start new parser

Author: h908714124

src/main/java/io/polypen/parse/Parser.java

@@ -0,0 +1,82 @@
+package io.polypen.parse;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.io.PushbackReader;
+import java.util.ArrayList;
+import java.util.List;
+
+public final class Parser {
+
+    private static ListExpr parse(PushbackReader reader) throws IOException {
+        consumeWhitespace(reader);
+        List<Expr> result = new ArrayList<>();
+        int c;
+        loop:
+        while ((c = reader.read()) != -1) {
+            switch (c) {
+                case '(' -> {
+                    ListExpr expr = parse(reader);
+                    result.add(expr);
+                }
+                case ')' -> {
+                    break loop;
+                }
+                default -> {
+                    reader.unread(c);
+                    Expr atom = readAtom(reader);
+                    result.add(atom);
+                }
+            }
+        }
+        return new ListExpr(result);
+    }
+
+    private static Expr readAtom(PushbackReader reader) throws IOException {
+        consumeWhitespace(reader);
+        StringBuilder sb = new StringBuilder();
+        int c;
+        while ((c = reader.read()) != -1) {
+            if (c != ' ' && c != '(' && c != ')') {
+                sb.append((char) c);
+            } else {
+                reader.unread(c);
+                break;
+            }
+        }
+        return new Atom(sb.toString());
+    }
+
+    private static void consumeWhitespace(PushbackReader reader) throws IOException {
+        while (true) {
+            int c = reader.read();
+            if (c != ' ') {
+                if (c != -1) {
+                    reader.unread(c);
+                }
+                break;
+            }
+        }
+    }
+
+    public static ListExpr parse(String s) {
+        try (PushbackReader reader = new PushbackReader(new InputStreamReader(new ByteArrayInputStream(s.getBytes())))) {
+            return parse(reader);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    public interface Expr {
+    }
+
+    public record Atom(String value) implements Expr {
+    }
+
+    public record ListExpr(List<Expr> values) implements Expr {
+    }
+
+    private Parser() {
+    }
+}

src/test/java/io/polypen/parse/ParserTest.java

@@ -0,0 +1,12 @@
+package io.polypen.parse;
+
+import org.junit.jupiter.api.Test;
+
+class ParserTest {
+
+    @Test
+    void parse() {
+        Parser.ListExpr result = Parser.parse("(a b) 2 3");
+        System.out.println(result);
+    }
+}