explain the following code in full. review it.

import json, os, webbrowser, tkinter as tk
from tkinterdnd2 import TkinterDnD
from tkinter import messagebox
from datetime import datetime
from dateparser import parse as dateparse
from cryptography.fernet import Fernet
from plyer import notification
import threading, re

class TrialManager:
    def __init__(self, master):
        self.master = master
        self.master.title("Trial Manager")
        self.key = Fernet.generate_key()
        self.cipher = Fernet(self.key)
        self.data_file = 'trials.json'
        self.data = self.load_data()
        self.dark_mode = False
        self.setup_ui()
        self.start_notification_thread()

    def setup_ui(self):
        self.master.geometry('650x550')
        self.master.configure(bg='lightgrey')
        tk.Label(self.master, text="Trial Manager", font=("Arial", 20, "bold"), bg='darkgrey', fg='white').pack(fill='x')
        frame = tk.Frame(self.master, bg='lightgrey')
        frame.pack(padx=20, pady=20, fill='both', expand=True)
        labels = ["Software Name:", "Email Used:", "Start Date (YYYY-MM-DD):", "End Date (YYYY-MM-DD):"]
        defaults = ["Enter Software Name", "Enter Email", "YYYY-MM-DD", "Enter End Date (e.g., 'aug 14')"]
        self.entries = []
        for i, (label, default) in enumerate(zip(labels, defaults)):
            tk.Label(frame, text=label, bg='lightgrey').grid(row=i, column=0, sticky='e', pady=5)
            entry = tk.Entry(frame)
            entry.grid(row=i, column=1, columnspan=2, sticky='ew', padx=5)
            entry.insert(0, default)
            entry.bind("<FocusIn>", lambda e, entry=entry, default=default: entry.delete(0, 'end') if entry.get() == default else None)
            entry.bind("<FocusOut>", lambda e, entry=entry, default=default: entry.insert(0, default) if entry.get() == '' else None)
            self.entries.append(entry)
        buttons = [("Add Trial", self.add_trial), ("Show Trials", self.show_trials), ("Quick Card Freeze", self.quick_links), ("Toggle Dark Mode", self.toggle_dark_mode)]
        for i, (text, cmd) in enumerate(buttons):
            tk.Button(frame, text=text, command=cmd, width=15).grid(row=4+i//3, column=i%3, pady=10)
        self.search_var = tk.StringVar()
        tk.Entry(frame, textvariable=self.search_var, width=30).grid(row=5, column=0, columnspan=2, pady=10, padx=5)
        search_icon = tk.PhotoImage(file='search-icon.png')
        tk.Button(frame, image=search_icon, command=self.search_trials).grid(row=5, column=2, pady=10)
        self.trial_listbox = tk.Listbox(frame, width=70, height=15, relief='sunken', bd=2, highlightthickness=0)
        self.trial_listbox.grid(row=6, column=0, columnspan=3, pady=10)
        self.master.dnd_bind('<<Drop>>', self.drop)

    def load_data(self):
        if os.path.exists(self.data_file):
            try:
                with open(self.data_file, 'rb') as file:
                    encrypted_data = file.read()
                    if encrypted_data:
                        decrypted_data = self.cipher.decrypt(encrypted_data)
                        return json.loads(decrypted_data)
            except (json.JSONDecodeError, OSError, ValueError) as e:
                self.show_error_message("Data Load Error", f"Failed to load data: {str(e)}")
        return []

    def save_data(self):
        try:
            with open(self.data_file, 'wb') as file:
                encrypted_data = self.cipher.encrypt(json.dumps(self.data, indent=2).encode())
                file.write(encrypted_data)
        except (OSError, ValueError) as e:
            self.show_error_message("Data Save Error", f"Failed to save data: {str(e)}")

    def show_error_message(self, title, message):
        messagebox.showerror(title, message)

    def parse_end_date_input(self, end_date_input):
        try:
            parsed_date = dateparse(end_date_input)
            if not parsed_date:
                raise ValueError("Unable to parse end date input")
            return parsed_date.strftime('%Y-%m-%d')
        except Exception as e:
            self.show_error_message("Date Parsing Error", str(e))
            return None

    def add_trial(self):
        trial_type = messagebox.askquestion("Trial Type", "Is this a new trial?")
        start_date = datetime.now().strftime('%Y-%m-%d') if trial_type == 'yes' else self.entries[2].get().strip()
        if not self.validate_date_format(start_date):
            return
        name = self.entries[0].get().strip()
        email = self.entries[1].get().strip()
        end_date_input = self.entries[3].get().strip()
        end_date = self.parse_end_date_input(end_date_input)
        if not end_date:
            return
        if not all([name, email, start_date, end_date]) or not self.validate_email_format(email):
            return
        try:
            trial_info = {"name": name, "email": email, "start_date": start_date, "end_date": end_date, "status": "Active" if trial_type == 'yes' else "Past"}
            self.data.append(trial_info)
            self.save_data()
            messagebox.showinfo("Success", "Trial added successfully!")
            self.clear_entries()
            self.show_trials()
        except Exception as e:
            self.show_error_message("Error", f"An error occurred: {str(e)}")

    def show_trials(self):
        self.trial_listbox.delete(0, 'end')
        for trial in sorted(self.data, key=lambda x: x['end_date']):
            end_date = datetime.strptime(trial['end_date'], '%Y-%m-%d')
            status_color = "green" if end_date > datetime.now() else "red"
            self.trial_listbox.insert('end', f"{trial['name']} ({trial['email']}) - Ends: {trial['end_date']}")
            self.trial_listbox.itemconfig('end', fg=status_color)

    def search_trials(self):
        query = self.search_var.get().strip().lower()
        self.trial_listbox.delete(0, 'end')
        for trial in sorted(self.data, key=lambda x: x['end_date']):
            if query in trial['name'].lower() or query in trial['email'].lower():
                end_date = datetime.strptime(trial['end_date'], '%Y-%m-%d')
                status_color = "green" if end_date > datetime.now() else "red"
                self.trial_listbox.insert('end', f"{trial['name']} ({trial['email']}) - Ends: {trial['end_date']}")
                self.trial_listbox.itemconfig('end', fg=status_color)

    def quick_links(self):
        quick_links_window = tk.Toplevel(self.master)
        quick_links_window.title("Quick Links")
        quick_links_window.geometry('300x150')
        tk.Button(quick_links_window, text="Open Chime", command=lambda: webbrowser.open('https://app.chime.com/login')).pack(fill='x', pady=5)
        tk.Button(quick_links_window, text="Open CashApp", command=lambda: webbrowser.open('https://cash.app/login')).pack(fill='x', pady=5)
        tk.Button(quick_links_window, text="Open SoFi", command=lambda: webbrowser.open('https://www.sofi.com/login/')).pack(fill='x', pady=5)

    def clear_entries(self):
        for entry in self.entries:
            entry.delete(0, 'end')

    def toggle_dark_mode(self):
        self.dark_mode = not self.dark_mode
        new_bg = 'black' if self.dark_mode else 'lightgrey'
        new_fg = 'white' if self.dark_mode else 'black'
        self.master.configure(bg=new_bg)
        for widget in self.master.winfo_children():
            self.set_widget_colors(widget, new_bg, new_fg)

    def set_widget_colors(self, widget, bg, fg):
        if isinstance(widget, tk.Frame):
            widget.config(bg=bg)
            for child in widget.winfo_children():
                self.set_widget_colors(child, bg, fg)
        elif isinstance(widget, (tk.Label, tk.Entry, tk.Listbox, tk.Button)):
            widget.config(bg=bg, fg=fg)
        widget.update_idletasks()

    def start_notification_thread(self):
        def notify():
            while True:
                for trial in self.data:
                    end_date = datetime.strptime(trial['end_date'], '%Y-%m-%d')
                    if 0 < (end_date - datetime.now()).days <= 3:
                        notification.notify(
                            title=f"Trial Expiring: {trial['name']}",
                            message=f"Trial '{trial['name']}' is expiring soon!",
                            timeout=10
                        )
                time.sleep(3600)
        threading.Thread(target=notify, daemon=True).start()

    def drop(self, event):
        files = self.master.tk.splitlist(event.data)
        for file_path in files:
            if os.path.isfile(file_path):
                try:
                    with open(file_path, 'r') as file:
                        for line in file:
                            if match := re.match(r'^(.*?),(.*?),(.*?),(.*?)$', line.strip()):
                                name, email, start_date, end_date = match.groups()
                                if (self.validate_date_format(start_date) and self.validate_date_format(end_date) and self.validate_email_format(email)):
                                    self.data.append({"name": name.strip(), "email": email.strip(), "start_date": start_date.strip(), "end_date": end_date.strip(), "status": "Active"})
                    self.save_data()
                except Exception as e:
                    self.show_error_message("Error", f"Error processing file {file_path}: {str(e)}")
        self.show_trials()

    @staticmethod
    def validate_date_format(date_text):
        try:
            datetime.strptime(date_text, "%Y-%m-%d")
            return True
        except ValueError:
            messagebox.showerror("Invalid Date", f"The date '{date_text}' is not valid. Please use YYYY-MM-DD format.")
            return False

    @staticmethod
    def validate_email_format(email):
        pattern = r"^[\w\.-]+@[\w\.-]+\.\w+$"
        if not re.match(pattern, email):
            messagebox.showerror("Invalid Email", f"The email '{email}' is not valid. Please enter a correct email format.")
            return False
        return True

if __name__ == '__main__':
    root = TkinterDnD.Tk()
    app = TrialManager(root)
    root.mainloop()