From cac74b1c10c6f788fa9d78551af452c88d49f444 Mon Sep 17 00:00:00 2001
From: Sebastian Hugentobler <sebastian@vanwa.ch>
Date: Sat, 8 Feb 2025 10:21:26 +0100
Subject: [PATCH] add ast generator in java

---
 README.md                                     |  4 +-
 java/lox-interpreter/pom.xml                  | 19 ++++-
 .../java/ch/vanwa/lox_interpreter/Expr.java   | 40 +++++++++++
 .../main/java/ch/vanwa/tool/GenerateAst.java  | 72 +++++++++++++++++++
 4 files changed, 131 insertions(+), 4 deletions(-)
 create mode 100644 java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Expr.java
 create mode 100644 java/lox-interpreter/src/main/java/ch/vanwa/tool/GenerateAst.java

diff --git a/README.md b/README.md
index b301b11..d18e422 100644
--- a/README.md
+++ b/README.md
@@ -13,7 +13,9 @@ Though my recommendation is to use the provided
 Implementation of the tree-walk interpreter in java after the book.
 
 Run `mvn package` to create an executable jar file in the `target` directory and
-`mvn exec:java` to get a repl.
+`mvn exec:java@jlox` to get a repl.
+
+Run `mvn exec:java@generate-ast` to generate the expression class.
 
 ## [Rust implementation](./rust/rox/)
 
diff --git a/java/lox-interpreter/pom.xml b/java/lox-interpreter/pom.xml
index 36e01d3..dabfda5 100644
--- a/java/lox-interpreter/pom.xml
+++ b/java/lox-interpreter/pom.xml
@@ -112,14 +112,27 @@
           <version>3.5.0</version>
           <executions>
             <execution>
+              <id>jlox</id>
               <goals>
                 <goal>java</goal>
               </goals>
+              <configuration>
+                <mainClass>ch.vanwa.lox_interpreter.App</mainClass>
+              </configuration>
+            </execution>
+            <execution>
+              <id>generate-ast</id>
+              <goals>
+                <goal>java</goal>
+              </goals>
+              <configuration>
+                <mainClass>ch.vanwa.tool.GenerateAst</mainClass>
+                  <arguments>
+                    <argument>src/main/java/ch/vanwa/lox_interpreter</argument>
+                  </arguments>
+              </configuration>
             </execution>
           </executions>
-          <configuration>
-            <mainClass>ch.vanwa.lox_interpreter.App</mainClass>
-          </configuration>
         </plugin>
       </plugins>
   </build>
diff --git a/java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Expr.java b/java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Expr.java
new file mode 100644
index 0000000..ab28035
--- /dev/null
+++ b/java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Expr.java
@@ -0,0 +1,40 @@
+package ch.vanwa.lox_interpreter;
+
+import java.util.List;
+
+abstract class Expr {
+  static class Binary extends Expr {
+    Binary(Expr left, Token operator, Expr right) {
+      this.left = left;
+      this.operator = operator;
+      this.right = right;
+    }
+
+    final Expr left;
+    final Token operator;
+    final Expr right;
+  }
+  static class Grouping extends Expr {
+    Grouping(Expr expression) {
+      this.expression = expression;
+    }
+
+    final Expr expression;
+  }
+  static class Literal extends Expr {
+    Literal(Object value) {
+      this.value = value;
+    }
+
+    final Object value;
+  }
+  static class Unary extends Expr {
+    Unary(Token operator, Expr right) {
+      this.operator = operator;
+      this.right = right;
+    }
+
+    final Token operator;
+    final Expr right;
+  }
+}
diff --git a/java/lox-interpreter/src/main/java/ch/vanwa/tool/GenerateAst.java b/java/lox-interpreter/src/main/java/ch/vanwa/tool/GenerateAst.java
new file mode 100644
index 0000000..ad6e201
--- /dev/null
+++ b/java/lox-interpreter/src/main/java/ch/vanwa/tool/GenerateAst.java
@@ -0,0 +1,72 @@
+package ch.vanwa.tool;
+
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.util.Arrays;
+import java.util.List;
+
+public class GenerateAst {
+  public static void main(String[] args) throws IOException {
+    if (args.length != 1) {
+      System.err.println("Usage: generate_ast <output directory>");
+      System.exit(64);
+    }
+    String outputDir = args[0];
+
+    defineAst(outputDir, "Expr", Arrays.asList(
+        "Binary   : Expr left, Token operator, Expr right",
+        "Grouping : Expr expression",
+        "Literal  : Object value",
+        "Unary    : Token operator, Expr right"));
+  }
+
+  private static void defineAst(
+      String outputDir, String baseName, List<String> types)
+      throws IOException {
+    String path = outputDir + "/" + baseName + ".java";
+    PrintWriter writer = new PrintWriter(path, "UTF-8");
+
+    writer.println("package ch.vanwa.lox_interpreter;");
+    writer.println();
+    writer.println("import java.util.List;");
+    writer.println();
+    writer.println("abstract class " + baseName + " {");
+
+    // The AST classes.
+    for (String type : types) {
+      String className = type.split(":")[0].trim();
+      String fields = type.split(":")[1].trim();
+      defineType(writer, baseName, className, fields);
+    }
+
+    writer.println("}");
+    writer.close();
+  }
+
+  private static void defineType(
+      PrintWriter writer, String baseName,
+      String className, String fieldList) {
+    writer.println("  static class " + className + " extends " +
+        baseName + " {");
+
+    // Constructor.
+    writer.println("    " + className + "(" + fieldList + ") {");
+
+    // Store parameters in fields.
+    String[] fields = fieldList.split(", ");
+    for (String field : fields) {
+      String name = field.split(" ")[1];
+      writer.println("      this." + name + " = " + name + ";");
+    }
+
+    writer.println("    }");
+
+    // Fields.
+    writer.println();
+    for (String field : fields) {
+      writer.println("    final " + field + ";");
+    }
+
+    writer.println("  }");
+  }
+}