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Java > Open Source Codes > com > go > trove > io > PushbackPositionReader


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52
53 package com.go.trove.io;
54
55 import java.io.*;
56
57 /******************************************************************************
58  * The PushbackPositionReader is a kind of pushback reader that tracks the
59  * postion in the stream of the next character to be read.
60  * The java.io.PushbackReader allows arbitrary characters
61  * to be pushed back. Since this Reader may need to keep track of how many
62  * characters were scanned from the underlying Reader to actually produce
63  * a character, the unread operation cannot accept any arbitrary character.
64  *
65  * @author Brian S O'Neill
66  * @version
67  * <!--$$Revision:--> 12 <!-- $-->, <!--$$JustDate:--> 12/11/00 <!-- $-->
68  * @see java.io.PushbackReader
69  */
70 public class PushbackPositionReader extends PositionReader {
71     /** Maximum pushback allowed. */
72     private int mMaxPushback;
73
74     /** Most recently read characters with escapes already processed. */
75     private int[] mCharacters;
76
77     /** The scan lengths of the most recently read characters. */
78     private int[] mPositions;
79
80     /** The cursor marks the position in the arrays of the last character. */
81     private int mCursor;
82
83     /** Amount of characters currently pushed back. */
84     private int mPushback;
85
86     public PushbackPositionReader(Reader reader) {
87         this(reader, 0);
88     }
89
90     public PushbackPositionReader(Reader reader, int pushback) {
91         super(reader);
92
93         // Two more are required for correct operation
94 pushback += 2;
95
96         mMaxPushback = pushback;
97         mCharacters = new int[pushback];
98         mPositions = new int[pushback];
99         mCursor = 0;
100         mPushback = 0;
101     }
102
103     /**
104      * @return the start position of the last read character.
105      */
106     public int getStartPosition() {
107         int back = mCursor - 2;
108         if (back < 0) back += mMaxPushback;
109
110         return mPositions[back];
111     }
112
113     public int read() throws IOException {
114         int c;
115
116         if (mPushback > 0) {
117             mPushback--;
118
119             mPosition = mPositions[mCursor];
120             c = mCharacters[mCursor++];
121             if (mCursor >= mMaxPushback) mCursor = 0;
122             
123             return c;
124         }
125
126         c = super.read();
127         
128         mPositions[mCursor] = mPosition;
129         mCharacters[mCursor++] = c;
130         if (mCursor >= mMaxPushback) mCursor = 0;
131
132         return c;
133     }
134
135     public int peek() throws IOException {
136         int c = read();
137         unread();
138         return c;
139     }
140
141     /**
142      * Unread the last several characters read.
143      *
144      * <p>Unlike PushbackReader, unread does not allow arbitrary characters to
145      * to be unread. Rather, it functions like an undo operation.
146      *
147      * @param amount Amount of characters to unread.
148      * @see java.io.PushbackReader#unread(int)
149      */
150     public void unread(int amount) throws IOException {
151         for (int i=0; i<amount; i++) {
152             unread();
153         }
154     }
155     
156     /**
157      * Unread the last character read.
158      *
159      * <p>Unlike PushbackReader, unread does not allow arbitrary characters to
160      * to be unread. Rather, it functions like an undo operation.
161      *
162      * @see java.io.PushbackReader#unread(int)
163      */
164     public void unread() throws IOException {
165         mPushback++;
166
167         if (mPushback > mMaxPushback - 2) {
168             throw new IOException(this.getClass().getName() +
169                                   ": pushback exceeded " + (mMaxPushback - 2));
170         }
171
172         if ((--mCursor) < 0) mCursor += mMaxPushback;
173
174         if (mCursor > 0) {
175             mPosition = mPositions[mCursor - 1];
176         }
177         else {
178             mPosition = mPositions[mMaxPushback - 1];
179         }
180
181         unreadHook(mCharacters[mCursor]);
182     }
183
184     /**
185      * A hook call from the unread method(s). Every unread character is
186      * passed to this method.
187      */
188     protected void unreadHook(int c) {
189     }
190 }
191
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