SmartList

SmartList/.gitignore

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+# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm, CLion, Android Studio and Webstorm
+# Reference: https://intellij-support.jetbrains.com/hc/en-us/articles/206544839
+
+# User-specific stuff:
+.idea/**/workspace.xml
+.idea/**/tasks.xml
+.idea/dictionaries
+
+# Sensitive or high-churn files:
+.idea/**/dataSources/
+.idea/**/dataSources.ids
+.idea/**/dataSources.xml
+.idea/**/dataSources.local.xml
+.idea/**/sqlDataSources.xml
+.idea/**/dynamic.xml
+.idea/**/uiDesigner.xml
+
+# Gradle:
+.idea/**/gradle.xml
+.idea/**/libraries
+
+# CMake
+cmake-build-debug/
+
+# Mongo Explorer plugin:
+.idea/**/mongoSettings.xml
+
+## File-based project format:
+*.iws
+
+## Plugin-specific files:
+
+# IntelliJ
+out/
+
+# mpeltonen/sbt-idea plugin
+.idea_modules/
+
+# JIRA plugin
+atlassian-ide-plugin.xml
+
+# Cursive Clojure plugin
+.idea/replstate.xml
+
+# Crashlytics plugin (for Android Studio and IntelliJ)
+com_crashlytics_export_strings.xml
+crashlytics.properties
+crashlytics-build.properties
+fabric.properties

SmartList/ds.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module org.jetbrains.idea.maven.project.MavenProjectsManager.isMavenModule="true" type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" LANGUAGE_LEVEL="JDK_1_8">
+    <output url="file://$MODULE_DIR$/target/classes" />
+    <output-test url="file://$MODULE_DIR$/target/test-classes" />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src/main/java" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/src/main/resources" type="java-resource" />
+      <sourceFolder url="file://$MODULE_DIR$/src/test/java" isTestSource="true" />
+      <excludeFolder url="file://$MODULE_DIR$/target" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+    <orderEntry type="library" name="Maven: junit:junit:4.12" level="project" />
+    <orderEntry type="library" name="Maven: org.hamcrest:hamcrest-core:1.3" level="project" />
+    <orderEntry type="library" name="Maven: org.jetbrains:annotations:13.0" level="project" />
+    <orderEntry type="library" name="Maven: org.apache.commons:commons-collections4:4.0" level="project" />
+  </component>
+</module>

SmartList/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>ru.spbau.mit.kazakov.SpiralMatrix</groupId>
+    <artifactId>SmartList</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>1.8</source>
+                    <target>1.8</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.12</version>
+        </dependency>
+        <dependency>
+            <groupId>org.jetbrains</groupId>
+            <artifactId>annotations</artifactId>
+            <version>13.0</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-collections4</artifactId>
+            <version>4.0</version>
+        </dependency>
+    </dependencies>
+
+</project>

SmartList/src/main/java/ru/spbau/mit/kazakov/SmartList/SmartList.java

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+package ru.spbau.mit.kazakov.SmartList;
+
+import org.apache.commons.collections4.iterators.EmptyIterator;
+import org.apache.commons.collections4.iterators.SingletonIterator;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.*;
+
+/**
+ * Smart list storage. Store one element using one reference, 2-5 element using an array, and more elements using ArrayList.
+ * @param <E> generic type
+ */
+public class SmartList<E> extends AbstractList<E> implements List<E> {
+    private int size;
+    private Object data;
+
+    /**
+     * Creates new empty list.
+     */
+    public SmartList() {
+        size = 0;
+        data = null;
+    }
+
+    /**
+     * Creates new list containing elements from specified collection.
+     *
+     * @param collection specified collection
+     */
+    public SmartList(@NotNull Collection<? extends E> collection) {
+        for (E elem : collection) {
+            add(elem);
+        }
+    }
+
+    /**
+     * Returns elements by specified index.
+     *
+     * @param i specified index
+     */
+    @SuppressWarnings("unchecked")
+    @Override
+    public E get(int i) {
+        if (i >= size) {
+            throw new IndexOutOfBoundsException();
+        }
+
+        if (size == 1) {
+            return (E) data;
+        } else if (size <= 5) {
+            return (E) ((Object[]) data)[i];
+        } else {
+            return (E) ((ArrayList<Object>) data).get(i);
+        }
+    }
+
+    @Override
+    public int size() {
+        return size;
+    }
+
+    /**
+     * Removes element by specified index
+     *
+     * @param i specified index
+     * @return removed element
+     * @throws IndexOutOfBoundsException if there is no element with specified index
+     */
+    @Override
+    @SuppressWarnings("unchecked")
+    public E remove(int i) throws IndexOutOfBoundsException {
+        if (i >= size) {
+            throw new IndexOutOfBoundsException();
+        }
+
+        E returnValue;
+        if (size == 1) {
+            returnValue = (E) data;
+            data = null;
+        } else if (size == 2) {
+            returnValue = (E) ((Object[]) data)[i];
+            data = i == 0 ? ((Object[]) data)[1] : ((Object[]) data)[0];
+        } else if (size <= 5) {
+            returnValue = (E) ((Object[]) data)[i];
+        } else if (size == 6) {
+            returnValue = (E) ((ArrayList<Object>) data).get(i);
+            Object[] array = new Object[5];
+            int j = 0;
+            while (j != 4) {
+                int cur = 0;
+                if (j != i) {
+                    array[cur] = ((ArrayList<Object>) data).get(j);
+                    cur++;
+                }
+                j++;
+            }
+            data = array;
+        } else {
+            returnValue = (E) ((ArrayList<Object>) data).remove(i);
+        }
+
+        size--;
+        return returnValue;
+    }
+
+    /**
+     * Adds specified element to list.
+     *
+     * @param elem specified element
+     * @return true
+     */
+    @Override
+    public boolean add(@NotNull E elem) {
+        size++;
+
+        if (size == 1) {
+            data = elem;
+        } else if (size == 2) {
+            Object[] array = new Object[5];
+            array[0] = this.data;
+            array[1] = elem;
+            data = array;
+        } else if (size <= 5) {
+            ((Object[]) data)[size - 1] = elem;
+        } else if (size == 6) {
+            ArrayList<Object> list = new ArrayList<>(Arrays.asList((Object[]) data));
+            list.add(elem);
+            data = list;
+        } else {
+            //noinspection unchecked
+            ((ArrayList<Object>) data).add(elem);
+        }
+
+        return true;
+    }
+
+    /**
+     * Checks if there is specified element in list
+     *
+     * @param elem specified element
+     * @return true if there is such element, and false otherwise
+     */
+    @Override
+    public boolean contains(@NotNull Object elem) {
+        if (size == 1) {
+            return data.equals(elem);
+        } else if (size <= 5) {
+            boolean result = false;
+            for (Object i : (Object[]) data) {
+                result |= i.equals(elem);
+            }
+            return result;
+        } else {
+            boolean result = false;
+            //noinspection unchecked
+            for (Object i : (ArrayList<Object>) data) {
+                result |= i.equals(elem);
+            }
+            return result;
+        }
+    }
+
+    /**
+     * Replaces the element at the specified index with the specified element.
+     *
+     * @param i    specified position
+     * @param elem specified element
+     * @return the element previously at the specified index
+     */
+    @Override
+    @SuppressWarnings("unchecked")
+    public E set(int i, @NotNull E elem) {
+        if (i >= size) {
+            throw new IndexOutOfBoundsException();
+        }
+
+        E returnValue;
+        if (size == 1) {
+            returnValue = (E) data;
+            data = elem;
+        } else if (size <= 5) {
+            returnValue = (E) ((Object[]) data)[i];
+            ((Object[]) data)[i] = elem;
+        } else {
+            returnValue = (E) ((ArrayList<Object>) data).get(i);
+            ((ArrayList<Object>) data).set(i, elem);
+        }
+
+        return returnValue;
+    }
+
+}