mirror of https://github.com/M66B/FairEmail.git
M66B
2 years ago
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package com.twitter.elephantbird.util;
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import java.io.IOException;
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import java.io.InputStream;
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import java.util.Arrays;
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/**
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* An efficient stream searching class based on the Knuth-Morris-Pratt algorithm.
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* For more on the algorithm works see: https://www.inf.fh-flensburg.de/lang/algorithmen/pattern/kmpen.htm.
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*/
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public class StreamSearcher {
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protected byte[] pattern_;
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protected int[] borders_;
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// An upper bound on pattern length for searching. Results are undefined for longer patterns.
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public static final int MAX_PATTERN_LENGTH = 1024;
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public StreamSearcher(byte[] pattern) {
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setPattern(pattern);
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}
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/**
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* Sets a new pattern for this StreamSearcher to use.
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* @param pattern
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* the pattern the StreamSearcher will look for in future calls to search(...)
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*/
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public void setPattern(byte[] pattern) {
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pattern_ = Arrays.copyOf(pattern, pattern.length);
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borders_ = new int[pattern_.length + 1];
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preProcess();
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}
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/**
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* Searches for the next occurrence of the pattern in the stream, starting from the current stream position. Note
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* that the position of the stream is changed. If a match is found, the stream points to the end of the match -- i.e. the
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* byte AFTER the pattern. Else, the stream is entirely consumed. The latter is because InputStream semantics make it difficult to have
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* another reasonable default, i.e. leave the stream unchanged.
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*
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* @return bytes consumed if found, -1 otherwise.
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* @throws IOException
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*/
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public long search(InputStream stream) throws IOException {
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long bytesRead = 0;
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int b;
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int j = 0;
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while ((b = stream.read()) != -1) {
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bytesRead++;
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while (j >= 0 && (byte)b != pattern_[j]) {
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j = borders_[j];
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}
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// Move to the next character in the pattern.
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++j;
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// If we've matched up to the full pattern length, we found it. Return,
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// which will automatically save our position in the InputStream at the point immediately
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// following the pattern match.
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if (j == pattern_.length) {
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return bytesRead;
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}
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}
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// No dice, Note that the stream is now completely consumed.
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return -1;
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}
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/**
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* Builds up a table of longest "borders" for each prefix of the pattern to find. This table is stored internally
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* and aids in implementation of the Knuth-Moore-Pratt string search.
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* <p>
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* For more information, see: https://www.inf.fh-flensburg.de/lang/algorithmen/pattern/kmpen.htm.
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*/
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protected void preProcess() {
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int i = 0;
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int j = -1;
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borders_[i] = j;
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while (i < pattern_.length) {
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while (j >= 0 && pattern_[i] != pattern_[j]) {
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j = borders_[j];
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}
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borders_[++i] = ++j;
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}
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}
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}
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