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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.javacutil.trees;

import com.sun.source.tree.ExpressionTree;
import com.sun.tools.javac.processing.JavacProcessingEnvironment;
import com.sun.tools.javac.tree.JCTree.JCExpression;
import com.sun.tools.javac.tree.TreeMaker;
import com.sun.tools.javac.util.Context;
import com.sun.tools.javac.util.List;
import com.sun.tools.javac.util.ListBuffer;
import com.sun.tools.javac.util.Names;
import java.util.StringTokenizer;
import javax.annotation.processing.ProcessingEnvironment;
import org.plumelib.util.IPair;

/**
 * A utility class for parsing Java expression snippets, and converting them to proper Javac AST
 * nodes.
 *
 * This is useful for parsing {@code EnsuresNonNull*}, and {@code KeyFor} values.
 *
 * 
Currently, it handles four tree types only:
 *
 * 

 *   Identifier tree (e.g. {@code id})
 *   
Literal tree (e.g. 2, 3)
 *   
Method invocation tree (e.g. {@code method(2, 3)})
 *   
Member select tree (e.g. {@code Class.field}, {@code instance.method()})
 *   
Array access tree (e.g. {@code array[id]})
 * 
 *
 * Notable limitation: Doesn't handle spaces, or non-method-argument parenthesis.
 *
 * It's implemented via a Recursive-Descend parser.
 */
public class TreeParser {
  /** Valid delimiters. */
  private static final String DELIMS = ".[](),";

  /** A sentinel value. */
  private static final String SENTINEL = "";

  /** The TreeMaker instance. */
  private final TreeMaker maker;

  /** The names instance. */
  private final Names names;

  /** Create a TreeParser. */
  public TreeParser(ProcessingEnvironment env) {
    Context context = ((JavacProcessingEnvironment) env).getContext();
    maker = TreeMaker.instance(context);
    names = Names.instance(context);
  }

  /**
   * Parses the snippet in the string as an internal Javac AST expression node.
   *
   * @param s the Java snippet
   * @return the AST corresponding to the snippet
   */
  public ExpressionTree parseTree(String s) {
    StringTokenizer tokenizer = new StringTokenizer(s, DELIMS, true);
    String token = tokenizer.nextToken();

    try {
      return parseExpression(tokenizer, token).first;
    } catch (Exception e) {
      throw new ParseError(e);
    } finally {
      tokenizer = null;
      token = null;
    }
  }

  /** The next token from the tokenizer, or the {@code SENTINEL} if none is available. */
  private String nextToken(StringTokenizer tokenizer) {
    return tokenizer.hasMoreTokens() ? tokenizer.nextToken() : SENTINEL;
  }

  /** The parsed expression tree for the given token. */
  private JCExpression fromToken(String token) {
    // Optimization
    if ("true".equals(token)) {
      return maker.Literal(true);
    } else if ("false".equals(token)) {
      return maker.Literal(false);
    }

    if (Character.isLetter(token.charAt(0))) {
      return maker.Ident(names.fromString(token));
    }

    Object value;
    try {
      value = Integer.valueOf(token);
    } catch (Exception e2) {
      try {
        value = Double.valueOf(token);
      } catch (Exception ef) {
        throw new Error("Can't parse as integer or double: " + token);
      }
    }
    return maker.Literal(value);
  }

  /**
   * Parse an expression.
   *
   * @param tokenizer the tokenizer
   * @param token the first token
   * @return a pair of a parsed expression and the next token
   */
  private IPair parseExpression(StringTokenizer tokenizer, String token) {
    JCExpression tree = fromToken(token);

    while (tokenizer.hasMoreTokens()) {
      String delim = nextToken(tokenizer);
      token = delim;
      if (".".equals(delim)) {
        token = nextToken(tokenizer);
        tree = maker.Select(tree, names.fromString(token));
      } else if ("(".equals(delim)) {
        token = nextToken(tokenizer);
        ListBuffer args = new ListBuffer<>();
        while (!")".equals(token)) {
          IPair p = parseExpression(tokenizer, token);
          JCExpression arg = p.first;
          token = p.second;
          args.append(arg);
          if (",".equals(token)) {
            token = nextToken(tokenizer);
          }
        }
        // For now, handle empty args only
        assert ")".equals(token) : "Unexpected token: " + token;
        tree = maker.Apply(List.nil(), tree, args.toList());
      } else if ("[".equals(token)) {
        token = nextToken(tokenizer);
        IPair p = parseExpression(tokenizer, token);
        JCExpression index = p.first;
        token = p.second;
        assert "]".equals(token) : "Unexpected token: " + token;
        tree = maker.Indexed(tree, index);
      } else {
        return IPair.of(tree, token);
      }
      assert tokenizer != null : "@AssumeAssertion(nullness): side effects";
    }

    return IPair.of(tree, token);
  }

  /** An internal error. */
  private static class ParseError extends RuntimeException {
    /** The serial version UID. */
    private static final long serialVersionUID = 1887754619522101929L;

    /** Create a ParseError. */
    ParseError(Throwable cause) {
      super(cause);
    }
  }
}