Package org.apache.groovy.contracts.ast

Class LoopVariantASTTransformation

java.lang.Object
org.apache.groovy.contracts.ast.LoopVariantASTTransformation
All Implemented Interfaces:
CompilationUnitAware, ASTTransformation
public class LoopVariantASTTransformation extends Object implements ASTTransformation, CompilationUnitAware
Handles Decreases annotations placed on loop statements (for, while, do-while). The closure must return a value that is non-negative at the start of each iteration and strictly decreases between consecutive iterations.

The transformation injects code to:

  1. Evaluate the expression at the start of each iteration.
  2. On the first iteration, assert a scalar measure is non-negative at loop entry (well-foundedness).
  3. On subsequent iterations, assert it has strictly decreased since the previous iteration start, and is non-negative (GROOVY-12128: comparing iteration starts — rather than the start and end of one pass — sees progress made by a classic for loop's update expression, which runs after the body, and does not demand non-negativity after the final body execution).
The final iteration's decrease is intentionally unchecked: a loop that has exited needs no further termination evidence.

Example: 

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

  • Constructor Details

    • LoopVariantASTTransformation

      public LoopVariantASTTransformation()

  • Method Details

    • setCompilationUnit

      public void setCompilationUnit(CompilationUnit unit)
      Description copied from interface: CompilationUnitAware
      Sets the compilation unit currently applying this transformation.
      Specified by:
      setCompilationUnit in interface CompilationUnitAware
      Parameters:
      unit - the active compilation unit

    • 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