struct Environment {
    struct Environment* parent;
    int       key_count;
    char**    keys;
    Ast_Node* values;
};

typedef struct Environment Environment;

Environment* create_empty_environment() {
    Environment* env = new(Environment);

    env->parent    = nullptr;
    env->key_count = 0;
    env->keys      = nullptr;
    env->values    = nullptr;

    return env;
}

Ast_Node* lookup_symbol(Symbol* sym, Environment* env) {
    for (int i = 0; i < env->key_count; ++i)
        if (string_equal(env->keys[i], sym->identifier))
            return env->values+i;

    if (env->parent)
        return lookup_symbol(sym, env->parent);

    char* built_in_names[] = {
        // Math stuff
        "+", "-", "*", "/",
        ">", "<", "=",
        // Conditional stuff
        "if", "and", "or", "not",
        // Cons stuff
        "first", "rest", "pair",
        // rest
        "load", "define",
        "lambda", "progn",
        "eval", "quote",
        "print", "read",
        "help"
    };
    int built_in_count = 23;

    for (int i = 0; i < built_in_count; ++i) {
        if (string_equal(built_in_names[i], sym->identifier)) {
            Ast_Node* ret = new(Ast_Node);
            ret->type = Ast_Node_Type_Built_In_Function;
            ret->value.built_in_function = new(Built_In_Function);
            ret->value.built_in_function->identifier = built_in_names[i];
            return ret;
        }
    }

    char* message;
    asprintf(&message, "Symbol not defined: %s\n", sym->identifier);
    panic(message);

    return nullptr;
}