typedef enum ObjectType {
    OBJ_TYPE_FIXNUM,
    OBJ_TYPE_BOOL,
    OBJ_TYPE_NIL,
    OBJ_TYPE_SYMBOL,
    OBJ_TYPE_STRING,
    OBJ_TYPE_PAIR,
    OBJ_TYPE_PROCEDURE,
    OBJ_TYPE_LAMBDA,
    OBJ_TYPE_ERR,
} ObjectType;

struct Environment;

typedef struct Object {
    ObjectType type;
    union {
        // OBJ_TYPE_FIXNUM
        ssize_t fixnum;

        // OBJ_TYPE_STRING
        struct {
            char *string;
            size_t string_n;
        };

        // OBJ_TYPE_PAIR
        struct {
            struct Object *car;
            struct Object *cdr;
        };

        // OBJ_TYPE_SYMBOL
        struct {
            char *symbol;
            size_t symbol_n;
        };

        // OBJ_TYPE_PROCEDURE
        struct Object *(*proc)(struct Environment *env, struct Object *args);

        // OBJ_TYPE_LAMBDA
        struct {
            struct Object *params;
            struct Object *body;
            struct Environment *env;
        };
    };
} Object;

//
// Singletons.
//

static Object *obj_nil;
static Object *obj_true;
static Object *obj_false;
static Object *obj_err;

//
// Constructors.
//

Object *
alloc_object(ObjectType type) {
    Object *obj = malloc(sizeof(Object));
    obj->type = type;
    return obj;
}

Object *
make_fixnum(ssize_t num) {
    Object *obj = alloc_object(OBJ_TYPE_FIXNUM);
    obj->fixnum = num;
    return obj;
}

Object *
make_procedure(Object *(*proc)(struct Environment *, struct Object *args)) {
    Object *obj = alloc_object(OBJ_TYPE_PROCEDURE);
    obj->proc = proc;
    return obj;
}

Object *
make_pair(Object *car, Object *cdr) {
    Object *obj = alloc_object(OBJ_TYPE_PAIR);
    obj->car = car;
    obj->cdr = cdr;
    return  obj;
}

Object *
make_symbol(StringView sv) {
    Object *obj = alloc_object(OBJ_TYPE_SYMBOL);
    obj->symbol = malloc(sizeof(char) * sv.n);
    memcpy(obj->symbol, sv.start, sv.n);
    obj->symbol_n = sv.n;
    return obj;
}

Object *
make_string(void) {
    Object *obj = alloc_object(OBJ_TYPE_STRING);
    obj->string = NULL;
    obj->string_n = 0;
    return obj;
}

void
append_string(Object *obj, const StringView sv) {
    assert(obj != NULL);
    assert(obj->type == OBJ_TYPE_STRING);
    if (sv.n == 0) {
        return;
    }
    obj->string = realloc(obj->string, (obj->string_n + sv.n) * sizeof(char));
    memcpy(obj->string + obj->string_n, sv.start, sv.n);
    obj->string_n += sv.n;
}

Object *
obj_duplicate(Object *obj) {
    Object *copy = obj_err;
    switch (obj->type) {
        case OBJ_TYPE_BOOL:
        case OBJ_TYPE_NIL:
        case OBJ_TYPE_PROCEDURE:
        case OBJ_TYPE_LAMBDA: // TODO: should we duplicate everything inside?
        case OBJ_TYPE_ERR: {
            copy = obj;
        } break;
        case OBJ_TYPE_FIXNUM: {
            copy = make_fixnum(obj->fixnum);
        } break;
        case OBJ_TYPE_SYMBOL: {
            copy = make_symbol((StringView){obj->symbol, obj->symbol_n});
        } break;
        case OBJ_TYPE_STRING: {
            copy = make_string();
            append_string(copy, (StringView){obj->string, obj->string_n});
        } break;
        case OBJ_TYPE_PAIR: {
            Object *root = make_pair(obj_duplicate(obj->car), obj_nil);
            copy = root;
            obj = obj->cdr;
            while (obj != obj_nil) {
                root->cdr = make_pair(obj_duplicate(obj->car), obj_nil);
                root = root->cdr;
                obj = obj->cdr;
            }
        } break;
    }
    return copy;
}

void
free_objects(Object *root) {
    switch (root->type) {
        case OBJ_TYPE_BOOL: break;
        case OBJ_TYPE_NIL: break;
        case OBJ_TYPE_ERR: break;
        case OBJ_TYPE_PROCEDURE:
        case OBJ_TYPE_FIXNUM: {
            free(root);
        } break;
        case OBJ_TYPE_SYMBOL: {
            if (root->symbol != NULL) {
                free(root->symbol);
            }
            free(root);
        } break;
        case OBJ_TYPE_STRING: {
            if (root->string != NULL) {
                free(root->string);
            }
            free(root);
        } break;
        case OBJ_TYPE_PAIR: {
            if (root->car != NULL) {
                free_objects(root->car);
            }
            if (root->cdr != NULL) {
                free_objects(root->cdr);
            }
            free(root);
        } break;
        case OBJ_TYPE_LAMBDA: {
            free_objects(root->params);
            free_objects(root->body);
            // TODO: free_env(root->env);
            free(root);
        } break;
    }
}

void display(Object *root);

void
display_pair(Object *root) {
    display(root->car);
    if (root->cdr->type == OBJ_TYPE_PAIR) {
        printf(" ");
        display_pair(root->cdr);
    } else if (root->cdr == obj_nil) {
        return;
    } else {
        printf(" . ");
        display(root->cdr);
    }
}

void
display(Object *root) {
    switch (root->type) {
        case OBJ_TYPE_FIXNUM: {
            printf("%zd", root->fixnum);
        } break;
        case OBJ_TYPE_BOOL: {
            if (root == obj_true) {
                printf("true");
            } else {
                printf("false");
            }
        } break;
        case OBJ_TYPE_NIL: {
            printf("()");
        } break;
        case OBJ_TYPE_STRING: {
            printf("\"%.*s\"", (int)root->string_n, root->string);
        } break;
        case OBJ_TYPE_SYMBOL: {
            printf(":%.*s", (int)root->symbol_n, root->symbol);
        } break;
        case OBJ_TYPE_PAIR: {
            printf("(");
            display_pair(root);
            printf(")");
        } break;
        case OBJ_TYPE_LAMBDA:
        case OBJ_TYPE_PROCEDURE: {
            printf("#{procedure}");
        } break;
        case OBJ_TYPE_ERR: {
            printf("#{error}");
        } break;
    }
    return;
}

bool
obj_eq(Object *a, Object* b) {
    if (a->type != b->type) {
        return false;
    }
    switch (a->type) {
        case OBJ_TYPE_FIXNUM: {
            return a->fixnum == b->fixnum;
        } break;
        case OBJ_TYPE_STRING: {
            if (a->string_n != b->string_n) {
                return false;
            }
            for (size_t i = 0; i < a->string_n; i++) {
                if (a->string[i] != b->string[i]) {
                    return false;
                }
            }
        } break;
        case OBJ_TYPE_SYMBOL: {
            if (a->symbol_n != b->symbol_n) {
                return false;
            }
            for (size_t i = 0; i < a->symbol_n; i++) {
                if (a->symbol[i] != b->symbol[i]) {
                    return false;
                }
            }
        } break;
        default: {
            return a == b;
        } break;
    }
    return true;
}