Function

Function/.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

Function/Function.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:4.8" level="project" />
+    <orderEntry type="library" name="Maven: org.jetbrains:annotations:13.0" level="project" />
+    <orderEntry type="library" name="Maven: com.google.guava:guava:23.2-jre" level="project" />
+    <orderEntry type="library" name="Maven: com.google.code.findbugs:jsr305:1.3.9" level="project" />
+    <orderEntry type="library" name="Maven: com.google.errorprone:error_prone_annotations:2.0.18" level="project" />
+    <orderEntry type="library" name="Maven: com.google.j2objc:j2objc-annotations:1.1" level="project" />
+    <orderEntry type="library" name="Maven: org.codehaus.mojo:animal-sniffer-annotations:1.14" level="project" />
+  </component>
+</module>

Function/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</groupId>
+    <artifactId>Function</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>2.3.2</version>
+                <configuration>
+                    <source>1.8</source>
+                    <target>1.8</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.8</version>
+        </dependency>
+        <dependency>
+            <groupId>org.jetbrains</groupId>
+            <artifactId>annotations</artifactId>
+            <version>13.0</version>
+        </dependency>
+        <dependency>
+            <groupId>com.google.guava</groupId>
+            <artifactId>guava</artifactId>
+            <version>23.2-jre</version>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
+    </properties>
+
+</project>

Function/src/main/java/ru/spbau/mit/kazakov/Function/Collections.java

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+package ru.spbau.mit.kazakov.Function;
+
+
+import com.google.common.collect.Lists;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.*;
+
+/**
+ * Class providing methods for working with collections using functions.
+ */
+public class Collections {
+    /**
+     * Applies specified function to specified container's elements and adds mapped elements in new list.
+     *
+     * @param function specified function
+     * @param elements specified container
+     * @param <T>      type of elements in specified container
+     * @param <U>      type of elements in result list
+     * @return result list
+     */
+    static public <T, U> List<U> map(@NotNull final Function1<? super T, ? extends U> function, @NotNull final Iterable<T> elements) {
+        List<U> resultList = new ArrayList<>();
+
+        for (T element : elements) {
+            resultList.add(function.apply(element));
+        }
+
+        return resultList;
+    }
+
+    /**
+     * Creates new list composed of elements from specified container that satisfy specified predicate.
+     *
+     * @param predicate specified predicate
+     * @param elements  specified container
+     * @param <T>       type of elements in specified container
+     * @return result list
+     */
+    static public <T> List<T> filter(@NotNull final Predicate<? super T> predicate, @NotNull final Iterable<T> elements) {
+        List<T> resultList = new ArrayList<>();
+
+        for (T element : elements) {
+            if (predicate.apply(element)) {
+                resultList.add(element);
+            }
+        }
+
+        return resultList;
+    }
+
+    /**
+     * Creates new list by adding elements from specified container while they satisfy specified predicate.
+     *
+     * @param predicate specified predicate
+     * @param elements  specified container
+     * @param <T>       type of elements in specified container
+     * @return result list
+     */
+    static public <T> List<T> takeWhile(@NotNull final Predicate<? super T> predicate, @NotNull final Iterable<T> elements) {
+        List<T> resultList = new ArrayList<>();
+
+        for (T element : elements) {
+            if (!predicate.apply(element)) {
+                break;
+            }
+            resultList.add(element);
+        }
+
+        return resultList;
+    }
+
+    /**
+     * Creates new list by adding elements from specified container unless they satisfy specified predicate.
+     *
+     * @param predicate specified predicate
+     * @param elements  specified container
+     * @param <T>       type of elements in specified container
+     * @return result list
+     */
+    static public <T> List<T> takeUnless(@NotNull final Predicate<? super T> predicate, @NotNull final Iterable<T> elements) {
+        return takeWhile(predicate.not(), elements);
+    }
+
+    /**
+     * Left-associative fold of specified collection. Reduces the list using specified function from left to right.
+     *
+     * @param function   specified function
+     * @param startValue first value for function's first element
+     * @param elements   specified collection
+     * @param <T>        type of elements in specified collection
+     * @param <U>        specified function's codomain
+     * @return result value
+     */
+    static public <T, U> U foldl(@NotNull final Function2<? super U, ? super T, ? extends U> function,
+                                 U startValue, @NotNull final Collection<T> elements) {
+        U resultValue = startValue;
+
+        for (T element : elements) {
+            resultValue = function.apply(resultValue, element);
+        }
+
+        return resultValue;
+    }
+
+    /**
+     * Right-associative fold of specified collection. Reduces the list using specified function from right to left.
+     *
+     * @param function   specified function
+     * @param startValue first value for function's second element
+     * @param elements   specified collection
+     * @param <T>        type of elements in specified collection
+     * @param <U>        specified function's codomain
+     * @return result value
+     */
+    static public <T, U> U foldr(@NotNull final Function2<? super T, ? super U, ? extends U> function,
+                                 U startValue, @NotNull final Collection<T> elements) {
+        U resultValue = startValue;
+
+        for (T element : Lists.reverse(new ArrayList<>(elements))) {
+            resultValue = function.apply(element, resultValue);
+        }
+
+        return resultValue;
+    }
+}

Function/src/main/java/ru/spbau/mit/kazakov/Function/Function1.java

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+package ru.spbau.mit.kazakov.Function;
+
+
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * The interface for representing a function with one argument.
+ *
+ * @param <T> domain's type
+ * @param <U> codomain's type
+ */
+@FunctionalInterface
+public interface Function1<T, U> {
+    /**
+     * Applies function to specified argument.
+     *
+     * @param argument for applying to
+     * @return mapped value
+     */
+    U apply(T argument);
+
+    /**
+     * Creates composition of functions.
+     *
+     * @param function for composition
+     * @param <V>      specified function's codomain's type
+     * @return result of function composition
+     */
+    default <V> Function1<T, V> compose(@NotNull final Function1<? super U, ? extends V> function) {
+        return argument -> function.apply(apply(argument));
+    }
+}

Function/src/main/java/ru/spbau/mit/kazakov/Function/Function2.java

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+package ru.spbau.mit.kazakov.Function;
+
+
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * The interface for representing a function with two arguments.
+ *
+ * @param <T> first argument's type
+ * @param <U> second argument's type
+ * @param <K> codomain's type
+ */
+@FunctionalInterface
+public interface Function2<T, U, K> {
+    /**
+     * Applies function to specified argument.
+     *
+     * @param firstArgument  for applying to
+     * @param secondArgument for applying to
+     * @return mapped value
+     */
+    K apply(T firstArgument, U secondArgument);
+
+    /**
+     * Creates composition of functions.
+     *
+     * @param <R> specified function's codomain's type
+     * @return result of function composition
+     */
+    default <R> Function2<T, U, R> compose(@NotNull final Function1<? super K, ? extends R> function) {
+        return (firstArgument, secondArgument) -> function.apply(apply(firstArgument, secondArgument));
+    }
+
+    /**
+     * Binds first argument to specified value.
+     *
+     * @param firstArgument specified value
+     * @return function of one argument
+     */
+    default Function1<U, K> bind1(T firstArgument) {
+        return secondArgument -> apply(firstArgument, secondArgument);
+    }
+
+    /**
+     * Binds second argument to specified value.
+     *
+     * @param secondArgument specified value
+     * @return function of one argument
+     */
+    default Function1<T, K> bind2(U secondArgument) {
+        return firstArgument -> apply(firstArgument, secondArgument);
+    }
+
+    /**
+     * Converts current function with two arguments into two functions with one argument.
+     *
+     * @return new function with one argument
+     */
+    default Function1<T, Function1<U, K>> curry() {
+        return this::bind1;
+    }
+}