001/*
002 *  Licensed to the Apache Software Foundation (ASF) under one or more
003 *  contributor license agreements.  See the NOTICE file distributed with
004 *  this work for additional information regarding copyright ownership.
005 *  The ASF licenses this file to You under the Apache License, Version 2.0
006 *  (the "License"); you may not use this file except in compliance with
007 *  the License.  You may obtain a copy of the License at
008 *
009 *     http://www.apache.org/licenses/LICENSE-2.0
010 *
011 *  Unless required by applicable law or agreed to in writing, software
012 *  distributed under the License is distributed on an "AS IS" BASIS,
013 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 *  See the License for the specific language governing permissions and
015 *  limitations under the License.
016 */
017package org.apache.commons.compress.harmony.pack200;
018
019import java.io.EOFException;
020import java.io.IOException;
021import java.io.InputStream;
022import java.util.ArrayList;
023import java.util.Arrays;
024import java.util.List;
025
026import org.apache.commons.compress.utils.ExactMath;
027
028/**
029 * A BHSD codec is a means of encoding integer values as a sequence of bytes or vice versa using a specified "BHSD"
030 * encoding mechanism. It uses a variable-length encoding and a modified sign representation such that small numbers are
031 * represented as a single byte, whilst larger numbers take more bytes to encode. The number may be signed or unsigned;
032 * if it is unsigned, it can be weighted towards positive numbers or equally distributed using a one's complement. The
033 * Codec also supports delta coding, where a sequence of numbers is represented as a series of first-order differences.
034 * So a delta encoding of the integers [1..10] would be represented as a sequence of 10x1s. This allows the absolute
035 * value of a coded integer to fall outside of the 'small number' range, whilst still being encoded as a single byte.
036 *
037 * A BHSD codec is configured with four parameters:
038 * <dl>
039 * <dt>B</dt>
040 * <dd>The maximum number of bytes that each value is encoded as. B must be a value between [1..5]. For a pass-through
041 * coding (where each byte is encoded as itself, aka {@link #BYTE1}, B is 1 (each byte takes a maximum of 1 byte).</dd>
042 * <dt>H</dt>
043 * <dd>The radix of the integer. Values are defined as a sequence of values, where value {@code n} is multiplied by
044 * {@code H^<sup>n</sup>}. So the number 1234 may be represented as the sequence 4 3 2 1 with a radix (H) of 10.
045 * Note that other permutations are also possible; 43 2 1 will also encode 1234. The co-parameter L is defined as 256-H.
046 * This is important because only the last value in a sequence may be &lt; L; all prior values must be &gt; L.</dd>
047 * <dt>S</dt>
048 * <dd>Whether the codec represents signed values (or not). This may have 3 values; 0 (unsigned), 1 (signed, one's
049 * complement) or 2 (signed, two's complement)</dd>
050 * <dt>D</dt>
051 * <dd>Whether the codec represents a delta encoding. This may be 0 (no delta) or 1 (delta encoding). A delta encoding
052 * of 1 indicates that values are cumulative; a sequence of {@code 1 1 1 1 1} will represent the sequence
053 * {@code 1 2 3 4 5}. For this reason, the codec supports two variants of decode; one
054 * {@link #decode(InputStream, long) with} and one {@link #decode(InputStream) without} a {@code last} parameter.
055 * If the codec is a non-delta encoding, then the value is ignored if passed. If the codec is a delta encoding, it is a
056 * run-time error to call the value without the extra parameter, and the previous value should be returned. (It was
057 * designed this way to support multi-threaded access without requiring a new instance of the Codec to be cloned for
058 * each use.)
059 * </dd>
060 * </dl>
061 *
062 * Codecs are notated as (B,H,S,D) and either D or S,D may be omitted if zero. Thus {@link #BYTE1} is denoted
063 * (1,256,0,0) or (1,256). The {@link #toString()} method prints out the condensed form of the encoding. Often, the last
064 * character in the name ({@link #BYTE1}, {@link #UNSIGNED5}) gives a clue as to the B value. Those that start with U
065 * ({@link #UDELTA5}, {@link #UNSIGNED5}) are unsigned; otherwise, in most cases, they are signed. The presence of the
066 * word Delta ({@link #DELTA5}, {@link #UDELTA5}) indicates a delta encoding is used.
067 */
068public final class BHSDCodec extends Codec {
069
070    /**
071     * The maximum number of bytes in each coding word
072     */
073    private final int b;
074
075    /**
076     * Whether delta encoding is used (0=false,1=true)
077     */
078    private final int d;
079
080    /**
081     * The radix of the encoding
082     */
083    private final int h;
084
085    /**
086     * The co-parameter of h; 256-h
087     */
088    private final int l;
089
090    /**
091     * Represents signed numbers or not (0=unsigned,1/2=signed)
092     */
093    private final int s;
094
095    private long cardinality;
096
097    private final long smallest;
098
099    private final long largest;
100
101    /**
102     * radix^i powers
103     */
104    private final long[] powers;
105
106    /**
107     * Constructs an unsigned, non-delta Codec with the given B and H values.
108     *
109     * @param b the maximum number of bytes that a value can be encoded as [1..5]
110     * @param h the radix of the encoding [1..256]
111     */
112    public BHSDCodec(final int b, final int h) {
113        this(b, h, 0, 0);
114    }
115
116    /**
117     * Constructs a non-delta Codec with the given B, H and S values.
118     *
119     * @param b the maximum number of bytes that a value can be encoded as [1..5]
120     * @param h the radix of the encoding [1..256]
121     * @param s whether the encoding represents signed numbers (s=0 is unsigned; s=1 is signed with 1s complement; s=2
122     *        is signed with ?)
123     */
124    public BHSDCodec(final int b, final int h, final int s) {
125        this(b, h, s, 0);
126    }
127
128    /**
129     * Constructs a Codec with the given B, H, S and D values.
130     *
131     * @param b the maximum number of bytes that a value can be encoded as [1..5]
132     * @param h the radix of the encoding [1..256]
133     * @param s whether the encoding represents signed numbers (s=0 is unsigned; s=1 is signed with 1s complement; s=2
134     *        is signed with ?)
135     * @param d whether this is a delta encoding (d=0 is non-delta; d=1 is delta)
136     */
137    public BHSDCodec(final int b, final int h, final int s, final int d) {
138        if (b < 1 || b > 5) {
139            throw new IllegalArgumentException("1<=b<=5");
140        }
141        if (h < 1 || h > 256) {
142            throw new IllegalArgumentException("1<=h<=256");
143        }
144        if (s < 0 || s > 2) {
145            throw new IllegalArgumentException("0<=s<=2");
146        }
147        if (d < 0 || d > 1) {
148            throw new IllegalArgumentException("0<=d<=1");
149        }
150        if (b == 1 && h != 256) {
151            throw new IllegalArgumentException("b=1 -> h=256");
152        }
153        if (h == 256 && b == 5) {
154            throw new IllegalArgumentException("h=256 -> b!=5");
155        }
156        this.b = b;
157        this.h = h;
158        this.s = s;
159        this.d = d;
160        this.l = 256 - h;
161        if (h == 1) {
162            cardinality = b * 255 + 1;
163        } else {
164            cardinality = (long) ((long) (l * (1 - Math.pow(h, b)) / (1 - h)) + Math.pow(h, b));
165        }
166        smallest = calculateSmallest();
167        largest = calculateLargest();
168
169        powers = new long[b];
170        Arrays.setAll(powers, c -> (long) Math.pow(h, c));
171    }
172
173    private long calculateLargest() {
174        long result;
175        // TODO This can probably be optimized into a better mathematical
176        // statement
177        if (d == 1) {
178            final BHSDCodec bh0 = new BHSDCodec(b, h);
179            return bh0.largest();
180        }
181        switch (s) {
182        case 0:
183            result = cardinality() - 1;
184            break;
185        case 1:
186            result = cardinality() / 2 - 1;
187            break;
188        case 2:
189            result = (3L * cardinality()) / 4 - 1;
190            break;
191        default:
192            throw new Error("Unknown s value");
193        }
194        return Math.min((s == 0 ? ((long) Integer.MAX_VALUE) << 1 : Integer.MAX_VALUE) - 1, result);
195    }
196
197    private long calculateSmallest() {
198        long result;
199        if (d == 1 || !isSigned()) {
200            if (cardinality >= 4294967296L) { // 2^32
201                result = Integer.MIN_VALUE;
202            } else {
203                result = 0;
204            }
205        } else {
206            result = Math.max(Integer.MIN_VALUE, -cardinality() / (1 << s));
207        }
208        return result;
209    }
210
211    /**
212     * Returns the cardinality of this codec; that is, the number of distinct values that it can contain.
213     *
214     * @return the cardinality of this codec
215     */
216    public long cardinality() {
217        return cardinality;
218    }
219
220    @Override
221    public int decode(final InputStream in) throws IOException, Pack200Exception {
222        if (d != 0) {
223            throw new Pack200Exception("Delta encoding used without passing in last value; this is a coding error");
224        }
225        return decode(in, 0);
226    }
227
228    @Override
229    public int decode(final InputStream in, final long last) throws IOException, Pack200Exception {
230        int n = 0;
231        long z = 0;
232        long x = 0;
233
234        do {
235            x = in.read();
236            lastBandLength++;
237            z += x * powers[n];
238            n++;
239        } while (x >= l && n < b);
240
241        if (x == -1) {
242            throw new EOFException("End of stream reached whilst decoding");
243        }
244
245        if (isSigned()) {
246            final int u = ((1 << s) - 1);
247            if ((z & u) == u) {
248                z = z >>> s ^ -1L;
249            } else {
250                z = z - (z >>> s);
251            }
252        }
253        // This algorithm does the same thing, but is probably slower. Leaving
254        // in for now for readability
255        // if (isSigned()) {
256        // long u = z;
257        // long twoPowS = (long)Math.pow(2, s);
258        // double twoPowSMinusOne = twoPowS-1;
259        // if (u % twoPowS < twoPowSMinusOne) {
260        // if (cardinality < Math.pow(2, 32)) {
261        // z = (long) (u - (Math.floor(u/ twoPowS)));
262        // } else {
263        // z = cast32((long) (u - (Math.floor(u/ twoPowS))));
264        // }
265        // } else {
266        // z = (long) (-Math.floor(u/ twoPowS) - 1);
267        // }
268        // }
269        if (isDelta()) {
270            z += last;
271        }
272        return (int) z;
273    }
274
275    // private long cast32(long u) {
276    // u = (long) ((long) ((u + Math.pow(2, 31)) % Math.pow(2, 32)) -
277    // Math.pow(2, 31));
278    // return u;
279    // }
280
281    @Override
282    public int[] decodeInts(final int n, final InputStream in) throws IOException, Pack200Exception {
283        final int[] band = super.decodeInts(n, in);
284        if (isDelta()) {
285            for (int i = 0; i < band.length; i++) {
286                while (band[i] > largest) {
287                    band[i] -= cardinality;
288                }
289                while (band[i] < smallest) {
290                    band[i] = ExactMath.add(band[i], cardinality);
291                }
292            }
293        }
294        return band;
295    }
296
297    @Override
298    public int[] decodeInts(final int n, final InputStream in, final int firstValue)
299        throws IOException, Pack200Exception {
300        final int[] band = super.decodeInts(n, in, firstValue);
301        if (isDelta()) {
302            for (int i = 0; i < band.length; i++) {
303                while (band[i] > largest) {
304                    band[i] -= cardinality;
305                }
306                while (band[i] < smallest) {
307                    band[i] = ExactMath.add(band[i], cardinality);
308                }
309            }
310        }
311        return band;
312    }
313
314    @Override
315    public byte[] encode(final int value) throws Pack200Exception {
316        return encode(value, 0);
317    }
318
319    @Override
320    public byte[] encode(final int value, final int last) throws Pack200Exception {
321        if (!encodes(value)) {
322            throw new Pack200Exception("The codec " + this + " does not encode the value " + value);
323        }
324
325        long z = value;
326        if (isDelta()) {
327            z -= last;
328        }
329        if (isSigned()) {
330            if (z < Integer.MIN_VALUE) {
331                z += 4294967296L;
332            } else if (z > Integer.MAX_VALUE) {
333                z -= 4294967296L;
334            }
335            if (z < 0) {
336                z = (-z << s) - 1;
337            } else if (s == 1) {
338                z = z << s;
339            } else {
340                z += (z - z % 3) / 3;
341            }
342        } else if (z < 0) {
343            // Need to use integer overflow here to represent negatives.
344            // 4294967296L is the 1 << 32.
345            z += Math.min(cardinality, 4294967296L);
346        }
347        if (z < 0) {
348            throw new Pack200Exception("unable to encode");
349        }
350
351        final List<Byte> byteList = new ArrayList<>();
352        for (int n = 0; n < b; n++) {
353            long byteN;
354            if (z < l) {
355                byteN = z;
356            } else {
357                byteN = z % h;
358                while (byteN < l) {
359                    byteN += h;
360                }
361            }
362            byteList.add(Byte.valueOf((byte) byteN));
363            if (byteN < l) {
364                break;
365            }
366            z -= byteN;
367            z /= h;
368        }
369        final byte[] bytes = new byte[byteList.size()];
370        for (int i = 0; i < bytes.length; i++) {
371            bytes[i] = byteList.get(i).byteValue();
372        }
373        return bytes;
374    }
375
376    /**
377     * True if this encoding can code the given value
378     *
379     * @param value the value to check
380     * @return {@code true} if the encoding can encode this value
381     */
382    public boolean encodes(final long value) {
383        return value >= smallest && value <= largest;
384    }
385
386    @Override
387    public boolean equals(final Object o) {
388        if (o instanceof BHSDCodec) {
389            final BHSDCodec codec = (BHSDCodec) o;
390            return codec.b == b && codec.h == h && codec.s == s && codec.d == d;
391        }
392        return false;
393    }
394
395    /**
396     * @return the b
397     */
398    public int getB() {
399        return b;
400    }
401
402    /**
403     * @return the h
404     */
405    public int getH() {
406        return h;
407    }
408
409    /**
410     * @return the l
411     */
412    public int getL() {
413        return l;
414    }
415
416    /**
417     * @return the s
418     */
419    public int getS() {
420        return s;
421    }
422
423    @Override
424    public int hashCode() {
425        return ((b * 37 + h) * 37 + s) * 37 + d;
426    }
427
428    /**
429     * Returns true if this codec is a delta codec
430     *
431     * @return true if this codec is a delta codec
432     */
433    public boolean isDelta() {
434        return d != 0;
435    }
436
437    /**
438     * Returns true if this codec is a signed codec
439     *
440     * @return true if this codec is a signed codec
441     */
442    public boolean isSigned() {
443        return s != 0;
444    }
445
446    /**
447     * Returns the largest value that this codec can represent.
448     *
449     * @return the largest value that this codec can represent.
450     */
451    public long largest() {
452        return largest;
453    }
454
455    /**
456     * Returns the smallest value that this codec can represent.
457     *
458     * @return the smallest value that this codec can represent.
459     */
460    public long smallest() {
461        return smallest;
462    }
463
464    /**
465     * Returns the codec in the form (1,256) or (1,64,1,1). Note that trailing zero fields are not shown.
466     */
467    @Override
468    public String toString() {
469        final StringBuilder buffer = new StringBuilder(11);
470        buffer.append('(');
471        buffer.append(b);
472        buffer.append(',');
473        buffer.append(h);
474        if (s != 0 || d != 0) {
475            buffer.append(',');
476            buffer.append(s);
477        }
478        if (d != 0) {
479            buffer.append(',');
480            buffer.append(d);
481        }
482        buffer.append(')');
483        return buffer.toString();
484    }
485}