# To test this file you could use your own implementation of the machine
# or mine. If you want to use mine, then here is a script:
# > java duckMachine.Text
# DM>> load fact.dm
# DM>> run
# 5
# 120
# DM>> quit
#

# Assembly program to read one number, compute its factorial, and print result

# Corresponding Java program:

# int factorial (int i) {
#  int result = 1;
#  while (i > 0) {
#    result = result * i;
#    i--;
#  }
#  return result;
# }

# Our assembly language does not have multiplication; let's rewrite 
# the program without multiplication:

# int factorial (int i) {
#  int result = 1;
#  while (i > 0) {
#
#    // The following 8 lines replace the line: result = result * i;
#    int left_multiply = result;
#    int right_multiply = i;
#    int multiply_result = 0;
#    while (right_multiply > 0) {
#      multiply_result += left_multiply;
#      right_multiply--;
#    }
#    result = multiply_result;
#
#    i--;
#  }
#  return result;
# }

# Now write the thing in assembly:

# result => Memory location 50
# i => Memory location 49
# left_multiply => Memory location 48
# right_multiply => Memory location 47
# multiply_result => Memory location 46
# 0 => Memory location 45

0	in 49		# read i
1	clear 50	# result = 0
2	increment 50	# result = 1
3	clear 45	# constant 0 used for various operations
4	load 45		# beginning of while (i > 0) loop
5	compare 49	# compare i and 0 
6	jumpgt 9	# if (i > 0) goto 9 else goto 7
7	out 50		# print result
8	halt		# DONE
9	load 50		# get result
10	store 48	# left_multiply = result
11	load 49		# get i
12	store 47	# right_multiply = i
13	clear 46	# multiply_result = 0
14	load 45		# beginning of while (right_multiply > 0) loop
15	compare 47	# compare right_multiply and 0
16	jumpgt 21	# if (right_multiply > 0) goto 21 else goto 17
17	load 46		# get multiply_result
18	store 50	# result = multiply_result
19	decrement 49	# i--
20	jump 4		# end of while (i > 0) loop
21	load 46		# get multiply_result
22 	add 48		# add to left_multiply
23	store 46	# multiply_result = multiply_result + left_multiply
24	decrement 47	# right_multiply--
25 	jump 14		# end of while (right_multiply > 0) loop