Solves string decompression problem with parser combinators
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parent
6048270224
commit
21edb50158
4
pom.xml
4
pom.xml
@ -13,8 +13,8 @@
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<configuration>
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<source>8</source>
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<target>8</target>
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<source>9</source>
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<target>9</target>
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</configuration>
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</plugin>
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</plugins>
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233
src/main/java/net/abhinavsarkar/former/StringDecompression.java
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233
src/main/java/net/abhinavsarkar/former/StringDecompression.java
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@ -0,0 +1,233 @@
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package net.abhinavsarkar.former;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.Optional;
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import java.util.function.BiFunction;
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import java.util.function.Function;
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import java.util.function.Predicate;
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import java.util.stream.Collectors;
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import java.util.stream.Stream;
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/**
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* https://techdevguide.withgoogle.com/resources/compress-decompression/
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*/
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public class StringDecompression {
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public static void main(String[] args) {
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System.out.println(decompress("10[a]"));
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System.out.println(decompress("3[abc]4[ab]c"));
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System.out.println(decompress("2[3[a]b]"));
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System.out.println(decompress("2[]b"));
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System.out.println(decompress("0[abc]v"));
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System.out.println(decompress("szs"));
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}
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static String decompress(String s) {
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return parse(s).toString();
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}
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private static CompressedString parse(String s) {
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Optional<Tuple<String, CompressedString>> parsed = CompressedString.PARSER.parse(s);
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if (!parsed.isPresent()) {
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throw new IllegalArgumentException("Unable to decompress");
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}
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return parsed.get().second;
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}
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}
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class Tuple<A, B> {
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A first;
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B second;
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Tuple(A first, B second) {
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this.first = first;
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this.second = second;
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}
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<C> Tuple<A, C> map(Function<B, C> f) {
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return new Tuple<>(first, f.apply(second));
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}
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}
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@FunctionalInterface
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interface Parser<I, O> {
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Optional<Tuple<I, O>> parse(I input);
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default <P> Parser<I, P> map(Function<O, P> mapper) {
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return input -> this.parse(input).map(tup -> tup.map(mapper));
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}
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default <P> Parser<I, P> seqApply(Parser<I, Function<O, P>> parser) {
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return input -> parser.parse(input)
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.flatMap(ftup -> this.parse(ftup.first).map(tup -> tup.map(ftup.second)));
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}
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default <P> Parser<I, P> leftSeqApply(Parser<I, P> parser) {
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return this.apply((x, y) -> y, parser);
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}
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default <P> Parser<I, O> rightSeqApply(Parser<I, P> parser) {
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return this.apply((x, y) -> x, parser);
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}
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default <P, Q> Parser<I, Q> apply(BiFunction<O, P, Q> function, Parser<I, P> parser) {
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return parser.seqApply(this.map(o -> p -> function.apply(o, p)));
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}
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default Parser<I, O> or(Parser<I, O> parser) {
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return input -> this.parse(input).or(() -> parser.parse(input));
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}
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default Parser<I, List<O>> many() {
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return input -> {
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I rest = input;
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List<O> output = new ArrayList<>();
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Optional<Tuple<I, O>> parsed = this.parse(rest);
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while (parsed.isPresent()) {
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Tuple<I, O> tup = parsed.get();
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rest = tup.first;
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output.add(tup.second);
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parsed = this.parse(rest);
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}
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return Optional.of(new Tuple<>(rest, output));
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};
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}
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default Parser<I, List<O>> some() {
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return input -> {
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Tuple<I, List<O>> output = many().parse(input).get();
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if (output.second.isEmpty()) {
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return Optional.empty();
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} else {
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return Optional.of(output);
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}
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};
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}
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}
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class CharPredicateParser implements Parser<String, Character> {
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private Predicate<Character> predicate;
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CharPredicateParser(Predicate<Character> predicate) {
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this.predicate = predicate;
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}
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@Override
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public Optional<Tuple<String, Character>> parse(String input) {
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if (input.isEmpty()) {
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return Optional.empty();
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}
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Character i = input.charAt(0);
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if (predicate.test(i)) {
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return Optional.of(new Tuple<>(input.substring(1), i));
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}
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return Optional.empty();
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}
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}
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class Parsers {
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public static final Parser<String, Character> DIGIT_PARSER =
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new CharPredicateParser(Character::isDigit);
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public static final Parser<String, Integer> NUMBER_PARSER =
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DIGIT_PARSER
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.some()
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.map(Parsers::digitsToInt);
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public static final Parser<String, Character> ENGLISH_LOWER_CASE_CHAR_PARSER =
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new CharPredicateParser(c -> c >= 97 && c <= 122);
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public static Parser<String, Character> charParser(Character c) {
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return new CharPredicateParser(c::equals);
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}
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private static Integer digitsToInt(List<Character> digits) {
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int i = 0;
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for (char c : digits) {
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i = i * 10 + (c - 48);
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}
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return i;
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}
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}
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class CompressedString {
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private List<Part> parts;
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CompressedString(List<Part> parts) {
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this.parts = parts;
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}
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interface Part {
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String toString();
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}
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@Override
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public String toString() {
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return parts.stream()
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.map(Object::toString)
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.collect(Collectors.joining());
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}
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public static final Parser<String, CompressedString> PARSER =
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CompressedPart.PARSER
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.or(StringPart.PARSER)
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.many()
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.map(CompressedString::new);
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}
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class StringPart implements CompressedString.Part {
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private String string;
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StringPart(String string) {
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this.string = string;
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}
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@Override
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public String toString() {
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return this.string;
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}
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public static final Parser<String, CompressedString.Part> PARSER =
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Parsers.ENGLISH_LOWER_CASE_CHAR_PARSER
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.some()
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.map(chars -> new StringPart(charListToString(chars)));
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private static String charListToString(List<Character> chars) {
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return chars.stream()
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.map(String::valueOf)
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.collect(Collectors.joining());
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}
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}
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class CompressedPart implements CompressedString.Part {
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private int repetition;
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private CompressedString string;
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CompressedPart(int repetition, CompressedString string) {
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this.repetition = repetition;
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this.string = string;
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}
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@Override
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public String toString() {
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String s = string.toString();
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return Stream.generate(() -> s)
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.limit(repetition)
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.collect(Collectors.joining());
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}
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private static final Parser<String, CompressedString> BRACKETED_STRING_PARSER =
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Parsers.charParser('[')
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.leftSeqApply(input -> CompressedString.PARSER
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.rightSeqApply(Parsers.charParser(']'))
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.parse(input));
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public static final Parser<String, CompressedString.Part> PARSER =
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Parsers.NUMBER_PARSER.apply(CompressedPart::new, BRACKETED_STRING_PARSER);
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}
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