Package org.apache.groovy.contracts.ast

Class LoopVariantASTTransformation

java.lang.Object
org.apache.groovy.contracts.ast.LoopVariantASTTransformation
All Implemented Interfaces:
ASTTransformation
public class LoopVariantASTTransformation extends Object implements ASTTransformation
Handles Decreases annotations placed on loop statements (for, while, do-while). The closure must return a value that strictly decreases on every iteration and remains non-negative.

The transformation injects code to:

  1. Save the expression value at the start of each iteration.
  2. Re-evaluate it at the end of the iteration.
  3. Assert the value has strictly decreased.
  4. Assert the value is non-negative.

Example: 

 int n = 10
 @Decreases({ n })
 while (n > 0) {
     n--
 }
Since:
6.0.0
See Also:

  • Constructor Details

    • LoopVariantASTTransformation

      public LoopVariantASTTransformation()

  • Method Details

    • visit

      public void visit(ASTNode[] nodes, SourceUnit source)
      Rewrites a loop-level Decreases annotation into variant bookkeeping and runtime checks.
      Specified by:
      visit in interface ASTTransformation
      Parameters:
      nodes - the annotated AST nodes supplied by the compiler
      source - the current source unit

    • checkDecreased

      public static void checkDecreased(Object prev, Object curr)
      Runtime check called from generated code. Throws LoopVariantViolation if the variant did not strictly decrease or became negative.

      If both values are Lists, they are compared lexicographically: the first position where values differ must show a strict decrease; all earlier positions must be equal. If all positions are equal, the variant has not decreased and a violation is thrown.