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| <h2 title="Class CompressedRecordArray" class="title">Class CompressedRecordArray</h2> |
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| <pre>public final class <span class="typeNameLabel">CompressedRecordArray</span> |
| extends <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true" title="class or interface in java.lang" class="externalLink" target="_blank">Object</a> |
| implements <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/Serializable.html?is-external=true" title="class or interface in java.io" class="externalLink" target="_blank">Serializable</a></pre> |
| <div class="block">This class represents an array of records which are stored in a compressed format whilst |
| still allowing random access to them. Each record in turn is simply an array of ints. Each |
| record must be the same length. To implement this we divide the array of records up into |
| blocks. There is an index and a bit stream. The index gives the start of each block in the |
| bit stream. Each block contains a set of records stored in an encoded format. A header at |
| the beginning defines the encoding used. The encoding is chosen dynamically to give the |
| best compression. Deltas (ie the differences between values in adjacent records) are stored |
| rather than the values themselves which gives good results for certain types of data. |
| The number of records per block is |
| configurable and there is a space/time trade-off to be made because a large number of |
| records per block will give better compression at the cost of more time to extract each |
| record (because you have to start at the beginning of the block and then uncompress each |
| record in turn until you reach the one you want). |
| <p> |
| I wrote a test to measure the performance on some real life data |
| (in fact this data is the reason I wrote this class in the first place). The data |
| consists of a file containing z/OS fpos_t objects obtained by calling fgetpos |
| sequentially for every block (4060 bytes) in an svcdump. Each fpos_t object is |
| actually an array of 8 ints containing obscure info about the disk geometry or |
| something, but the important thing is that it changes in a reasonably regular |
| fashion and so is a good candidate for compression via deltas. The original file |
| had a length of 3401088. Here are the results which suggest that a block size of |
| 32 (log2 of 5) is a good choice (the time is that taken to write the data and then read it |
| back again to check): |
| <p> |
| <table border="1"><tr><th>log2</th><th>block size</th><th>memory usage</th><th>time (ms)</th></tr> |
| <tr><td>0</td><td>1</td><td>4191388</td><td>782</td></tr> |
| <tr><td>1</td><td>2</td><td>2706992</td><td>691</td></tr> |
| <tr><td>2</td><td>4</td><td>1217920</td><td>621</td></tr> |
| <tr><td>3</td><td>8</td><td>790472</td><td>620</td></tr> |
| <tr><td>4</td><td>16</td><td>516772</td><td>721</td></tr> |
| <tr><td>5</td><td>32</td><td>340448</td><td>942</td></tr> |
| <tr><td>6</td><td>64</td><td>334304</td><td>1362</td></tr> |
| <tr><td>7</td><td>128</td><td>334304</td><td>2223</td></tr> |
| <tr><td>8</td><td>256</td><td>340448</td><td>3966</td></tr> |
| <tr><td>9</td><td>512</td><td>355808</td><td>7470</td></tr> |
| </table> |
| <p></div> |
| <dl> |
| <dt><span class="seeLabel">See Also:</span></dt> |
| <dd><a href="../../../../../../../../serialized-form.html#com.ibm.j9ddr.corereaders.tdump.zebedee.util.CompressedRecordArray">Serialized Form</a></dd> |
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| <th class="colConstructorName" scope="row"><code><span class="memberNameLink"><a href="#%3Cinit%3E(int,int)">CompressedRecordArray</a></span>​(int blockSizeLog2, |
| int recordSize)</code></th> |
| <td class="colLast"> |
| <div class="block">Create a new CompressedRecordArray.</div> |
| </td> |
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| <td class="colFirst"><code>void</code></td> |
| <th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#add(int%5B%5D)">add</a></span>​(int[] record)</code></th> |
| <td class="colLast"> |
| <div class="block">Add a new record.</div> |
| </td> |
| </tr> |
| <tr id="i1" class="rowColor"> |
| <td class="colFirst"><code>void</code></td> |
| <th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#close()">close</a></span>()</code></th> |
| <td class="colLast"> |
| <div class="block">Close this CompressedRecordArray.</div> |
| </td> |
| </tr> |
| <tr id="i2" class="altColor"> |
| <td class="colFirst"><code>void</code></td> |
| <th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#get(int,int%5B%5D)">get</a></span>​(int recordNumber, |
| int[] record)</code></th> |
| <td class="colLast"> |
| <div class="block">Get the given record number.</div> |
| </td> |
| </tr> |
| <tr id="i3" class="rowColor"> |
| <td class="colFirst"><code>static void</code></td> |
| <th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#main(java.lang.String%5B%5D)">main</a></span>​(<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html?is-external=true" title="class or interface in java.lang" class="externalLink" target="_blank">String</a>[] args)</code></th> |
| <td class="colLast"> |
| <div class="block">This method is provided to test the CompressedRecordArray.</div> |
| </td> |
| </tr> |
| <tr id="i4" class="altColor"> |
| <td class="colFirst"><code>int</code></td> |
| <th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#memoryUsage()">memoryUsage</a></span>()</code></th> |
| <td class="colLast"> |
| <div class="block">Give a rough estimate of how many bytes of storage we use.</div> |
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| <code><a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#clone()" title="class or interface in java.lang" class="externalLink" target="_blank">clone</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#equals(java.lang.Object)" title="class or interface in java.lang" class="externalLink" target="_blank">equals</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#finalize()" title="class or interface in java.lang" class="externalLink" target="_blank">finalize</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#getClass()" title="class or interface in java.lang" class="externalLink" target="_blank">getClass</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#hashCode()" title="class or interface in java.lang" class="externalLink" target="_blank">hashCode</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#notify()" title="class or interface in java.lang" class="externalLink" target="_blank">notify</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#notifyAll()" title="class or interface in java.lang" class="externalLink" target="_blank">notifyAll</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#toString()" title="class or interface in java.lang" class="externalLink" target="_blank">toString</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#wait()" title="class or interface in java.lang" class="externalLink" target="_blank">wait</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#wait(long)" title="class or interface in java.lang" class="externalLink" target="_blank">wait</a>, <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html?is-external=true#wait(long,int)" title="class or interface in java.lang" class="externalLink" target="_blank">wait</a></code></li> |
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| <h4>CompressedRecordArray</h4> |
| <pre>public CompressedRecordArray​(int blockSizeLog2, |
| int recordSize)</pre> |
| <div class="block">Create a new CompressedRecordArray. A size of 5 for blockSizeLog2 gives good results.</div> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>blockSizeLog2</code> - the number of records in each block expressed as a power of 2</dd> |
| <dd><code>recordSize</code> - the number of ints in each record</dd> |
| </dl> |
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| <pre class="methodSignature">public void add​(int[] record)</pre> |
| <div class="block">Add a new record. Data is copied from the given array.</div> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>record</code> - an array of ints which forms the record to be added</dd> |
| </dl> |
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| <h4>close</h4> |
| <pre class="methodSignature">public void close()</pre> |
| <div class="block">Close this CompressedRecordArray. This must be called before any reading is done |
| and no more records may be added afterwards.</div> |
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| <pre class="methodSignature">public void get​(int recordNumber, |
| int[] record)</pre> |
| <div class="block">Get the given record number. To save on GC overhead the user supplies the |
| int array to copy the record into.</div> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>recordNumber</code> - the sequential number of the record to read</dd> |
| <dd><code>record</code> - the array to copy the record into</dd> |
| </dl> |
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| <pre class="methodSignature">public int memoryUsage()</pre> |
| <div class="block">Give a rough estimate of how many bytes of storage we use. This is the actual storage |
| allocated so may be more that what is in use at any one time.</div> |
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| <pre class="methodSignature">public static void main​(<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html?is-external=true" title="class or interface in java.lang" class="externalLink" target="_blank">String</a>[] args) |
| throws <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Exception.html?is-external=true" title="class or interface in java.lang" class="externalLink" target="_blank">Exception</a></pre> |
| <div class="block">This method is provided to test the CompressedRecordArray. We need to change this to use |
| unit tests at some point.</div> |
| <dl> |
| <dt><span class="throwsLabel">Throws:</span></dt> |
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