This is a minimal implementation of the PostScript language supporting all operators defined in the PDF Spec for "Operators in Type 4 Functions".
It's not useful for much more than evaluating Type4 functions, but perhaps it will be some fun to play around with -- PostScript is a rather interesting language!
It includes a very simple parser (see PostScript::Parser), and a runtime (see PostScript::Runtime).
PostScript is a stack-based language, and this library supports (nearly) all of its stack, arithmetic, boolean, and conditional operators.
You can refer to the PostScript Language Reference for more details.
The runtime can be driven manually (through ruby) or by evaluating a snippet of PostScript.
Here's some trivial examples, first driven manually:
ps = PostScript::Runtime.new
ps.stack # => []
ps.push 2 # => [2]
ps.push 2 # => [2, 2]
ps.mul # => [4]
ps.push 2 # => [4, 2]
ps.div # => [2]
ps.stack # => [2]
And then as a bit of PostScript code:
ps = PostScript::Runtime.new
ps.stack # => []
ps.eval "{ 2 2 mul 2 div }"
ps.stack # => [2]
Of course, the manual stack manipulation can be combined with PostScript code:
ps = PostScript::Runtime.new
# Set up the initial state
ps.push -4.0, 2.0
# Execute the "Rhomboid" spot function as defined in the PDF Spec.
ps.eval "{ abs exch abs 0.9 mul add 2 div }" # => [2.8]