ast: Create AST visitor to traverse the tree

In the future we want to have the possibility of traverse the tree and
pretty print it or generate binary for other platform like LLVM or
transpile to C.

This solution also implements the gas assembly x86_64 Linux code
generation by using the visitor interface.

Signed-off-by: Johnny Richard <johnny@johnnyrichard.com>

7 files changed, 231 insertions, 32 deletions

diff --git a/src/ast.c b/src/ast.c
new file mode 100644
index 0000000..f8c2713
--- /dev/null
+++ b/src/ast.c
@@ -0,0 +1,63 @@
+/*
+* Copyright (C) 2023 Johnny Richard
+*
+* This program is free software: you can redistribute it and/or modify
+* it under the terms of the GNU General Public License as published by
+* the Free Software Foundation, either version 3 of the License, or
+* (at your option) any later version.
+*
+* This program is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU General Public License for more details.
+*
+* You should have received a copy of the GNU General Public License
+* along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+#include <assert.h>
+#include "ast.h"
+
+void
+ast_node_accept_visitor(ast_node_t *node, ast_visitor_t *visitor)
+{
+  assert(node);
+  assert(node->accept_visitor);
+  node->accept_visitor(node, visitor);
+}
+
+static void
+ast_function_accept_visitor(ast_node_t *node, ast_visitor_t *visitor)
+{
+  visitor->visit_function(visitor, (ast_function_t *) node);
+}
+
+static void
+ast_return_stmt_accept_visitor(ast_node_t *node, ast_visitor_t *visitor)
+{
+  visitor->visit_return_stmt(visitor, (ast_return_stmt_t *) node);
+}
+
+ast_return_stmt_t
+ast_return_stmt_create(uint32_t number)
+{
+  return (ast_return_stmt_t)  {
+    .super = (ast_node_t) {
+      .accept_visitor = &ast_return_stmt_accept_visitor
+    },
+    .number = number
+  };
+}
+
+ast_function_t
+ast_function_create(string_view_t name, type_t return_type, ast_return_stmt_t body)
+{
+  return (ast_function_t) {
+    .super = (ast_node_t) {
+      .accept_visitor = &ast_function_accept_visitor
+    },
+    .name = name,
+    .return_type = return_type,
+    .body = body
+  };
+}
+
diff --git a/src/ast.h b/src/ast.h
new file mode 100644
index 0000000..7f5a198
--- /dev/null
+++ b/src/ast.h
@@ -0,0 +1,57 @@
+/*
+* Copyright (C) 2023 Johnny Richard
+*
+* This program is free software: you can redistribute it and/or modify
+* it under the terms of the GNU General Public License as published by
+* the Free Software Foundation, either version 3 of the License, or
+* (at your option) any later version.
+*
+* This program is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU General Public License for more details.
+*
+* You should have received a copy of the GNU General Public License
+* along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+#ifndef AST_H
+#define AST_H
+#include <stdint.h>
+#include "string_view.h"
+
+#define ast_visitor_visit(visitor, node) ast_node_accept_visitor((ast_node_t *) node, (ast_visitor_t *) visitor);
+
+typedef enum {
+  TYPE_I32
+} type_t;
+
+typedef struct ast_visitor_t ast_visitor_t;
+
+typedef struct ast_node_t {
+  void (*accept_visitor)(struct ast_node_t *, ast_visitor_t *);
+} ast_node_t;
+
+typedef struct ast_return_stmt_t {
+  struct ast_node_t super;
+  uint32_t number;
+} ast_return_stmt_t;
+
+typedef struct ast_function_t {
+  struct ast_node_t super;
+  string_view_t name;
+  type_t  return_type;
+  ast_return_stmt_t body;
+} ast_function_t;
+
+typedef struct ast_visitor_t {
+  void (*visit_function)(struct ast_visitor_t *, ast_function_t *);
+  void (*visit_return_stmt)(struct ast_visitor_t *, ast_return_stmt_t *);
+} ast_visitor_t;
+
+void ast_node_accept_visitor(ast_node_t *node, ast_visitor_t *visitor);
+
+ast_function_t ast_function_create(string_view_t name, type_t return_type, ast_return_stmt_t body);
+ast_return_stmt_t ast_return_stmt_create(uint32_t number);
+
+#endif /* AST_H */
+
diff --git a/src/gas_assembly_generator.c b/src/gas_assembly_generator.c
new file mode 100644
index 0000000..d164b7d
--- /dev/null
+++ b/src/gas_assembly_generator.c
@@ -0,0 +1,66 @@
+/*
+* Copyright (C) 2023 Johnny Richard
+*
+* This program is free software: you can redistribute it and/or modify
+* it under the terms of the GNU General Public License as published by
+* the Free Software Foundation, either version 3 of the License, or
+* (at your option) any later version.
+*
+* This program is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU General Public License for more details.
+*
+* You should have received a copy of the GNU General Public License
+* along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+#include "gas_assembly_generator.h"
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+static void gas_assembly_generator_visit_function(ast_visitor_t *visitor, ast_function_t *func);
+static void gas_assembly_generator_visit_return_stmt(ast_visitor_t *visitor, ast_return_stmt_t *return_stmt);
+
+void
+gas_assembly_generator_init(gas_assembly_generator_t *gen, FILE *out)
+{
+  assert(gen && out);
+  gen->super = (ast_visitor_t) {
+    .visit_function = &gas_assembly_generator_visit_function,
+    .visit_return_stmt = &gas_assembly_generator_visit_return_stmt
+  };
+  gen->out = out;
+}
+
+static void
+gas_assembly_generator_visit_function(ast_visitor_t *visitor, ast_function_t *func)
+{
+  assert(visitor && func);
+
+  if (!string_view_eq(func->name, string_view_from_str("main"))) {
+    fprintf(stderr, "[ERROR]: no main function has been defined!\n");
+    exit(EXIT_FAILURE);
+  }
+
+  gas_assembly_generator_t *gen = (gas_assembly_generator_t *) visitor;
+
+  fprintf(gen->out,".global _start\n");
+  fprintf(gen->out,".text\n");
+  fprintf(gen->out,"_start:\n");
+
+  ast_visitor_visit(visitor, &func->body);
+}
+
+static void
+gas_assembly_generator_visit_return_stmt(ast_visitor_t *visitor, ast_return_stmt_t *return_stmt)
+{
+  assert(visitor && return_stmt);
+  gas_assembly_generator_t *gen = (gas_assembly_generator_t *) visitor;
+
+  fprintf(gen->out, "  mov $1, %%al\n");
+  fprintf(gen->out, "  mov $%d, %%ebx\n", return_stmt->number);
+  fprintf(gen->out, "  int $0x80\n");
+}
+
diff --git a/src/gas_assembly_generator.h b/src/gas_assembly_generator.h
new file mode 100644
index 0000000..64f861e
--- /dev/null
+++ b/src/gas_assembly_generator.h
@@ -0,0 +1,33 @@
+/*
+* Copyright (C) 2023 Johnny Richard
+*
+* This program is free software: you can redistribute it and/or modify
+* it under the terms of the GNU General Public License as published by
+* the Free Software Foundation, either version 3 of the License, or
+* (at your option) any later version.
+*
+* This program is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU General Public License for more details.
+*
+* You should have received a copy of the GNU General Public License
+* along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+#ifndef GAS_ASSEMBLY_GENERATOR_H
+#define GAS_ASSEMBLY_GENERATOR_H
+
+#include "ast.h"
+#include <stdio.h>
+
+typedef struct gas_assembly_generator_t {
+  ast_visitor_t super;
+  FILE *out;
+} gas_assembly_generator_t;
+
+void gas_assembly_generator_init(gas_assembly_generator_t *, FILE *);
+
+void gas_assembly_generator_visit(gas_assembly_generator_t * gen, ast_node_t* ast);
+
+#endif /* GAS_ASSEMBLY_GENERATOR_H */
+
diff --git a/src/parser.c b/src/parser.c
index 593f75a..b956c4e 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -20,6 +20,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "ast.h"
 #include "lexer.h"
 #include "parser.h"
 
@@ -92,9 +93,7 @@ parser_parse_return_stmt(parser_t *parser)
   char number_as_str[number_token.value.size];
   string_view_to_str(&number_token.value, number_as_str);
 
-  return (ast_return_stmt_t) {
-    .number = atoi(number_as_str)
-  };
+  return ast_return_stmt_create(atoi(number_as_str));
 }
 
 ast_function_t
@@ -106,9 +105,9 @@ parser_parse_function(parser_t *parser)
   expected_token(parser, TOKEN_COLON);
   type_t return_type = parser_parse_type(parser);
 
-  return (ast_function_t) {
-    .name = func_name_token.value,
-    .return_type = return_type,
-    .body = parser_parse_return_stmt(parser)
-  };
+  return ast_function_create(
+    func_name_token.value,
+    return_type,
+    parser_parse_return_stmt(parser)
+  );
 }
diff --git a/src/parser.h b/src/parser.h
index d44a4df..b846ae1 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -17,6 +17,7 @@
 #ifndef PARSER_H
 #define PARSER_H
 
+#include "ast.h"
 #include "lexer.h"
 #include "string_view.h"
 
@@ -24,20 +25,6 @@ typedef struct parser_t {
   lexer_t *lexer;
 } parser_t;
 
-typedef enum {
-  TYPE_I32
-} type_t;
-
-typedef struct ast_return_stmt_t {
-  uint32_t number;
-} ast_return_stmt_t;
-
-typedef struct ast_function_t {
-  string_view_t name;
-  type_t  return_type;
-  ast_return_stmt_t body;
-} ast_function_t;
-
 void parser_init(parser_t *parser, lexer_t *lexer);
 
 ast_function_t parser_parse_function(parser_t *parser);
diff --git a/src/pipac.c b/src/pipac.c
index 6e6df9e..e56d8a1 100644
--- a/src/pipac.c
+++ b/src/pipac.c
@@ -21,21 +21,15 @@
 #include "lexer.h"
 #include "parser.h"
 #include "string_view.h"
+#include "gas_assembly_generator.h"
 
 void
 generate_gas_x86_64_linux(ast_function_t *func)
 {
-  if (!string_view_eq(func->name, string_view_from_str("main"))) {
-    fprintf(stderr, "[ERROR]: no main function has been defined!\n");
-    exit(EXIT_FAILURE);
-  }
+  gas_assembly_generator_t gen;
+  gas_assembly_generator_init(&gen, stdout);
 
-  printf(".global _start\n");
-  printf(".text\n");
-  printf("_start:\n");
-  printf("  mov $1, %%al\n");
-  printf("  mov $%d, %%ebx\n", func->body.number);
-  printf("  int $0x80\n");
+  ast_visitor_visit(&gen, func);
 }
 
 void