/*
 * Custom Filesystem Module - Implements the described behavior for "books" and "authors" directories.
 */

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/pagemap.h>
#include <linux/mount.h>
#include <linux/namei.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/hashtable.h>
#include <linux/fs_context.h>

#define FS_MAGIC 0x20240615
#define BOOKS_MODE 01777
#define AUTHORS_MODE 0777
#define AUTHOR_SUBDIR_MODE 01777

MODULE_LICENSE("GPL");
MODULE_AUTHOR("user");
MODULE_DESCRIPTION("Custom FS with books and authors directories");

static struct inode *books_inode;
static struct inode *authors_inode;

struct link_entry {
    struct hlist_node node;
    struct dentry *link;
    struct inode *target;
};

DEFINE_HASHTABLE(link_table, 6);  // 64 buckets

static void remove_links_to_inode(struct inode *inode) {
    struct link_entry *entry;
    struct hlist_node *tmp;
    int bkt;
    hash_for_each_safe(link_table, bkt, tmp, entry, node) {
        if (entry->target == inode) {
            d_delete(entry->link);
            hash_del(&entry->node);
            kfree(entry);
        }
    }
}

static void fs_evict_inode(struct inode *inode) {
    if (inode->i_sb && inode->i_sb->s_magic == FS_MAGIC) {
        if (S_ISREG(inode->i_mode)) {
            remove_links_to_inode(inode);
        }
    }
    truncate_inode_pages_final(&inode->i_data);
    clear_inode(inode);
}

static ssize_t books_file_read(struct file *file, char __user *buf, size_t len, loff_t *ppos) {
    return simple_read_from_buffer(buf, len, ppos, file->private_data, PAGE_SIZE);
}

static ssize_t books_file_write(struct file *file, const char __user *buf, size_t len, loff_t *ppos) {
    return simple_write_to_buffer(file->private_data, PAGE_SIZE, ppos, buf, len);
}

static const struct file_operations books_file_fops = {
    .read = books_file_read,
    .write = books_file_write,
    .llseek = generic_file_llseek,
};

static const struct inode_operations books_file_iops = {
    .getattr = simple_getattr,
    .setattr = simple_setattr,
};

static const struct inode_operations author_subdir_iops;

static struct dentry *author_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags) {
    return simple_lookup(dir, dentry, flags);
}

static int author_mkdir(struct user_namespace *ns, struct inode *dir, struct dentry *dentry, umode_t mode) {
    struct inode *inode;
    inode = new_inode(dir->i_sb);
    if (!inode)
        return -ENOMEM;
    inode->i_ino = get_next_ino();
    inode_init_owner(ns, inode, dir, S_IFDIR | AUTHOR_SUBDIR_MODE);
    inode->i_op = &author_subdir_iops;
    inode->i_fop = &simple_dir_operations;
    d_add(dentry, inode);
    return 0;
}

static int author_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry) {
    int err;
    struct inode *inode = d_inode(old_dentry);
    if (!S_ISREG(inode->i_mode)) return -EPERM;
    err = simple_link(old_dentry, dir, new_dentry);
    if (err == 0) {
        struct link_entry *entry = kmalloc(sizeof(*entry), GFP_KERNEL);
        if (entry) {
            entry->link = dget(new_dentry);
            entry->target = inode;
            hash_add(link_table, &entry->node, inode->i_ino);
        }
    }
    return err;
}

static const struct inode_operations author_subdir_iops = {
    .lookup = author_lookup,
    .mkdir = author_mkdir,
    .rmdir = simple_rmdir,
    .link = author_link,
};

static const struct super_operations fs_ops = {
    .evict_inode = fs_evict_inode,
};

static struct inode *fs_make_inode(struct super_block *sb, umode_t mode) {
    struct inode *inode = new_inode(sb);
    if (!inode)
        return NULL;
    inode->i_ino = get_next_ino();
    inode_init_owner(&init_user_ns, inode, NULL, mode);
    inode->i_sb = sb;
    inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);

    if (S_ISDIR(mode) && !inode->i_op) {
        inode->i_op = &simple_dir_inode_operations;
        inode->i_fop = &simple_dir_operations;
    } else if (S_ISREG(mode)) {
        inode->i_fop = &books_file_fops;
        inode->i_op = &books_file_iops;
        inode->i_private = kzalloc(PAGE_SIZE, GFP_KERNEL);
    }
    return inode;
}

static int fs_fill_super(struct super_block *sb, void *data, int silent) {
    struct inode *root_inode;
    struct dentry *root_dentry;
    struct dentry *books_dentry;
    struct dentry *authors_dentry;

    sb->s_magic = FS_MAGIC;
    sb->s_op = &fs_ops;

    root_inode = fs_make_inode(sb, S_IFDIR | 0755);
    if (!root_inode)
        return -ENOMEM;

    root_dentry = d_make_root(root_inode);
    if (!root_dentry)
        return -ENOMEM;

    sb->s_root = root_dentry;

    books_inode = fs_make_inode(sb, S_IFDIR | BOOKS_MODE);
    if (!books_inode)
        return -ENOMEM;
    books_inode->i_op = &simple_dir_inode_operations;
    books_inode->i_fop = &simple_dir_operations;
    books_inode->i_mode |= S_IWUSR | S_IWGRP | S_IWOTH;
    books_dentry = d_alloc_name(root_dentry, "books");
    if (!books_dentry)
        return -ENOMEM;
    d_add(books_dentry, books_inode);

    authors_inode = fs_make_inode(sb, S_IFDIR | AUTHORS_MODE);
    if (!authors_inode)
        return -ENOMEM;
    authors_dentry = d_alloc_name(root_dentry, "authors");
    if (!authors_dentry)
        return -ENOMEM;
    authors_inode->i_op = &author_subdir_iops;
    d_add(authors_dentry, authors_inode);

    return 0;
}

static struct dentry *fs_mount(struct file_system_type *type, int flags,
                               const char *dev, void *data) {
    return mount_nodev(type, flags, data, fs_fill_super);
}

static struct file_system_type fs_type = {
    .owner = THIS_MODULE,
    .name = "customfs",
    .mount = fs_mount,
    .kill_sb = kill_litter_super,
};

static int __init fs_init(void) {
    hash_init(link_table);
    return register_filesystem(&fs_type);
}

static void __exit fs_exit(void) {
    unregister_filesystem(&fs_type);
}

module_init(fs_init);
module_exit(fs_exit);