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package org.checkerframework.checker.index.qual;

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;
import org.checkerframework.framework.qual.JavaExpression;

/**
 * The annotated sequence contains a subsequence that is equal to the value of some other
 * expression. This annotation permits the Upper Bound Checker to translate indices for one sequence
 * into indices for the other sequence.
 *
 * Consider the following example:
 *
 * 

 *  class IntSubArray {
 *  {@literal @}HasSubsequence(subsequence = "this", from = "this.start", to = "this.end")
 *    int [] array;
 *    int @IndexFor("array") int start;
 *    int @IndexOrHigh("array") int end;
 *  }
 * 
 *
 * The above annotations mean that the value of an {@code IntSubArray} object is equal to a
 * subsequence of its {@code array} field.
 *
 * These annotations imply the following relationships among {@code @}{@link IndexFor}
 * annotations:
 *
 * 

 *   If {@code i} is {@code @IndexFor("this")}, then {@code start + i} is
 *       {@code @IndexFor("array")}.
 *   
If {@code j} is {@code @IndexFor("array")}, then {@code j - start } is
 *       {@code @IndexFor("this")}.
 * 
 *
 * When assigning an array {@code a} to {@code array}, 4 facts need to be true:
 *
 * 
 *   {@code start} is {@code @NonNegative}.
 *   
{@code end} is {@code @LTEqLengthOf("a")}.
 *   
{@code start} is {@code @LessThan("end + 1")}.
 *   
the value of {@code this} equals {@code array[start..end-1]}
 * 
 *
 * The Index Checker verifies the first 3 facts, but always issues a warning because it cannot prove
 * the 4th fact. The programmer should should manually verify that the {@code subsequence} field is
 * equal to the given subsequence and then suppress the warning.
 *
 * For an example of how this annotation is used in practice, see the test GuavaPrimitives.java
 * in /checker/tests/index/.
 *
 * This annotation may only be written on fields.
 *
 * @checker_framework.manual #index-checker Index Checker
 */
@Target({ElementType.FIELD})
public @interface HasSubsequence {
    /** An expression that evaluates to the subsequence. */
    @JavaExpression
    String subsequence();

    /** The index into this where the subsequence starts. */
    @JavaExpression
    String from();

    /** The index into this, immediately past where the subsequence ends. */
    @JavaExpression
    String to();
}