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# QuickPiet commands listed below with their effect/explanation to the right after the arrow (-->)
# These commands follow the actions available with the Piet programming languages with some
# small changes to allow algorithms to be tested without the need of creating valid Piet images.
# Original Piet information can be found at http://www.dangermouse.net/esoteric/piet.html
# Just as in Piet, this language spec assumes a single "infinite" stack and a linear command execution order.
# Blank lines should be ignored.
# An implicit "end" command is present at the bottom of the document.
# Improperly formatted or typed commands should be ignored, allowing for future commands to be added.
push X Y Z ... --> Pushes 1 or more space separated positive integer values onto the stack (X first, then Y, then Z and so on).
--> Positive integer values may represent ASCII character codes or plain integers
--> Do not include commas (,) in large values
pop X --> Pops the top X value(s) of the stack and discards. If X is omitted, 1 is assumed.
duplicate --> Pushes a copy of the top value of the stack onto the stack
roll --> Pops the top two values, and "rolls" the remaining stack entries to a depth equal to the second value popped
--> By a number of rolls equal to the first value popped
--> A single roll to depth n is defined as burying the top value on the stack n deep
--> And bringing all values above it up by 1 place
--> A negative number of rolls rolls in the opposite direction
--> Rolling the stack [1,2,3,4,5] 2 rolls to a depth of 3 results in [1,2,5,3,4]
--> Another roll of value 2, with depth 3, results in [1,2,4,5,3]
input --> Read a single value from STDIN and push it onto the stack; characters are read as their ASCII value
output --> Pop the top value from the stack and output it to STDOUT in it's ASCII character value
add --> Pops the top two values, adds them, and pushes the result
subtract --> Pops the top two values, subtracts the top value from the second top value, and pushes the result
multiply --> Pops the top two values, multiplies them, and pushes the result
divide --> Pops the top two values, integer divides the second top value by the top value, and pushes the result
mod --> Pops the top two values, calculates the second top value modulo the top value, and pushes the result
not --> Replaces the top value of the stack with 0 if it is non-zero, and 1 if it is zero
greater --> Pops the top two values
--> Pushes 1 on to the stack if the second top value is greater than the top value, 0 otherwise
end --> Stop program execution, values left on the stack are discarded
# LINE COMMENT --> Line comment must begin wit a # and may contains zero or more characters of any type
:label --> Line label must begin with a ":" character and one or more alpha-numeric characters (no whitespace)
goto label label --> Pops the top value from the stack
--> Label names should not include the ":" symbol
--> If the value modded by 4 is equal to 1, program execution switches to the first label (e.g. 1, 5, etc)
--> If the value modded by 4 equals 3, program execution switches to the second label (e.g. 3, 7, etc)
--> If the value modded by 4 equals 0 or 2, execution continues to the next command
--> Negative values go in the opposite "direction". -1 = 3, -2 = 0, -3 = 1, -4 = 0
--> If a label name is not found within the program, execution will continue on the next line
--> Label names that do not exist can be used in situations where a goto split is not needed otherwise
assert X Y Z ... --> Pops all values from the stack, checking that each equals the given value in order
--> Right most value is "top" of stack: Z = top of stack, Y second top, X third top
--> If any value is not equal to the value on the stack, execution stops and error message is presented to STDOUT
--> If there are different numbers of stack elements, execution stops and error message is presented to STDOUT
--> This may be used in testing
--> This could be created through a series of smaller commands,
--> But is included as a convenience to test the current state of the stack