| 1 | #!/usr/bin/env python2
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| 2 | """pretty.py - Pretty printing library.
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| 3 |
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| 4 | Pretty printing means intelligently choosing whitespace including indentation
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| 5 | and newline placement, to attempt to display data nicely while staying within a
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| 6 | maximum line width.
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| 7 | """
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| 8 |
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| 9 | # ~~~ Architecture ~~~
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| 10 | #
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| 11 | # Based on a version of the algorithm from Wadler's "A Prettier Printer".
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| 12 | #
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| 13 | # Pretty printing proceeds in two phases:
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| 14 | #
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| 15 | # 1. Convert the thing you want to print into a `doc`.
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| 16 | # 2. Print the `doc` using a standard algorithm.
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| 17 | #
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| 18 | # This separation keeps the details of the data you want to print separate from
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| 19 | # the printing algorithm.
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| 20 |
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| 21 | # ~~~ Pretty Printing Overview ~~~
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| 22 | #
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| 23 | # If you're just using this file, you don't need to know how pretty printing
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| 24 | # works. Just call `PrettyPrinter().PrintValue()`. However if you want to change
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| 25 | # or extend how values are printed, you'll need to know, so here's an overview.
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| 26 | #
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| 27 | # You may want to first read Walder's "A Prettier Printer", which this is based
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| 28 | # off of:
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| 29 | # https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf
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| 30 | #
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| 31 | # Some additional reading, though only tangentially related:
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| 32 | #
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| 33 | # - https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf
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| 34 | # - https://lindig.github.io/papers/strictly-pretty-2000.pdf
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| 35 | # - https://justinpombrio.net/2024/02/23/a-twist-on-Wadlers-printer.html
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| 36 | # - https://lobste.rs/s/1r0aak/twist_on_wadler_s_printer
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| 37 | # - https://lobste.rs/s/aevptj/why_is_prettier_rock_solid
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| 38 | #
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| 39 | # ~ Constructors ~
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| 40 | #
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| 41 | # There are just a few constructors for `doc`, from which everything else is
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| 42 | # built from.
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| 43 | #
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| 44 | # Text(string) prints like:
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| 45 | # |string
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| 46 | #
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| 47 | # Break(string) prints like:
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| 48 | # |string
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| 49 | # or like a newline:
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| 50 | # |
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| 51 | # |
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| 52 | # (It does the latter if printed in "flat" mode, and the former otherwise. See
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| 53 | # Group for details.)
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| 54 | #
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| 55 | # Concat(a, b) prints like:
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| 56 | # |AAAAA
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| 57 | # |AAABBB
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| 58 | # |BBBBB
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| 59 | #
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| 60 | # Indent(3, a) prints like:
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| 61 | # |AAAAA
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| 62 | # | AAAAA
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| 63 | # | AAAAA
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| 64 | #
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| 65 | # Group(a) makes a decision. It either:
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| 66 | # - Prints `a` "flat", meaning that (i) every Break inside of it is printed as a
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| 67 | # string instead of as a newline, and (ii) every Group nested inside of it is
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| 68 | # printed flat.
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| 69 | # - Prints `a` normally, meaning that (i) the Breaks inside of it are printed as
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| 70 | # newlines, and (ii) the Groups inside of it make their own decision about
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| 71 | # whether to be flat.
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| 72 | # It makes this decision greedily. If the current line would not overflow if the
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| 73 | # group printed flat, then it will print flat. This takes into account not only
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| 74 | # the group itself, but the content before and after it on the same line.
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| 75 | #
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| 76 | # IfFlat(a, b) prints a if in flat mode or b otherwise.
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| 77 | #
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| 78 | # Flat(a) prints a in flat mode. You should generally not need to use it.
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| 79 | #
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| 80 | # ~ Measures ~
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| 81 | #
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| 82 | # The algorithm used here is close to the one originally described by Wadler,
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| 83 | # but it precomputes a "measure" for each node in the `doc`. This "measure"
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| 84 | # allows each Groups to decide whether to print flat or not without needing to
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| 85 | # look ahead per Wadler's algorithm. A measure has two pieces of information:
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| 86 | #
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| 87 | # - Measure.flat is the width of the doc if it's printed flat.
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| 88 | # - Measure.nonflat is the width of the doc until the _earliest possible_
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| 89 | # newline, or -1 if it doesn't contain a Break.
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| 90 | #
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| 91 | # Measures are used in two steps:
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| 92 | # (1) First, they're computed bottom-up on the `doc`, measuring the size of each
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| 93 | # node.
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| 94 | # (2) Later, PrintDoc() stores a measure in each DocFragment. These Measures
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| 95 | # measure something different: the width from the doc _to the end of the
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| 96 | # entire doc tree_. This second set of Measures (the ones in the
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| 97 | # DocFragments) are computed top-down, and they're used to decide for each
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| 98 | # Group whether to use flat mode or not, without needing to scan ahead.
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| 99 |
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| 100 | from __future__ import print_function
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| 101 |
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| 102 | from _devbuild.gen.pretty_asdl import (doc, doc_e, DocFragment, Measure,
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| 103 | MeasuredDoc, List_Measured)
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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 2 concatenated docs.
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| 129 |
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| 130 | If m1 and m2 are the measures of doc1 and doc2, then _ConcatMeasure(m1, m2)
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| 131 | is the measure of doc.Concat([doc1, doc2]). This concatenation is
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| 132 | associative but not commutative.
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| 133 | """
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| 134 | if m1.nonflat != -1:
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| 135 | return Measure(m1.flat + m2.flat, m1.nonflat)
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| 136 | elif m2.nonflat != -1:
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| 137 | return Measure(m1.flat + m2.flat, m1.flat + m2.nonflat)
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| 138 | else:
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| 139 | return Measure(m1.flat + m2.flat, -1)
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| 140 |
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| 141 |
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| 142 | def _SuffixLen(measure):
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| 143 | # type: (Measure) -> int
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| 144 | """The width until the earliest possible newline, or end of document."""
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| 145 | if measure.nonflat != -1:
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| 146 | return measure.nonflat
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| 147 | else:
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| 148 | return measure.flat
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| 149 |
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| 150 |
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| 151 | #
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| 152 | # Doc Construction
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| 153 | #
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| 154 |
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| 155 |
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| 156 | def AsciiText(string):
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| 157 | # type: (str) -> MeasuredDoc
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| 158 | """Print string (which must not contain a newline)."""
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| 159 | return MeasuredDoc(doc.Text(string), Measure(len(string), -1))
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| 160 |
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| 161 |
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| 162 | def _Break(string):
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| 163 | # type: (str) -> MeasuredDoc
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| 164 | """If in flat mode, print string, otherwise print `\n`.
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| 165 |
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| 166 | Note: Doesn't try to compute Unicode width, since we control these strings.
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| 167 | """
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| 168 | return MeasuredDoc(doc.Break(string), Measure(len(string), 0))
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| 169 |
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| 170 |
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| 171 | def _Indent(indent, mdoc):
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| 172 | # type: (int, MeasuredDoc) -> MeasuredDoc
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| 173 | """Add 'indent' spaces after every newline in mdoc."""
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| 174 | return MeasuredDoc(doc.Indent(indent, mdoc), mdoc.measure)
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| 175 |
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| 176 |
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| 177 | def _Splice(out, mdocs):
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| 178 | # type: (List[MeasuredDoc], List[MeasuredDoc]) -> Measure
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| 179 | """Optimization for _Concat.
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| 180 |
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| 181 | This reduces the total size of the doc_t tree, and thus the memory usage
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| 182 | (as long as we mylib.MaybeCollect() in _HNode!)
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| 183 |
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| 184 | Example of optimizing _Concat nodes together: _Field() concatenates
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| 185 | AsciiText("name:") and _HNode(), and the latter is often a doc.Concat node.
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| 186 | """
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| 187 | measure = _EmptyMeasure()
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| 188 | for mdoc in mdocs:
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| 189 | with tagswitch(mdoc.doc) as case:
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| 190 | if case(doc_e.Concat):
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| 191 | child = cast(List_Measured, mdoc.doc)
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| 192 | # ignore return value, because the parent has the measure already
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| 193 | _Splice(out, child)
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| 194 | else:
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| 195 | out.append(mdoc)
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| 196 | measure = _ConcatMeasure(measure, mdoc.measure)
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| 197 | return measure
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| 198 |
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| 199 |
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| 200 | def _Concat(mdocs):
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| 201 | # type: (List[MeasuredDoc]) -> MeasuredDoc
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| 202 | """Print the mdocs in order (with no space in between)."""
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| 203 | flattened = List_Measured.New()
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| 204 | measure = _Splice(flattened, mdocs)
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| 205 | return MeasuredDoc(flattened, measure)
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| 206 |
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| 207 |
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| 208 | def _Group(mdoc):
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| 209 | # type: (MeasuredDoc) -> MeasuredDoc
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| 210 | """Print mdoc. Use flat mode if mdoc will fit on the current line."""
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| 211 | return MeasuredDoc(mdoc, mdoc.measure)
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| 212 |
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| 213 |
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| 214 | def _IfFlat(flat_mdoc, nonflat_mdoc):
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| 215 | # type: (MeasuredDoc, MeasuredDoc) -> MeasuredDoc
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| 216 | """If in flat mode, print flat_mdoc; otherwise print nonflat_mdoc."""
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| 217 | return MeasuredDoc(
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| 218 | doc.IfFlat(flat_mdoc, nonflat_mdoc),
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| 219 | Measure(flat_mdoc.measure.flat, nonflat_mdoc.measure.nonflat))
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| 220 |
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| 221 |
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| 222 | def _Flat(mdoc):
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| 223 | # type: (MeasuredDoc) -> MeasuredDoc
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| 224 | """Prints mdoc in flat mode."""
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| 225 | return MeasuredDoc(doc.Flat(mdoc), _FlattenMeasure(mdoc.measure))
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| 226 |
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| 227 |
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| 228 | class PrettyPrinter(object):
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| 229 |
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| 230 | def __init__(self, max_width):
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| 231 | # type: (int) -> None
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| 232 | self.max_width = max_width
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| 233 |
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| 234 | def _Fits(self, prefix_len, group, suffix_measure):
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| 235 | # type: (int, MeasuredDoc, Measure) -> bool
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| 236 | """Will group fit flat on the current line?"""
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| 237 | measure = _ConcatMeasure(_FlattenMeasure(group.measure),
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| 238 | suffix_measure)
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| 239 | return prefix_len + _SuffixLen(measure) <= self.max_width
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| 240 |
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| 241 | def PrintDoc(self, document, buf):
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| 242 | # type: (MeasuredDoc, BufWriter) -> None
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| 243 | """Pretty print a doc_t to a BufWriter."""
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| 244 |
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| 245 | # The width of the text we've printed so far on the current line
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| 246 | prefix_len = 0
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| 247 | # A _stack_ of document fragments to print. Each fragment contains:
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| 248 | # - A MeasuredDoc (doc node and its measure, saying how "big" it is)
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| 249 | # - The indentation level to print this doc node at.
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| 250 | # - Is this doc node being printed in flat mode?
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| 251 | # - The measure _from just after the doc node, to the end of the entire document_.
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| 252 | # (Call this the suffix_measure)
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| 253 | fragments = [DocFragment(_Group(document), 0, False, _EmptyMeasure())]
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| 254 |
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| 255 | max_stack = len(fragments)
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| 256 |
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| 257 | while len(fragments) > 0:
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| 258 | max_stack = max(max_stack, len(fragments))
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| 259 |
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| 260 | frag = fragments.pop()
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| 261 | UP_doc = frag.mdoc.doc
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| 262 | with tagswitch(UP_doc) as case:
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| 263 |
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| 264 | if case(doc_e.Text):
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| 265 | text = cast(doc.Text, UP_doc)
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| 266 | buf.write(text.string)
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| 267 | prefix_len += frag.mdoc.measure.flat
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| 268 |
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| 269 | elif case(doc_e.Break):
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| 270 | break_ = cast(doc.Break, UP_doc)
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| 271 | if frag.is_flat:
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| 272 | buf.write(break_.string)
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| 273 | prefix_len += frag.mdoc.measure.flat
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| 274 | else:
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| 275 | buf.write('\n')
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| 276 | buf.write_spaces(frag.indent)
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| 277 | prefix_len = frag.indent
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| 278 |
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| 279 | elif case(doc_e.Indent):
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| 280 | indented = cast(doc.Indent, UP_doc)
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| 281 | fragments.append(
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| 282 | DocFragment(indented.mdoc,
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| 283 | frag.indent + indented.indent,
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| 284 | frag.is_flat, frag.measure))
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| 285 |
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| 286 | elif case(doc_e.Concat):
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| 287 | concat = cast(List_Measured, UP_doc)
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| 288 |
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| 289 | # If we encounter Concat([A, B, C]) with a suffix measure M,
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| 290 | # we need to push A,B,C onto the stack in reverse order:
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| 291 | # - C, with suffix_measure = B.measure + A.measure + M
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| 292 | # - B, with suffix_measure = A.measure + M
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| 293 | # - A, with suffix_measure = M
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| 294 | measure = frag.measure
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| 295 | for mdoc in reversed(concat):
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| 296 | fragments.append(
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| 297 | DocFragment(mdoc, frag.indent, frag.is_flat,
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| 298 | measure))
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| 299 | # TODO: this algorithm allocates too much!
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| 300 | measure = _ConcatMeasure(mdoc.measure, measure)
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| 301 |
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| 302 | elif case(doc_e.Group):
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| 303 | # If the group would fit on the current line when printed
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| 304 | # flat, do so. Otherwise, print it non-flat.
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| 305 | group = cast(MeasuredDoc, UP_doc)
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| 306 | is_flat = self._Fits(prefix_len, group, frag.measure)
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| 307 | fragments.append(
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| 308 | DocFragment(group, frag.indent, is_flat, frag.measure))
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| 309 |
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| 310 | elif case(doc_e.IfFlat):
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| 311 | if_flat = cast(doc.IfFlat, UP_doc)
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| 312 | if frag.is_flat:
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| 313 | subdoc = if_flat.flat_mdoc
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| 314 | else:
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| 315 | subdoc = if_flat.nonflat_mdoc
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| 316 | fragments.append(
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| 317 | DocFragment(subdoc, frag.indent, frag.is_flat,
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| 318 | frag.measure))
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| 319 |
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| 320 | elif case(doc_e.Flat):
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| 321 | flat_doc = cast(doc.Flat, UP_doc)
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| 322 | fragments.append(
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| 323 | DocFragment(flat_doc.mdoc, frag.indent, True,
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| 324 | frag.measure))
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| 325 |
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| 326 | if 0:
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| 327 | log('')
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| 328 | log('___ MAX DocFragment stack: %d', max_stack)
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| 329 | log('')
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| 330 |
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| 331 |
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| 332 | # vim: sw=4
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