| 1 | #!/usr/bin/env python2
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| 2 | """
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| 3 | Pretty printing library.
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| 4 |
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| 5 | Pretty printing means intelligently choosing whitespace including indentation
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| 6 | and newline placement, to attempt to display data nicely while staying within a
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| 7 | maximum line width.
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| 8 | """
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| 9 |
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| 10 | # ~~~ Architecture ~~~
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| 11 | #
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| 12 | # Based on a version of the algorithm from Wadler's "A Prettier Printer".
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| 13 | #
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| 14 | # Pretty printing proceeds in two phases:
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| 15 | #
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| 16 | # 1. Convert the thing you want to print into a `doc`.
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| 17 | # 2. Print the `doc` using a standard algorithm.
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| 18 | #
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| 19 | # This separation keeps the details of the data you want to print separate from
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| 20 | # the printing algorithm.
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| 21 |
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| 22 | # ~~~ Pretty Printing Overview ~~~
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| 23 | #
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| 24 | # If you're just using this file, you don't need to know how pretty printing
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| 25 | # works. Just call `PrettyPrinter().PrintValue()`. However if you want to change
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| 26 | # or extend how values are printed, you'll need to know, so here's an overview.
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| 27 | #
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| 28 | # You may want to first read Walder's "A Prettier Printer", which this is based
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| 29 | # off of:
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| 30 | # https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf
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| 31 | #
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| 32 | # Some additional reading, though only tangentially related:
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| 33 | #
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| 34 | # - https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf
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| 35 | # - https://lindig.github.io/papers/strictly-pretty-2000.pdf
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| 36 | # - https://justinpombrio.net/2024/02/23/a-twist-on-Wadlers-printer.html
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| 37 | # - https://lobste.rs/s/1r0aak/twist_on_wadler_s_printer
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| 38 | # - https://lobste.rs/s/aevptj/why_is_prettier_rock_solid
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| 39 | #
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| 40 | # ~ Constructors ~
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| 41 | #
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| 42 | # There are just a few constructors for `doc`, from which everything else is
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| 43 | # built from.
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| 44 | #
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| 45 | # Text(string) prints like:
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| 46 | # |string
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| 47 | #
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| 48 | # Break(string) prints like:
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| 49 | # |string
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| 50 | # or like a newline:
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| 51 | # |
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| 52 | # |
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| 53 | # (It does the latter if printed in "flat" mode, and the former otherwise. See
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| 54 | # Group for details.)
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| 55 | #
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| 56 | # Concat(a, b) prints like:
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| 57 | # |AAAAA
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| 58 | # |AAABBB
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| 59 | # |BBBBB
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| 60 | #
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| 61 | # Indent(3, a) prints like:
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| 62 | # |AAAAA
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| 63 | # | AAAAA
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| 64 | # | AAAAA
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| 65 | #
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| 66 | # Group(a) makes a decision. It either:
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| 67 | # - Prints `a` "flat", meaning that (i) every Break inside of it is printed as a
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| 68 | # string instead of as a newline, and (ii) every Group nested inside of it is
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| 69 | # printed flat.
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| 70 | # - Prints `a` normally, meaning that (i) the Breaks inside of it are printed as
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| 71 | # newlines, and (ii) the Groups inside of it make their own decision about
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| 72 | # whether to be flat.
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| 73 | # It makes this decision greedily. If the current line would not overflow if the
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| 74 | # group printed flat, then it will print flat. This takes into account not only
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| 75 | # the group itself, but the content before and after it on the same line.
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| 76 | #
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| 77 | # IfFlat(a, b) prints a if in flat mode or b otherwise.
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| 78 | #
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| 79 | # Flat(a) prints a in flat mode. You should generally not need to use it.
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| 80 | #
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| 81 | # ~ Measures ~
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| 82 | #
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| 83 | # The algorithm used here is close to the one originally described by Wadler,
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| 84 | # but it precomputes a "measure" for each node in the `doc`. This "measure"
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| 85 | # allows each Groups to decide whether to print flat or not without needing to
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| 86 | # look ahead per Wadler's algorithm. A measure has two pieces of information:
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| 87 | #
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| 88 | # - Measure.flat is the width of the doc if it's printed flat.
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| 89 | # - Measure.nonflat is the width of the doc until the _earliest possible_
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| 90 | # newline, or -1 if it doesn't contain a Break.
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| 91 | #
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| 92 | # Measures are used in two steps:
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| 93 | # (1) First, they're computed bottom-up on the `doc`, measuring the size of each
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| 94 | # node.
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| 95 | # (2) Later, PrintDoc() stores a measure in each DocFragment. These Measures
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| 96 | # measure something different: the width from the doc _to the end of the
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| 97 | # entire doc tree_. This second set of Measures (the ones in the
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| 98 | # DocFragments) are computed top-down, and they're used to decide for each
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| 99 | # Group whether to use flat mode or not, without needing to scan ahead.
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| 100 |
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| 101 | from __future__ import print_function
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| 102 |
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| 103 | from _devbuild.gen.pretty_asdl import doc, doc_e, DocFragment, Measure, MeasuredDoc
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| 104 | from mycpp.mylib import log, tagswitch, BufWriter
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| 105 | from typing import cast, List
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| 106 |
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| 107 | _ = log
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| 108 |
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| 109 | ################
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| 110 | # Measurements #
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| 111 | ################
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| 112 |
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| 113 |
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| 114 | def _EmptyMeasure():
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| 115 | # type: () -> Measure
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| 116 | """The measure of an empty doc."""
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| 117 | return Measure(0, -1)
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| 118 |
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| 119 |
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| 120 | def _FlattenMeasure(measure):
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| 121 | # type: (Measure) -> Measure
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| 122 | """The measure if its document is rendered flat."""
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| 123 | return Measure(measure.flat, -1)
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| 124 |
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| 125 |
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| 126 | def _ConcatMeasure(m1, m2):
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| 127 | # type: (Measure, Measure) -> Measure
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| 128 | """Compute the measure of concatenated docs.
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| 129 |
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| 130 | If m1 and m2 are the measures of doc1 and doc2,
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| 131 | then _ConcatMeasure(m1, m2) is the measure of doc.Concat([doc1, doc2]).
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| 132 | This concatenation is associative but not commutative."""
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| 133 | if m1.nonflat != -1:
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| 134 | return Measure(m1.flat + m2.flat, m1.nonflat)
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| 135 | elif m2.nonflat != -1:
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| 136 | return Measure(m1.flat + m2.flat, m1.flat + m2.nonflat)
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| 137 | else:
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| 138 | return Measure(m1.flat + m2.flat, -1)
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| 139 |
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| 140 |
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| 141 | def _SuffixLen(measure):
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| 142 | # type: (Measure) -> int
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| 143 | """The width until the earliest possible newline, or end of document."""
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| 144 | if measure.nonflat != -1:
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| 145 | return measure.nonflat
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| 146 | else:
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| 147 | return measure.flat
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| 148 |
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| 149 |
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| 150 | ####################
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| 151 | # Doc Construction #
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| 152 | ####################
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| 153 |
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| 154 |
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| 155 | def AsciiText(string):
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| 156 | # type: (str) -> MeasuredDoc
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| 157 | """Print `string` (which must not contain a newline)."""
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| 158 | return MeasuredDoc(doc.Text(string), Measure(len(string), -1))
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| 159 |
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| 160 |
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| 161 | def _Break(string):
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| 162 | # type: (str) -> MeasuredDoc
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| 163 | """If in `flat` mode, print `string`, otherwise print `\n`.
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| 164 |
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| 165 | Note: Doesn't try to compute Unicode width, since we control these strings.
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| 166 | """
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| 167 | return MeasuredDoc(doc.Break(string), Measure(len(string), 0))
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| 168 |
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| 169 |
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| 170 | def _Indent(indent, mdoc):
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| 171 | # type: (int, MeasuredDoc) -> MeasuredDoc
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| 172 | """Add `indent` spaces after every newline in `mdoc`."""
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| 173 | return MeasuredDoc(doc.Indent(indent, mdoc), mdoc.measure)
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| 174 |
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| 175 |
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| 176 | def _Concat(mdocs):
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| 177 | # type: (List[MeasuredDoc]) -> MeasuredDoc
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| 178 | """Print the mdocs in order (with no space in between)."""
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| 179 | measure = _EmptyMeasure()
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| 180 | for mdoc in mdocs:
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| 181 | measure = _ConcatMeasure(measure, mdoc.measure)
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| 182 | return MeasuredDoc(doc.Concat(mdocs), measure)
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| 183 |
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| 184 |
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| 185 | def _Group(mdoc):
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| 186 | # type: (MeasuredDoc) -> MeasuredDoc
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| 187 | """Print `mdoc`. Use flat mode if `mdoc` will fit on the current line."""
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| 188 | return MeasuredDoc(doc.Group(mdoc), mdoc.measure)
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| 189 |
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| 190 |
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| 191 | def _IfFlat(flat_mdoc, nonflat_mdoc):
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| 192 | # type: (MeasuredDoc, MeasuredDoc) -> MeasuredDoc
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| 193 | """If in flat mode, print `flat_mdoc` otherwise print `nonflat_mdoc`."""
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| 194 | return MeasuredDoc(
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| 195 | doc.IfFlat(flat_mdoc, nonflat_mdoc),
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| 196 | Measure(flat_mdoc.measure.flat, nonflat_mdoc.measure.nonflat))
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| 197 |
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| 198 |
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| 199 | def _Flat(mdoc):
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| 200 | # type: (MeasuredDoc) -> MeasuredDoc
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| 201 | """Prints `mdoc` in flat mode."""
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| 202 | return MeasuredDoc(doc.Flat(mdoc), _FlattenMeasure(mdoc.measure))
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| 203 |
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| 204 |
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| 205 | ###################
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| 206 | # Pretty Printing #
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| 207 | ###################
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| 208 |
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| 209 |
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| 210 | class PrettyPrinter(object):
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| 211 |
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| 212 | def __init__(self, max_width):
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| 213 | # type: (int) -> None
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| 214 | self.max_width = max_width
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| 215 |
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| 216 | def _Fits(self, prefix_len, group, suffix_measure):
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| 217 | # type: (int, doc.Group, Measure) -> bool
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| 218 | """Will `group` fit flat on the current line?"""
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| 219 | measure = _ConcatMeasure(_FlattenMeasure(group.mdoc.measure),
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| 220 | suffix_measure)
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| 221 | return prefix_len + _SuffixLen(measure) <= self.max_width
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| 222 |
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| 223 | def PrintDoc(self, document, buf):
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| 224 | # type: (MeasuredDoc, BufWriter) -> None
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| 225 | """Pretty print a `pretty.doc` to a BufWriter."""
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| 226 |
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| 227 | # The width of the text we've printed so far on the current line
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| 228 | prefix_len = 0
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| 229 | # A _stack_ of document fragments to print. Each fragment contains:
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| 230 | # - A MeasuredDoc (doc node and its measure, saying how "big" it is)
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| 231 | # - The indentation level to print this doc node at.
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| 232 | # - Is this doc node being printed in flat mode?
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| 233 | # - The measure _from just after the doc node, to the end of the entire document_.
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| 234 | # (Call this the suffix_measure)
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| 235 | fragments = [DocFragment(_Group(document), 0, False, _EmptyMeasure())]
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| 236 |
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| 237 | while len(fragments) > 0:
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| 238 | frag = fragments.pop()
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| 239 | with tagswitch(frag.mdoc.doc) as case:
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| 240 |
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| 241 | if case(doc_e.Text):
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| 242 | text = cast(doc.Text, frag.mdoc.doc)
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| 243 | buf.write(text.string)
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| 244 | prefix_len += frag.mdoc.measure.flat
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| 245 |
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| 246 | elif case(doc_e.Break):
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| 247 | if frag.is_flat:
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| 248 | break_str = cast(doc.Break, frag.mdoc.doc).string
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| 249 | buf.write(break_str)
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| 250 | prefix_len += frag.mdoc.measure.flat
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| 251 | else:
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| 252 | buf.write('\n')
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| 253 | buf.write_spaces(frag.indent)
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| 254 | prefix_len = frag.indent
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| 255 |
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| 256 | elif case(doc_e.Indent):
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| 257 | indented = cast(doc.Indent, frag.mdoc.doc)
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| 258 | fragments.append(
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| 259 | DocFragment(indented.mdoc,
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| 260 | frag.indent + indented.indent,
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| 261 | frag.is_flat, frag.measure))
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| 262 |
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| 263 | elif case(doc_e.Concat):
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| 264 | # If we encounter Concat([A, B, C]) with a suffix measure M,
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| 265 | # we need to push A,B,C onto the stack in reverse order:
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| 266 | # - C, with suffix_measure = B.measure + A.measure + M
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| 267 | # - B, with suffix_measure = A.measure + M
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| 268 | # - A, with suffix_measure = M
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| 269 | concat = cast(doc.Concat, frag.mdoc.doc)
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| 270 | measure = frag.measure
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| 271 | for mdoc in reversed(concat.mdocs):
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| 272 | fragments.append(
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| 273 | DocFragment(mdoc, frag.indent, frag.is_flat,
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| 274 | measure))
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| 275 | measure = _ConcatMeasure(mdoc.measure, measure)
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| 276 |
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| 277 | elif case(doc_e.Group):
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| 278 | # If the group would fit on the current line when printed
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| 279 | # flat, do so. Otherwise, print it non-flat.
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| 280 | group = cast(doc.Group, frag.mdoc.doc)
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| 281 | flat = self._Fits(prefix_len, group, frag.measure)
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| 282 | fragments.append(
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| 283 | DocFragment(group.mdoc, frag.indent, flat,
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| 284 | frag.measure))
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| 285 |
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| 286 | elif case(doc_e.IfFlat):
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| 287 | if_flat = cast(doc.IfFlat, frag.mdoc.doc)
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| 288 | if frag.is_flat:
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| 289 | subdoc = if_flat.flat_mdoc
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| 290 | else:
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| 291 | subdoc = if_flat.nonflat_mdoc
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| 292 | fragments.append(
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| 293 | DocFragment(subdoc, frag.indent, frag.is_flat,
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| 294 | frag.measure))
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| 295 |
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| 296 | elif case(doc_e.Flat):
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| 297 | flat_doc = cast(doc.Flat, frag.mdoc.doc)
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| 298 | fragments.append(
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| 299 | DocFragment(flat_doc.mdoc, frag.indent, True,
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| 300 | frag.measure))
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| 301 |
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| 302 |
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| 303 | # vim: sw=4
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