Recursive S-expression parsing

lisp.c

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "sprite.h"

#define GAME_FILE "config/game.lisp"

enum NodeType {
  LIST = 0,
  SYMBOL = 1
};

struct Node {
  enum NodeType type;
  
  struct Node *next;
  
  union {
    struct Node *list;
    char *symbol;
  };
};

void free_node(struct Node *node);
struct Node *parse_expr(FILE *fp, struct Node *head, struct Node *tail);

int main(int argc, char *argv[]) {
  FILE *fp;
  fp = fopen(GAME_FILE, "r");
  
  struct Node *root = parse_expr(fp, NULL, NULL);

  fclose(fp);

  free_node(root);

  return 0;
}

void free_node(struct Node *node) {
  struct Node *tmp;
  
  while (node != NULL) {
    tmp = node;
    
    if (node->type == SYMBOL) {
      free(node->symbol);
      node->symbol = NULL;
    }
    
    if (node->type == LIST) {
      free_node(node->list);
    }
    
    node = node->next;
    
    free(tmp);
    tmp = NULL;
  }
}

struct Node *parse_expr(FILE *fp, struct Node *head, struct Node *tail) {
  struct Node *node;
  
  int c;
  char buffer[32];
  
  while ((c = fgetc(fp)) != EOF) {
    if (c == '(') {
      node = calloc(1, sizeof(struct Node));
      node->type = LIST;
      node->list = parse_expr(fp, NULL, NULL);
      
      if (head == NULL) {
        return parse_expr(fp, node, node);
      } else {
        tail->next = node;
        return parse_expr(fp, head, node);
      }
    }
    else if (c == ')') {
      break;
    }
    else if (!isspace(c)) {
      ungetc(c, fp);
      
      if (c == '"') {
        fscanf(fp, "\"%31[^\"]\"", buffer);
      } else {
        fscanf(fp, "%31[^() \t\n\v\f\r]", buffer);
      }
      
      node = calloc(1, sizeof(struct Node));
      node->type = SYMBOL;
      node->symbol = calloc(strlen(buffer), sizeof(char));
      strcpy(node->symbol, buffer);
      
      if (head == NULL) {
        return parse_expr(fp, node, node);
      } else {
        tail->next = node;
        return parse_expr(fp, head, node);
      }
    }
  }
  
  return head;
}