Commit a922ebfd authored by Taddeus Kroes's avatar Taddeus Kroes

Documentation

parent f143b684
......@@ -61,47 +61,28 @@ Compression phases
==================
To achieve the features listed above, the input stylesheet is rewritten in a
number of phases:
1. `parse`: Transform the input into an Abstract Syntax Tree (AST) in
accordance with the CSS syntax definition. This eliminates unnecessary
whitespaces and catches syntax errors.
2. `unfold`:
- Transform shorthand declarations into a separate declaration for each
expression in the shorthand.
- Duplicate rulesets for each of its selectors, so that every ruleset has
exactly one block of declarations.
3. `concat`: Concatenate different declaration blocks for the same selector.
Now, every selector has exacly one block of declarations associated with it.
4. `optimize`: Optimize individual declaration blocks by generating shorthand
declarations and simple compression on expressions (color compression,
removal of unnecessary string quotes, etc.).
5. `combine`: Try to combine declarations for different selectors, by comparing
each pair of rulesets and checking if a shorter representation is possible
by generating a shared ruleset.
Motivation
----------
The use of a proper CSS syntax definition for the parser, rather than a simple
ad-hoc tokenizer, assures that all unnecessary whitespace is ignored. The AST
definition also provides a convenient infrastructure for the subsequent
transformations.
The unfolding phase eliminates any clever constructions used by the programmer,
so that the following phases can make assumptions about the format of the
stylesheet and need not deal with, for example, shorthand declarations.
Concatenation of declaration blocks for the same selector is necessary in order
to generate a single minimal representation for each selector in the `optimize`
phase.
The `combine` phase is tricky to implement, since it is difficult to prove if
the resulting combinations are in fact the shortest possible representation of
the stylesheet. If this is not the case, the risk exists that while the
programmer efficiently merged some declarations for different selectors, the
compressor unfolds the ruleset and generates somethning longer.
*FIXME*: how do we solve this?
number of steps:
1. Parse the input CSS, producing an Abstract Syntax Tree (AST) in accordance
with the CSS syntax definition. This eliminates unnecessary whitespaces and
catches syntax errors.
2. Transform shorthand declarations into a separate declaration for each
expression in the shorthand.
3. Duplicate rulesets for each of its selectors, so that every ruleset has
exactly one block of declarations.
4. Create a new declaration block for each property declaration on each
selector, e.g. `a,p{color:red;border:blue}` becomes `a{color:red}
a{border:blue} p{color:red} p{border:blue}`.
5. Prune duplicate declarations (when the same property is overwritten for the
same selector). Note: this may be disabled with `--no-prune` for stylesheets
that contain duplication hacks.
6. Combine selectors for identical blocks (as long as the correct order of
declarations is preserved).
7. Concatenate blocks for identical (combinations of) selectors.
8. Optimize individual declaration blocks by generating shorthand declarations
and simple compression on expressions (i.e. color compression, removal of
unnecessary string quotes, etc.).
9. Output the resulting AST with minimal whitespace.
Building mincss
......
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