Implemented PoolOptimizer

.github/workflows/continuous-integration.yml

@@ -11,7 +11,6 @@ on:
 env:
   COMPOSER_FLAGS: "--ansi --no-interaction --no-progress --prefer-dist"
   COMPOSER_UPDATE_FLAGS: ""
-  COMPOSER_TESTS_ARE_RUNNING: "1"
 
 jobs:
   tests:

doc/articles/troubleshooting.md

@@ -353,3 +353,27 @@ See also https://github.com/composer/composer/issues/4180 for more information.
 Composer can unpack zipballs using either a system-provided `unzip` or `7z` (7-Zip) utility, or PHP's
 native `ZipArchive` class. On OSes where ZIP files can contain permissions and symlinks, we recommend
 installing `unzip` or `7z` as these features are not supported by `ZipArchive`.
+
+
+## Disabling the pool optimizer
+
+In Composer, the `Pool` class contains all the packages that are relevant for the dependency
+resolving process. That is what is used to generate all the rules which are then
+passed on to the dependency solver.
+In order to improve performance, Composer tries to optimize this `Pool` by removing useless
+package information early on.
+
+If all goes well, you should never notice any issues with it but in case you run into
+an unexpected result such as an unresolvable set of dependencies or conflicts where you
+think Composer is wrong, you might want to disable the optimizer by using the environment
+variable `COMPOSER_POOL_OPTIMIZER` and run the update again like so:
+
+```bash
+COMPOSER_POOL_OPTIMIZER=0 php composer.phar update
+```
+
+Now double check if the result is still the same. It will take significantly longer and use
+a lot more memory to run the dependency resolving process.
+
+If the result is different, you likely hit a problem in the pool optimizer.
+Please [report this issue](https://github.com/composer/composer/issues) so it can be fixed.

src/Composer/DependencyResolver/Pool.php

@@ -221,6 +221,14 @@ class Pool implements \Countable
         return \in_array($package, $this->unacceptableFixedOrLockedPackages, true);
     }
 
+    /**
+     * @return BasePackage[]
+     */
+    public function getUnacceptableFixedOrLockedPackages()
+    {
+        return $this->unacceptableFixedOrLockedPackages;
+    }
+
     public function __toString()
     {
         $str = "Pool:\n";

src/Composer/DependencyResolver/PoolBuilder.php

@@ -61,6 +61,10 @@ class PoolBuilder
      * @var ?EventDispatcher
      */
     private $eventDispatcher;
+    /**
+     * @var PoolOptimizer|null
+     */
+    private $poolOptimizer;
     /**
      * @var IOInterface
      */
@@ -128,13 +132,14 @@ class PoolBuilder
      * @param string[] $rootReferences an array of package name => source reference
      * @phpstan-param array<string, string> $rootReferences
      */
-    public function __construct(array $acceptableStabilities, array $stabilityFlags, array $rootAliases, array $rootReferences, IOInterface $io, EventDispatcher $eventDispatcher = null)
+    public function __construct(array $acceptableStabilities, array $stabilityFlags, array $rootAliases, array $rootReferences, IOInterface $io, EventDispatcher $eventDispatcher = null, PoolOptimizer $poolOptimizer = null)
     {
         $this->acceptableStabilities = $acceptableStabilities;
         $this->stabilityFlags = $stabilityFlags;
         $this->rootAliases = $rootAliases;
         $this->rootReferences = $rootReferences;
         $this->eventDispatcher = $eventDispatcher;
+        $this->poolOptimizer = $poolOptimizer;
         $this->io = $io;
     }
 
@@ -259,6 +264,8 @@ class PoolBuilder
         $this->skippedLoad = array();
         $this->indexCounter = 0;
 
+        $pool = $this->runOptimizer($request, $pool);
+
         Intervals::clear();
 
         return $pool;
@@ -572,4 +579,33 @@ class PoolBuilder
             unset($this->aliasMap[spl_object_hash($package)]);
         }
     }
+
+    /**
+     * @return Pool
+     */
+    private function runOptimizer(Request $request, Pool $pool)
+    {
+        if (null === $this->poolOptimizer) {
+            return $pool;
+        }
+
+        $total = \count($pool->getPackages());
+
+        $pool = $this->poolOptimizer->optimize($request, $pool);
+
+        $filtered = $total - \count($pool->getPackages());
+
+        if (0 === $filtered) {
+            return $pool;
+        }
+
+        $this->io->write(sprintf(
+            '<info>Found %s package versions referenced in your dependency graph. %s (%d%%) were optimized away.</info>',
+            number_format($total),
+            number_format($filtered),
+            round(100/$total*$filtered)
+        ), true, IOInterface::VERY_VERBOSE);
+
+        return $pool;
+    }
 }

src/Composer/DependencyResolver/PoolOptimizer.php

@@ -0,0 +1,345 @@
+<?php
+
+/*
+ * 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;
+
+use Composer\Package\AliasPackage;
+use Composer\Package\BasePackage;
+use Composer\Semver\CompilingMatcher;
+use Composer\Semver\Constraint\ConstraintInterface;
+use Composer\Semver\Constraint\Constraint;
+use Composer\Semver\Constraint\MultiConstraint;
+use Composer\Semver\Intervals;
+
+/**
+ * Optimizes a given pool
+ *
+ * @author Yanick Witschi <yanick.witschi@terminal42.ch>
+ */
+class PoolOptimizer
+{
+    /**
+     * @var PolicyInterface
+     */
+    private $policy;
+
+    /**
+     * @var array<int, true>
+     */
+    private $irremovablePackages = array();
+
+    /**
+     * @var array<string, array<string, ConstraintInterface>>
+     */
+    private $requireConstraintsPerPackage = array();
+
+    /**
+     * @var array<string, array<string, ConstraintInterface>>
+     */
+    private $conflictConstraintsPerPackage = array();
+
+    /**
+     * @var array<int, true>
+     */
+    private $packagesToRemove = array();
+
+    /**
+     * @var array<int, BasePackage[]>
+     */
+    private $aliasesPerPackage = array();
+
+    public function __construct(PolicyInterface $policy)
+    {
+        $this->policy = $policy;
+    }
+
+    /**
+     * @return Pool
+     */
+    public function optimize(Request $request, Pool $pool)
+    {
+        $this->prepare($request, $pool);
+
+        $optimizedPool = $this->optimizeByIdenticalDependencies($pool);
+
+        // No need to run this recursively at the moment
+        // because the current optimizations cannot provide
+        // even more gains when ran again. Might change
+        // in the future with additional optimizations.
+
+        $this->irremovablePackages = array();
+        $this->requireConstraintsPerPackage = array();
+        $this->conflictConstraintsPerPackage = array();
+        $this->packagesToRemove = array();
+        $this->aliasesPerPackage = array();
+
+        return $optimizedPool;
+    }
+
+    /**
+     * @return void
+     */
+    private function prepare(Request $request, Pool $pool)
+    {
+        $irremovablePackageConstraintGroups = array();
+
+        // Mark fixed or locked packages as irremovable
+        foreach ($request->getFixedOrLockedPackages() as $package) {
+            $irremovablePackageConstraintGroups[$package->getName()][] = new Constraint('==', $package->getVersion());
+        }
+
+        // Extract requested package requirements
+        foreach ($request->getRequires() as $require => $constraint) {
+            $constraint = Intervals::compactConstraint($constraint);
+            $this->requireConstraintsPerPackage[$require][(string) $constraint] = $constraint;
+        }
+
+        // First pass over all packages to extract information and mark package constraints irremovable
+        foreach ($pool->getPackages() as $package) {
+            // Extract package requirements
+            foreach ($package->getRequires() as $link) {
+                $constraint = Intervals::compactConstraint($link->getConstraint());
+                $this->requireConstraintsPerPackage[$link->getTarget()][(string) $constraint] = $constraint;
+            }
+            // Extract package conflicts
+            foreach ($package->getConflicts() as $link) {
+                $constraint = Intervals::compactConstraint($link->getConstraint());
+                $this->conflictConstraintsPerPackage[$link->getTarget()][(string) $constraint] = $constraint;
+            }
+
+            // Keep track of alias packages for every package so if either the alias or aliased is kept
+            // we keep the others as they are a unit of packages really
+            if ($package instanceof AliasPackage) {
+                $this->aliasesPerPackage[$package->getAliasOf()->id][] = $package;
+            }
+        }
+
+        $irremovablePackageConstraints = array();
+        foreach ($irremovablePackageConstraintGroups as $packageName => $constraints) {
+            $irremovablePackageConstraints[$packageName] = 1 === \count($constraints) ? $constraints[0] : new MultiConstraint($constraints, false);
+        }
+        unset($irremovablePackageConstraintGroups);
+
+        // Mark the packages as irremovable based on the constraints
+        foreach ($pool->getPackages() as $package) {
+            if (!isset($irremovablePackageConstraints[$package->getName()])) {
+                continue;
+            }
+
+            if (CompilingMatcher::match($irremovablePackageConstraints[$package->getName()], Constraint::OP_EQ, $package->getVersion())) {
+                $this->markPackageIrremovable($package);
+            }
+        }
+    }
+
+    /**
+     * @return void
+     */
+    private function markPackageIrremovable(BasePackage $package)
+    {
+        $this->irremovablePackages[$package->id] = true;
+        if ($package instanceof AliasPackage) {
+            // recursing here so aliasesPerPackage for the aliasOf can be checked
+            // and all its aliases marked as irremovable as well
+            $this->markPackageIrremovable($package->getAliasOf());
+        }
+        if (isset($this->aliasesPerPackage[$package->id])) {
+            foreach ($this->aliasesPerPackage[$package->id] as $aliasPackage) {
+                $this->irremovablePackages[$aliasPackage->id] = true;
+            }
+        }
+    }
+
+    /**
+     * @return Pool Optimized pool
+     */
+    private function applyRemovalsToPool(Pool $pool)
+    {
+        $packages = array();
+        foreach ($pool->getPackages() as $package) {
+            if (!isset($this->packagesToRemove[$package->id])) {
+                $packages[] = $package;
+            }
+        }
+
+        $optimizedPool = new Pool($packages, $pool->getUnacceptableFixedOrLockedPackages());
+
+        // Reset package removals
+        $this->packagesToRemove = array();
+
+        return $optimizedPool;
+    }
+
+    /**
+     * @return Pool
+     */
+    private function optimizeByIdenticalDependencies(Pool $pool)
+    {
+        $identicalDefinitionPerPackage = array();
+        $packageIdsToRemove = array();
+
+        foreach ($pool->getPackages() as $package) {
+
+            // If that package was already marked irremovable, we can skip
+            // the entire process for it
+            if (isset($this->irremovablePackages[$package->id])) {
+                continue;
+            }
+
+            $packageIdsToRemove[$package->id] = true;
+
+            $dependencyHash = $this->calculateDependencyHash($package);
+
+            foreach ($package->getNames(false) as $packageName) {
+
+                if (!isset($this->requireConstraintsPerPackage[$packageName])) {
+                    continue;
+                }
+
+                foreach ($this->requireConstraintsPerPackage[$packageName] as $requireConstraint) {
+
+                    $groupHashParts = array();
+
+                    if (CompilingMatcher::match($requireConstraint, Constraint::OP_EQ, $package->getVersion())) {
+                        $groupHashParts[] = 'require:' . (string) $requireConstraint;
+                    }
+
+                    if ($package->getReplaces()) {
+                        foreach ($package->getReplaces() as $link) {
+                            if (CompilingMatcher::match($link->getConstraint(), Constraint::OP_EQ, $package->getVersion())) {
+                                // Use the same hash part as the regular require hash because that's what the replacement does
+                                $groupHashParts[] = 'require:' . (string) $link->getConstraint();
+                            }
+                        }
+                    }
+
+                    if (isset($this->conflictConstraintsPerPackage[$packageName])) {
+                        foreach ($this->conflictConstraintsPerPackage[$packageName] as $conflictConstraint) {
+                            if (CompilingMatcher::match($conflictConstraint, Constraint::OP_EQ, $package->getVersion())) {
+                                $groupHashParts[] = 'conflict:' . (string) $conflictConstraint;
+                            }
+                        }
+                    }
+
+                    if (!$groupHashParts) {
+                        continue;
+                    }
+
+                    $identicalDefinitionPerPackage[$packageName][implode('', $groupHashParts)][$dependencyHash][] = $package;
+                }
+            }
+        }
+
+        $keepPackage = function (BasePackage $package, $aliasesPerPackage) use (&$packageIdsToRemove, &$keepPackage) {
+            unset($packageIdsToRemove[$package->id]);
+            if ($package instanceof AliasPackage) {
+                // recursing here so aliasesPerPackage for the aliasOf can be checked
+                // and all its aliases marked to be kept as well
+                $keepPackage($package->getAliasOf(), $aliasesPerPackage);
+            }
+            if (isset($aliasesPerPackage[$package->id])) {
+                foreach ($aliasesPerPackage[$package->id] as $aliasPackage) {
+                    unset($packageIdsToRemove[$aliasPackage->id]);
+                }
+            }
+        };
+
+        foreach ($identicalDefinitionPerPackage as $package => $constraintGroups) {
+            foreach ($constraintGroups as $constraintGroup) {
+                foreach ($constraintGroup as $hash => $packages) {
+
+                    // Only one package in this constraint group has the same requirements, we're not allowed to remove that package
+                    if (1 === \count($packages)) {
+                        $keepPackage($packages[0], $this->aliasesPerPackage);
+                        continue;
+                    }
+
+                    // Otherwise we find out which one is the preferred package in this constraint group which is
+                    // then not allowed to be removed either
+                    $literals = array();
+
+                    foreach ($packages as $package) {
+                        $literals[] = $package->id;
+                    }
+
+                    foreach ($this->policy->selectPreferredPackages($pool, $literals) as $preferredLiteral) {
+                        $keepPackage($pool->literalToPackage($preferredLiteral), $this->aliasesPerPackage);
+                    }
+                }
+            }
+        }
+
+        foreach ($packageIdsToRemove as $id => $dummy) {
+            $this->markPackageForRemoval($id);
+        }
+
+        // Apply removals
+        return $this->applyRemovalsToPool($pool);
+    }
+
+    /**
+     * @return string
+     */
+    private function calculateDependencyHash(BasePackage $package)
+    {
+        $hash = '';
+
+        $hashRelevantLinks = array(
+            'requires' => $package->getRequires(),
+            'conflicts' => $package->getConflicts(),
+            'replaces' => $package->getReplaces(),
+            'provides' => $package->getProvides()
+        );
+
+        foreach ($hashRelevantLinks as $key => $links) {
+            if (0 === \count($links)) {
+                continue;
+            }
+
+            // start new hash section
+            $hash .= $key . ':';
+
+            $subhash = array();
+
+            foreach ($links as $link) {
+                // To get the best dependency hash matches we should use Intervals::compactConstraint() here.
+                // However, the majority of projects are going to specify their constraints already pretty
+                // much in the best variant possible. In other words, we'd be wasting time here and it would actually hurt
+                // performance more than the additional few packages that could be filtered out would benefit the process.
+                $subhash[$link->getTarget()] = (string) $link->getConstraint();
+            }
+
+            // Sort for best result
+            ksort($subhash);
+
+            foreach ($subhash as $target => $constraint) {
+                $hash .= $target . '@' . $constraint;
+            }
+        }
+
+        return $hash;
+    }
+
+    /**
+     * @param int $id
+     * @return void
+     */
+    private function markPackageForRemoval($id)
+    {
+        // We are not allowed to remove packages if they have been marked as irremovable
+        if (isset($this->irremovablePackages[$id])) {
+            throw new \LogicException('Attempted removing a package which was previously marked irremovable');
+        }
+
+        $this->packagesToRemove[$id] = true;
+    }
+}

src/Composer/Installer.php

@@ -20,6 +20,7 @@ use Composer\DependencyResolver\LockTransaction;
 use Composer\DependencyResolver\Operation\UpdateOperation;
 use Composer\DependencyResolver\Operation\InstallOperation;
 use Composer\DependencyResolver\Operation\UninstallOperation;
+use Composer\DependencyResolver\PoolOptimizer;
 use Composer\DependencyResolver\Pool;
 use Composer\DependencyResolver\Request;
 use Composer\DependencyResolver\Solver;
@@ -430,7 +431,7 @@ class Installer
             $request->setUpdateAllowList($this->updateAllowList, $this->updateAllowTransitiveDependencies);
         }
 
-        $pool = $repositorySet->createPool($request, $this->io, $this->eventDispatcher);
+        $pool = $repositorySet->createPool($request, $this->io, $this->eventDispatcher, $this->createPoolOptimizer($policy));
 
         $this->io->writeError('<info>Updating dependencies</info>');
 
@@ -999,6 +1000,23 @@ class Installer
         );
     }
 
+    /**
+     * @return PoolOptimizer|null
+     */
+    private function createPoolOptimizer(PolicyInterface $policy)
+    {
+        // Not the best architectural decision here, would need to be able
+        // to configure from the outside of Installer but this is only
+        // a debugging tool and should never be required in any other use case
+        if ('0' === Platform::getEnv('COMPOSER_POOL_OPTIMIZER')) {
+            $this->io->write('Pool Optimizer was disabled for debugging purposes.', true, IOInterface::DEBUG);
+
+            return null;
+        }
+
+        return new PoolOptimizer($policy);
+    }
+
     /**
      * Create Installer
      *

src/Composer/Repository/RepositorySet.php

@@ -12,6 +12,8 @@
 
 namespace Composer\Repository;
 
+use Composer\DependencyResolver\PoolOptimizer;
+use Composer\DependencyResolver\PolicyInterface;
 use Composer\DependencyResolver\Pool;
 use Composer\DependencyResolver\PoolBuilder;
 use Composer\DependencyResolver\Request;
@@ -244,9 +246,9 @@ class RepositorySet
      *
      * @return Pool
      */
-    public function createPool(Request $request, IOInterface $io, EventDispatcher $eventDispatcher = null)
+    public function createPool(Request $request, IOInterface $io, EventDispatcher $eventDispatcher = null, PoolOptimizer $poolOptimizer = null)
     {
-        $poolBuilder = new PoolBuilder($this->acceptableStabilities, $this->stabilityFlags, $this->rootAliases, $this->rootReferences, $io, $eventDispatcher);
+        $poolBuilder = new PoolBuilder($this->acceptableStabilities, $this->stabilityFlags, $this->rootAliases, $this->rootReferences, $io, $eventDispatcher, $poolOptimizer);
 
         foreach ($this->repositories as $repo) {
             if (($repo instanceof InstalledRepositoryInterface || $repo instanceof InstalledRepository) && !$this->allowInstalledRepositories) {

tests/Composer/Test/DependencyResolver/Fixtures/poolbuilder/multi-repo-replace-partial-update.test

@@ -105,3 +105,13 @@ Check that replacers from additional repositories are loaded when doing a partia
     "shared/dep-1.0.0.0",
     "shared/dep-1.2.0.0"
 ]
+
+--EXPECT-OPTIMIZED--
+[
+    "indirect/replacer-1.2.0.0",
+    "indirect/replacer-1.0.0.0",
+    "replacer/package-1.2.0.0",
+    "replacer/package-1.0.0.0",
+    "base/package-1.0.0.0",
+    "shared/dep-1.2.0.0"
+]

tests/Composer/Test/DependencyResolver/Fixtures/poolbuilder/multi-repo-replace.test

@@ -96,3 +96,13 @@ Check that replacers from additional repositories are loaded
     "replacer/package-1.0.0.0",
     "shared/dep-1.0.0.0"
 ]
+
+--EXPECT-OPTIMIZED--
+[
+    "base/package-1.0.0.0",
+    "indirect/replacer-1.2.0.0",
+    "indirect/replacer-1.0.0.0",
+    "shared/dep-1.2.0.0",
+    "replacer/package-1.2.0.0",
+    "replacer/package-1.0.0.0"
+]

tests/Composer/Test/DependencyResolver/Fixtures/poolbuilder/partial-update-unfixing-locked-deps.test

@@ -48,3 +48,13 @@ locked packages still need to be taking into account for loading all necessary v
     "dep/pkg1-1.0.1.0",
     "dep/pkg1-2.0.0.0"
 ]
+
+--EXPECT-OPTIMIZED--
+[
+    "root/req1-1.0.0.0 (locked)",
+    "root/req2-1.0.0.0 (locked)",
+    "dep/pkg2-1.0.0.0",
+    "dep/pkg2-1.2.0.0",
+    "dep/pkg1-1.0.1.0",
+    "dep/pkg1-2.0.0.0"
+]

tests/Composer/Test/DependencyResolver/Fixtures/poolbuilder/partial-update-unfixing-with-replacers.test

@@ -50,3 +50,12 @@ Fixed packages and replacers get unfixed correctly (refs https://github.com/comp
     "replaced/pkg-1.2.3.0",
     "replaced/pkg-1.2.4.0"
 ]
+
+--EXPECT-OPTIMIZED--
+[
+    "root/req3-1.0.0.0 (locked)",
+    "dep/dep-2.3.5.0 (locked)",
+    "root/req1-1.1.0.0",
+    "replacer/pkg-1.1.0.0",
+    "replaced/pkg-1.2.4.0"
+]

tests/Composer/Test/DependencyResolver/Fixtures/poolbuilder/stability-flags-take-over-minimum-stability-and-filter-packages.test

@@ -46,3 +46,10 @@ Stability flags apply
     6,
     "default/pkg-1.2.0.0 (alias of 6)"
 ]
+
+--EXPECT-OPTIMIZED--
+[
+    1,
+    6,
+    "default/pkg-1.2.0.0 (alias of 6)"
+]

tests/Composer/Test/DependencyResolver/Fixtures/pooloptimizer/aliases.test

@@ -0,0 +1,99 @@
+--TEST--
+Test aliased and aliasees remain untouched if either is required, but are still optimized away otherwise.
+
+--REQUEST--
+{
+    "require": {
+        "package/a": "^1.0",
+        "package/required-aliasof-and-alias": "dev-main-both",
+        "package/required-aliasof": "dev-main-direct",
+        "package/required-alias": "1.*"
+    }
+}
+
+
+--POOL-BEFORE--
+[
+    {
+        "name": "package/a",
+        "version": "1.0.0",
+        "require": {
+            "package/required-aliasof-and-alias": "^1.0"
+        }
+    },
+    {
+        "name": "package/required-aliasof-and-alias",
+        "version": "dev-main-both",
+        "extra": {
+            "branch-alias": {
+                "dev-main-both": "1.x-dev"
+            }
+        }
+    },
+    {
+        "name": "package/required-aliasof",
+        "version": "dev-main-direct",
+        "extra": {
+            "branch-alias": {
+                "dev-main-direct": "1.x-dev"
+            }
+        }
+    },
+    {
+        "name": "package/required-alias",
+        "version": "dev-main-alias",
+        "extra": {
+            "branch-alias": {
+                "dev-main-alias": "1.x-dev"
+            }
+        }
+    },
+    {
+        "name": "package/not-referenced",
+        "version": "dev-lonesome-pkg",
+        "extra": {
+            "branch-alias": {
+                "dev-lonesome-pkg": "1.x-dev"
+            }
+        }
+    }
+]
+
+
+--POOL-AFTER--
+[
+    {
+        "name": "package/a",
+        "version": "1.0.0",
+        "require": {
+            "package/b": "^1.0"
+        }
+    },
+    {
+        "name": "package/required-aliasof-and-alias",
+        "version": "dev-main-both",
+        "extra": {
+            "branch-alias": {
+                "dev-main-both": "1.x-dev"
+            }
+        }
+    },
+    {
+        "name": "package/required-aliasof",
+        "version": "dev-main-direct",
+        "extra": {
+            "branch-alias": {
+                "dev-main-direct": "1.x-dev"
+            }
+        }
+    },
+    {
+        "name": "package/required-alias",
+        "version": "dev-main-alias",
+        "extra": {
+            "branch-alias": {
+                "dev-main-alias": "1.x-dev"
+            }
+        }
+    }
+]

tests/Composer/Test/DependencyResolver/Fixtures/pooloptimizer/basic-prefer-highest.test

@@ -0,0 +1,46 @@
+--TEST--
+Test filters irrelevant package "package/b" in version 1.0.0
+
+--REQUEST--
+{
+    "require": {
+        "package/a": "^1.0"
+    }
+}
+
+
+--POOL-BEFORE--
+[
+        {
+            "name": "package/a",
+            "version": "1.0.0",
+            "require": {
+                "package/b": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.0"
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.1"
+        }
+]
+
+
+--POOL-AFTER--
+[
+        {
+            "name": "package/a",
+            "version": "1.0.0",
+            "require": {
+                "package/b": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.1"
+        }
+]
+

tests/Composer/Test/DependencyResolver/Fixtures/pooloptimizer/basic-prefer-lowest.test

@@ -0,0 +1,47 @@
+--TEST--
+Test filters irrelevant package "package/b" in version 1.0.1 because prefer-lowest
+
+--REQUEST--
+{
+    "require": {
+        "package/a": "^1.0"
+    },
+    "preferLowest": true
+}
+
+
+--POOL-BEFORE--
+[
+        {
+            "name": "package/a",
+            "version": "1.0.0",
+            "require": {
+                "package/b": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.0"
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.1"
+        }
+]
+
+
+--POOL-AFTER--
+[
+        {
+            "name": "package/a",
+            "version": "1.0.0",
+            "require": {
+                "package/b": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.0"
+        }
+]
+

tests/Composer/Test/DependencyResolver/Fixtures/pooloptimizer/conflict.test

@@ -0,0 +1,107 @@
+--TEST--
+We have to make sure, conflicts are considered in the grouping so we do not remove packages
+from the pool which might end up being part of the solution.
+
+--REQUEST--
+{
+    "require": {
+        "nesty/nest": "^1.0"
+    }
+}
+
+--POOL-BEFORE--
+[
+        {
+            "name": "nesty/nest",
+            "version": "1.0.0",
+            "require": {
+                "conflicter/pkg": "^1.0",
+                "victim/pkg": "^1 <1.2"
+            }
+        },
+        {
+            "name": "conflicter/pkg",
+            "version": "1.0.1",
+            "conflict": {
+                "victim/pkg": "1.1.0 || 1.1.1"
+            }
+        },
+        {
+            "name": "conflicter/pkg",
+            "version": "1.0.2",
+            "conflict": {
+                "victim/pkg": "1.1.1 || 1.1.2"
+            }
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.0.0"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.0.1"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.0.2"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.0"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.1"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.2"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.2.0"
+        }
+]
+
+
+--POOL-AFTER--
+[
+        {
+            "name": "nesty/nest",
+            "version": "1.0.0",
+            "require": {
+                "conflicter/pkg": "^1.0",
+                "victim/pkg": "^1 <1.2"
+            }
+        },
+        {
+            "name": "conflicter/pkg",
+            "version": "1.0.1",
+            "conflict": {
+                "victim/pkg": "1.1.0 || 1.1.1"
+            }
+        },
+        {
+            "name": "conflicter/pkg",
+            "version": "1.0.2",
+            "conflict": {
+                "victim/pkg": "1.1.2"
+            }
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.0.2"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.0"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.1"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.2"
+        }
+]

tests/Composer/Test/DependencyResolver/Fixtures/pooloptimizer/conflict2.test

@@ -0,0 +1,103 @@
+--TEST--
+We have to make sure, conflicts are considered in the grouping so we do not remove packages
+from the pool which might end up being part of the solution.
+
+--REQUEST--
+{
+    "require": {
+        "nesty/nest": "^1.0"
+    }
+}
+
+--POOL-BEFORE--
+[
+        {
+            "name": "nesty/nest",
+            "version": "1.0.0",
+            "require": {
+                "conflicter/pkg": "^1.0",
+                "victim/pkg": "^1 <1.2"
+            }
+        },
+        {
+            "name": "conflicter/pkg",
+            "version": "1.0.1",
+            "conflict": {
+                "victim/pkg": "1.1.0"
+            }
+        },
+        {
+            "name": "conflicter/pkg",
+            "version": "1.0.2",
+            "conflict": {
+                "victim/pkg": "1.1.2"
+            }
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.0.0"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.0.1"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.0.2"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.0"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.1"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.2"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.2.0"
+        }
+]
+
+
+--POOL-AFTER--
+[
+        {
+            "name": "nesty/nest",
+            "version": "1.0.0",
+            "require": {
+                "conflicter/pkg": "^1.0",
+                "victim/pkg": "^1 <1.2"
+            }
+        },
+        {
+            "name": "conflicter/pkg",
+            "version": "1.0.1",
+            "conflict": {
+                "victim/pkg": "1.1.0 || 1.1.1"
+            }
+        },
+        {
+            "name": "conflicter/pkg",
+            "version": "1.0.2",
+            "conflict": {
+                "victim/pkg": "1.1.2"
+            }
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.0"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.1"
+        },
+        {
+            "name": "victim/pkg",
+            "version": "1.1.2"
+        }
+]

tests/Composer/Test/DependencyResolver/Fixtures/pooloptimizer/group-by-required.test

@@ -0,0 +1,99 @@
+--TEST--
+We are not allowed to group packages only by their dependency definition. It's also relevant what other
+packages require (package/b@1.0.1 must not be dropped although it has the very same definition as 2.0.0 and both are
+allowed by the request). However, package/b@1.0.0 can be removed.
+
+--REQUEST--
+{
+    "require": {
+        "package/a": "^1.0"
+    }
+}
+
+
+--POOL-BEFORE--
+[
+        {
+            "name": "package/a",
+            "version": "1.0.0",
+            "require": {
+                "package/b": "^1.0 || ^2.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.0",
+            "require": {
+                "package/c": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.1",
+            "require": {
+                "package/c": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "2.0.0",
+            "require": {
+                "package/c": "^1.0"
+            }
+        },
+        {
+            "name": "package/c",
+            "version": "1.0.0",
+            "require": {
+                "package/d": "^1.0"
+            }
+        },
+        {
+            "name": "package/d",
+            "version": "1.0.0",
+            "require": {
+                "package/b": ">=1.0 <1.1"
+            }
+        }
+]
+
+
+--POOL-AFTER--
+[
+        {
+             "name": "package/a",
+             "version": "1.0.0",
+             "require": {
+                 "package/b": "^1.0 || ^2.0"
+             }
+         },
+         {
+             "name": "package/b",
+             "version": "1.0.1",
+             "require": {
+                 "package/c": "^1.0"
+             }
+         },
+         {
+             "name": "package/b",
+             "version": "2.0.0",
+             "require": {
+                 "package/c": "^1.0"
+             }
+         },
+         {
+             "name": "package/c",
+             "version": "1.0.0",
+             "require": {
+                 "package/d": "^1.0"
+             }
+         },
+         {
+             "name": "package/d",
+             "version": "1.0.0",
+             "require": {
+                 "package/b": ">=1.0 <1.1"
+             }
+         }
+]
+

tests/Composer/Test/DependencyResolver/Fixtures/pooloptimizer/locked-fixed-untouched.test

@@ -0,0 +1,46 @@
+--TEST--
+Test locked and fixed packages remain untouched.
+
+--REQUEST--
+{
+    "require": {
+    },
+    "locked": [
+        {
+            "name": "package/a",
+            "version": "1.0.0"
+        }
+    ],
+    "fixed": [
+        {
+            "name": "package/c",
+            "version": "2.0.0"
+        }
+    ]
+}
+
+
+--POOL-BEFORE--
+[
+    {
+        "name": "package/a",
+        "version": "1.0.0"
+    },
+    {
+        "name": "package/c",
+        "version": "2.0.0"
+    }
+]
+
+
+--POOL-AFTER--
+[
+    {
+        "name": "package/a",
+        "version": "1.0.0"
+    },
+    {
+        "name": "package/c",
+        "version": "2.0.0"
+    }
+]

tests/Composer/Test/DependencyResolver/Fixtures/pooloptimizer/replaces.test

@@ -0,0 +1,59 @@
+--TEST--
+Test replaced packages are correctly removed.
+
+--REQUEST--
+{
+    "require": {
+        "package/a": "^1.0"
+    }
+}
+
+
+--POOL-BEFORE--
+[
+        {
+            "name": "package/a",
+            "version": "1.0.0",
+            "require": {
+                "package/b": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.0",
+            "replace": {
+                "package/c": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.1",
+            "replace": {
+                "package/c": "^1.0"
+            }
+        },
+        {
+            "name": "package/c",
+            "version": "1.0.0"
+        }
+]
+
+
+--POOL-AFTER--
+[
+        {
+            "name": "package/a",
+            "version": "1.0.0",
+            "require": {
+                "package/b": "^1.0"
+            }
+        },
+        {
+            "name": "package/b",
+            "version": "1.0.1",
+            "replace": {
+                "package/c": "^1.0"
+            }
+        }
+]
+

tests/Composer/Test/DependencyResolver/PoolBuilderTest.php

@@ -12,6 +12,9 @@
 
 namespace Composer\Test\DependencyResolver;
 
+use Composer\DependencyResolver\DefaultPolicy;
+use Composer\DependencyResolver\Pool;
+use Composer\DependencyResolver\PoolOptimizer;
 use Composer\IO\NullIO;
 use Composer\Repository\ArrayRepository;
 use Composer\Repository\FilterRepository;
@@ -31,13 +34,14 @@ class PoolBuilderTest extends TestCase
      * @dataProvider getIntegrationTests
      * @param string $file
      * @param string $message
-     * @param mixed[] $expect
+     * @param string[] $expect
+     * @param string[] $expectOptimized
      * @param mixed[] $root
      * @param mixed[] $requestData
      * @param mixed[] $packageRepos
      * @param mixed[] $fixed
      */
-    public function testPoolBuilder($file, $message, $expect, $root, $requestData, $packageRepos, $fixed)
+    public function testPoolBuilder($file, $message, $expect, $expectOptimized, $root, $requestData, $packageRepos, $fixed)
     {
         $rootAliases = !empty($root['aliases']) ? $root['aliases'] : array();
         $minimumStability = !empty($root['minimum-stability']) ? $root['minimum-stability'] : 'stable';
@@ -56,6 +60,8 @@ class PoolBuilderTest extends TestCase
         $loader = new ArrayLoader();
         $packageIds = array();
         $loadPackage = function ($data) use ($loader, &$packageIds) {
+            /** @var ?int $id */
+            $id = null;
             if (!empty($data['id'])) {
                 $id = $data['id'];
                 unset($data['id']);
@@ -115,12 +121,28 @@ class PoolBuilderTest extends TestCase
         }
 
         $pool = $repositorySet->createPool($request, new NullIO());
+
+        $result = $this->getPackageResultSet($pool, $packageIds);
+
+        $this->assertSame($expect, $result, 'Unoptimized pool does not match expected package set');
+
+        $optimizer = new PoolOptimizer(new DefaultPolicy());
+        $result = $this->getPackageResultSet($optimizer->optimize($request, $pool), $packageIds);
+        $this->assertSame($expectOptimized, $result, 'Optimized pool does not match expected package set');
+    }
+
+    /**
+     * @param array<int, BasePackage> $packageIds
+     * @return string[]
+     */
+    private function getPackageResultSet(Pool $pool, $packageIds)
+    {
         $result = array();
         for ($i = 1, $count = count($pool); $i <= $count; $i++) {
             $result[] = $pool->packageById($i);
         }
 
-        $result = array_map(function ($package) use ($packageIds) {
+        return array_map(function (BasePackage $package) use ($packageIds) {
             if ($id = array_search($package, $packageIds, true)) {
                 return $id;
             }
@@ -143,8 +165,6 @@ class PoolBuilderTest extends TestCase
 
             return (string) $package->getName().'-'.$package->getVersion() . $suffix;
         }, $result);
-
-        $this->assertSame($expect, $result);
     }
 
     /**
@@ -173,11 +193,12 @@ class PoolBuilderTest extends TestCase
                     $fixed = JsonFile::parseJson($testData['FIXED']);
                 }
                 $expect = JsonFile::parseJson($testData['EXPECT']);
+                $expectOptimized = !empty($testData['EXPECT-OPTIMIZED']) ? JsonFile::parseJson($testData['EXPECT-OPTIMIZED']) : $expect;
             } catch (\Exception $e) {
                 die(sprintf('Test "%s" is not valid: '.$e->getMessage(), str_replace($fixturesDir.'/', '', $file)));
             }
 
-            $tests[basename($file)] = array(str_replace($fixturesDir.'/', '', $file), $message, $expect, $root, $request, $packageRepos, $fixed);
+            $tests[basename($file)] = array(str_replace($fixturesDir.'/', '', $file), $message, $expect, $expectOptimized, $root, $request, $packageRepos, $fixed);
         }
 
         return $tests;
@@ -199,6 +220,7 @@ class PoolBuilderTest extends TestCase
             'FIXED' => false,
             'PACKAGE-REPOS' => true,
             'EXPECT' => true,
+            'EXPECT-OPTIMIZED' => false,
         );
 
         $section = null;

tests/Composer/Test/DependencyResolver/PoolOptimizerTest.php

@@ -0,0 +1,197 @@
+<?php
+
+/*
+ * 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\Test\DependencyResolver;
+
+use Composer\DependencyResolver\DefaultPolicy;
+use Composer\DependencyResolver\Pool;
+use Composer\DependencyResolver\PoolOptimizer;
+use Composer\DependencyResolver\Request;
+use Composer\Json\JsonFile;
+use Composer\Package\AliasPackage;
+use Composer\Package\BasePackage;
+use Composer\Package\Loader\ArrayLoader;
+use Composer\Package\Version\VersionParser;
+use Composer\Repository\LockArrayRepository;
+use Composer\Test\TestCase;
+
+class PoolOptimizerTest extends TestCase
+{
+    /**
+     * @dataProvider provideIntegrationTests
+     * @param mixed[] $requestData
+     * @param BasePackage[] $packagesBefore
+     * @param BasePackage[] $expectedPackages
+     * @param string $message
+     */
+    public function testPoolOptimizer(array $requestData, array $packagesBefore, array $expectedPackages, $message)
+    {
+        $lockedRepo = new LockArrayRepository();
+
+        $request = new Request($lockedRepo);
+        $parser = new VersionParser();
+
+        if (isset($requestData['locked'])) {
+            foreach ($requestData['locked'] as $package) {
+                $request->lockPackage($this->loadPackage($package));
+            }
+        }
+        if (isset($requestData['fixed'])) {
+            foreach ($requestData['fixed'] as $package) {
+                $request->fixPackage($this->loadPackage($package));
+            }
+        }
+
+        foreach ($requestData['require'] as $package => $constraint) {
+            $request->requireName($package, $parser->parseConstraints($constraint));
+        }
+
+        $preferStable = isset($requestData['preferStable']) ? $requestData['preferStable'] : false;
+        $preferLowest = isset($requestData['preferLowest']) ? $requestData['preferLowest'] : false;
+
+        $pool = new Pool($packagesBefore);
+        $poolOptimizer = new PoolOptimizer(new DefaultPolicy($preferStable, $preferLowest));
+
+        $pool = $poolOptimizer->optimize($request, $pool);
+
+        $this->assertSame(
+            $this->reducePackagesInfoForComparison($expectedPackages),
+            $this->reducePackagesInfoForComparison($pool->getPackages()),
+            $message
+        );
+    }
+
+    public function provideIntegrationTests()
+    {
+        $fixturesDir = realpath(__DIR__.'/Fixtures/pooloptimizer/');
+        $tests = array();
+        foreach (new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator($fixturesDir), \RecursiveIteratorIterator::LEAVES_ONLY) as $file) {
+            if (!preg_match('/\.test$/', $file)) {
+                continue;
+            }
+
+            try {
+                $testData = $this->readTestFile($file, $fixturesDir);
+                $message = $testData['TEST'];
+                $requestData = JsonFile::parseJson($testData['REQUEST']);
+                $packagesBefore = $this->loadPackages(JsonFile::parseJson($testData['POOL-BEFORE']));
+                $expectedPackages = $this->loadPackages(JsonFile::parseJson($testData['POOL-AFTER']));
+
+            } catch (\Exception $e) {
+                die(sprintf('Test "%s" is not valid: '.$e->getMessage(), str_replace($fixturesDir.'/', '', $file)));
+            }
+
+            $tests[basename($file)] = array($requestData, $packagesBefore, $expectedPackages, $message);
+        }
+
+        return $tests;
+    }
+
+    /**
+     * @param  string $fixturesDir
+     * @return mixed[]
+     */
+    protected function readTestFile(\SplFileInfo $file, $fixturesDir)
+    {
+        $tokens = preg_split('#(?:^|\n*)--([A-Z-]+)--\n#', file_get_contents($file->getRealPath()), -1, PREG_SPLIT_DELIM_CAPTURE);
+
+        /** @var array<string, bool> $sectionInfo */
+        $sectionInfo = array(
+            'TEST' => true,
+            'REQUEST' => true,
+            'POOL-BEFORE' => true,
+            'POOL-AFTER' => true,
+        );
+
+        $section = null;
+        $data = array();
+        foreach ($tokens as $i => $token) {
+            if (null === $section && empty($token)) {
+                continue; // skip leading blank
+            }
+
+            if (null === $section) {
+                if (!isset($sectionInfo[$token])) {
+                    throw new \RuntimeException(sprintf(
+                        'The test file "%s" must not contain a section named "%s".',
+                        str_replace($fixturesDir.'/', '', $file),
+                        $token
+                    ));
+                }
+                $section = $token;
+                continue;
+            }
+
+            $sectionData = $token;
+
+            $data[$section] = $sectionData;
+            $section = $sectionData = null;
+        }
+
+        foreach ($sectionInfo as $section => $required) {
+            if ($required && !isset($data[$section])) {
+                throw new \RuntimeException(sprintf(
+                    'The test file "%s" must have a section named "%s".',
+                    str_replace($fixturesDir.'/', '', $file),
+                    $section
+                ));
+            }
+        }
+
+        return $data;
+    }
+
+    /**
+     * @param BasePackage[] $packages
+     * @return string[]
+     */
+    private function reducePackagesInfoForComparison(array $packages)
+    {
+        $packagesInfo = array();
+
+        foreach ($packages as $package) {
+            $packagesInfo[] = $package->getName() . '@' . $package->getVersion() . ($package instanceof AliasPackage ? ' (alias of '.$package->getAliasOf()->getVersion().')' : '');
+        }
+
+        sort($packagesInfo);
+
+        return $packagesInfo;
+    }
+
+    /**
+     * @param mixed[][] $packagesData
+     * @return BasePackage[]
+     */
+    private function loadPackages(array $packagesData)
+    {
+        $packages = array();
+
+        foreach ($packagesData as $packageData) {
+            $packages[] = $package = $this->loadPackage($packageData);
+            if ($package instanceof AliasPackage) {
+                $packages[] = $package->getAliasOf();
+            }
+        }
+
+        return $packages;
+    }
+
+    /**
+     * @param mixed[] $packageData
+     * @return BasePackage
+     */
+    private function loadPackage(array $packageData)
+    {
+        $loader = new ArrayLoader();
+        return $loader->load($packageData);
+    }
+}

tests/Composer/Test/Fixtures/installer/conflict-downgrade-nested.test

@@ -0,0 +1,39 @@
+--TEST--
+
+Test that a package which has a conflict does not get installed and has to be downgraded
+
+--COMPOSER--
+{
+    "repositories": [
+        {
+            "type": "package",
+            "package": [
+                { "name": "nesty/nest", "version": "1.0.0", "require": {
+                    "conflicter/pkg": "^1.0",
+                    "victim/pkg": "^1 <1.2"
+                } },
+                { "name": "conflicter/pkg", "version": "1.0.1", "conflict": { "victim/pkg": "1.1.0"} },
+                { "name": "victim/pkg", "version": "1.0.0" },
+                { "name": "victim/pkg", "version": "1.0.1" },
+                { "name": "victim/pkg", "version": "1.0.2" },
+                { "name": "victim/pkg", "version": "1.1.0" },
+                { "name": "victim/pkg", "version": "1.2.0" }
+            ]
+        }
+    ],
+    "require": {
+        "nesty/nest": "*"
+    }
+}
+
+
+--RUN--
+update
+
+--EXPECT-EXIT-CODE--
+0
+
+--EXPECT--
+Installing victim/pkg (1.0.2)
+Installing conflicter/pkg (1.0.1)
+Installing nesty/nest (1.0.0)

tests/Composer/Test/Fixtures/installer/conflict-downgrade.test

@@ -0,0 +1,35 @@
+--TEST--
+
+Test that a package which has a conflict does not get installed and has to be downgraded
+
+--COMPOSER--
+{
+    "repositories": [
+        {
+            "type": "package",
+            "package": [
+                { "name": "conflicter/pkg", "version": "1.0.1", "conflict": { "victim/pkg": "1.1.0"} },
+                { "name": "victim/pkg", "version": "1.0.0" },
+                { "name": "victim/pkg", "version": "1.0.1" },
+                { "name": "victim/pkg", "version": "1.0.2" },
+                { "name": "victim/pkg", "version": "1.1.0" },
+                { "name": "victim/pkg", "version": "1.2.0" }
+            ]
+        }
+    ],
+    "require": {
+        "conflicter/pkg": "^1.0",
+        "victim/pkg": "^1 <1.2"
+    }
+}
+
+
+--RUN--
+update
+
+--EXPECT-EXIT-CODE--
+0
+
+--EXPECT--
+Installing conflicter/pkg (1.0.1)
+Installing victim/pkg (1.0.2)

tests/Composer/Test/Fixtures/installer/deduplicate-solver-problems.test

@@ -44,5 +44,14 @@ Your requirements could not be resolved to an installable set of packages.
     - package/a[2.0.0, ..., 2.6.0] require missing/dep ^1.0 -> found missing/dep[2.0.0] but it does not match the constraint.
     - Root composer.json requires package/a * -> satisfiable by package/a[2.0.0, ..., 2.6.0].
 
+--EXPECT-OUTPUT-OPTIMIZED--
+Loading composer repositories with package information
+Updating dependencies
+Your requirements could not be resolved to an installable set of packages.
+
+  Problem 1
+    - Root composer.json requires package/a * -> satisfiable by package/a[2.6.0].
+    - package/a 2.6.0 requires missing/dep ^1.0 -> found missing/dep[2.0.0] but it does not match the constraint.
+
 --EXPECT--
 

tests/Composer/Test/Fixtures/installer/github-issues-7051.test

@@ -128,6 +128,21 @@ Your requirements could not be resolved to an installable set of packages.
     - You can only install one version of a package, so only one of these can be installed: symfony/console[v2.8.7, v2.8.8, v3.1.9, ..., v3.4.29].
     - illuminate/console v5.2.25 requires symfony/console 3.1.* -> satisfiable by symfony/console[v3.1.9, v3.1.10].
     - Conclusion: don't install symfony/console v3.1.10 (conflict analysis result)
+
+--EXPECT-OUTPUT-OPTIMIZED--
+Loading composer repositories with package information
+Updating dependencies
+Your requirements could not be resolved to an installable set of packages.
+
+  Problem 1
+    - Root composer.json requires illuminate/queue * -> satisfiable by illuminate/queue[v5.2.0].
+    - illuminate/queue v5.2.0 requires illuminate/console 5.2.* -> satisfiable by illuminate/console[v5.2.25, v5.2.26].
+    - illuminate/console v5.2.25 requires symfony/console 3.1.* -> satisfiable by symfony/console[v3.1.10].
+    - illuminate/console v5.2.26 requires symfony/console 2.8.* -> satisfiable by symfony/console[v2.8.8].
+    - You can only install one version of a package, so only one of these can be installed: symfony/console[v2.8.8, v3.1.10, v3.4.29].
+    - friendsofphp/php-cs-fixer v2.10.5 requires symfony/console ^3.2 || ^4.0 -> satisfiable by symfony/console[v3.4.29].
+    - Root composer.json requires friendsofphp/php-cs-fixer * -> satisfiable by friendsofphp/php-cs-fixer[v2.10.5].
+
 --EXPECT--
 
 --EXPECT-EXIT-CODE--

tests/Composer/Test/Fixtures/installer/provider-conflicts3.test

@@ -44,5 +44,15 @@ Your requirements could not be resolved to an installable set of packages.
     - Root composer.json requires regular/pkg 1.* -> satisfiable by regular/pkg[1.0.0, 1.0.1, 1.0.2, 1.0.3].
     - Root composer.json requires replacer/pkg 2.* -> satisfiable by replacer/pkg[2.0.0, 2.0.1, 2.0.2, 2.0.3].
 
+--EXPECT-OUTPUT-OPTIMIZED--
+Loading composer repositories with package information
+Updating dependencies
+Your requirements could not be resolved to an installable set of packages.
+
+  Problem 1
+    - Only one of these can be installed: regular/pkg[1.0.3], replacer/pkg[2.0.0, 2.0.1, 2.0.2, 2.0.3]. replacer/pkg replaces regular/pkg and thus cannot coexist with it.
+    - Root composer.json requires regular/pkg 1.* -> satisfiable by regular/pkg[1.0.3].
+    - Root composer.json requires replacer/pkg 2.* -> satisfiable by replacer/pkg[2.0.0, 2.0.1, 2.0.2, 2.0.3].
+
 --EXPECT--
 

tests/Composer/Test/InstallerTest.php

@@ -36,6 +36,7 @@ use Composer\Package\Locker;
 use Composer\Test\Mock\FactoryMock;
 use Composer\Test\Mock\InstalledFilesystemRepositoryMock;
 use Composer\Test\Mock\InstallationManagerMock;
+use Composer\Util\Platform;
 use Symfony\Component\Console\Input\StringInput;
 use Symfony\Component\Console\Output\StreamOutput;
 use Symfony\Component\Console\Output\OutputInterface;
@@ -56,6 +57,8 @@ class InstallerTest extends TestCase
 
     public function tearDown()
     {
+        Platform::clearEnv('COMPOSER_POOL_OPTIMIZER');
+
         chdir($this->prevCwd);
         if (isset($this->tempComposerHome) && is_dir($this->tempComposerHome)) {
             $fs = new Filesystem;
@@ -228,12 +231,15 @@ class InstallerTest extends TestCase
      * @param mixed[]|false $expectLock
      * @param ?mixed[] $expectInstalled
      * @param ?string $expectOutput
+     * @param ?string $expectOutputOptimized
      * @param string $expect
      * @param int|string $expectResult
      */
-    public function testSlowIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult)
+    public function testSlowIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult)
     {
-        return $this->testIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult);
+        Platform::putEnv('COMPOSER_POOL_OPTIMIZER', '0');
+
+        $this->doTestIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult);
     }
 
     /**
@@ -248,10 +254,56 @@ class InstallerTest extends TestCase
      * @param mixed[]|false $expectLock
      * @param ?mixed[] $expectInstalled
      * @param ?string $expectOutput
+     * @param ?string $expectOutputOptimized
      * @param string $expect
      * @param int|string $expectResult
      */
-    public function testIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult)
+    public function testIntegrationWithPoolOptimizer($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult)
+    {
+        Platform::putEnv('COMPOSER_POOL_OPTIMIZER', '1');
+
+        $this->doTestIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutputOptimized ?: $expectOutput, $expect, $expectResult);
+    }
+
+    /**
+     * @dataProvider provideIntegrationTests
+     * @param string $file
+     * @param string $message
+     * @param ?string $condition
+     * @param Config $composerConfig
+     * @param ?mixed[] $lock
+     * @param ?mixed[] $installed
+     * @param string $run
+     * @param mixed[]|false $expectLock
+     * @param ?mixed[] $expectInstalled
+     * @param ?string $expectOutput
+     * @param ?string $expectOutputOptimized
+     * @param string $expect
+     * @param int|string $expectResult
+     */
+    public function testIntegrationWithRawPool($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult)
+    {
+        Platform::putEnv('COMPOSER_POOL_OPTIMIZER', '0');
+
+        $this->doTestIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult);
+    }
+
+    /**
+     * @param string $file
+     * @param string $message
+     * @param ?string $condition
+     * @param Config $composerConfig
+     * @param ?mixed[] $lock
+     * @param ?mixed[] $installed
+     * @param string $run
+     * @param mixed[]|false $expectLock
+     * @param ?mixed[] $expectInstalled
+     * @param ?string $expectOutput
+     * @param string $expect
+     * @param int|string $expectResult
+     * @return void
+     */
+    private function doTestIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult)
     {
         if ($condition) {
             eval('$res = '.$condition.';');
@@ -518,6 +570,7 @@ class InstallerTest extends TestCase
                     $expectInstalled = JsonFile::parseJson($testData['EXPECT-INSTALLED']);
                 }
                 $expectOutput = isset($testData['EXPECT-OUTPUT']) ? $testData['EXPECT-OUTPUT'] : null;
+                $expectOutputOptimized = isset($testData['EXPECT-OUTPUT-OPTIMIZED']) ? $testData['EXPECT-OUTPUT-OPTIMIZED'] : null;
                 $expect = $testData['EXPECT'];
                 if (!empty($testData['EXPECT-EXCEPTION'])) {
                     $expectResult = $testData['EXPECT-EXCEPTION'];
@@ -533,7 +586,7 @@ class InstallerTest extends TestCase
                 die(sprintf('Test "%s" is not valid: '.$e->getMessage(), str_replace($fixturesDir.'/', '', $file)));
             }
 
-            $tests[basename($file)] = array(str_replace($fixturesDir.'/', '', $file), $message, $condition, $composer, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult);
+            $tests[basename($file)] = array(str_replace($fixturesDir.'/', '', $file), $message, $condition, $composer, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult);
         }
 
         return $tests;
@@ -557,6 +610,7 @@ class InstallerTest extends TestCase
             'EXPECT-LOCK' => false,
             'EXPECT-INSTALLED' => false,
             'EXPECT-OUTPUT' => false,
+            'EXPECT-OUTPUT-OPTIMIZED' => false,
             'EXPECT-EXIT-CODE' => false,
             'EXPECT-EXCEPTION' => false,
             'EXPECT' => true,

tests/bootstrap.php

@@ -10,6 +10,8 @@
  * file that was distributed with this source code.
  */
 
+use Composer\Util\Platform;
+
 error_reporting(E_ALL);
 
 if (function_exists('date_default_timezone_set') && function_exists('date_default_timezone_get')) {
@@ -19,3 +21,5 @@ if (function_exists('date_default_timezone_set') && function_exists('date_defaul
 require __DIR__.'/../src/bootstrap.php';
 require __DIR__.'/../src/Composer/InstalledVersions.php';
 require __DIR__.'/Composer/Test/TestCase.php';
+
+Platform::putEnv('COMPOSER_TESTS_ARE_RUNNING', '1');

tests/deprecations-8.1.json

@@ -75,9 +75,14 @@
         "count": 1
     },
     {
-        "location": "Composer\\Test\\InstallerTest::testIntegration",
+        "location": "Composer\\Test\\InstallerTest::testIntegrationWithRawPool",
         "message": "preg_match(): Passing null to parameter #4 ($flags) of type int is deprecated",
-        "count": 1640
+        "count": 1728
+    },
+    {
+        "location": "Composer\\Test\\InstallerTest::testIntegrationWithPoolOptimizer",
+        "message": "preg_match(): Passing null to parameter #4 ($flags) of type int is deprecated",
+        "count": 1728
     },
     {
         "location": "Composer\\Test\\Package\\Archiver\\ArchivableFilesFinderTest::testManualExcludes",