## Token Spacing

First, consider an issue that only concerns the stand-alone preprocessor: there needs to be a guarantee that re-reading its preprocessed output results in an identical token stream. Without taking special measures, this might not be the case because of macro substitution. For example:

    #define PLUS +
    #define EMPTY
    #define f(x) =x=
    +PLUS -EMPTY- PLUS+ f(=)
            → + + - - + + = = =
    not
            → ++ -- ++ ===

One solution would be to simply insert a space between all adjacent tokens. However, we would like to keep space insertion to a minimum, both for aesthetic reasons and because it causes problems for people who still try to abuse the preprocessor for things like Fortran source and Makefiles.

For now, just notice that when tokens are added (or removed, as shown by the `EMPTY` example) from the original lexed token stream, we need to check for accidental token pasting. We call this paste avoidance. Token addition and removal can only occur because of macro expansion, but accidental pasting can occur in many places: both before and after each macro replacement, each argument replacement, and additionally each token created by the ‘`#`’ and ‘`##`’ operators.

Look at how the preprocessor gets whitespace output correct normally. The `cpp_token` structure contains a flags byte, and one of those flags is `PREV_WHITE`. This is flagged by the lexer, and indicates that the token was preceded by whitespace of some form other than a new line. The stand-alone preprocessor can use this flag to decide whether to insert a space between tokens in the output.

Now consider the result of the following macro expansion:

    #define add(x, y, z) x + y +z;
    sum = add (1,2, 3);
            → sum = 1 + 2 +3;

The interesting thing here is that the tokens ‘`1`’ and ‘`2`’ are output with a preceding space, and ‘`3`’ is output without a preceding space, but when lexed none of these tokens had that property. Careful consideration reveals that ‘`1`’ gets its preceding whitespace from the space preceding ‘`add`’ in the macro invocation, not replacement list. ‘`2`’ gets its whitespace from the space preceding the parameter ‘`y`’ in the macro replacement list, and ‘`3`’ has no preceding space because parameter ‘`z`’ has none in the replacement list.

Once lexed, tokens are effectively fixed and cannot be altered, since pointers to them might be held in many places, in particular by in-progress macro expansions. So instead of modifying the two tokens above, the preprocessor inserts a special token, which I call a padding token, into the token stream to indicate that spacing of the subsequent token is special. The preprocessor inserts padding tokens in front of every macro expansion and expanded macro argument. These point to a source token from which the subsequent real token should inherit its spacing. In the above example, the source tokens are ‘`add`’ in the macro invocation, and ‘`y`’ and ‘`z`’ in the macro replacement list, respectively.

It is quite easy to get multiple padding tokens in a row, for example if a macro’s first replacement token expands straight into another macro.

    #define foo bar
    #define bar baz
    [foo]
            → [baz]

Here, two padding tokens are generated with sources the ‘`foo`’ token between the brackets, and the ‘`bar`’ token from foo’s replacement list, respectively. Clearly the first padding token is the one to use, so the output code should contain a rule that the first padding token in a sequence is the one that matters.

But what if a macro expansion is left? Adjusting the above example slightly:

    #define foo bar
    #define bar EMPTY baz
    #define EMPTY
    [foo] EMPTY;
            → [ baz] ;

As shown, now there should be a space before ‘`baz`’ and the semicolon in the output.

The rules we decided above fail for ‘`baz`’: we generate three padding tokens, one per macro invocation, before the token ‘`baz`’. We would then have it take its spacing from the first of these, which carries source token ‘`foo`’ with no leading space.

It is vital that cpplib get spacing correct in these examples since any of these macro expansions could be stringized, where spacing matters.

So, this demonstrates that not just entering macro and argument expansions, but leaving them requires special handling too. I made cpplib insert a padding token with a `NULL` source token when leaving macro expansions, as well as after each replaced argument in a macro’s replacement list. It also inserts appropriate padding tokens on either side of tokens created by the ‘`#`’ and ‘`##`’ operators. I expanded the rule so that, if we see a padding token with a `NULL` source token, and that source token has no leading space, then we behave as if we have seen no padding tokens at all. A quick check shows this rule will then get the above example correct as well.

Now a relationship with paste avoidance is apparent: we have to be careful about paste avoidance in exactly the same locations we have padding tokens in order to get white space correct. This makes implementation of paste avoidance easy: wherever the stand-alone preprocessor is fixing up spacing because of padding tokens, and it turns out that no space is needed, it has to take the extra step to check that a space is not needed after all to avoid an accidental paste. The function `cpp_avoid_paste` advises whether a space is required between two consecutive tokens. To avoid excessive spacing, it tries hard to only require a space if one is likely to be necessary, but for reasons of efficiency it is slightly conservative and might recommend a space where one is not strictly needed.