Source Code Cross Referenced for LetExpression.java in  » XML » saxonb » net » sf » saxon » expr » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » XML » saxonb » net.sf.saxon.expr 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        package net.sf.saxon.expr;
002:
003:        import net.sf.saxon.instruct.TailCall;
004:        import net.sf.saxon.instruct.TailCallReturner;
005:        import net.sf.saxon.om.Item;
006:        import net.sf.saxon.om.NamePool;
007:        import net.sf.saxon.om.SequenceIterator;
008:        import net.sf.saxon.om.ValueRepresentation;
009:        import net.sf.saxon.trans.XPathException;
010:        import net.sf.saxon.type.ItemType;
011:        import net.sf.saxon.type.SchemaType;
012:        import net.sf.saxon.type.TypeHierarchy;
013:        import net.sf.saxon.value.Value;
014:
015:        import java.io.PrintStream;
016:
017:        /**
018:         * A LetExpression is modelled on the XQuery syntax let $x := expr return expr. This syntax
019:         * is not available in the surface XPath language, but it is used internally in an optimized
020:         * expression tree.
021:         */
022:
023:        public class LetExpression extends Assignation implements 
024:                TailCallReturner {
025:
026:            // This integer holds an approximation to the number of times that the declared variable is referenced.
027:            // The value 1 means there is only one reference and it is not in a loop, which means that the value will
028:            // not be retained in memory. If there are multiple references or references within a loop, the value will
029:            // be a small integer > 1. The special value FILTERED indicates that there is a reference within a loop
030:            // in the form $x[predicate], which indicates that the value should potentially be indexable.
031:            int refCount;
032:
033:            public LetExpression() {
034:            }
035:
036:            /**
037:             * Type-check the expression
038:             */
039:
040:            public Expression typeCheck(StaticContext env,
041:                    ItemType contextItemType) throws XPathException {
042:
043:                if (declaration == null) {
044:                    // we've already done the type checking, no need to do it again
045:                    return this ;
046:                }
047:
048:                // The order of events is critical here. First we ensure that the type of the
049:                // sequence expression is established. This is used to establish the type of the variable,
050:                // which in turn is required when type-checking the action part.
051:
052:                sequence = sequence.typeCheck(env, contextItemType);
053:
054:                RoleLocator role = new RoleLocator(RoleLocator.VARIABLE,
055:                        new Integer(nameCode), 0, env.getNamePool());
056:                role.setSourceLocator(this );
057:                sequence = TypeChecker.strictTypeCheck(sequence, declaration
058:                        .getRequiredType(), role, env);
059:                final TypeHierarchy th = env.getNamePool().getTypeHierarchy();
060:                final ItemType actualItemType = sequence.getItemType(th);
061:                declaration.refineTypeInformation(actualItemType, sequence
062:                        .getCardinality(),
063:                        (sequence instanceof  Value ? (Value) sequence : null),
064:                        sequence.getSpecialProperties(), env);
065:
066:                action = action.typeCheck(env, contextItemType);
067:                return this ;
068:            }
069:
070:            /**
071:             * Perform optimisation of an expression and its subexpressions.
072:             * <p/>
073:             * <p>This method is called after all references to functions and variables have been resolved
074:             * to the declaration of the function or variable, and after all type checking has been done.</p>
075:             *
076:             * @param opt             the optimizer in use. This provides access to supporting functions; it also allows
077:             *                        different optimization strategies to be used in different circumstances.
078:             * @param env             the static context of the expression
079:             * @param contextItemType the static type of "." at the point where this expression is invoked.
080:             *                        The parameter is set to null if it is known statically that the context item will be undefined.
081:             *                        If the type of the context item is not known statically, the argument is set to
082:             *                        {@link net.sf.saxon.type.Type#ITEM_TYPE}
083:             * @return the original expression, rewritten if appropriate to optimize execution
084:             * @throws net.sf.saxon.trans.StaticError if an error is discovered during this phase
085:             *                                        (typically a type error)
086:             */
087:
088:            public Expression optimize(Optimizer opt, StaticContext env,
089:                    ItemType contextItemType) throws XPathException {
090:                if (declaration != null) {
091:                    refCount = declaration.getReferenceCount(this , env);
092:                    if (refCount == 0) {
093:                        // variable is not used - no need to evaluate it
094:                        return action;
095:                    }
096:                    declaration = null; // let the garbage collector take it
097:                }
098:
099:                int tries = 0;
100:                while (tries++ < 5) {
101:                    Expression seq2 = sequence.optimize(opt, env,
102:                            contextItemType);
103:                    if (seq2 == sequence) {
104:                        break;
105:                    }
106:                    sequence = seq2;
107:                    adoptChildExpression(sequence);
108:                    resetStaticProperties();
109:                }
110:
111:                tries = 0;
112:                while (tries++ < 5) {
113:                    Expression act2 = action
114:                            .optimize(opt, env, contextItemType);
115:                    if (act2 == action) {
116:                        break;
117:                    }
118:                    action = act2;
119:                    adoptChildExpression(action);
120:                    resetStaticProperties();
121:                }
122:
123:                // Try to promote any WHERE clause appearing within the LET expression
124:
125:                Expression p = promoteWhereClause(null);
126:                if (p != null) {
127:                    return p;
128:                }
129:
130:                return this ;
131:            }
132:
133:            /**
134:             * Check that any elements and attributes constructed or returned by this expression are acceptable
135:             * in the content model of a given complex type. It's always OK to say yes, since the check will be
136:             * repeated at run-time. The process of checking element and attribute constructors against the content
137:             * model of a complex type also registers the type of content expected of those constructors, so the
138:             * static validation can continue recursively.
139:             */
140:
141:            public void checkPermittedContents(SchemaType parentType,
142:                    StaticContext env, boolean whole) throws XPathException {
143:                action.checkPermittedContents(parentType, env, whole);
144:            }
145:
146:            /**
147:             * Iterate over the sequence of values
148:             */
149:
150:            public SequenceIterator iterate(XPathContext context)
151:                    throws XPathException {
152:                // minimize stack consumption by evaluating nested LET expressions iteratively
153:                LetExpression let = this ;
154:                while (true) {
155:                    ValueRepresentation val = let.eval(context);
156:                    context.setLocalVariable(let.slotNumber, val);
157:                    if (let.action instanceof  LetExpression) {
158:                        let = (LetExpression) let.action;
159:                    } else {
160:                        break;
161:                    }
162:                }
163:                return let.action.iterate(context);
164:            }
165:
166:            /**
167:             * Evaluate the variable. (This is overridden in a subclass).
168:             */
169:
170:            protected ValueRepresentation eval(XPathContext context)
171:                    throws XPathException {
172:                //System.err.println("Let line " + getLineNumber());
173:                return ExpressionTool.lazyEvaluate(sequence, context, refCount);
174:            }
175:
176:            /**
177:             * Evaluate the expression as a singleton
178:             */
179:
180:            public Item evaluateItem(XPathContext context)
181:                    throws XPathException {
182:                // minimize stack consumption by evaluating nested LET expressions iteratively
183:                LetExpression let = this ;
184:                while (true) {
185:                    ValueRepresentation val = let.eval(context);
186:                    context.setLocalVariable(let.slotNumber, val);
187:                    if (let.action instanceof  LetExpression) {
188:                        let = (LetExpression) let.action;
189:                    } else {
190:                        break;
191:                    }
192:                }
193:                return let.action.evaluateItem(context);
194:            }
195:
196:            /**
197:             * Process this expression as an instruction, writing results to the current
198:             * outputter
199:             */
200:
201:            public void process(XPathContext context) throws XPathException {
202:                // minimize stack consumption by evaluating nested LET expressions iteratively
203:                LetExpression let = this ;
204:                while (true) {
205:                    ValueRepresentation val = let.eval(context);
206:                    context.setLocalVariable(let.slotNumber, val);
207:                    if (let.action instanceof  LetExpression) {
208:                        let = (LetExpression) let.action;
209:                    } else {
210:                        break;
211:                    }
212:                }
213:                let.action.process(context);
214:            }
215:
216:            /**
217:             * Determine the data type of the items returned by the expression, if possible
218:             * @return one of the values Type.STRING, Type.BOOLEAN, Type.NUMBER, Type.NODE,
219:             * or Type.ITEM (meaning not known in advance)
220:             * @param th
221:             */
222:
223:            public ItemType getItemType(TypeHierarchy th) {
224:                return action.getItemType(th);
225:            }
226:
227:            /**
228:             * Determine the static cardinality of the expression
229:             */
230:
231:            public int computeCardinality() {
232:                return action.getCardinality();
233:            }
234:
235:            /**
236:             * Get the static properties of this expression (other than its type). The result is
237:             * bit-signficant. These properties are used for optimizations. In general, if
238:             * property bit is set, it is true, but if it is unset, the value is unknown.
239:             */
240:
241:            public int computeSpecialProperties() {
242:                return action.getSpecialProperties();
243:            }
244:
245:            /**
246:             * Mark tail function calls
247:             */
248:
249:            public boolean markTailFunctionCalls() {
250:                return ExpressionTool.markTailFunctionCalls(action);
251:            }
252:
253:            /**
254:             * Promote this expression if possible
255:             */
256:
257:            public Expression promote(PromotionOffer offer)
258:                    throws XPathException {
259:                Expression exp = offer.accept(this );
260:                if (exp != null) {
261:                    return exp;
262:                } else {
263:                    // pass the offer on to the sequence expression
264:                    sequence = doPromotion(sequence, offer);
265:                    if (offer.action == PromotionOffer.INLINE_VARIABLE_REFERENCES
266:                            || offer.action == PromotionOffer.UNORDERED
267:                            || offer.action == PromotionOffer.REPLACE_CURRENT) {
268:                        action = doPromotion(action, offer);
269:                    } else if (offer.action == PromotionOffer.RANGE_INDEPENDENT
270:                    //                  ||  offer.action == PromotionOffer.WHERE_CLAUSE
271:                    ) {
272:                        // Pass the offer to the action expression only if the action isn't depending on the
273:                        // variable bound by this let expression
274:                        Binding[] savedBindingList = offer.bindingList;
275:                        Binding[] newBindingList = new Binding[offer.bindingList.length + 1];
276:                        System.arraycopy(offer.bindingList, 0, newBindingList,
277:                                0, offer.bindingList.length);
278:                        newBindingList[offer.bindingList.length] = this ;
279:                        offer.bindingList = newBindingList;
280:                        action = doPromotion(action, offer);
281:                        offer.bindingList = savedBindingList;
282:                    }
283:                    // if this results in the expression (let $x := $y return Z), replace all references to
284:                    // to $x by references to $y in the Z part, and eliminate this LetExpression by
285:                    // returning the action part.
286:                    if (sequence instanceof  VariableReference) {
287:                        replaceVariable(offer.getOptimizer(), sequence);
288:                        return action;
289:                    }
290:                    // similarly, for (let $x := lazy($y) return Z)
291:                    if (sequence instanceof  LazyExpression
292:                            && ((LazyExpression) sequence).getBaseExpression() instanceof  VariableReference) {
293:                        replaceVariable(offer.getOptimizer(),
294:                                ((LazyExpression) sequence).getBaseExpression());
295:                        return action;
296:                    }
297:
298:                    return this ;
299:                }
300:            }
301:
302:            private void replaceVariable(Optimizer opt, Expression seq)
303:                    throws XPathException {
304:                PromotionOffer offer2 = new PromotionOffer(opt);
305:                offer2.action = PromotionOffer.INLINE_VARIABLE_REFERENCES;
306:                Binding[] bindingList = { this  };
307:                offer2.bindingList = bindingList;
308:                offer2.containingExpression = seq;
309:                action = doPromotion(action, offer2);
310:                if (offer2.accepted) {
311:                    // there might be further references to the variable
312:                    offer2.accepted = false;
313:                    replaceVariable(opt, seq);
314:                }
315:            }
316:
317:            /**
318:             * ProcessLeavingTail: called to do the real work of this instruction.
319:             * The results of the instruction are written
320:             * to the current Receiver, which can be obtained via the Controller.
321:             *
322:             * @param context The dynamic context of the transformation, giving access to the current node,
323:             *                the current variables, etc.
324:             * @return null if the instruction has completed execution; or a TailCall indicating
325:             *         a function call or template call that is delegated to the caller, to be made after the stack has
326:             *         been unwound so as to save stack space.
327:             */
328:
329:            public TailCall processLeavingTail(XPathContext context)
330:                    throws XPathException {
331:                // minimize stack consumption by evaluating nested LET expressions iteratively
332:                LetExpression let = this ;
333:                while (true) {
334:                    ValueRepresentation val = let.eval(context);
335:                    context.setLocalVariable(let.slotNumber, val);
336:                    if (let.action instanceof  LetExpression) {
337:                        let = (LetExpression) let.action;
338:                    } else {
339:                        break;
340:                    }
341:                }
342:                if (let.action instanceof  TailCallReturner) {
343:                    return ((TailCallReturner) let.action)
344:                            .processLeavingTail(context);
345:                } else {
346:                    let.action.process(context);
347:                    return null;
348:                }
349:            }
350:
351:            /**
352:             * Diagnostic print of expression structure
353:             */
354:
355:            public void display(int level, NamePool pool, PrintStream out) {
356:                out.println(ExpressionTool.indent(level) + "let $"
357:                        + getVariableName(pool) + "[refCount=" + refCount
358:                        + "] :=");
359:                sequence.display(level + 1, pool, out);
360:                out.println(ExpressionTool.indent(level) + "return");
361:                action.display(level + 1, pool, out);
362:            }
363:
364:        }
365:
366:        //
367:        // The contents of this file are subject to the Mozilla Public License Version 1.0 (the "License");
368:        // you may not use this file except in compliance with the License. You may obtain a copy of the
369:        // License at http://www.mozilla.org/MPL/
370:        //
371:        // Software distributed under the License is distributed on an "AS IS" basis,
372:        // WITHOUT WARRANTY OF ANY KIND, either express or implied.
373:        // See the License for the specific language governing rights and limitations under the License.
374:        //
375:        // The Original Code is: all this file.
376:        //
377:        // The Initial Developer of the Original Code is Michael H. Kay
378:        //
379:        // Portions created by (your name) are Copyright (C) (your legal entity). All Rights Reserved.
380:        //
381:        // Contributor(s): none.
382:        //
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