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

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


001:        package net.sf.saxon.value;
002:
003:        import net.sf.saxon.Controller;
004:        import net.sf.saxon.event.SequenceOutputter;
005:        import net.sf.saxon.event.SequenceReceiver;
006:        import net.sf.saxon.event.TeeOutputter;
007:        import net.sf.saxon.expr.LastPositionFinder;
008:        import net.sf.saxon.expr.XPathContext;
009:        import net.sf.saxon.om.*;
010:        import net.sf.saxon.trans.DynamicError;
011:        import net.sf.saxon.trans.XPathException;
012:
013:        import java.util.List;
014:
015:        /**
016:         * A MemoClosure represents a value that has not yet been evaluated: the value is represented
017:         * by an expression, together with saved values of all the context variables that the
018:         * expression depends on.
019:         * <p/>
020:         * <p>The MemoClosure is designed for use when the value is only read several times. The
021:         * value is saved on the first evaluation and remembered for later use.</p>
022:         * <p/>
023:         * <p>The MemoClosure maintains a reservoir containing those items in the value that have
024:         * already been read. When a new iterator is requested to read the value, this iterator
025:         * first examines and returns any items already placed in the reservoir by previous
026:         * users of the MemoClosure. When the reservoir is exhausted, it then uses an underlying
027:         * Input Iterator to read further values of the underlying expression. If the value is
028:         * not read to completion (for example, if the first user did exists($expr), then the
029:         * Input Iterator is left positioned where this user abandoned it. The next user will read
030:         * any values left in the reservoir by the first user, and then pick up iterating the
031:         * base expression where the first user left off. Eventually, all the values of the
032:         * expression will find their way into the reservoir, and future users simply iterate
033:         * over the reservoir contents. Alternatively, of course, the values may be left unread.</p>
034:         * <p/>
035:         * <p>Delayed evaluation is used only for expressions with a static type that allows
036:         * more than one item, so the evaluateItem() method will not normally be used, but it is
037:         * supported for completeness.</p>
038:         * <p/>
039:         * <p>The expression may depend on local variables and on the context item; these values
040:         * are held in the saved XPathContext object that is kept as part of the Closure, and they
041:         * will always be read from that object. The expression may also depend on global variables;
042:         * these are unchanging, so they can be read from the Bindery in the normal way. Expressions
043:         * that depend on other contextual information, for example the values of position(), last(),
044:         * current(), current-group(), should not be evaluated using this mechanism: they should
045:         * always be evaluated eagerly. This means that the Closure does not need to keep a copy
046:         * of these context variables.</p>
047:         */
048:
049:        public class MemoClosure extends Closure {
050:
051:            private Item[] reservoir = null;
052:            private int used;
053:            protected int state;
054:
055:            // State in which no items have yet been read
056:            private static final int UNREAD = 0;
057:
058:            // State in which zero or more items are in the reservoir and it is not known
059:            // whether more items exist
060:            private static final int MAYBE_MORE = 1;
061:
062:            // State in which all the items are in the reservoir
063:            private static final int ALL_READ = 3;
064:
065:            // State in which we are getting the base iterator. If the closure is called in this state,
066:            // it indicates a recursive entry, which is only possible on an error path
067:            private static final int BUSY = 4;
068:
069:            // State in which we know that the value is an empty sequence
070:            protected static final int EMPTY = 5;
071:
072:            /**
073:             * Constructor should not be called directly, instances should be made using the make() method.
074:             */
075:
076:            public MemoClosure() {
077:            }
078:
079:            /**
080:             * Evaluate the expression in a given context to return an iterator over a sequence
081:             *
082:             * @param context the evaluation context. This is ignored; we use the context saved
083:             *                as part of the Closure instead.
084:             */
085:
086:            public SequenceIterator iterate(XPathContext context)
087:                    throws XPathException {
088:
089:                switch (state) {
090:                case UNREAD:
091:                    state = BUSY;
092:                    inputIterator = expression.iterate(savedXPathContext);
093:                    if (inputIterator instanceof  EmptyIterator) {
094:                        state = EMPTY;
095:                        return inputIterator;
096:                    }
097:                    // TODO: following optimization looks OK, but it throws func20 into an infinite loop
098:                    //                if (inputIterator instanceof GroundedIterator) {
099:                    //                    state = UNREAD;
100:                    //                    return inputIterator.getAnother();
101:                    //                }
102:                    reservoir = new Item[50];
103:                    used = 0;
104:                    state = MAYBE_MORE;
105:                    return new ProgressiveIterator();
106:
107:                case MAYBE_MORE:
108:                    return new ProgressiveIterator();
109:
110:                case ALL_READ:
111:                    return new ArrayIterator(reservoir, 0, used);
112:
113:                case BUSY:
114:                    // this indicates a recursive entry, probably on an error path while printing diagnostics
115:                    throw new DynamicError(
116:                            "Attempt to access a lazily-evaluated variable while it is being evaluated");
117:
118:                case EMPTY:
119:                    return EmptyIterator.getInstance();
120:
121:                default:
122:                    throw new IllegalStateException("Unknown iterator state");
123:
124:                }
125:            }
126:
127:            /**
128:             * Process the instruction, without returning any tail calls
129:             *
130:             * @param context The dynamic context, giving access to the current node,
131:             *                the current variables, etc.
132:             */
133:
134:            public void process(XPathContext context) throws XPathException {
135:                // To evaluate the closure in push mode, we need to use the original context of the
136:                // expression for everything except the current output destination, which is taken from the
137:                // context supplied at evaluation time
138:                if (state == EMPTY) {
139:                    return; // we know there is nothing to do
140:                }
141:                if (reservoir != null) {
142:                    SequenceIterator iter = iterate(context);
143:                    SequenceReceiver out = context.getReceiver();
144:                    while (true) {
145:                        Item it = iter.next();
146:                        if (it == null)
147:                            break;
148:                        out.append(it, 0, NodeInfo.ALL_NAMESPACES);
149:                    }
150:                } else {
151:                    Controller controller = context.getController();
152:                    XPathContext c2 = savedXPathContext.newMinorContext();
153:                    //c2.setOrigin(this);
154:                    // Fork the output: one copy goes to a SequenceOutputter which remembers the contents for
155:                    // use next time the variable is referenced; another copy goes to the current output destination.
156:                    SequenceOutputter seq = new SequenceOutputter();
157:                    seq.setPipelineConfiguration(controller
158:                            .makePipelineConfiguration());
159:                    seq.open();
160:                    TeeOutputter tee = new TeeOutputter(context.getReceiver(),
161:                            seq);
162:                    tee.setPipelineConfiguration(controller
163:                            .makePipelineConfiguration());
164:                    c2.setTemporaryReceiver(tee);
165:
166:                    expression.process(c2);
167:
168:                    seq.close();
169:                    List list = seq.getList();
170:                    if (list.size() == 0) {
171:                        state = EMPTY;
172:                    } else {
173:                        reservoir = new Item[list.size()];
174:                        reservoir = (Item[]) list.toArray(reservoir);
175:                        used = list.size();
176:                        state = ALL_READ;
177:                    }
178:                    // give unwanted stuff to the garbage collector
179:                    savedXPathContext = null;
180:                    //            inputIterator = null;
181:                    //            expression = null;
182:                }
183:
184:            }
185:
186:            /**
187:             * Get the n'th item in the sequence (starting from 0). This is defined for all
188:             * SequenceValues, but its real benefits come for a SequenceValue stored extensionally
189:             */
190:
191:            public Item itemAt(int n) throws XPathException {
192:                if (n < 0) {
193:                    return null;
194:                }
195:                if (reservoir != null && n < used) {
196:                    return reservoir[n];
197:                }
198:                if (state == ALL_READ || state == EMPTY) {
199:                    return null;
200:                }
201:                if (state == UNREAD) {
202:                    return super .itemAt(n);
203:                    // this will read from the start of the sequence
204:                }
205:                // We have read some items from the input sequence but not enough. Read as many more as are needed.
206:                int diff = n - used + 1;
207:                while (diff-- > 0) {
208:                    Item i = inputIterator.next();
209:                    if (i == null) {
210:                        state = ALL_READ;
211:                        condense();
212:                        return itemAt(n);
213:                    }
214:                    append(i);
215:                    state = MAYBE_MORE;
216:                }
217:                return reservoir[n];
218:            }
219:
220:            /**
221:             * Get the length of the sequence
222:             */
223:
224:            public int getLength() throws XPathException {
225:                if (state == ALL_READ) {
226:                    return used;
227:                } else if (state == EMPTY) {
228:                    return 0;
229:                } else {
230:                    return super .getLength();
231:                }
232:            }
233:
234:            /**
235:             * Append an item to the reservoir
236:             */
237:
238:            private void append(Item item) {
239:                if (used >= reservoir.length) {
240:                    Item[] r2 = new Item[used * 2];
241:                    System.arraycopy(reservoir, 0, r2, 0, used);
242:                    reservoir = r2;
243:                }
244:                reservoir[used++] = item;
245:            }
246:
247:            /**
248:             * Release unused space in the reservoir (provided the amount of unused space is worth reclaiming)
249:             */
250:
251:            private void condense() {
252:                if (reservoir.length - used > 30) {
253:                    Item[] r2 = new Item[used];
254:                    System.arraycopy(reservoir, 0, r2, 0, used);
255:                    reservoir = r2;
256:                }
257:                // give unwanted stuff to the garbage collector
258:                savedXPathContext = null;
259:                //        inputIterator = null;
260:                //        expression = null;
261:            }
262:
263:            /**
264:             * Determine whether the contents of the MemoClosure have been fully read
265:             */
266:
267:            public boolean isFullyRead() {
268:                return state == EMPTY || state == ALL_READ;
269:            }
270:
271:            /**
272:             * Return a value containing all the items in the sequence returned by this
273:             * SequenceIterator
274:             *
275:             * @return the corresponding value
276:             */
277:
278:            public Value materialize() throws XPathException {
279:                if (state == ALL_READ) {
280:                    return new SequenceExtent(reservoir);
281:                } else if (state == EMPTY) {
282:                    return EmptySequence.getInstance();
283:                }
284:                return new SequenceExtent(iterate(null));
285:            }
286:
287:            /**
288:             * A ProgressiveIterator starts by reading any items already held in the reservoir;
289:             * when the reservoir is exhausted, it reads further items from the inputIterator,
290:             * copying them into the reservoir as they are read.
291:             */
292:
293:            public final class ProgressiveIterator implements  SequenceIterator,
294:                    LastPositionFinder, GroundedIterator {
295:
296:                int position = -1; // zero-based position in the reservoir of the
297:
298:                // item most recently read
299:
300:                public ProgressiveIterator() {
301:
302:                }
303:
304:                public Item next() throws XPathException {
305:                    if (position == -2) { // means we've already return null once, keep doing so if called again.
306:                        return null;
307:                    }
308:                    if (++position < used) {
309:                        return reservoir[position];
310:                        //            } else if (state == ALL_READ) {
311:                        //                return null;
312:                    } else {
313:                        Item i = inputIterator.next();
314:                        if (i == null) {
315:                            state = ALL_READ;
316:                            condense();
317:                            //position--;     // leave position at last item
318:                            position = -2;
319:                            return null;
320:                        }
321:                        position = used;
322:                        append(i);
323:                        state = MAYBE_MORE;
324:                        return i;
325:                    }
326:                }
327:
328:                public Item current() {
329:                    if (position < 0) {
330:                        return null;
331:                    }
332:                    return reservoir[position];
333:                }
334:
335:                public int position() {
336:                    return position + 1; // return one-based position
337:                }
338:
339:                public SequenceIterator getAnother() {
340:                    return new ProgressiveIterator();
341:                }
342:
343:                /**
344:                 * Get the last position (that is, the number of items in the sequence)
345:                 */
346:
347:                public int getLastPosition() throws XPathException {
348:                    if (state == ALL_READ) {
349:                        return used;
350:                    } else if (state == EMPTY) {
351:                        return 0;
352:                    } else {
353:                        // save the current position
354:                        int savePos = position;
355:                        // fill the reservoir
356:                        while (true) {
357:                            Item item = next();
358:                            if (item == null) {
359:                                break;
360:                            }
361:                        }
362:                        // reset the current position
363:                        position = savePos;
364:                        // return the total number of items
365:                        return used;
366:                    }
367:                }
368:
369:                /**
370:                 * Return a value containing all the items in the sequence returned by this
371:                 * SequenceIterator
372:                 *
373:                 * @return the corresponding value
374:                 */
375:
376:                public Value materialize() throws XPathException {
377:                    if (state == ALL_READ) {
378:                        return new SequenceExtent(reservoir);
379:                    } else if (state == EMPTY) {
380:                        return EmptySequence.getInstance();
381:                    }
382:                    return new SequenceExtent(iterate(null));
383:
384:                }
385:
386:                /**
387:                 * Get properties of this iterator, as a bit-significant integer.
388:                 *
389:                 * @return the properties of this iterator. This will be some combination of
390:                 *         properties such as {@link GROUNDED} and {@link LAST_POSITION_FINDER}. It is always
391:                 *         acceptable to return the value zero, indicating that there are no known special properties.
392:                 */
393:
394:                public int getProperties() {
395:                    if (state == EMPTY || state == ALL_READ) {
396:                        return GROUNDED | LAST_POSITION_FINDER;
397:                    } else {
398:                        return 0;
399:                    }
400:                }
401:            }
402:
403:        }
404:
405:        //
406:        // The contents of this file are subject to the Mozilla Public License Version 1.0 (the "License");
407:        // you may not use this file except in compliance with the License. You may obtain a copy of the
408:        // License at http://www.mozilla.org/MPL/
409:        //
410:        // Software distributed under the License is distributed on an "AS IS" basis,
411:        // WITHOUT WARRANTY OF ANY KIND, either express or implied.
412:        // See the License for the specific language governing rights and limitations under the License.
413:        //
414:        // The Original Code is: all this file.
415:        //
416:        // The Initial Developer of the Original Code is Michael H. Kay.
417:        //
418:        // Portions created by (your name) are Copyright (C) (your legal entity). All Rights Reserved.
419:        //
420:        // Contributor(s): none.
421:        //
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