- combSort133m(): gap factor 1.33, modified for gaps 9 and 10
Quick Sort
- quickSortMiddle(): pivot on middle element (recommended)
- quickSortMedian(): pivot on median of low, mid, high
- quickSortMedianSwapped(): pivot on median and swap low, mid, high

tl;dr

In most cases, use shellSortKnuth().
- It costs only 142 bytes on an AVR and 80-112 bytes on 32-bit processors.
- It is faster than any O(N^2) algorithm while consuming only 34-82 extra bytes of flash over insertionSort().
If N >= ~1000, and you have sufficient static memory for recursive functions, and shellSortKnuth() is not fast enough, use:
- quickSortMiddle() on 8-bit AVR processors, and
- quickSortMedianSwapped() on 32-bit processors.
Use combSort133() or shellSortClassic() to get the smallest sorting function faster than O(N^2).
Use insertionSort() if you need a stable sort.
Don't use the C library qsort().
- It is 2-3X slower than the quickSortXxx() functions in this library, and consumes 4-5X more in flash bytes.
Never use Bubble Sort.
- Insertion Sort is 5-6X faster with only a handful (0-32) bytes of extra flash.

Version: 1.0.0 (2021-12-04)

Changelog: CHANGELOG.md

Hello Sorting
Installation
- Source Code
- Dependencies
Documentation
- Examples
Usage
Advanced Usage
Resource Consumption
- Flash And Static Memory
- CPU Cycles
System Requirements
Bugs and Limitations
Alternative Libraries
License
Feedback and Support
Authors

Hello Sorting

This is a simplified version of the examples/HelloSorting demo:

#include <Arduino.h>
#include <AceSorting.h>

using ace_sorting::shellSortKnuth;

const uint16_t ARRAY_SIZE = 20;
int array[ARRAY_SIZE];

void printArray(int* array, uint16_t arraySize) {
  for (uint16_t i = 0; i < arraySize; i++) {
    Serial.print(array[i]);
    Serial.print(' ');
  }
  Serial.println();
}

void fillArray(int* array, uint16_t arraySize) {
  for (uint16_t i = 0; i < arraySize; i++) {
    array[i] = random(256);
  }
}

void setup() {
  delay(1000);
  Serial.begin(115200);
  while (!Serial); // Leonardo/Micro

  // Attempt to get a random seed using the floating analog pin.
  randomSeed(analogRead(A0));

  fillArray();
  Serial.println("Unsorted:");
  printArray(array, arraySize);

  // The compiler automatically generates the correct version of
  // shellSortKnuth() based on the type of `array`.
  shellSortKnuth(array, arraySize);
  Serial.println("Sorted:");
  printArray(array, arraySize);
}

void loop() {}

Installation

The latest stable release is available in the Arduino IDE Library Manager. Search for "AceSorting". Click install.

The development version can be installed by cloning the GitHub repository, checking out the develop branch, then manually copying over to or creating a symlink from the ./libraries directory used by the Arduino IDE. (The result is a directory or link named ./libraries/AceSorting.)

The master branch contains the stable releases.

Source Code

The source files are organized as follows:

src/AceSorting.h - main header file
src/ace_sorting/ - implementation files
tests/ - unit tests which require AUnit
examples/ - example sketches
docs/ - contains the doxygen docs and additional manual docs

Dependencies

This library itself does not depend on any other library.

The unit tests depend on:

AUnit (https://github.com/bxparks/AUnit)

Some of the examples may depend on:

AceCommon (https://github.com/bxparks/AceCommon)

Documentation

this README.md file
Doxygen docs on Github pages.

Examples

examples/HelloSorting
- A simple demo of one of the sorting functions.
examples/CompoundSortingDemo
- A more complex example of sorting by a compound key, first by score, then breaking any ties by name.
Benchmarks
- examples/MemoryBenchmark
  - Determine flash and static RAM consumption of various algorithms.
- examples/AutoBenchmark
  - Determine CPU runtime of various algorithms.
- examples/WorstCaseBenchmark
  - Determine CPU runtime of worst case input data (e.g. sorted, reverse sorted).

Usage

Include Header and Namespace

Only a single header file AceSorting.h is required to use this library. To prevent name clashes with other libraries that the calling code may use, all classes are defined in the ace_sorting namespace. To use the code without prepending the ace_sorting:: prefix, use the using directive to select the specific algorithm. For example, to use the shellSortKnut() function, use something like this:

#include <Arduino.h>
#include <AceSorting.h>
using ace_sorting::shellSortKnuth;

Bubble Sort

See https://en.wikipedia.org/wiki/Bubble_sort.

namespace ace_sorting {

template <typename T>
void bubbleSort(T data[], uint16_t n);

}

Flash consumption: 44 bytes on AVR
Additional ram consumption: none
Runtime complexity: O(N^2)
Stable sort: Yes
Performance Notes:
- If data[] is already sorted, this is O(N).
- Worst performance O(N^2) when data is reverse sorted.
Not recommended

Insertion Sort

See https://en.wikipedia.org/wiki/Insertion_sort.

namespace ace_sorting {

template <typename T>
void insertionSort(T data[], uint16_t n);

}

Flash consumption, 60 bytes on AVR
Additional ram consumption: none
Runtime complexity: O(N^2) but 5-6X faster than bubbleSort()
Stable sort: Yes
Performance Notes:
- If data[] is already sorted, this algorithm is O(N).
- Worst performance O(N^2) when data is reverse sorted.
Recommendation: Use for N smaller than about 100 and only if you need a stable sort

Selection Sort

See https://en.wikipedia.org/wiki/Selection_sort.

namespace ace_sorting {

template <typename T>
void selectionSort(T data[], uint16_t n);

}

Flash consumption, 100 bytes on AVR
Additional ram consumption: none
Runtime complexity: O(N^2) but 2X slower than insertionSort()
Stable sort: No
Performance Notes:
- Little change in performance when data is already sorted or reverse sorted.
Not recommended:
- Larger and slower than insertionSort() but is not a stable sort.
- The only thing it has going for it is that it has the least number of writes versus reads. Might be worth looking into if writing the data is much more expensive than reading the data.

Shell Sort

See https://en.wikipedia.org/wiki/Shellsort. Three versions are provided in this library:

namespace ace_sorting {

template <typename T>
void shellSortClassic(T data[], uint16_t n);

template <typename T>
void shellSortKnuth(T data[], uint16_t n);

template <typename T>
void shellSortTokuda(T data[], uint16_t n);

}

Flash consumption: 100-180 bytes of flash on AVR
Additional ram consumption: none
Runtime complexity: O(N^k) where k=1.3 to 1.5
Stable sort: No
Performance Notes:
- As fast as Quick Sort for N <= ~300.
- Little change in performance when data is already sorted or reverse sorted.
Recomendation: Use shellSortKnuth(), which seems consistently faster than shellSortClassic(), just as fast as shellSortTokuda() but is simpler and takes less flash memory than shellSortTokuda().

Comb Sort

See https://en.wikipedia.org/wiki/Comb_sort and https://rosettacode.org/wiki/Sorting_algorithms/Comb_sort. Four versions are provided in this library:

namespace ace_sorting {

template <typename T>
void combSort13(T data[], uint16_t n);

template <typename T>
void combSort13m(T data[], uint16_t n);

template <typename T>
void combSort133(T data[], uint16_t n);

template <typename T>
void combSort133m(T data[], uint16_t n);

}

Flash consumption: 106-172 bytes on AVR
Additional ram consumption: none
Runtime complexity: O(N^k) where k seems similar to Shell Sort
Stable sort: No
Performance Notes:
- Slightly slower than Shell Sort over similar data sets.
- Little change in performance when data is already sorted or reverse sorted.
Upper limits:
- combSort13() and combSort13m(): n = 6553 on 8-bit AVR processors due to integer overflow.
- combSort133() and combSort133m(): n = 21845 on 8-bit AVR processors due to integer overflow.
Recommendation:
- Prefer Shell Sort over Comb Sort in most situations.
- Use combSort133() on 8-bit processors.
- Use combSort13m() on 32-bit processors.

The combSort13() and combSort13m() functions use a gap factor of 10/13 = 1.3. The combSort13m() function modifies the gap sequence when the gap is 9 or 10. That apparently make the algorithm faster for reasons that I have not been able to find on the internet.

The combSort133() function uses a gap factor of 4/3 = 1.33 instead of 10/13. The 4/3 ratio means that the gap size is multiplied by 3/4 on each iteration, which allows the compiler to replace the integer division by 4 with a right bit shift operation, so that code is smaller and faster on 8-bit processors without hardware integer division.

The combSort133m() function essentially the same as combSort133() but modifies the gap sequence when the gap is 9 or 10.

In terms of performance, examples/AutoBenchmark shows that Comb Sort is consistently slower than Shell Sort so it is difficult to recommend it over Shell Sort.

Quick Sort

See https://en.wikipedia.org/wiki/Quicksort. Three versions are provided in this library:

namespace ace_sorting {

template <typename T>
void quickSortMiddle(T data[], uint16_t n);

template <typename T>
void quickSortMedian(T data[], uint16_t n);

template <typename T>
void quickSortMedianSwapped(T data[], uint16_t n);

}

quickSortMiddle()
- The pivot is the middle element in each partition, regardless of its value.
quickSortMedian()
- The pivot is the median element among the 3 elements on the left, middle, and right slots of each partition.
quickSortMedianSwapped()
- The pivot is the median element among the 3 elements on the left, middle, and right slots of each partition.
- The 3 elements are swapped so that they are sorted.
Flash consumption: 178-278 bytes on AVR
Additional ram consumption: O(log(N)) bytes on stack due to recursion
Runtime complexity: O(N log(N))
Stable sort: No
Performance Notes:
- Fastest algorithm for large (N >= ~1000) data sets.
- Little change in performance when data is already sorted or reverse sorted.
Recommendation
- Avoid on 8-bit processors with limited ram due to extra stack usage by recursion.
- Use quickSortMiddle() if you have to, which is the smallest among the 3 versions, but only slightly slower.

C Library Qsort

The C library <stdlib.h> provides a qsort() function that implements Quick Sort. Since this is a standard C library function, it should be available on all Arduino platforms. The signature looks like this:

#include <stdlib.h>

void qsort(void *base, size_t nmemb, size_t size,
    int (*compar)(const void *, const void *));

It has the following characteristics:

Flash consumption: 1084 bytes on AVR
Additional ram consumption: O(log(N)) bytes on stack due to recursion
Runtime complexity: O(N log(N)), but 2-3X slower than C++ versions in this library
Stable sort: No
Not recommended due to excessive flash consumption and slowness.

According to benchmarks, qsort() is 2-3X slower than the C++ quickSortXxx() functions provided by this library, and consumes 4-5X more flash memory. The qsort() function is probably more sophisticated in the handling of edge cases, but it suffers from being a general function that uses a call-back function. The overhead of that function call makes it 2-3X slower than the C++ template functions in this library where the comparison function call is usually inlined by the compiler. For these reasons, it is difficult to recommend the C-library qsort() function.

Advanced Usage

Each sorting algorithm comes in another variant that takes 3 arguments instead of 2 arguments. For completeness, here are the signatures of the 3-argument variants:

namespace ace_sorting {

template <typename T, typename F>
void bubbleSort(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void insertionSort(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void selectionSort(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void shellSortClassic(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void shellSortKnuth(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void shellSortTokuda(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void combSort13(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void combSort13m(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void combSort133(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void combSort133m(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void quickSortMiddle(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void quickSortMedian(T data[], uint16_t n, F&& lessThan);

template <typename T, typename F>
void quickSortMedianSwapped(T data[], uint16_t n, F&& lessThan);

}

The 3rd argument lessThan is a user-defined function pointer or a lambda expression that implements the "less than" comparison operator between 2 elements in the data[] array. (The F&& is a C++11 syntax that means "rvalue reference to a templatized type of F".) In the context of this library, the lessThan parameter can be thought of as a function-like thing that accepts 2 elements of type T and returns a bool. Two types of this lessThan parameter is explained below.

Function Pointer

If lessThan is a function pointer, then its signature should look something like one of these:

bool lessThan(const T& a, const T& b);

bool lessThan(T a, T b);

The examples/CompoundSortingDemo example shows what this looks like to sort an array of pointers to Record entries by score, then by name, to break any ties:

struct Record {
  const char* name;
  int score;
};

bool sortByScoreThenName(const Record* a, const Record* b) {
  if (a->score < b->score) {
    return true;
  } else if (a->score > b->score) {
    return false;
  } else {
    return strcmp(a->name, b->name) < 0;
  }
}

const uint16_t ARRAY_SIZE = 10;
const Record RECORDS[ARRAY_SIZE] = {
  ...
};

const Record* recordPtrs[ARRAY_SIZE];

void doSorting() {
  shellSortKnuth(recordPtrs, ARRAY_SIZE, sortByScoreThenName);
  ...
}

Lambda Expression

Through the magic of C++ templates, the lessThan parameter can also be a lambda expression. When the comparison operator is simple and short, this feature can save us the hassle of creating a separate function that is used only once.

The examples/HelloSorting example shows how to sort an array of integers in reverse order using an inlined lambda expression that reverses the comparison operator:

const uint16_t ARRAY_SIZE = 20;
int array[ARRAY_SIZE];

void doSorting() {
  shellSortKnuth(array, ARRAY_SIZE, [](int a, int b) { return a > b; });
  ...
}

Compiler Optimizations

All 2-argument variants of the sorting functions (except for the Quick Sort functions quickSortXxx(), see below) are implemented by simply calling the 3-argument sorting functions using a default lambda expression using the < operator like this:

template <typename T>
void shellSortKnuth(T data[], uint16_t n) {
  auto&& lessThan = [](const T& a, const T& b) -> bool { return a < b; };
  shellSortKnuth(data, n, lessThan);
}

The benchmarks in examples/MemoryBenchmark show that the compiler is able to completely optimize away the overhead of the inlined lambda expression and produce code that is equivalent to inserting the < binary operator directly into the code that implements the 3-argument variant. No additional flash memory is consumed.

The only exception to this compiler optimization occurs with the Quick Sort algorithms. The compiler seems unable to optimize away the extra layer of indirection, probably due to the recursive function calls in the Quick Sort algorithms. Therefore, the 2-argument variants of quickSortXxx() algorithms are duplicated from the 3-argument variants with the simple < operator hardcoded, so that the simple case of ascending sorting consumes as little flash memory as possible. (In the source code, the ACE_SORTING_DIRECT_QUICK_SORT macro is set to 1 by default to achieve this, in contrast to all other sorting functions where the equivalent macro is set to 0.)

Resource Consumption

Flash And Static Memory

The full details of flash and static memory consumptions are given in examples/MemoryBenchmark. Here are 2 samples.

SparkFun Pro Micro (ATmega32U4)

+---------------------------------------------------------------------+
| Functionality                          |  flash/  ram |       delta |
|----------------------------------------+--------------+-------------|
| Baseline                               |   4060/  354 |     0/    0 |
|----------------------------------------+--------------+-------------|
| bubbleSort()                           |   4104/  354 |    44/    0 |
| insertionSort()                        |   4120/  354 |    60/    0 |
| selectionSort()                        |   4148/  354 |    88/    0 |
|----------------------------------------+--------------+-------------|
| shellSortClassic()                     |   4156/  354 |    96/    0 |
| shellSortKnuth()                       |   4202/  354 |   142/    0 |
| shellSortTokuda()                      |   4242/  380 |   182/   26 |
|----------------------------------------+--------------+-------------|
| combSort13()                           |   4214/  354 |   154/    0 |
| combSort13m()                          |   4232/  354 |   172/    0 |
| combSort133()                          |   4166/  354 |   106/    0 |
| combSort133m()                         |   4188/  354 |   128/    0 |
|----------------------------------------+--------------+-------------|
| quickSortMiddle()                      |   4238/  354 |   178/    0 |
| quickSortMedian()                      |   4290/  354 |   230/    0 |
| quickSortMedianSwapped()               |   4336/  354 |   276/    0 |
|----------------------------------------+--------------+-------------|
| qsort()                                |   5144/  354 |  1084/    0 |
+---------------------------------------------------------------------+

ESP8266

+---------------------------------------------------------------------+
| Functionality                          |  flash/  ram |       delta |
|----------------------------------------+--------------+-------------|
| Baseline                               | 260497/28108 |     0/    0 |
|----------------------------------------+--------------+-------------|
| bubbleSort()                           | 260545/28108 |    48/    0 |
| insertionSort()                        | 260545/28108 |    48/    0 |
| selectionSort()                        | 260561/28108 |    64/    0 |
|----------------------------------------+--------------+-------------|
| shellSortClassic()                     | 260577/28108 |    80/    0 |
| shellSortKnuth()                       | 260577/28108 |    80/    0 |
| shellSortTokuda()                      | 260637/28128 |   140/   20 |
|----------------------------------------+--------------+-------------|
| combSort13()                           | 260609/28108 |   112/    0 |
| combSort13m()                          | 260625/28108 |   128/    0 |
| combSort133()                          | 260593/28108 |    96/    0 |
| combSort133m()                         | 260609/28108 |   112/    0 |
|----------------------------------------+--------------+-------------|
| quickSortMiddle()                      | 260641/28108 |   144/    0 |
| quickSortMedian()                      | 260673/28108 |   176/    0 |
| quickSortMedianSwapped()               | 260689/28108 |   192/    0 |
|----------------------------------------+--------------+-------------|
| qsort()                                | 261617/28108 |  1120/    0 |
+---------------------------------------------------------------------+

CPU Cycles

The CPU benchmark numbers can be seen in examples/AutoBenchmark. All times in milliseconds. Here are 2 samples.

SparkFun Pro Micro (ATmega32U4)

+---------------------+------------------------+---------+---------+---------+
|            \      N |    10 |    30 |    100 |     300 |    1000 |    3000 |
| Function    \       |       |       |        |         |         |         |
|---------------------+-------+-------+--------+---------+---------+---------|
| bubbleSort()        | 0.083 | 1.211 | 13.093 | 118.403 |         |         |
| insertionSort()     | 0.044 | 0.259 |  2.515 |  21.941 | 240.911 |         |
| selectionSort()     | 0.084 | 0.555 |  5.388 |  46.517 | 509.037 |         |
|---------------------+-------+-------+--------+---------+---------+---------|
| shellSortClassic()  | 0.076 | 0.311 |  1.738 |   6.895 |  28.869 |         |
| shellSortKnuth()    | 0.100 | 0.336 |  1.450 |   5.681 |  25.273 |         |
| shellSortTokuda()   | 0.076 | 0.339 |  1.636 |   6.543 |  28.090 |         |
|---------------------+-------+-------+--------+---------+---------+---------|
| combSort13()        | 0.163 | 0.532 |  2.235 |   8.240 |  36.409 |         |
| combSort13m()       | 0.166 | 0.559 |  2.225 |   8.279 |  34.916 |         |
| combSort133()       | 0.085 | 0.385 |  2.030 |   7.787 |  34.229 |         |
| combSort133m()      | 0.088 | 0.422 |  1.991 |   7.713 |  34.179 |         |
|---------------------+-------+-------+--------+---------+---------+---------|
| quickSortMiddle()   | 0.098 | 0.367 |  1.601 |   5.659 |  22.104 |         |
| quickSortMedian()   | 0.118 | 0.432 |  1.701 |   5.984 |  22.732 |         |
| quickSortMdnSwppd() | 0.087 | 0.335 |  1.385 |   4.871 |  18.914 |         |
|---------------------+-------+-------+--------+---------+---------+---------|
| qsort()             | 0.202 | 0.876 |  3.685 |  13.098 |         |         |
+---------------------+-------+-------+--------+---------+---------+---------+

ESP8266

+---------------------+------------------------+---------+---------+---------+
|            \      N |    10 |    30 |    100 |     300 |    1000 |    3000 |
| Function    \       |       |       |        |         |         |         |
|---------------------+-------+-------+--------+---------+---------+---------|
| bubbleSort()        | 0.017 | 0.145 |  1.604 |  14.477 | 169.490 |         |
| insertionSort()     | 0.008 | 0.036 |  0.338 |   2.895 |  31.947 |         |
| selectionSort()     | 0.013 | 0.071 |  0.715 |   6.270 |  69.023 |         |
|---------------------+-------+-------+--------+---------+---------+---------|
| shellSortClassic()  | 0.008 | 0.032 |  0.180 |   0.719 |   3.030 |  11.560 |
| shellSortKnuth()    | 0.009 | 0.031 |  0.149 |   0.588 |   2.608 |  10.006 |
| shellSortTokuda()   | 0.007 | 0.029 |  0.137 |   0.552 |   2.358 |   8.532 |
|---------------------+-------+-------+--------+---------+---------+---------|
| combSort13()        | 0.014 | 0.052 |  0.238 |   0.899 |   4.001 |  14.567 |
| combSort13m()       | 0.015 | 0.056 |  0.240 |   0.903 |   3.903 |  14.197 |
| combSort133()       | 0.010 | 0.043 |  0.209 |   0.852 |   3.840 |  13.288 |
| combSort133m()      | 0.010 | 0.047 |  0.222 |   0.850 |   3.733 |  13.341 |
|---------------------+-------+-------+--------+---------+---------+---------|
| quickSortMiddle()   | 0.013 | 0.044 |  0.175 |   0.616 |   2.353 |   8.087 |
| quickSortMedian()   | 0.015 | 0.049 |  0.189 |   0.634 |   2.370 |   7.764 |
| quickSortMdnSwppd() | 0.012 | 0.037 |  0.149 |   0.513 |   1.951 |   6.597 |
|---------------------+-------+-------+--------+---------+---------+---------|
| qsort()             | 0.028 | 0.094 |  0.405 |   1.472 |   5.826 |  19.875 |
+---------------------+-------+-------+--------+---------+---------+---------+

System Requirements

Hardware

This library has Tier 1 support on the following boards:

Arduino Nano (16 MHz ATmega328P)
SparkFun Pro Micro (16 MHz ATmega32U4)
SAMD21 M0 Mini (48 MHz ARM Cortex-M0+)
STM32 Blue Pill (STM32F103C8, 72 MHz ARM Cortex-M3)
NodeMCU 1.0 (ESP-12E module, 80MHz ESP8266)
WeMos D1 Mini (ESP-12E module, 80 MHz ESP8266)
ESP32 dev board (ESP-WROOM-32 module, 240 MHz dual core Tensilica LX6)
Teensy 3.2 (72 MHz ARM Cortex-M4)

Tier 2 support can be expected on the following boards, mostly because I don't test these as often:

ATtiny85 (8 MHz ATtiny85)
Arduino Pro Mini (16 MHz ATmega328P)
Teensy LC (48 MHz ARM Cortex-M0+)
Mini Mega 2560 (Arduino Mega 2560 compatible, 16 MHz ATmega2560)

The following boards are not supported:

Any platform using the ArduinoCore-API (https://github.com/arduino/ArduinoCore-api).
- For example, Nano Every, MKRZero, and Raspberry Pi Pico RP2040.

Tool Chain

Operating System

I use Ubuntu 20.04 for the vast majority of my development. I expect that the library will work fine under MacOS and Windows, but I have not explicitly tested them.

Bugs and Limitations

The number of elements n of the input data array is of type uint16_t.
- The maximum size of the input array is 65535.
- If you need bigger, copy the sorting algorithm that you want and change the uint16_t n to a uint32_t n.
- Using a fixed uint16_t means that the edge case behavior of these algorithms are consistent across all platforms.
- Certain implementation choices and optimizations can be made if we know that n cannot exceed a bounded value.
  - For example, the shellSortTokuda() function can use a pre-generated sequence of gaps which is a reasonable size because it only needs to go up to 65535.
- The alternative was to use the size_t type whose size is different on different platforms: 2 bytes on 8-bit processors, 4 bytes on 32-bit processors, and 8 bytes on 64-bit processors.
  - However, I did not want to worry about edge case behavior of some of these algorithms for extremely large values of n.
The behavior of the sorting algorithms with a data size of exactly n = 65535 has not been validated.
- Some algorithms may be buggy because this edge case may trigger an integer overflow.
- The actual maximum value of n may actually be 65534 for some algorithms.
Some of the Comb Sort algorithms have even lower limits of n due to integer overflows.
- See remarks above and in the source code.
No hybrid sorting algorithms.
- Different sorting algorithms are more efficient at different ranges of N. So hybrid algorithms will use different sorting algorithms at different points in their iteration. An example is the Introsort which uses Quick Sort, then switches to Heap Sort when the recursion depth becomes too high, then switches to Insertion Sort when N is below a threshold.
- This library currently does not implement such sorting algorithms, partly because they are more difficult to write and validate, and partly because these hybrid algorithms will inevitably consume more flash memory, which is usually a scarce resource in embedded environments.

Alternative Libraries

When I looked for sorting algorithm libraries for Arduino I found a small handful, but I found them unsuitable for me.

https://github.com/LinnesLab/KickSort
- Much of the AceSorting library was inspired by the KickSort library.
- The problem is that its shellSort() implementation was copied from this post (https://forum.arduino.cc/index.php?topic=280486.0) from 2014. Unfortunately, that implementation of Shell Sort is incorrect.
- No unit tests provided.
https://github.com/emilv/ArduinoSort
- Implements the Insertion Sort, which is an O(N^2) algorithm.
- Some unit testing provided.
- It is not suitable for array sizes greater than about 100.
https://github.com/luisllamasbinaburo/Arduino-QuickSort
- Documentation is in Spanish which I unfortunately do not understand.
- Provides a single version of Quick Sort, the equivalent of quickSortMiddle() provided by this library.
- No unit tests provided.
https://github.com/arduino-libraries/Arduino_AVRSTL
- Provides a template sort() function, which delegates to stable_sort().
- But this implementation is a bubble sort which is O(N^2) and in practice much slower than insertionSort().
  - In fairness, there is a "FIXME" note in the code which implies that a better algorithm ought to be provided.
- The library is configured to target only the avr and megaavr platforms.
- No unit tests provided.

Here are some of the reasons that I created my own library:

Shell Sort is one of my favorite sorting algorithms because it can be very fast and very small. But the KickSort library provides an incorrect implementation which is even slower than Bubble Sort.
Most libraries do not come with unit tests, so I do not know what other bugs are lurking in the code.
Most libraries do not evaluate the memory and CPU consumption of their sorting algorithms. I wanted to know these numbers precisely so that I could make informed trade off decisions.
I did not want to deal with the complexity of the C++ STL library. It is just too painful in small embedded programming projects.
Lastly, I wanted to implement my own sorting routines to make sure that I had a complete understanding of each algorithm. I had forgotten so much since learning many of these in my undergraduate years.

License

MIT License

Feedback and Support

If you have any questions, comments, or feature requests for this library, please use the GitHub Discussions for this project. If you have bug reports, please file a ticket in GitHub Issues. Feature requests should go into Discussions first because they often have alternative solutions which are useful to remain visible, instead of disappearing from the default view of the Issue tracker after the ticket is closed.

Please refrain from emailing me directly unless the content is sensitive. The problem with email is that I cannot reference the email conversation when other people ask similar questions later.

Authors

Created by Brian T. Park (brian@xparks.net).