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+# CSCI 424, Homework 3
+# Adam Carpenter - username acarpent - acarpenter@email.wm.edu
+	.text
+	.globl	main
+main:
+	la	$s0, array1 		# create array
+	la	$t0, ($s0) 		# create pointer
+	add	$s1, $zero, $zero 	# represents n-1 of array
+	la 	$a0, startMsg 
+	jal	Print_string 		# print message
+InputLoop: # Take in only positive integers and store them in array until user enters -1
+	la 	$a0, prompt
+	jal 	Print_string 		# Print prompt
+	jal 	Read_integer 		# read integer
+	beq	$v0, -1, isArrayEmpty 	# Exit if user enters -1
+	sw	$v0, ($t0) 		# Store integer
+	add	$t0, $t0, 4 		# increment pointer
+	add	$s1, $s1, 1 		# increment counter
+	j 	InputLoop
+isArrayEmpty: # If the user only enters -1, there is nothing to sort
+	bne 	$t0, $s0, arrayNotEmpty
+	la	$a0, emptyMsg
+	jal 	Print_string	
+	jal 	Exit
+arrayNotEmpty: # The array isn't empty, continue execution
+	add	$t0, $s0, $zero
+	add	$t1, $zero, $zero
+	la	$a0, unsortedMsg
+	jal 	Print_string
+printLoopUnsorted:
+	lw	$a0, ($t0)
+	jal 	Print_integer
+	la	$a0, separator
+	jal 	Print_string
+	add	$t0, $t0, 4
+	add	$t1, $t1, 1
+	bne	$s1, $t1, printLoopUnsorted
+
+	# Call MergeSort for first time
+	sub	$s1, $s1, 1		# set s1 to length-1
+	add	$a0, $zero, $zero 	# low stored in $a0
+	add	$a1, $zero, $zero 	# mid stored in $a1
+	add	$a2, $zero, $s1		# high stored in $a2
+	jal	MergeSort
+			
+	# Finish execution in main by printing sorted array
+	add	$t0, $s0, $zero
+	add	$t1, $zero, $zero
+	la 	$a0, sortedMsg
+	jal	Print_string
+	addi	$s1, $s1, 1		# set s1 to length
+printLoopSorted:
+	lw	$a0, ($t0)
+	jal 	Print_integer
+	la	$a0, separator
+	jal 	Print_string
+	add	$t0, $t0, 4
+	add	$t1, $t1, 1
+	bne	$s1, $t1, printLoopSorted
+	jal 	Exit 			# End the program
+# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+# How it works: This mergesort algorithm does not follow the algorithm listed in the
+# homework prompt. I believe it is simpler. Mergesort() takes in low and high arguments
+# (the array is globally saved and accessed) and then makes recursive calls to 
+# mergesort(low, mid) and then mergesort(mid+1, high) and then calls merge(low, mid, high). 
+# Merge() creates L1 and L2 to look at the elements of the array. It creates a second 
+# array (b) of the required size and places elements from a into b based on the 
+# divide and conquer conditions passed. Once the merge is done, control is returned
+# to Mergesort which continues the algorithm. For more information, visit 
+# https://www.tutorialspoint.com/data_structures_algorithms/merge_sort_program_in_c.htm
+
+# --- MergeSort Procedure ---
+MergeSort:
+	addi	$sp, $sp, -16		# adjust stack, push return address
+	sw	$ra, 12($sp)
+	
+	bge	$a0, $a2, MergeSortEnd 	# if low >= high, end procedure
+	add	$a1, $zero, $zero 	# reset mid
+	add	$t0, $a0, $a2 		# temp = low + high
+	div 	$a1, $t0, 2		# mid = temp / 2 
+	
+	# push arguments onto stack
+	sw	$a0, 0($sp)
+	sw	$a1, 4($sp)
+	sw	$a2, 8($sp)
+	
+	add	$a2, $a1, $zero		# Call MergeSort(low, mid)
+	jal 	MergeSort
+	
+	# load arguments back
+	lw	$a0, 0($sp)
+	lw	$a1, 4($sp)
+	lw	$a2, 8($sp)
+	# lw	$ra, 12($sp)
+
+	addi	$a0, $a1, 1		# Call MergeSort(mid+1, high)
+	jal MergeSort
+	
+	# load arguments back
+	lw	$a0, 0($sp)
+	lw	$a1, 4($sp)
+	lw	$a2, 8($sp)
+	lw	$ra, 12($sp)
+
+	jal 	Merge			# Call Merge
+MergeSortEnd:
+	lw	$ra, 12($sp)		# Load return address
+	addi	$sp, $sp, 16 		# Readjust stack pointer
+	jr	$ra			# Return
+# ---------------------------
+
+# --- Merge Procedure ---
+Merge:
+	add	$s2, $zero, $zero	# initialize
+	add	$t3, $zero, $zero
+	add	$t9, $zero, $zero
+	add	$t0, $a0, $zero		# i = low
+	add	$t1, $a0, $zero		# L1 = low
+	addi	$t2, $a1, 1		# L2 = mid + 1
+	
+	add	$s2, $a0, $zero		# Protect a0 from being overwritten permanently
+	add	$a0, $a2, $zero		# create array b for merging
+	li	$v0, 9
+	syscall
+
+	add	$t3, $v0, $zero 	# tmp3 points to arrayb
+	add	$a0, $s2, $zero
+	add	$s2, $t3, $zero		# s2 points to arrayb as backup
+	
+While1:
+	mul	$t9, $t1, 4		# tmp9 is the array index converter (turn i into address increment)
+	add	$t5, $t9, $s0		# tmp5 = &a[L1]
+	lw	$t5, ($t5)		# tmp5 = *a[L1]
+	mul	$t9, $t2, 4
+	add	$t6, $t9, $s0		# tmp6 = &a[L2]
+	lw	$t6, ($t6)		# tmp6 = *a[L2]
+	add	$t3, $s2, $zero		# tmp3 = &b[0]
+	mul	$t9, $t0, 4
+	add	$t3, $t3, $t9		# tmp3 = &b[i]
+	
+	bgt	$t5, $t6, Else1		# if *a[L1] <= *a[L2] then
+	sw	$t5, ($t3)		# *b[i] = *a[L1]
+	addi	$t1, $t1, 1		# L1 = L1 + 1
+	j	Final1
+Else1:					# else
+	sw	$t6, ($t3)		# *b[i] = *a[L2]
+	addi	$t2, $t2, 1		# L2 = L2 + 1
+Final1:
+	addi	$t0, $t0, 1		# i = i + 1
+	bgt	$t1, $a1, End1		# loop only if L1 <= mid
+	bgt	$t2, $a2, End1		# loop only if L2 <= high
+	# addi	$t0, $t0, 1		# i = i + 1
+	j	While1
+End1:
+	bgt	$t1, $a1, End2		# loop only if L1 <= mid
+	
+While2:
+	mul	$t9, $t1, 4
+	add	$t5, $t9, $s0		# tmp5 = &a[L1]
+	lw	$t5, ($t5)		# tmp5 = *a[L1]
+	add	$t3, $s2, $zero		# tmp3 = &b[0]
+	mul	$t9, $t0, 4
+	add	$t3, $t3, $t9		# tmp3 = &b[i]
+	sw	$t5, ($t3)		# *b[i] = *a[L1]
+	addi	$t1, $t1, 1		# L1 = L1 + 1
+	addi	$t0, $t0, 1		# i = i + 1	
+	bgt	$t1, $a1, End2		# loop only if L1 <= mid
+	j	While2
+End2:
+	bgt	$t2, $a2, End3		# loop only if L2 <= high
+	
+While3:
+	mul	$t9, $t2, 4
+	add	$t6, $t9, $s0		# tmp6 = &a[L2]
+	lw	$t6, ($t6)		# tmp6 = *a[L2]
+	add	$t3, $s2, $zero		# tmp3 = &b[0]
+	mul	$t9, $t0, 4
+	add	$t3, $t3, $t9		# tmp3 = &b[i]
+	sw	$t6, ($t3)		# *b[i] = *a[L2]
+	addi	$t2, $t2, 1		# L2 = L2 + 1
+	addi	$t0, $t0, 1		# i = i + 1
+	bgt	$t2, $a2, End3		# loop only if L2 <= high
+	j	While3
+	
+End3:
+	add	$t0, $a0, $zero		# i = low
+	add	$t7, $s0, $zero		# tmp7 = &a[0]
+	
+While4:
+	add	$t3, $s2, $zero		# tmp3 = &b[0] 
+	mul	$t9, $t0, 4
+	add	$t3, $t3, $t9		# tmp3 = &b[i]
+	lw	$t3, ($t3)		# tmp3 = *b[i]
+	add	$t7, $s0, $zero		# tmp7 = &a[0]
+	add	$t7, $t7, $t9		# tmp7 = &a[i]
+	sw	$t3, ($t7)		# *a[i] = *b[i]
+	addi	$t0, $t0, 1		# i = i + 1
+	bgt	$t0, $a2, MergeEnd	# loop only if i <= high
+	j	While4
+	
+MergeEnd:
+	jr	$ra			# Return
+# ---------------------------
+
+# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+	.data
+startMsg:
+	.asciiz "\nPlease enter positive integers. When you are finished enter -1.\n"
+prompt:
+	.asciiz "Enter integer: "
+emptyMsg:
+	.asciiz "Nothing to sort -- the array is empty!\n"
+unsortedMsg:
+	.asciiz "\nUnsorted array: "
+sortedMsg:
+	.asciiz "\nSorted array:   "
+separator:
+	.asciiz " "
+	
+	.align 4
+array1: 
+	.space 60 # 60 bytes for max 15 integers
+	
+	.align 4
+arrayL:
+	.space 32 # 32 bytes for max 8 integers
+	
+	.align 4
+arrayR:
+	.space 32 # 32 bytes for max 8 integers
+	
+# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+# Wrapper functions around some of the system calls
+# From P&H COD, Fig. A.9.1
+	.text
+
+	.globl Read_integer
+Read_integer: 	# input an integer to be stored in v0
+	addi	$v0, $zero, 5
+	syscall
+	jr	$ra
+
+	.globl Print_integer
+Print_integer: # print the integer in register a0
+	addi 	$v0, $zero, 1
+	syscall
+	jr 	$ra
+
+	.globl	Print_string
+Print_string:	# print the string whose starting address is in register a0
+	addi	$v0, $zero, 4
+	syscall
+	jr	$ra
+
+	.globl	Exit
+Exit:		# end the program, no explicit return status
+	addi	$v0, $zero, 10
+	syscall
+	jr	$ra
+
+	.globl	Exit2
+Exit2:		# end the program, with return status from register a0
+	addi	$v0, $zero, 17
+	syscall
+	jr	$ra