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<?php declare(strict_types=1);
/*
* This file is part of Composer.
*
* (c) Nils Adermann <naderman@naderman.de>
* Jordi Boggiano <j.boggiano@seld.be>
*
* For the full copyright and license information, please view the LICENSE
* file that was distributed with this source code.
*/
namespace Composer\DependencyResolver;
/**
* The RuleWatchGraph efficiently propagates decisions to other rules
*
* All rules generated for solving a SAT problem should be inserted into the
* graph. When a decision on a literal is made, the graph can be used to
* propagate the decision to all other rules involving the literal, leading to
* other trivial decisions resulting from unit clauses.
*
* @author Nils Adermann <naderman@naderman.de>
*/
class RuleWatchGraph
{
/** @var array<int, RuleWatchChain> */
protected $watchChains = [];
/**
* Inserts a rule node into the appropriate chains within the graph
*
* The node is prepended to the watch chains for each of the two literals it
* watches.
*
* Assertions are skipped because they only depend on a single package and
* have no alternative literal that could be true, so there is no need to
* watch changes in any literals.
*
* @param RuleWatchNode $node The rule node to be inserted into the graph
*/
public function insert(RuleWatchNode $node): void
{
if ($node->getRule()->isAssertion()) {
return;
}
if (!$node->getRule() instanceof MultiConflictRule) {
foreach ([$node->watch1, $node->watch2] as $literal) {
if (!isset($this->watchChains[$literal])) {
$this->watchChains[$literal] = new RuleWatchChain;
}
$this->watchChains[$literal]->unshift($node);
}
} else {
foreach ($node->getRule()->getLiterals() as $literal) {
if (!isset($this->watchChains[$literal])) {
$this->watchChains[$literal] = new RuleWatchChain;
}
$this->watchChains[$literal]->unshift($node);
}
}
}
/**
* Propagates a decision on a literal to all rules watching the literal
*
* If a decision, e.g. +A has been made, then all rules containing -A, e.g.
* (-A|+B|+C) now need to satisfy at least one of the other literals, so
* that the rule as a whole becomes true, since with +A applied the rule
* is now (false|+B|+C) so essentially (+B|+C).
*
* This means that all rules watching the literal -A need to be updated to
* watch 2 other literals which can still be satisfied instead. So literals
* that conflict with previously made decisions are not an option.
*
* Alternatively it can occur that a unit clause results: e.g. if in the
* above example the rule was (-A|+B), then A turning true means that
* B must now be decided true as well.
*
* @param int $decidedLiteral The literal which was decided (A in our example)
* @param int $level The level at which the decision took place and at which
* all resulting decisions should be made.
* @param Decisions $decisions Used to check previous decisions and to
* register decisions resulting from propagation
* @return Rule|null If a conflict is found the conflicting rule is returned
*/
public function propagateLiteral(int $decidedLiteral, int $level, Decisions $decisions): ?Rule
{
// we invert the decided literal here, example:
// A was decided => (-A|B) now requires B to be true, so we look for
// rules which are fulfilled by -A, rather than A.
$literal = -$decidedLiteral;
if (!isset($this->watchChains[$literal])) {
return null;
}
$chain = $this->watchChains[$literal];
$chain->rewind();
while ($chain->valid()) {
$node = $chain->current();
if (!$node->getRule() instanceof MultiConflictRule) {
$otherWatch = $node->getOtherWatch($literal);
if (!$node->getRule()->isDisabled() && !$decisions->satisfy($otherWatch)) {
$ruleLiterals = $node->getRule()->getLiterals();
$alternativeLiterals = array_filter($ruleLiterals, static function ($ruleLiteral) use ($literal, $otherWatch, $decisions): bool {
return $literal !== $ruleLiteral &&
$otherWatch !== $ruleLiteral &&
!$decisions->conflict($ruleLiteral);
});
if (\count($alternativeLiterals) > 0) {
reset($alternativeLiterals);
$this->moveWatch($literal, current($alternativeLiterals), $node);
continue;
}
if ($decisions->conflict($otherWatch)) {
return $node->getRule();
}
$decisions->decide($otherWatch, $level, $node->getRule());
}
} else {
foreach ($node->getRule()->getLiterals() as $otherLiteral) {
if ($literal !== $otherLiteral && !$decisions->satisfy($otherLiteral)) {
if ($decisions->conflict($otherLiteral)) {
return $node->getRule();
}
$decisions->decide($otherLiteral, $level, $node->getRule());
}
}
}
$chain->next();
}
return null;
}
/**
* Moves a rule node from one watch chain to another
*
* The rule node's watched literals are updated accordingly.
*
* @param int $fromLiteral A literal the node used to watch
* @param int $toLiteral A literal the node should watch now
* @param RuleWatchNode $node The rule node to be moved
*/
protected function moveWatch(int $fromLiteral, int $toLiteral, RuleWatchNode $node): void
{
if (!isset($this->watchChains[$toLiteral])) {
$this->watchChains[$toLiteral] = new RuleWatchChain;
}
$node->moveWatch($fromLiteral, $toLiteral);
$this->watchChains[$fromLiteral]->remove();
$this->watchChains[$toLiteral]->unshift($node);
}
}