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

struct node_type;
struct sh_context;


struct node
{
    struct node_type *type;
};

struct node_type {
    const char *name;
    void (*free)(struct node *node);
    // a real world example would have an output stream
    void (*print)(struct node *node);
    int (*exec)(struct node *node, struct sh_context *context);
};

static void node_free(struct node *node) {
    if (node)
        node->type->free(node);
}

static void node_print(struct node *node) {
    if (node == NULL)
        printf("<null node>");
    else
        node->type->print(node);
}

static int node_exec(struct node *node, struct sh_context *context) {
    return node->type->exec(node, context);
}

static void *alloc_node(size_t size, struct node_type *type) {
    struct node *res = malloc(size);
    res->type = type;
    return res;
}

// type must be both a node_type instance, and a struct
// inheriting from node
#define make_node(Type) \
    ((struct Type*[]){alloc_node(sizeof(struct Type), &(Type))})[0]

struct node_if
{
    struct node base;
    struct node *condition;
    struct node *branch;
    struct node *else_branch;
};

static void node_if_free(struct node *base_node) {
    struct node_if *node = (void*)base_node;
    node_free(node->condition);
    node_free(node->branch);
    node_free(node->else_branch);
    free(node);
}

static void node_if_print(struct node *base_node) {
    struct node_if *node = (void*)base_node;
    printf("if (");
    node_print(node->condition);
    printf(") {");
    node_print(node->branch);
    printf("}");

    if (node->else_branch) {
        printf("{");
        node_print(node->else_branch);
        printf("}");
    }
}

static int node_if_exec(struct node *base_node, struct sh_context *context) {
    struct node_if *node = (void*)base_node;
    int cond_res = node_exec(node->condition, context);
    if (cond_res == 0)
        return node_exec(node->branch, context);

    if (node->else_branch == NULL)
        return cond_res;

    return node_exec(node->else_branch, context);
}

struct node_type node_if = {
    .name = "if",
    .free = node_if_free,
    .print = node_if_print,
    .exec = node_if_exec,
};


int main(void) {
    struct node_if *node = make_node(node_if);
    node->condition = NULL;
    node->branch = NULL;
    node->else_branch = NULL;
    node_print(&node->base);
    node_free(&node->base);
}