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+########################################################################
+# COMP1521 20T2 --- assignment 1: a cellular automaton renderer
+
+# Complex logical statements are marked with horizontal bars for readability.
+
+# Maximum and minimum values for the 3 parameters.
+
+MIN_WORLD_SIZE	=    1
+MAX_WORLD_SIZE	=  128
+MIN_GENERATIONS	= -256
+MAX_GENERATIONS	=  256
+MIN_RULE	=    0
+MAX_RULE	=  255
+
+# Characters used to print alive/dead cells.
+
+ALIVE_CHAR	= '#'
+DEAD_CHAR	= '.'
+
+# Maximum number of bytes needs to store all generations of cells.
+
+MAX_CELLS_BYTES	= (MAX_GENERATIONS + 1) * MAX_WORLD_SIZE
+
+	.data
+
+# `cells' is used to store successive generations.  Each byte will be 1
+# if the cell is alive in that generation, and 0 otherwise.
+
+cells:	.space MAX_CELLS_BYTES
+#cells:	.byte 1:MAX_CELLS_BYTES           # Very useful for debugging.
+
+# Some strings you'll need to use:
+
+prompt_world_size:	.asciiz "Enter world size: "
+error_world_size:	.asciiz "Invalid world size\n"
+prompt_rule:		.asciiz "Enter rule: "
+error_rule:		.asciiz "Invalid rule\n"
+prompt_n_generations:	.asciiz "Enter how many generations: "
+error_n_generations:	.asciiz "Invalid number of generations\n"
+
+	.text
+
+    # REGISTERS IN MAIN
+    # $s0: int world_size    (runtime input)
+    # $s1: int rule          (runtime input)
+    # $s2: int n_generations (runtime input)
+    # $s3: int reverse       (0 or 1)
+    # $s4: int g             (loop counters)
+    # $t0 to $t2             (temp variables)
+    # $v0 to $v2             (syscalls and functions)
+
+#--------------------------------BEGIN_MAIN-------------------------------------
+
+main:
+    # No need to store $ra on the stack until we call our own functions.
+
+    la      $a0, prompt_world_size      # printf("Enter world size: ");
+    li      $v0, 4
+    syscall
+    
+    move    $s0, $zero                  # $s0: int world_size = 0;
+    li      $v0, 5                      # scanf("%d", &world_size);
+    syscall
+    move    $s0, $v0
+
+    blt     $s0, MIN_WORLD_SIZE, if0    # if (world_size < MIN_WORLD_SIZE)
+    bgt     $s0, MAX_WORLD_SIZE, if0    # ^|| (world_size > MAX_WORLD_SIZE);
+    b       skip0
+if0:
+    la      $a0, error_world_size       # printf("Invalid world size\n");
+    li      $v0, 4
+    syscall
+    li      $v0, 1                      # return 1;
+    jr      $ra
+    # Despite returning 1, echo $? says '0'. I suspect this is due to spim
+    # returning 0 on the "successful" execution of a program. I am unaware
+    # of how to extract the return value of an emulated mips program.
+
+skip0:
+    la      $a0, prompt_rule            # printf("Enter rule: ");
+    li      $v0, 4
+    syscall
+    
+    move    $s1, $zero                  # $s1: int rule = 0;
+    li      $v0, 5                      # scanf("%d", &world_size);
+    syscall
+    move    $s1, $v0
+
+    blt     $s1, MIN_RULE, if1          # if (world_size < MIN_RULE)
+    bgt     $s1, MAX_RULE, if1          # ^|| (world_size > MAX_RULE);
+    b       skip1
+if1:
+    la      $a0, error_rule             # printf("Invalid world size\n");
+    li      $v0, 4
+    syscall
+    li      $v0, 1                      # return 1;
+    jr      $ra
+
+skip1:
+    la      $a0, prompt_n_generations   # printf("Enter how many generations ");
+    li      $v0, 4
+    syscall
+    
+    move    $s2, $zero                  # $s2: n_generations = 0;
+    li      $v0, 5                      # scanf("%d", &world_size);
+    syscall
+    move    $s2, $v0
+
+    blt     $s2, MIN_GENERATIONS, if2   # if (n_generations < MIN_GENERATIONS)
+    bgt     $s2, MAX_GENERATIONS, if2   # ^|| (n_generations > MAX_GENERATIONS);
+    b       skip2
+if2:
+    la      $a0, error_n_generations    # printf("Invalid world size\n");
+    li      $v0, 4
+    syscall
+    li      $v0, 1                      # return 1;
+    jr      $ra
+
+skip2:
+    li      $a0, '\n'                   # putchar('\n');
+    li      $v0, 11
+    syscall
+
+    move    $s3, $zero                  # $s3: int reverse = 0;
+    blt     $s2, 0, if3                 # if (n_generations < 0);
+    b       skip3
+if3:
+    li      $s3, 1                      # reverse = 1;
+    mul     $s2, $s2, -1                # n_generations = -n_generations;
+
+skip3:
+    la      $t0, cells                  # cells[0][world_size / 2] = 1;
+    li      $t1, 2
+    div     $s0, $t1
+    mflo    $t1
+    add     $t0, $t0, $t1
+    li      $t2, 1
+    sb      $t2, ($t0)
+
+    # After this point we begin to use our functions, so save the stack pointer.
+    sub     $sp, $sp, 4
+    sw      $ra, ($sp)
+
+
+#-------------------------------FIRST_LOOP_START--------------------------------
+    
+    li      $s4, 1                      # for (int g = 1; ($s4: int g = 1)
+loop0:
+    bgt     $s4, $s2, end0              # ^g <= n_generations; (test condition)
+    
+    move    $a0, $s0                    #run_generation(world_size, g, rule);
+    move    $a1, $s4
+    move    $a2, $s1
+    jal     run_generation
+    
+    add     $s4, $s4, 1                 # g++;);   (increment)
+    b       loop0
+end0:
+
+#--------------------------------FIRST_LOOP_END---------------------------------
+#--------------------------------------IF---------------------------------------
+
+    beqz    $s3, skip4                  # if (reverse)
+
+#------------------------------SECOND_LOOP_START--------------------------------
+    move    $s4, $s2                    # for (int g = n_generations; 
+                                        # ($s4: int g = n_generations)
+
+loop1:
+    blt     $s4, 0, end1                # ^g >= 0; (test condition)
+
+    move    $a0, $s0                    # print_generation(world_size, g);
+    move    $a1, $s4
+    jal     print_generation 
+    
+    sub     $s4, $s4, 1                 # g--;);   (decrement)
+    b       loop1
+end1:
+#------------------------------SECOND_LOOP_END----------------------------------
+    b       skip5 
+#------------------------------------ELSE---------------------------------------
+skip4:                                  # else
+#------------------------------THIRD_LOOP_START---------------------------------
+    li      $s4, 0                      # for (int g = 0; ($s4: int g = 0)
+loop2:
+    bgt     $s4, $s2, end2              # ^g <= n_generations; (test_condition) 
+
+    move    $a0, $s0                    # print_generation(world_size, g);
+    move    $a1, $s4
+    jal     print_generation 
+
+    add     $s4, $s4, 1                 # g++;) (increment)
+    b       loop2
+
+end2:
+#-------------------------------THIRD_LOOP_END----------------------------------
+skip5:
+#----------------------------------ELSE_END-------------------------------------
+
+	lw      $ra, ($sp)                  # Load $ra from the stack.
+    add     $sp, $sp, 4                 # Deallocate from the stack, now empty.
+    li      $v0, 0                      # Return 0.
+	jr      $ra
+#----------------------------------MAIN_END-------------------------------------
+
+
+	# Given `world_size', `which_generation', and `rule', calculate
+	# a new generation according to `rule' and store it in `cells'.
+
+    # REGISTERS IN RUN_GENERATION
+    # $a0: int world_size                (passed to function)
+    # $a1: int which_generation          (passed to function)
+    # $a2: int rule                      (passed to function)   
+    # $t0: int x                         (copy of arguments)    CHANGED!
+    # $t1: int left                      (copy of arguments)    CHANGED!
+    # $t2: int centre                    (copy of arguments)    CHANGED!
+    # $t3: int right                     (increment variable)   CHANGED!
+    # $t4 to $t5:                        (temp variables)       CHANGED!
+    # $v0: int                           (syscalls)             CHANGED!
+
+#-----------------------------BEGIN_FUNCTION_0----------------------------------
+run_generation:
+
+    # In this function we are pressed for variables. We will modify $a variables
+    # but continue to preserve them without using the stack as they are
+    # easily reversible changes which we may keep track of and fix before we
+    # return.
+
+#---------------------------------BEGIN_LOOP------------------------------------
+    li      $t0, 0                      # for(int x = 0; ($t0: int x = 0)
+f0loop0:
+    bge     $t0, $a0, f0end0            # ^x < world_size
+
+    li      $t1, 0                      # $t1: int left = 0;
+
+    sub     $a1, $a1, 1                 # which_generation--
+    sub     $t0, $t0, 1                 # x--
+
+    la      $t2, cells                  # $t3 = &cells[which_generation-1][x-1]
+    mul     $t3, $a1, MAX_WORLD_SIZE
+    add     $t3, $t2, $t3
+    add     $t3, $t3, $t0
+
+    add     $t0, $t0, 1                 # x++
+    ble     $t0, 0, f0skip0             # if (x > 0)
+    lb      $t1, ($t3)                  # left = cells[which_generation-1][x-1]
+
+f0skip0:
+    add     $t3, $t3, 1                 # ++t3, access [x] not [x-1]
+    lb      $t2, ($t3)                  # $t2 = cells[which_generation-1][x]
+    
+    add     $t4, $t3, 1                 # copy useful ++$t3 before we use $t3
+
+    li      $t3, 0                      # $t3: int right = 0
+    sub     $a0, $a0, 1                 # world_size--
+    bge     $t0, $a0, f0skip1
+    
+    lb      $t3, ($t4)                  # right = cells[which_generation-1][x+1]
+f0skip1:
+    add     $a0, $a0, 1                 # world_size++
+    add     $a1, $a1, 1                 # which_generation++
+    
+    sll     $t1, $t1, 2                 # int state = left << 2 | centre << 1
+    sll     $t2, $t2, 1                 # ^ | right << 0;
+    or      $t5, $t1, $t2               
+    or      $t5, $t5, $t3
+
+    li      $t1, 1
+    sll     $t5, $t1, $t5               # int bit = 1 << state;
+    and     $t5, $a2, $t5               # int set = rule & bit;
+
+    la      $t2, cells                  # $t2 = &cells[which_generation][x]
+    mul     $t3, $a1, MAX_WORLD_SIZE    
+    add     $t3, $t2, $t3
+    add     $t3, $t3, $t0
+    
+#--------------------------------------IF---------------------------------------
+    beqz    $t5, f0skip2                # if (set)
+
+    li      $t1, 1                      # cells[which_generation][x] = 1;
+    sb      $t1, ($t3)
+
+    b       f0skip3
+f0skip2:
+#------------------------------------ELSE---------------------------------------
+    li      $t1, 0                      # cells[which_generation][x] = 0;
+    sb      $t1, ($t3)
+f0skip3:
+
+    add     $t0, $t0, 1                 # x++ (increment)
+    b       f0loop0
+#-----------------------------------END_ELSE------------------------------------
+f0end0:
+#-----------------------------------END_LOOP------------------------------------
+	jr	$ra                             # return;
+#-------------------------------END_FUNCTION_0----------------------------------
+
+	# Given `world_size', and `which_generation', print out the
+	# specified generation.
+
+    # REGISTERS IN PRINT_GENERATION
+    # $a0: int world_size                (passed to function)   CHANGED!
+    # $a1: int which_generation          (passed to function)
+    # $t0: int world_size                (copy of arguments)    CHANGED!
+    # $t1: int which_generation          (copy of arguments)    CHANGED!
+    # $t2: int x                         (0++, loop counter)    CHANGED!
+    # $t3 to $t4:                        (temp variables)       CHANGED!
+    # $v0                                (syscalls)             CHANGED!
+
+#-----------------------------BEGIN_FUNCTION_1----------------------------------
+print_generation:
+    move    $t0, $a0                    # Copy arguments, to use syscalls.
+    move    $t1, $a1
+
+    move    $a0, $t1                    # printf("%d", which_generation);
+    li      $v0, 1
+    syscall
+
+    li      $a0, '\t'                   # putchar('\t');
+    li      $v0, 11
+    syscall
+
+#-------------------------------FOR_LOOP_START----------------------------------
+    li      $t2, 0                      # for (int x = 0; ($t2: int x = 0)
+f1loop0:
+    bge     $t2, $t0, f1end0            # ^x < world_size; (test condition)
+
+#-------------------------------------IF----------------------------------------
+    la      $t3, cells                  # if(cells[which_generation][x])
+    mul     $t4, $t1, MAX_WORLD_SIZE
+    add     $t4, $t4, $t2
+    add     $t4, $t4, $t3
+    lb      $t4, ($t4)
+    beqz    $t4, f1skip0
+
+    li      $a0, ALIVE_CHAR             # putchar(ALIVE_CHAR);
+    li      $v0, 11
+    syscall
+    b       f1skip1
+
+
+f1skip0:
+#------------------------------------ELSE---------------------------------------
+    li      $a0, DEAD_CHAR              # putchar(DEAD_CHAR);
+    li      $v0, 11
+    syscall
+
+
+f1skip1:
+#----------------------------------ELSE_END-------------------------------------
+    add     $t2, $t2, 1                 # x++;);   (increment)
+    b       f1loop0
+f1end0:
+
+#--------------------------------FOR_LOOP_END-----------------------------------
+
+    li      $a0, '\n'                   # putchar('\n');
+    li      $v0, 11
+    syscall
+
+	jr	$ra                             # return;
+#-------------------------------END_FUNCTION_1----------------------------------