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antlr-tree-rewriter-cs's Introduction

ANTLR tree rewriter

Version 4 of ANTLR produces parse trees (contrary to abstract syntax trees). Depending on how your grammar is written, this can cause the parse tree to become large. This library might help compact the parse tree and adds the possibility to serialize the parse tree to JSON (and the other way around).

TOC

Install

NuGet:

<PackageReference Include="AntlrTreeRewriter" Version="1.0.6" />

Example

Given the following ANTLR grammar:

grammar Expr;

parse
 : expr EOF
 ;

expr
 : or_expr
 ;

or_expr
 : and_expr ('||' expr)?
 ;

and_expr
 : add_expr ('&&' expr)?
 ;

add_expr
 : mult_expr (('+' | '-') expr)?
 ;

mult_expr
 : unary_expr (('*' | '/' | '%') expr)?
 ;

unary_expr
 : '-' atom
 | atom
 ;

atom
 : '(' expr ')'
 | ID
 | NUM
 ;

ADD  : '+';
MIN  : '-';
MUL  : '*';
DIV  : '/';
MOD  : '%';
AND  : '&&';
OR   : '||';
OPAR : '(';
CPAR : ')';
ID   : [a-zA-Z_] [a-zA-Z_0-9]*;
NUM  : [0-9]+ ('.' [0-9]+)?;
WS   : [ \t\r\n]+ -> skip;

If you now generate a parser and parse the input (3.14159265 + Mu) * 42 ANTLR will give you the following parse tree:

graph TD
  N_1179381257["parse"] --> N_258754732["expr"]
  N_1179381257["parse"] --> N_333362446["&lt;EOF&gt;"]
  N_258754732["expr"] --> N_597255128["or_expr"]
  N_597255128["or_expr"] --> N_985397764["and_expr"]
  N_985397764["and_expr"] --> N_1476394199["add_expr"]
  N_1476394199["add_expr"] --> N_837764579["mult_expr"]
  N_837764579["mult_expr"] --> N_1501587365["unary_expr"]
  N_837764579["mult_expr"] --> N_1007603019["*"]
  N_837764579["mult_expr"] --> N_348100441["expr"]
  N_348100441["expr"] --> N_1597249648["or_expr"]
  N_1597249648["or_expr"] --> N_89387388["and_expr"]
  N_89387388["and_expr"] --> N_1333592072["add_expr"]
  N_1333592072["add_expr"] --> N_655381473["mult_expr"]
  N_655381473["mult_expr"] --> N_1486371051["unary_expr"]
  N_1486371051["unary_expr"] --> N_1121647253["atom"]
  N_1121647253["atom"] --> N_1694556038["42"]
  N_1501587365["unary_expr"] --> N_1076496284["atom"]
  N_1076496284["atom"] --> N_1508646930["("]
  N_1076496284["atom"] --> N_1291286504["expr"]
  N_1076496284["atom"] --> N_795372831[")"]
  N_1291286504["expr"] --> N_1072601481["or_expr"]
  N_1072601481["or_expr"] --> N_121295574["and_expr"]
  N_121295574["and_expr"] --> N_1887813102["add_expr"]
  N_1887813102["add_expr"] --> N_485041780["mult_expr"]
  N_1887813102["add_expr"] --> N_1459672753["+"]
  N_1887813102["add_expr"] --> N_117244645["expr"]
  N_117244645["expr"] --> N_1540011289["or_expr"]
  N_1540011289["or_expr"] --> N_239465106["and_expr"]
  N_239465106["and_expr"] --> N_1596000437["add_expr"]
  N_1596000437["add_expr"] --> N_832947102["mult_expr"]
  N_832947102["mult_expr"] --> N_1061804750["unary_expr"]
  N_1061804750["unary_expr"] --> N_507084503["atom"]
  N_507084503["atom"] --> N_1225439493["Mu"]
  N_485041780["mult_expr"] --> N_1454127753["unary_expr"]
  N_1454127753["unary_expr"] --> N_667026744["atom"]
  N_667026744["atom"] --> N_1926764753["3.14159265"]

Flattened tree

This library can be used to "flatten" the generated parse tree as follows:

var source = "(3.14159265 + Mu) * 42";
var lexer = new ExprLexer(CharStreams.fromString(source));
var parser = new ExprParser(new CommonTokenStream(lexer));
var root = parser.parse();

var node = new TreeRewriter(root).Rewrite();

and node will now represent the following tree:

graph TD
  N_1845904670["parse"] --> N_1497973285["mult_expr"]
  N_1845904670["parse"] --> N_1846896625["&lt;EOF&gt;"]
  N_1497973285["mult_expr"] --> N_1555690610["atom"]
  N_1497973285["mult_expr"] --> N_13329486["*"]
  N_1497973285["mult_expr"] --> N_327177752["42"]
  N_1555690610["atom"] --> N_1458540918["("]
  N_1555690610["atom"] --> N_1164371389["add_expr"]
  N_1555690610["atom"] --> N_517210187[")"]
  N_1164371389["add_expr"] --> N_267760927["3.14159265"]
  N_1164371389["add_expr"] --> N_633070006["+"]
  N_1164371389["add_expr"] --> N_1459794865["Mu"]

Ignoring tokens

If you want to ignore certain tokens, like (, ), and EOF for example, you can do the following:

var source = "(3.14159265 + Mu) * 42";
var lexer = new ExprLexer(CharStreams.fromString(source));
var parser = new ExprParser(new CommonTokenStream(lexer));
var root = parser.parse();

var node = new TreeRewriter(root)
  .Ignore(ExprLexer.Eof, ExprLexer.OPAR, ExprLexer.CPAR)
  .Rewrite();

and now node will represent the following tree:

graph TD
  N_1845904670["mult_expr"] --> N_1497973285["add_expr"]
  N_1845904670["mult_expr"] --> N_1846896625["*"]
  N_1845904670["mult_expr"] --> N_1555690610["42"]
  N_1497973285["add_expr"] --> N_13329486["3.14159265"]
  N_1497973285["add_expr"] --> N_327177752["+"]
  N_1497973285["add_expr"] --> N_1458540918["Mu"]

Promoting tokens

When you want to "promote" certain tokens, for example if you want to rewrite:

graph TD
  a["rule"] --> b1["1"]
  a["rule"] --> b2["+"]
  a["rule"] --> b3["2"]

into:

graph TD
  a["+"] --> b1["1"]
  a["+"] --> b2["2"]

you can do the following:

var source = "(3.14159265 + Mu) * 42";
var lexer = new ExprLexer(CharStreams.fromString(source));
var parser = new ExprParser(new CommonTokenStream(lexer));
var root = parser.parse();

var node = new TreeRewriter(root)
  .Ignore(ExprLexer.Eof, ExprLexer.OPAR, ExprLexer.CPAR)
  .Promote(ExprLexer.ADD, ExprLexer.MIN, ExprLexer.MUL, ExprLexer.DIV, ExprLexer.MOD, ExprLexer.AND, ExprLexer.OR)
  .Rewrite();

which will result in node looking likt this:

graph TD
  N_1845904670["*"] --> N_1497973285["+"]
  N_1845904670["*"] --> N_1846896625["42"]
  N_1497973285["+"] --> N_1555690610["3.14159265"]
  N_1497973285["+"] --> N_13329486["Mu"]

Note that if your parse tree can produce the following:

graph TD
  a["add_expr"] --> b["a"]
  a["add_expr"] --> c["+"]
  a["add_expr"] --> d["b"]
  a["add_expr"] --> e["-"]
  a["add_expr"] --> f["c"]

and you do promote both the + and - tokens:

var node = new TreeRewriter(root)
  .Promote(ExprLexer.ADD, ExprLexer.MIN)
  .Rewrite();

then the first token that is encountered (+ in this case) will become the promoted token:

graph TD
  a["+"] --> b["a"]
  a["+"] --> d["-"]
  d["-"] --> e["b"]
  d["-"] --> f["c"]

JSON

The TreeNode class can be easily used to (de) serialize from and to JSON:

[Fact]
public void JsonDemo()
{
  var source = "(3.14159265 + Mu) * 42";

  var lexer = new ExprLexer(CharStreams.fromString(source));
  var parser = new ExprParser(new CommonTokenStream(lexer));
  var root = parser.parse();

  var node = new TreeRewriter(root)
    .Ignore(ExprLexer.Eof, ExprLexer.OPAR, ExprLexer.CPAR)
    .Promote(ExprLexer.ADD, ExprLexer.MIN, ExprLexer.MUL, ExprLexer.DIV, ExprLexer.MOD, ExprLexer.AND, ExprLexer.OR)
    .Rewrite();
  
  var json = JsonConvert.SerializeObject(node);

  Assert.Equal("{\"Label\":\"*\",\"TokenType\":3,\"Line\":1,\"StartIndex\":18,\"StopIndex\":18,\"Children\":[{\"Label\":\"+\",\"TokenType\":1,\"Line\":1,\"StartIndex\":12,\"StopIndex\":12,\"Children\":[{\"Label\":\"3.14159265\",\"TokenType\":11,\"Line\":1,\"StartIndex\":1,\"StopIndex\":10,\"Children\":[]},{\"Label\":\"Mu\",\"TokenType\":10,\"Line\":1,\"StartIndex\":14,\"StopIndex\":15,\"Children\":[]}]},{\"Label\":\"42\",\"TokenType\":11,\"Line\":1,\"StartIndex\":20,\"StopIndex\":21,\"Children\":[]}]}", json);

  var deserializedNode = JsonConvert.DeserializeObject<TreeNode>(json);

  Assert.Equal("*", deserializedNode.Label);
  Assert.Equal("+", deserializedNode.Children[0].Label);
  Assert.Equal("3.14159265", deserializedNode.Children[0].Children[0].Label);
  Assert.Equal("Mu", deserializedNode.Children[0].Children[1].Label);
  Assert.Equal("42", deserializedNode.Children[1].Label);
}

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