572 lines
20 KiB
Rust
572 lines
20 KiB
Rust
import std::io;
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import std::vec;
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import std::str;
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/*
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* This pretty-printer is a direct reimplementation of Philip Karlton's
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* Mesa pretty-printer, as described in appendix A of
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*
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* STAN-CS-79-770: "Pretty Printing", by Derek C. Oppen.
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* Stanford Department of Computer Science, 1979.
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*
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* The algorithm's aim is to break a stream into as few lines as possible
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* while respecting the indentation-consistency requirements of the enclosing
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* block, and avoiding breaking at silly places on block boundaries, for
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* example, between "x" and ")" in "x)".
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*
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* I am implementing this algorithm because it comes with 20 pages of
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* documentation explaining its theory, and because it addresses the set of
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* concerns I've seen other pretty-printers fall down on. Weirdly. Even though
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* it's 32 years old and not written in Haskell. What can I say?
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*
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* Despite some redundancies and quirks in the way it's implemented in that
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* paper, I've opted to keep the implementation here as similar as I can,
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* changing only what was blatantly wrong, a typo, or sufficiently
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* non-idiomatic rust that it really stuck out.
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*
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* In particular you'll see a certain amount of churn related to INTEGER vs.
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* CARDINAL in the Mesa implementation. Mesa apparently interconverts the two
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* somewhat readily? In any case, I've used uint for indices-in-buffers and
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* ints for character-sizes-and-indentation-offsets. This respects the need
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* for ints to "go negative" while carrying a pending-calculation balance, and
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* helps differentiate all the numbers flying around internally (slightly).
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*
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* I also inverted the indentation arithmetic used in the print stack, since
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* the Mesa implementation (somewhat randomly) stores the offset on the print
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* stack in terms of margin-col rather than col itself. I store col.
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*
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* I also implemented a small change in the STRING token, in that I store an
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* explicit length for the string. For most tokens this is just the length of
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* the accompanying string. But it's necessary to permit it to differ, for
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* encoding things that are supposed to "go on their own line" -- certain
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* classes of comment and blank-line -- where relying on adjacent
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* hardbreak-like BREAK tokens with long blankness indication doesn't actually
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* work. To see why, consider when there is a "thing that should be on its own
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* line" between two long blocks, say functions. If you put a hardbreak after
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* each function (or before each) and the breaking algorithm decides to break
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* there anyways (because the functions themselves are long) you wind up with
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* extra blank lines. If you don't put hardbreaks you can wind up with the
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* "thing which should be on its own line" not getting its own line in the
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* rare case of "really small functions" or such. This re-occurs with comments
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* and explicit blank lines. So in those cases we use a string with a payload
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* we want isolated to a line and an explicit length that's huge, surrounded
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* by two zero-length breaks. The algorithm will try its best to fit it on a
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* line (which it can't) and so naturally place the content on its own line to
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* avoid combining it with other lines and making matters even worse.
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*/
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tag breaks { consistent; inconsistent; }
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type break_t = rec(int offset, int blank_space);
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type begin_t = rec(int offset, breaks breaks);
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tag token {
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STRING(str,int);
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BREAK(break_t);
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BEGIN(begin_t);
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END;
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EOF;
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}
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fn tok_str(token t) -> str {
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alt (t) {
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case (STRING(?s, ?len)) { ret #fmt("STR(%s,%d)", s, len); }
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case (BREAK(_)) { ret "BREAK"; }
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case (BEGIN(_)) { ret "BEGIN"; }
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case (END) { ret "END"; }
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case (EOF) { ret "EOF"; }
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}
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}
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fn buf_str(vec[token] toks, vec[int] szs,
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uint left, uint right, uint lim) -> str {
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auto n = vec::len(toks);
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assert n == vec::len(szs);
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auto i = left;
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auto L = lim;
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auto s = "[";
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while (i != right && L != 0u) {
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L -= 1u;
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if (i != left) {
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s += ", ";
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}
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s += #fmt("%d=%s", szs.(i), tok_str(toks.(i)));
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i += 1u;
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i %= n;
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}
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s += "]";
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ret s;
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}
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tag print_stack_break { fits; broken(breaks); }
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type print_stack_elt = rec(int offset, print_stack_break pbreak);
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const int size_infinity = 0xffff;
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fn mk_printer(io::writer out, uint linewidth) -> printer {
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// Yes 3, it makes the ring buffers big enough to never
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// fall behind.
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let uint n = 3u * linewidth;
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log #fmt("mk_printer %u", linewidth);
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let vec[token] token = vec::init_elt[token](EOF, n);
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let vec[int] size = vec::init_elt[int](0, n);
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let vec[uint] scan_stack = vec::init_elt[uint](0u, n);
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let vec[print_stack_elt] print_stack = [];
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ret printer(out,
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n,
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linewidth as int, // margin
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linewidth as int, // space
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0u, // left
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0u, // right
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token,
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size,
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0, // left_total
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0, // right_total
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scan_stack,
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true, // scan_stack_empty
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0u, // top
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0u, // bottom
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print_stack);
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}
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/*
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* In case you do not have the paper, here is an explanation of what's going
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* on.
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*
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* There is a stream of input tokens flowing through this printer.
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*
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* The printer buffers up to 3N tokens inside itself, where N is linewidth.
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* Yes, linewidth is chars and tokens are multi-char, but in the worst
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* case every token worth buffering is 1 char long, so it's ok.
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*
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* Tokens are STRING, BREAK, and BEGIN/END to delimit blocks.
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*
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* BEGIN tokens can carry an offset, saying "how far to indent when you break
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* inside here", as well as a flag indicating "consistent" or "inconsistent"
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* breaking. Consistent breaking means that after the first break, no attempt
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* will be made to flow subsequent breaks together onto lines. Inconsistent
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* is the opposite. Inconsistent breaking example would be, say:
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*
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* foo(hello, there, good, friends)
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*
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* breaking inconsistently to become
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*
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* foo(hello, there
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* good, friends);
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*
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* whereas a consistent breaking would yield:
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*
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* foo(hello,
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* there
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* good,
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* friends);
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*
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* That is, in the consistent-break blocks we value vertical alignment
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* more than the ability to cram stuff onto a line. But in all cases if it
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* can make a block a one-liner, it'll do so.
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*
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* Carrying on with high-level logic:
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*
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* The buffered tokens go through a ring-buffer, 'tokens'. The 'left' and
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* 'right' indices denote the active portion of the ring buffer as well as
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* describing hypothetical points-in-the-infinite-stream at most 3N tokens
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* apart (i.e. "not wrapped to ring-buffer boundaries"). The paper will switch
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* between using 'left' and 'right' terms to denote the wrapepd-to-ring-buffer
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* and point-in-infinite-stream senses freely.
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*
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* There is a parallel ring buffer, 'size', that holds the calculated size of
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* each token. Why calculated? Because for BEGIN/END pairs, the "size"
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* includes everything betwen the pair. That is, the "size" of BEGIN is
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* actually the sum of the sizes of everything between BEGIN and the paired
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* END that follows. Since that is arbitrarily far in the future, 'size' is
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* being rewritten regularly while the printer runs; in fact most of the
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* machinery is here to work out 'size' entries on the fly (and give up when
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* they're so obviously over-long that "infinity" is a good enough
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* approximation for purposes of line breaking).
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*
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* The "input side" of the printer is managed as an abstract process called
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* SCAN, which uses 'scan_stack', 'scan_stack_empty', 'top' and 'bottom', to
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* manage calculating 'size'. SCAN is, in other words, the process of
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* calculating 'size' entries.
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*
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* The "output side" of the printer is managed by an abstract process called
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* PRINT, which uses 'print_stack', 'margin' and 'space' to figure out what to
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* do with each token/size pair it consumes as it goes. It's trying to consume
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* the entire buffered window, but can't output anything until the size is >=
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* 0 (sizes are set to negative while they're pending calculation).
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*
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* So SCAN takeks input and buffers tokens and pending calculations, while
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* PRINT gobbles up completed calculations and tokens from the buffer. The
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* theory is that the two can never get more than 3N tokens apart, because
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* once there's "obviously" too much data to fit on a line, in a size
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* calculation, SCAN will write "infinity" to the size and let PRINT consume
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* it.
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*
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* In this implementation (following the paper, again) the SCAN process is
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* the method called 'pretty_print', and the 'PRINT' process is the method
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* called 'print'.
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*/
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obj printer(io::writer out,
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uint buf_len,
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mutable int margin, // width of lines we're constrained to
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mutable int space, // number of spaces left on line
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mutable uint left, // index of left side of input stream
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mutable uint right, // index of right side of input stream
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mutable vec[token] token, // ring-buffer stream goes through
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mutable vec[int] size, // ring-buffer of calculated sizes
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mutable int left_total, // running size of stream "...left"
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mutable int right_total, // running size of stream "...right"
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// pseudo-stack, really a ring too. Holds the primary-ring-buffers
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// index of the BEGIN that started the current block, possibly
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// with the most recent BREAK after that BEGIN (if there is any)
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// on top of it. Stuff is flushed off the bottom as it becomes
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// irrelevant due to the primary ring-buffer advancing.
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mutable vec[uint] scan_stack,
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mutable bool scan_stack_empty, // top==bottom disambiguator
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mutable uint top, // index of top of scan_stack
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mutable uint bottom, // index of bottom of scan_stack
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// stack of blocks-in-progress being flushed by print
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mutable vec[print_stack_elt] print_stack
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) {
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fn pretty_print(token t) {
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log #fmt("pp [%u,%u]", left, right);
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alt (t) {
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case (EOF) {
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if (!scan_stack_empty) {
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self.check_stack(0);
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self.advance_left(token.(left), size.(left));
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}
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self.indent(0);
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}
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case (BEGIN(?b)) {
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if (scan_stack_empty) {
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left_total = 1;
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right_total = 1;
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left = 0u;
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right = 0u;
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} else {
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self.advance_right();
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}
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log #fmt("pp BEGIN/buffer [%u,%u]", left, right);
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token.(right) = t;
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size.(right) = -right_total;
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self.scan_push(right);
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}
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case (END) {
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if (scan_stack_empty) {
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log #fmt("pp END/print [%u,%u]", left, right);
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self.print(t, 0);
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} else {
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log #fmt("pp END/buffer [%u,%u]", left, right);
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self.advance_right();
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token.(right) = t;
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size.(right) = -1;
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self.scan_push(right);
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}
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}
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case (BREAK(?b)) {
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if (scan_stack_empty) {
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left_total = 1;
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right_total = 1;
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left = 0u;
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right = 0u;
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} else {
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self.advance_right();
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}
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log #fmt("pp BREAK/buffer [%u,%u]", left, right);
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self.check_stack(0);
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self.scan_push(right);
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token.(right) = t;
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size.(right) = -right_total;
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right_total += b.blank_space;
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}
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case (STRING(?s, ?len)) {
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if (scan_stack_empty) {
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log #fmt("pp STRING/print [%u,%u]", left, right);
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self.print(t, len);
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} else {
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log #fmt("pp STRING/buffer [%u,%u]", left, right);
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self.advance_right();
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token.(right) = t;
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size.(right) = len;
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right_total += len;
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self.check_stream();
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}
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}
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}
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}
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fn check_stream() {
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log #fmt("check_stream [%u, %u] with left_total=%d, right_total=%d",
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left, right, left_total, right_total);;
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if (right_total - left_total > space) {
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log #fmt("scan window is %d, longer than space on line (%d)",
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right_total - left_total, space);
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if (!scan_stack_empty) {
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if (left == scan_stack.(bottom)) {
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log #fmt("setting %u to infinity and popping", left);
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size.(self.scan_pop_bottom()) = size_infinity;
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}
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}
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self.advance_left(token.(left), size.(left));
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if (left != right) {
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self.check_stream();
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}
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}
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}
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fn scan_push(uint x) {
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log #fmt("scan_push %u", x);
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if (scan_stack_empty) {
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scan_stack_empty = false;
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} else {
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top += 1u;
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top %= buf_len;
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assert top != bottom;
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}
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scan_stack.(top) = x;
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}
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fn scan_pop() -> uint {
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assert !scan_stack_empty;
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auto x = scan_stack.(top);
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if (top == bottom) {
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scan_stack_empty = true;
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} else {
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top += (buf_len - 1u);
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top %= buf_len;
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}
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ret x;
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}
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fn scan_top() -> uint {
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assert !scan_stack_empty;
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ret scan_stack.(top);
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}
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fn scan_pop_bottom() -> uint {
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assert !scan_stack_empty;
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auto x = scan_stack.(bottom);
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if (top == bottom) {
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scan_stack_empty = true;
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} else {
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bottom += 1u;
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bottom %= buf_len;
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}
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ret x;
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}
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fn advance_right() {
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right += 1u;
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right %= buf_len;
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assert right != left;
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}
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fn advance_left(token x, int L) {
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log #fmt("advnce_left [%u,%u], sizeof(%u)=%d", left, right, left, L);
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if (L >= 0) {
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self.print(x, L);
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alt (x) {
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case (BREAK(?b)) {
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left_total += b.blank_space;
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}
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case (STRING(_, ?len)) {
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assert len == L;
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left_total += len;
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}
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case (_) {}
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}
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if (left != right) {
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left += 1u;
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left %= buf_len;
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self.advance_left(token.(left), size.(left));
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}
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}
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}
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fn check_stack(int k) {
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if (!scan_stack_empty) {
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auto x = self.scan_top();
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alt (token.(x)) {
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case (BEGIN(?b)) {
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if (k > 0) {
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size.(self.scan_pop()) = size.(x) + right_total;
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self.check_stack(k - 1);
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}
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}
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case (END) {
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// paper says + not =, but that makes no sense.
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size.(self.scan_pop()) = 1;
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self.check_stack(k + 1);
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}
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case (_) {
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size.(self.scan_pop()) = size.(x) + right_total;
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if (k > 0) {
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self.check_stack(k);
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}
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}
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}
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}
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}
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fn print_newline(int amount) {
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log #fmt("NEWLINE %d", amount);
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out.write_str("\n");
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self.indent(amount);
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}
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fn indent(int amount) {
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log #fmt("INDENT %d", amount);
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auto u = 0;
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while (u < amount) {
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out.write_str(" ");
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u += 1;
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}
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}
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fn print(token x, int L) {
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log #fmt("print %s %d (remaining line space=%d)",
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tok_str(x), L, space);
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log buf_str(token, size, left, right, 6u);
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alt (x) {
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case (BEGIN(?b)) {
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if (L > space) {
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auto col = (margin - space) + b.offset;
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log #fmt("print BEGIN -> push broken block at col %d",
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col);
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vec::push(print_stack,
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rec(offset = col,
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pbreak = broken(b.breaks)));
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} else {
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log "print BEGIN -> push fitting block";
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vec::push(print_stack,
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rec(offset = 0,
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pbreak = fits));
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}
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}
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case (END) {
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log "print END -> pop END";
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assert vec::len(print_stack) != 0u;
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vec::pop(print_stack);
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}
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case (BREAK(?b)) {
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auto n = vec::len(print_stack);
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let print_stack_elt top =
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rec(offset=0, pbreak=broken(inconsistent));;
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if (n != 0u) {
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top = print_stack.(n - 1u);
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}
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alt (top.pbreak) {
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case (fits) {
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log "print BREAK in fitting block";
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space -= b.blank_space;
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self.indent(b.blank_space);
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}
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case (broken(consistent)) {
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log "print BREAK in consistent block";
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self.print_newline(top.offset + b.offset);
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space = margin - (top.offset + b.offset);
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}
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case (broken(inconsistent)) {
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if (L > space) {
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log "print BREAK w/ newline in inconsistent";
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self.print_newline(top.offset + b.offset);
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space = margin - (top.offset + b.offset);
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} else {
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log "print BREAK w/o newline in inconsistent";
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self.indent(b.blank_space);
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space -= b.blank_space;
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}
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}
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}
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}
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case (STRING(?s, ?len)) {
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log "print STRING";
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assert L == len;
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// assert L <= space;
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space -= len;
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out.write_str(s);
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}
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|
|
case (EOF) {
|
|
// EOF should never get here.
|
|
fail;
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
|
|
// Convenience functions to talk to the printer.
|
|
|
|
fn box(printer p, uint indent, breaks b) {
|
|
p.pretty_print(BEGIN(rec(offset = indent as int,
|
|
breaks = b)));
|
|
}
|
|
|
|
|
|
fn ibox(printer p, uint indent) {
|
|
box(p, indent, inconsistent);
|
|
}
|
|
|
|
fn cbox(printer p, uint indent) {
|
|
box(p, indent, consistent);
|
|
}
|
|
|
|
|
|
fn break_offset(printer p, uint n, int off) {
|
|
p.pretty_print(BREAK(rec(offset = off,
|
|
blank_space = n as int)));
|
|
}
|
|
|
|
fn end(printer p) { p.pretty_print(END); }
|
|
fn eof(printer p) { p.pretty_print(EOF); }
|
|
fn word(printer p, str wrd) {
|
|
p.pretty_print(STRING(wrd, str::char_len(wrd) as int));
|
|
}
|
|
fn huge_word(printer p, str wrd) {
|
|
p.pretty_print(STRING(wrd, 0xffff));
|
|
}
|
|
fn spaces(printer p, uint n) { break_offset(p, n, 0); }
|
|
fn zerobreak(printer p) { spaces(p, 0u); }
|
|
fn space(printer p) { spaces(p, 1u); }
|
|
fn hardbreak(printer p) { spaces(p, 0xffffu); }
|
|
|
|
|
|
|
|
//
|
|
// Local Variables:
|
|
// mode: rust
|
|
// fill-column: 78;
|
|
// indent-tabs-mode: nil
|
|
// c-basic-offset: 4
|
|
// buffer-file-coding-system: utf-8-unix
|
|
// compile-command: "make -k -C $RBUILD 2>&1 | sed -e 's/\\/x\\//x:\\//g'";
|
|
// End:
|
|
//
|