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edb48ffec2
If we want to match greedy or reluctant regular expressions, we have to make sure that certain threads are split off with a higher priority than others. We will use the ThreadQueues' natural order as priority order: high to low. To support splitting into different-priority threads, let's introduce a second SPLIT opcode: SPLIT_JMP. The latter prefers to jump while the former prefers to execute the opcode directly after the SPLIT opcode. There is a subtle challenge here, though: let's assume that there are two current threads and the higher-priority one wants to jump where the lower-priority one is already. In the PikeVM implementation before this change, queueImmediately() would see that there is already a thread queued for that program counter and *not* queue the higher-priority one. Example: when matching the pattern '(a?)(a??)(a?)' against the string 'aa', after the first character, the first (high priority) thread will have matched the first group while the second thread matched the second group. In the following step, therefore, the first thread will want to SPLIT_JMP to match the final 'a' to the third group but the second thread already queued that program counter. The proposed solution is to introduce a third thread queue: 'queued'. When queuing threads to be executed after reading the next character from the string to match, they are not directly queued into 'next' but into 'queued'. Every thread requiring immediate execution (i.e. before reading the next character) will be queued into 'current'. Whenever 'current' is drained, the next thread from 'queued' that has not been queued to 'current' yet will be executed. That way, we can guarantee that 1) no lower-priority thread can override a higher-priority thread and 2) infinite loop are prevented. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
31 lines
733 B
Java
31 lines
733 B
Java
/* Copyright (c) 2008-2013, Avian Contributors
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Permission to use, copy, modify, and/or distribute this software
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for any purpose with or without fee is hereby granted, provided
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that the above copyright notice and this permission notice appear
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in all copies.
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There is NO WARRANTY for this software. See license.txt for
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details. */
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package regex;
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/**
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* Opcodes for the Pike VM.
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* <p>
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* See {@link PikeVM}.
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* </p>
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*
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* @author Johannes Schindelin
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*/
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interface PikeVMOpcodes {
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final static int DOT = -1;
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final static int DOTALL = -2;
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final static int SAVE_OFFSET = -40;
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final static int SPLIT = -50;
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final static int SPLIT_JMP = -51; // this split prefers to jump
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final static int JMP = -52;
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}
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