# ARM Assembly: Sorting

After taking a hiatus for two years, I’ve started working with ARM assembly language again. I realized that the code I had been working on before had become a kind of utility library, so I rearranged the git repository to reflect that.

While doing so, I noticed that my sorting libraries were in an incomplete state, so I decided to work on finishing them. The result is that I now have four working sort functions that all operate in place on an existing array of 32bit signed integers.

Below you’ll find more details about each of the sorting algorithms I implemented, along with the code. All of the following code snippets are licensed under the GPLv3.

# Bubble Sort

Bubble sort is often one of the first sorting algorithms people learn. It works by iterating through the list of items to be sorted and swapping items that are out of order. After each iteration, if any swaps were made it iterates again. This results in the largest (or smallest) value «bubbling» up to the end of the list.

The algorithm is very simple, but both its average and worst case time complexities are O(N²) (quadratic), which make it fairly inefficient. Because of this, bubble sort is almost never used in real applications. However, one nice thing about bubble sort is that when applied to an already sorted list it only requires a single iteration to verify that the list is sorted. In this best case scenario the time complexity is O(1) (constant).

The version provided below includes an optimization that skips the already sorted elements at the end of the array during each successive iteration. This improves actual performance but does not change the overall time complexity.

```
/*
Copyright 2019, Andrew C. Young <andrew@vaelen.org>
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
// Bubble Sort
// Exported Methods
.global bsort
// Code Section
bsort:
// Bubble sort an array of 32bit integers in place
// Arguments: R0 = Array location, R1 = Array size
PUSH {R0-R6,LR} // Push registers on to the stack
bsort_next: // Check for a sorted array
MOV R2,#0 // R2 = Current Element Number
MOV R6,#0 // R6 = Number of swaps
bsort_loop: // Start loop
ADD R3,R2,#1 // R3 = Next Element Number
CMP R3,R1 // Check for the end of the array
BGE bsort_check // When we reach the end, check for changes
LDR R4,[R0,R2,LSL #2] // R4 = Current Element Value
LDR R5,[R0,R3,LSL #2] // R5 = Next Element Value
CMP R4,R5 // Compare element values
STRGT R5,[R0,R2,LSL #2] // If R4 > R5, store current value at next
STRGT R4,[R0,R3,LSL #2] // If R4 > R5, Store next value at current
ADDGT R6,R6,#1 // If R4 > R5, Increment swap counter
MOV R2,R3 // Advance to the next element
B bsort_loop // End loop
bsort_check: // Check for changes
CMP R6,#0 // Were there changes this iteration?
SUBGT R1,R1,#1 // Optimization: skip last value in next loop
BGT bsort_next // If there were changes, do it again
bsort_done: // Return
POP {R0-R6,PC} // Pop the registers off of the stack
```

# Quicksort

Quicksort is often the second sorting algorithm one learns. It works by first choosing a pivot value and then dividing the list of items to be sorted into two buckets based on whether they are smaller than or larger than the pivot value. It then repeats this process on each bucket until the entire list is sorted.

The average time complexity of quicksort is O(n log n) (loglinear), making it much faster than bubble sort. However, if the pivot selected at each step is either the largest or smallest value in the list, then it effectively becomes a bubble sort and its time complexity becomes O(N²). This can be avoided by improving the pivot selection process so that it never selects either the largest or smallest value.

The simplest implementation simply chooses either the first value in the list or the last value to use as the pivot. With this implementation, applying the algorithm to an already sorted list results in the worst case scenario in terms of time complexity. The code below avoids this by using the «median-of-three» pivot selection algorithm. This works by looking at the first, middle, and last values in the list and selecting the one that is the median of these three values as the pivot. In this way the code is guaranteed to never choose either the largest or smallest value in the list.

```
/*
Copyright 2019, Andrew C. Young <andrew@vaelen.org>
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
// Quick Sort
// Exported Methods
.global qsort
// Code Section
qsort:
// Quick sort an array of 32bit integers
// Arguments: R0 = Array location, R1 = Array size
PUSH {R0-R10,LR} // Push registers on to the stack
MOV R4,R0 // R4 = Array Location
MOV R5,R1 // R5 - Array Size
CMP R5,#1 // Check for an array of size <= 1
BLE qsort_done // If array size <= 1, return
CMP R5,#2 // Check for an array of size == 2
BEQ qsort_check // If array size == 2, check values
qsort_partition:
MOV R1,#2 // Find the middle element
SDIV R2,R5,R1 // R2 = The middle element index
LDR R6,[R4] // R6 = Beginning of array value
LDR R7,[R4,R2,LSL #2] // R7 = Middle of array value
SUB R8,R5,#1 // R8 = Upper array bound index (len -1 1)
LDR R8,[R4,R8,LSL #2] // R8 = End of the array value
CMP R6,R7 // Sort the values
MOVGT R9,R6
MOVGT R6,R7
MOVGT R7,R9
CMP R7,R8
MOVGT R9,R7
MOVGT R7,R8
MOVGT R8,R9
CMP R6,R7
MOVGT R9,R6
MOVGT R6,R7
MOVGT R7,R9
MOV R6,R7 // R6 = Pivot
MOV R7,#0 // R7 = Lower array bounds index
SUB R8,R5,#1 // R8 = Upper array bounds index (len - 1)
qsort_loop:
LDR R0,[R4,R7,LSL #2] // R0 = Lower value
LDR R1,[R4,R8,LSL #2] // R1 = Upper value
CMP R0,R6 // Compare lower value to pivot
BEQ qsort_loop_u // If == pivot, do nothing
ADDLT R7,R7,#1 // If < pivot, increment lower index
STRGT R0,[R4,R8,LSL #2] // If > pivot, swap values
STRGT R1,[R4,R7,LSL #2]
SUBGT R8,R8,#1 // if > pivot, decrement upper index
CMP R7,R8 // if indexes are the same, recurse
BEQ qsort_recurse
LDR R0,[R4,R7,LSL #2] // R0 = Lower value
LDR R1,[R4,R8,LSL #2] // R1 = Upper value
qsort_loop_u:
CMP R1,R6 // Compare upper value to pivot
SUBGT R8,R8,#1 // if > pivot, decrement upper index
STRLT R0,[R4,R8,LSL #2] // If < pivot, swap values
STRLT R1,[R4,R7,LSL #2]
ADDLT R7,R7,#1 // if < pivot, increment lower index
CMP R7,R8 // if indexes are the same, recurse
BEQ qsort_recurse
B qsort_loop // Continue loop
qsort_recurse:
MOV R0,R4 // R0 = Location of the first bucket
MOV R1,R7 // R1 = Length of the first bucket
BL qsort // Sort first bucket
ADD R8,R8,#1 // R8 = 1 index past final index
CMP R8,R5 // Compare final index to original length
BGE qsort_done // If equal, return
ADD R0,R4,R8,LSL #2 // R0 = Location of the second bucket
SUB R1,R5,R8 // R1 = Length of the second bucket
BL qsort // Sort second bucket
B qsort_done // return
qsort_check:
LDR R0,[R4] // Load first value into R0
LDR R1,[R4,#4] // Load second value into R1
CMP R0,R1 // Compare R0 and R1
BLE qsort_done // If R1 <= R0, then we are done
STR R1,[R4] // Otherwise, swap values
STR R0,[R4,#4] //
qsort_done:
POP {R0-R10,PC} // Pop registers off of the stack and return
```

# Heapsort

Heapsort works by building a heap out of the array. Once an array has been converted to a heap, the first item is guaranteed to be either the smallest or largest value depending on whether the heap is a minimum or maximum heap, respectively. The heapsort algorithm uses a maximum heap and therefore the first value in the array will always be the largest value after the heap is created. It then swaps this value with the value at the end of the array and re-creates the heap ignoring the now properly sorted last element. The algorithm thus builds the sorted array in place from the end forward. When there are only two items left to be sorted it performs a simple comparison and swap.

The best, worst, and average time complexities of heapsort are all O(N log N). This is because building the heap has an average time complexity of O(log N) and this process is performed N times as the array is sorted.

```
/*
Copyright 2019, Andrew C. Young <andrew@vaelen.org>
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
// Heapsort
// Exported Methods
.global hsort
// Code Section
hsort:
// R0 = Array location
// R1 = Array length
PUSH {R2-R12,LR} // Push registers on to the stack
CMP R1, #1 // If the array has size 0 or 1, return
BLE hsort_done
CMP R1, #2 // If the array has size 2, swap if needed
BLE hsort_simple
MOV R11, #1 // R11 = 1 (constant)
MOV R12, #2 // R12 = 2 (constant)
MOV R10, #-1 // R10 = -1 (temporarily)
UDIV R10, R1, R12 // R10 = (len / 2) - 1 (start heapify index)
SUB R10, R10, #1
SUB R9, R1, #1 // R9 = len - 1 (start pop index)
MOV R2, R10 // R2 = Starting heapify index
heapify:
// R2 = Index
MOV R3, R2 // R3 = Index of largest value
MLA R4, R2, R12, R11 // R4 = Left index (2i+1)
MLA R5, R2, R12, R12 // R5 = Right index (2i+2)
LDR R6, [R0, R2, LSL#2] // R6 = Index value (R0 + (R2 * 4))
MOV R7, R6 // R7 = Current largest value
heapify_check_left:
CMP R4, R9 // Check if left node is in array bounds
BGT heapify_check_right // If not then check the right node
LDR R8, [R0, R4, LSL#2] // R8 = Left value (R0 + (R4 * 4)
CMP R8, R7 // Compare left value to largest value
BLE heapify_check_right // If left <= largest check right
MOV R3, R4 // If left > largest, change largest index
MOV R7, R8 // and copy the value
heapify_check_right:
CMP R5, R9 // Check if right node is in array bounds
BGT heapify_swap // If not then continue
LDR R8, [R0, R5, LSL#2] // R8 = Right value (R0 + (R5 * 4))
CMP R8, R7 // Compare right value to largest value
BLE heapify_swap // If right <= largest then continue
MOV R3, R5 // If right > largest, change largest index
MOV R7, R8 // and copy the value
heapify_swap:
CMP R2, R3 // Check if largest is the initial value
BEQ heapify_next // If so, exit the heapify loop
STR R7, [R0, R2, LSL#2] // If not, swap largest and initial values
STR R6, [R0, R3, LSL#2]
MOV R2, R3 // and change index to largest index
B heapify // and recurse
heapify_next:
CMP R10, #0 // If last index was 0, heap is finished
BEQ heapify_pop
SUB R10, R10, #1 // Else, heapify next index
MOV R2, R10
B heapify
heapify_pop:
LDR R3, [R0] // R3 = Largest value (front of heap)
LDR R4, [R0, R9, LSL#2] // R4 = Value from end of heap
STR R3, [R0, R9, LSL#2] // Store largest value at end of heap
STR R4, [R0] // Store end value at start of heap
SUB R9, #1 // Decrement end of heap index
CMP R9, #1 // If there are only two items left, compare
BEQ hsort_simple
MOV R2, #0 // Else, heapify from the start of the heap
B heapify
hsort_simple:
// Sort a list of exactly 2 elements
LDR R2, [R0] // First value
LDR R3, [R0, #4] // Second value
CMP R2, R3 // Compare values
STRGT R3, [R0] // Swap if out of order
STRGT R2, [R0, #4]
hsort_done:
POP {R2-R12,PC} // Return
```

# Radix Sort

Radix sort works a bit differently from other sorting algorithms. Like quicksort it works by first dividing the unsorted list into two buckets and then iterating over each bucket. However, rather than using a comparison function to determine whether a given value is larger or smaller than another value it uses the radix (or base) to determine which bucket each value should go in.

For example, if we were going to sort a deck of 100 cards numbered 0-99, we might perform the sort like this:

- Divide the cards into piles based on their first digit (0-9), left-to-right. This results in 10 piles.
- For each pile, divide those cards into piles based on their second digit (0-9), left-to-right. This results in 10 new piles for each of the first piles, each with only a single card in it.
- Reassemble the deck by picking up the cards from left to right and the cards will be in order.

As shown by the example, the number of buckets used at any given step depends on the radix (or base) being used. If the numbers are represented in base 10 (decimal), then each iteration produces 10 buckets. However, if the numbers are stored in base 2 (binary), then only two buckets are produced. Likewise, the data can be sorted either by the most significant digit (MSD) as in the example above, or by the least significant digit (LSD).

The code below implements an in-place binary MSD radix sort, also called a binary quicksort. Although sorting unsigned integers works as expected, sorting signed integers will result in the negative integers being sorted after the positive integers. This is due to the fact that negative signed integers have their most significant bit set to `1`

. This code could also probably be further optimized by moving the swap logic in-line, but I think it is more readable this way.

```
/*
Copyright 2019, Andrew C. Young <andrew@vaelen.org>
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
// Radix Most Significant Bit In-Place Sorting Algorithm
// Exported Methods
.global rsort
.global swap
// Code Section
rsort:
// Radix MSD sort an array of 32bit integers
// Arguments: R0 = Array location, R1 = Array size
PUSH {R0-R2,LR} // Push existing registers on to the stack
CMP R1,#1 // Check for an empty or single member array
POPLE {R0-R7,PC} // If so, return to where we came from
ADD R1,R0,R1,LSL #2 // R1 = End of the array (R0 + (R1*4))
SUB R1,R1,#4 //
MOV R2,#1 // R2 = Bitmask
LSL R2,R2,#31 // most significant bit
BL rsort_recurse // Begin recursion
POP {R0-R2,PC} // Pop the registers off of the stack
rsort_recurse:
// Radix MSD sort an array of 32bit integers (recursive helper)
// Arguments: R0 = Array start, R1 = Array end, R2 = Bitmask
PUSH {R0-R8,LR} // Store previous values
SUB R8,R1,R0 // Check array length
CMP R8,#4 //
BLT rr_done // Return if array is empty or has 1 entry
MOV R3,R0 // R3 = 0s bin pointer (start of array)
MOV R4,R1 // R4 = 1s bin pointer (end of array)
MOV R5,#0 // R5 = Current value
MOV R6,R0 // Set original array start (debug)
MOV R7,R1 // Set original array end (debug)
rr_0loop:
LDR R5,[R3] // Load next value from pointer
TST R5,R2 // Check bitmask
BEQ rr_0loop_next // If the value is 0, loop
BL rr_swap // If not, swap values
B rr_1loop_next // Switch to the 1s bin
rr_0loop_next:
ADD R3,R3,#4 // Increment the pointer
CMP R3,R4 // Check if pointers are the same
BGT rr_next_bit // If so, move to the next bit
B rr_0loop // If not, check the next value
rr_1loop:
ADD R12,#1 // Increment loop counter (debug)
LDR R5,[R4] // Load current value from pointer
TST R5,R2 // Check bitmask
BNE rr_1loop_next // If the value is 1, loop
BL rr_swap // If not, swap values
B rr_0loop_next // Switch to the 0s bin
rr_1loop_next:
SUB R4,R4,#4 // Decrement the pointer
CMP R3,R4 // Check if pointers are the same
BGT rr_next_bit // If so, move to the next bit
B rr_1loop // If not, check the next value
rr_next_bit:
LSR R2,R2,#1 // Update bitmask to next bit
CMP R2,#0 // Check for all zeros
BEQ rr_done // If so, do not recurse anymore
MOV R0,R6 // Array start
MOV R1,R3 // 0s bin array end
SUB R1,#4
BL rsort_recurse // Sort 0s bin
MOV R0,R4 // 1s bin array start
ADD R0,#4
MOV R1,R7 // Array end
BL rsort_recurse // Sort 1s bin
B rr_done // All done
rr_swap:
PUSH {LR} // Store LR
MOV R0,R3 // End of 0s bin
MOV R1,R4 // End of 1s bin
BL swap // Swap values
POP {PC} // Return
rr_done:
POP {R0-R8,PC} // Restore previous values
swap:
// Swaps the values from 2 memory locations
// R0 = 1st memory location
// R1 = 2nd memory location
PUSH {R0-R4,LR} // Store previous values
LDR R3,[R0] // Load value from the first memory location
LDR R4,[R1] // Load value from the second memory location
STR R3,[R1] // Store value 2 in memory location 1
STR R4,[R0] // Store value 1 in memory location 2
POP {R0-R4,PC} // Return
```

# Additional Links and Videos

For more details on common sorting algorithms, including visualizations, I suggest checking out the Sorting Algorithm Animations page by Toptal. I also recommend the Sound of Sorting website, which is where I found many of the videos in this article. They have a YouTube playlist that includes visualizations of many different sorting algorithms. Finally, check out AlgoRythmics, who produced the dancing videos.