The decoder should not pollute a global scope unless it's fine

[lifthrasiir]

<canvas id=a><script>/* Roadroller output from a JS code using a */</script>

This won't work, because a (among others) gets assigned during the decompression and it shadows window.a.

The problem is that this might be okay depending on the input, and we definitely want to exploit that whenever possible. There are ~16 variables used by Roadroller, carefully chosen so that the decoder gets compressed more, and changing any of them can have undesirable effects. One possible resolution is to put everything into a giant function:

eval(((A='....', t=1234, a, c, f, l, ...) => {
    /* code */
})())

But I'm not sure how to make it compatible with multiple inputs (#8).

Workaround

For now you should avoid using the following ids in your HTML, either outside <script> or written with document.write.

A M a c e f l m o p r t u w x y

If you can't avoid using them there are two other workarounds:

• Prepend window. to all uses of such variables.
• Put delete VARIABLE; for all affected variables at the beginning of your code.

[lifthrasiir]

I've experimented with multiple implementation strategies:

• Insert delete VARIABLE;s at the beginning of the input. This turned out to be unworkable if the variable was only used locally and got hoisted (delete f; function f() { ... } will be incorrect for example). So this workaround only works when we do the escape analysis, which is unreasonable for Roadroller.
• Insert delete VARIABLE;s right before the actual eval. This is less invalid than before but still can cause a problem if there is other uncompressed code around. I'm not sure if we should support this mode or not though.
• eval(((A='COMPRESSED DATA', VARIABLES...) => { VAR=INIT; ...; return c })()) was very lengthy.
• eval(eval("let A='COMPRESSED DATA'," + [...'VARIABLES'] + "; VAR=INIT; ...; c")) avoids return but VARIABLES should be unique, limiting the optimization possibility. A variant using var would allow duplicates but pollutes the global scope.
• eval(Function("A='COMPRESSED DATA'", ...'VARIABLES', "VAR=INIT; ...; return c")()) still has the same problem but seems to be a bit smaller.
• eval(Function("A='COMPRESSED DATA'", ...'SOME VARIABLES', "VAR=INIT", "...; return c")()) combines initial assignments with argument definitions.
• eval(Function("[A='COMPRESSED DATA'", ...']VARIABLES', "...; return c")([], INIT...)) eliminates initial assignments by reusing arguments. The first variable has to be undefined, hence a weird argument definition (translates to [A=...,],V,A,R,I,A,B,L,E,S). This seems to be the best alternative so far, only ~25B larger than before. (But see below.)

Some of these experiments required another variable renaming, and honestly I'm sick of it. So the next step is to make variable names fully configurable.