# CSCI 424, Homework 2
# Adam Carpenter - username acarpent - acarpenter@email.wm.edu
	.text
	
	.globl	main
main:
	# Get the string to test and store it.
	jal 	Input_string
	la	$s0, ($a0)
	
	# ---Make a reversed copy of the string---
	la 	$a0, buffer2 # set aside space for new string
	la 	$s1, ($a0) # set s1 to space for new string
	la 	$t0, ($s0) # create address pointer
	add 	$t1, $zero, 10 # create index
copier:
	add	$t0, $s0, $t1 # point to character at index
	lb	$t2, ($t0) # load byte located at index
	beq	$t2, 0, skipJunk  # Skip null characters
	beq	$t2, 10, skipJunk # Skip new lines
	sb	$t2, ($a0) # store byte in reversed string
	addi	$a0, $a0, 1 # increment reversed string pointer
skipJunk:
	addi	$t1, $t1, -1 # decrement string pointer
	bne	$t1, -1, copier # if $t1 != -1, repeat the process
	
	# Now s0 points to the original string and
	# s1 points to the reverse of the original 
	# string, which has been stripped of 
	# whitespace and null characters
	# ----------------------------------------
	
	# Start printing out the response
	la	$a0, str1
	jal	Print_string
	
	la	$a0, ($s0)
	jal	Print_string
	
	la	$a0, str2
	jal	Print_string
	
	# ---Loop to check if strings match---
	add	$t0, $zero, $zero # flag marks whether string is palindrome
	add 	$t1, $zero, $zero # create index
checkPalindrome:
	add	$t2, $s0, $t1 # point to character at next index
	lb	$s2, ($t2) # load character into t2
	beq	$s2, 10, endLoop # end loop if newline
	add	$t3, $s1, $t1 # point to character at same index
	lb	$s3, ($t3) # load character into t3
	sub	$t0, $s2, $s3 # if the two characters match, flag will remain 0
	bne	$t0, $zero, notPalindrome # if flag is not zero, it's not a palindrome
	addi	$t1, $t1, 1 # increment index
	jal	checkPalindrome
endLoop:
	beq	$t0, $zero, isPalindrome
	# This loop determines if the string is a 
	# palindrome by comparing it to the 
	# reversed duplicate string. The loop terminates
	# when it finds the carriage return in the
	# original string and jumps based on 
	# how the flag was set.
	# ------------------------------------
	
notPalindrome: # The string is NOT a palindrome
	la	$a0, str3 
	jal	Print_string
	jal 	Exit
	
isPalindrome: # The string is a palindrome
	la	$a0, str4 
	jal	Print_string
	jal	Exit	
	
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

	.data
	.align	0
str1:
	.asciiz "\nThe string "
str2: 
	.asciiz "is "
str3:
	.asciiz	"not a palindrome!\n"
str4:
	.asciiz "a palindrome!\n"
buffer: 
	.space 12
buffer2: 
	.space 12

# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

# Wrapper functions around some of the system calls
# From P&H COD, Fig. A.9.1
	.text
	
	.globl Input_string
Input_string:  # take in a string from keyboard. String is in $a0 and length is in $a1
	la	$a0, buffer 	 # Place result in location of myString
	li 	$a1, 12		 # Maximum length for string
	addi	$v0, $zero, 8
	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