package net.sf.saxon.expr;

import net.sf.saxon.trace.ExpressionPresenter;
import net.sf.saxon.om.Item;
import net.sf.saxon.sort.AtomicComparer;
import net.sf.saxon.sort.DoubleSortComparer;
import net.sf.saxon.trans.XPathException;
import net.sf.saxon.type.BuiltInAtomicType;
import net.sf.saxon.type.ItemType;
import net.sf.saxon.type.TypeHierarchy;
import net.sf.saxon.value.BooleanValue;
import net.sf.saxon.value.Int64Value;
import net.sf.saxon.value.NumericValue;

import java.util.Iterator;

/**
 * This class implements a comparison of a numeric value to an integer constant using one of the operators
 * eq, ne, lt, gt, le, ge. The semantics are identical to ValueComparison, but this is a fast path for an
 * important common case.
 */

public class CompareToIntegerConstant extends Expression implements ComparisonExpression {

    private Expression operand;
    private long comparand;
    private int operator;

    /**
     * Create the expression
     * @param operand the operand to be compared with an integer constant
     * @param operator the comparison operator,
     *         one of {@link Token#FEQ}, {@link Token#FNE}, {@link Token#FGE},
     *                {@link Token#FGT}, {@link Token#FLE}, {@link Token#FLT}
     * @param comparand the integer constant
     */

    public CompareToIntegerConstant(Expression operand, int operator, long comparand) {
        this.operand = operand;
        this.operator = operator;
        this.comparand = comparand;
        adoptChildExpression(operand);
    }

    /**
     * Get the expression on the lhs of the comparison
     * @return the left hand operand
     */

    public Expression getOperand() {
        return operand;
    }

    /**
     * Get the integer value on the rhs of the expression
     * @return the integer constant
     */

    public long getComparand() {
        return comparand;
    }

    /**
     * Get the comparison operator
     * @return one of {@link Token#FEQ}, {@link Token#FNE}, {@link Token#FGE},
     *                {@link Token#FGT}, {@link Token#FLE}, {@link Token#FLT}
     */

    public int getComparisonOperator() {
        return operator;
    }

    /**
     * An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
     * This method indicates which of these methods is provided directly. The other methods will always be available
     * indirectly, using an implementation that relies on one of the other methods.
     * @return the value {@link #EVALUATE_METHOD}
     */

    public int getImplementationMethod() {
        return EVALUATE_METHOD;
    }

    /**
     * Simplify an expression. This performs any static optimization (by rewriting the expression
     * as a different expression). The default implementation does nothing.
     * @param visitor the expression visitor
     * @return the simplified expression
     * @throws XPathException
     *          if an error is discovered during expression
     *          rewriting
     */

    public Expression simplify(ExpressionVisitor visitor) throws XPathException {
        operand = visitor.simplify(operand);
        return this;
    }

    /**
     * Offer promotion for this subexpression. The offer will be accepted if the subexpression
     * is not dependent on the factors (e.g. the context item) identified in the PromotionOffer.
     * By default the offer is not accepted - this is appropriate in the case of simple expressions
     * such as constant values and variable references where promotion would give no performance
     * advantage. This method is always called at compile time.
     *
     * @param offer details of the offer, for example the offer to move
     *              expressions that don't depend on the context to an outer level in
     *              the containing expression
     * @return if the offer is not accepted, return this expression unchanged.
     *         Otherwise return the result of rewriting the expression to promote
     *         this subexpression
     * @throws net.sf.saxon.trans.XPathException
     *          if any error is detected
     */

    public Expression promote(PromotionOffer offer) throws XPathException {
        Expression exp = offer.accept(this);
        if (exp != null) {
            return exp;
        } else {
            operand = doPromotion(operand, offer);
            return this;
        }
    }

    public int computeSpecialProperties() {
        return StaticProperty.NON_CREATIVE;
    }

    /**
     * Compute the dependencies of an expression, as the union of the
     * dependencies of its subexpressions. (This is overridden for path expressions
     * and filter expressions, where the dependencies of a subexpression are not all
     * propogated). This method should be called only once, to compute the dependencies;
     * after that, getDependencies should be used.
     *
     * @return the depencies, as a bit-mask
     */

    public int computeDependencies() {
        return operand.getDependencies();
    }

    /**
     * Copy an expression. This makes a deep copy.
     *
     * @return the copy of the original expression
     */

    public Expression copy() {
        return new CompareToIntegerConstant(operand.copy(), operator, comparand);
    }

    /**
     * Get the immediate sub-expressions of this expression. Default implementation
     * returns a zero-length array, appropriate for an expression that has no
     * sub-expressions.
     *
     * @return an iterator containing the sub-expressions of this expression
     */

    public Iterator iterateSubExpressions() {
        return new MonoIterator(operand);
    }

    /**
     * Replace one subexpression by a replacement subexpression
     *
     * @param original    the original subexpression
     * @param replacement the replacement subexpression
     * @return true if the original subexpression is found
     */

    public boolean replaceSubExpression(Expression original, Expression replacement) {
        if (original == operand) {
            operand = replacement;
            return true;
        }
        return false;
    }

    /**
     * Evaluate an expression as a single item. This always returns either a single Item or
     * null (denoting the empty sequence). No conversion is done. This method should not be
     * used unless the static type of the expression is a subtype of "item" or "item?": that is,
     * it should not be called if the expression may return a sequence. There is no guarantee that
     * this condition will be detected.
     *
     * @param context The context in which the expression is to be evaluated
     * @return the node or atomic value that results from evaluating the
     *         expression; or null to indicate that the result is an empty
     *         sequence
     * @throws net.sf.saxon.trans.XPathException
     *          if any dynamic error occurs evaluating the
     *          expression
     */

    public Item evaluateItem(XPathContext context) throws XPathException {
        return BooleanValue.get(effectiveBooleanValue(context));
    }

    /**
     * Get the effective boolean value of the expression. This returns false if the value
     * is the empty sequence, a zero-length string, a number equal to zero, or the boolean
     * false. Otherwise it returns true.
     *
     * @param context The context in which the expression is to be evaluated
     * @return the effective boolean value
     * @throws net.sf.saxon.trans.XPathException
     *          if any dynamic error occurs evaluating the
     *          expression
     */

    public boolean effectiveBooleanValue(XPathContext context) throws XPathException {
        NumericValue n = (NumericValue)operand.evaluateItem(context);
        if (n.isNaN()) {
            return (operator == Token.FNE);
        }
        int c = n.compareTo(comparand);
        switch (operator) {
            case Token.FEQ:
                return c == 0;
            case Token.FNE:
                return c != 0;
            case Token.FGT:
                return c > 0;
            case Token.FLT:
                return c < 0;
            case Token.FGE:
                return c >= 0;
            case Token.FLE:
                return c <= 0;
            default:
                throw new UnsupportedOperationException("Unknown operator " + operator);
        }
    }

    protected int computeCardinality() {
        return StaticProperty.EXACTLY_ONE;
    }

    /**
     * Perform type checking of an expression and its subexpressions. This is the second phase of
     * static optimization.
     * <p/>
     * <p>This checks statically that the operands of the expression have
     * the correct type; if necessary it generates code to do run-time type checking or type
     * conversion. A static type error is reported only if execution cannot possibly succeed, that
     * is, if a run-time type error is inevitable. The call may return a modified form of the expression.</p>
     * <p/>
     * <p>This method is called after all references to functions and variables have been resolved
     * to the declaration of the function or variable. However, the types of such functions and
     * variables may not be accurately known if they have not been explicitly declared.</p>
     * <p/>
     * <p>If the implementation returns a value other than "this", then it is required to ensure that
     * the parent pointer and location information in the returned expression have been set up correctly.
     * It should not rely on the caller to do this, although for historical reasons many callers do so.</p>
     *
     * @param visitor the expession visitor
     * @param contextItemType the static type of "." at the point where this expression is invoked.
     *                        The parameter is set to null if it is known statically that the context item will be undefined.
     *                        If the type of the context item is not known statically, the argument is set to
     *                        {@link net.sf.saxon.type.Type#ITEM_TYPE}
     * @return the original expression, rewritten to perform necessary run-time type checks,
     *         and to perform other type-related optimizations
     * @throws XPathException if an error is discovered during this phase
     *                                        (typically a type error)
     */

    public Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException {
        operand = visitor.typeCheck(operand, contextItemType);
        return this;
    }

    /**
     * Perform optimisation of an expression and its subexpressions. This is the third and final
     * phase of static optimization.
     * <p/>
     * <p>This method is called after all references to functions and variables have been resolved
     * to the declaration of the function or variable, and after all type checking has been done.</p>
     *
     * @param visitor the expression visitor
     * @param contextItemType the static type of "." at the point where this expression is invoked.
     *                        The parameter is set to null if it is known statically that the context item will be undefined.
     *                        If the type of the context item is not known statically, the argument is set to
     *                        {@link net.sf.saxon.type.Type#ITEM_TYPE}
     * @return the original expression, rewritten if appropriate to optimize execution
     * @throws XPathException if an error is discovered during this phase
     *                                        (typically a type error)
     */

    public Expression optimize(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException {
        operand = visitor.optimize(operand, contextItemType);
        return this;
    }

    /**
     * Determine the data type of the expression, if possible. All expression return
     * sequences, in general; this method determines the type of the items within the
     * sequence, assuming that (a) this is known in advance, and (b) it is the same for
     * all items in the sequence.
     * <p/>
     * <p>This method should always return a result, though it may be the best approximation
     * that is available at the time.</p>
     *
     * @param th the type hierarchy cache
     * @return a value such as Type.STRING, Type.BOOLEAN, Type.NUMBER,
     *         Type.NODE, or Type.ITEM (meaning not known at compile time)
     */

    public ItemType getItemType(TypeHierarchy th) {
        return BuiltInAtomicType.BOOLEAN;
    }

    /**
     * Diagnostic print of expression structure. The abstract expression tree
     * is written to the supplied output destination.
     */

    public void explain(ExpressionPresenter destination) {
        destination.startElement("compareToInteger");
        destination.emitAttribute("op", Token.tokens[operator]);
        destination.emitAttribute("value", comparand+"");
        operand.explain(destination);
        destination.endElement();
    }

    /**
     * Get the AtomicComparer used to compare atomic values. This encapsulates any collation that is used
     */

    public AtomicComparer getAtomicComparer() {
        return DoubleSortComparer.getInstance();
        // Note: this treats NaN=NaN as true, but it doesn't matter, because the rhs will never be NaN.
    }

    /**
     * Get the primitive (singleton) operator used: one of Token.FEQ, Token.FNE, Token.FLT, Token.FGT,
     * Token.FLE, Token.FGE
     */

    public int getSingletonOperator() {
        return operator;
    }

    /**
     * Get the two operands of the comparison
     * @return the two operands
     */

    public Expression[] getOperands() {
        return new Expression[] {operand, Literal.makeLiteral(Int64Value.makeIntegerValue(comparand))};
    }

    /**
     * Determine whether untyped atomic values should be converted to the type of the other operand
     *
     * @return true if untyped values should be converted to the type of the other operand, false if they
     *         should be converted to strings.
     */

    public boolean convertsUntypedToOther() {
        return true;
    }
}

//
// The contents of this file are subject to the Mozilla Public License Version 1.0 (the "License");
// you may not use this file except in compliance with the License. You may obtain a copy of the
// License at http://www.mozilla.org/MPL/
//
// Software distributed under the License is distributed on an "AS IS" basis,
// WITHOUT WARRANTY OF ANY KIND, either express or implied.
// See the License for the specific language governing rights and limitations under the License.
//
// The Original Code is: all this file
//
// The Initial Developer of the Original Code is Michael H. Kay.
//
// Contributor(s):
//

