//! The bootable kernel binary. All the real code is in the library
//! (src/lib.rs and friends); this file just runs the demos in order.
//!
//! Boot flow: BIOS -> `bootloader` crate (switches to 64-bit long mode, sets up
//! paging, maps all physical memory) -> `kernel_main` below, with a
//! `BootInfo` describing the memory map.

#![no_std] // don't link the Rust standard library (it needs an OS)
#![no_main] // no normal `main`: the bootloader calls our entry point

extern crate alloc; // Box, Vec, Rc... once the heap is set up

use alloc::{boxed::Box, rc::Rc, vec, vec::Vec};
use bootloader::{BootInfo, entry_point};
use core::panic::PanicInfo;
use rust_kernel::{allocator, kprintln, memory, println, serial_println};
use x86_64::{VirtAddr, structures::paging::{Page, Translate}};

// `entry_point!` defines the real `_start` for us and checks at compile time
// that `kernel_main` has the right signature (takes `&'static BootInfo`).
entry_point!(kernel_main);

fn kernel_main(boot_info: &'static BootInfo) -> ! {
    kprintln!("Hello from Tyler's Rust kernel!");

    rust_kernel::init(); // GDT, IDT, PICs, enable interrupts

    // Fire a breakpoint exception on purpose: the handler prints a message,
    // returns, and we carry on as if nothing happened.
    x86_64::instructions::interrupts::int3();
    kprintln!("Survived the breakpoint, still running.");

    #[cfg(feature = "stack-overflow")]
    {
        serial_println!("Overflowing the stack on purpose...");
        stack_overflow();
    }

    paging_demo(boot_info);

    kprintln!("Type something! (keyboard echo)");
    serial_println!("Timer + keyboard interrupts enabled.");

    // Nothing else to do: sleep until the next interrupt, forever. The timer
    // and keyboard handlers do their work in between.
    rust_kernel::hlt_loop();
}

/// Chapters "Introduction to Paging", "Paging Implementation", "Heap Allocation".
fn paging_demo(boot_info: &'static BootInfo) {
    use x86_64::registers::control::Cr3;

    // --- Introduction to Paging -------------------------------------------
    // CR3 = physical address of the level-4 page table currently in use.
    let (level_4_frame, _) = Cr3::read();
    kprintln!("[paging] CR3 (level 4 table) at phys {:#x}", level_4_frame.start_address().as_u64());

    #[cfg(feature = "page-fault")]
    {
        kprintln!("[paging] writing to unmapped address 0xdeadbeaf...");
        unsafe { *(0xdeadbeaf as *mut u8) = 42 }; // -> page fault handler
    }

    // --- Paging Implementation --------------------------------------------
    let phys_mem_offset = VirtAddr::new(boot_info.physical_memory_offset);
    serial_println!("[paging] physical memory mapped at virt {:#x}", phys_mem_offset.as_u64());
    serial_println!(
        "[memory] {} memory map regions, {} KiB usable RAM",
        boot_info.memory_map.iter().count(),
        memory::usable_memory_bytes(&boot_info.memory_map) / 1024
    );

    // Safety: the bootloader mapped all physical memory at that offset, and
    // this is the only place we call `init`.
    let mut mapper = unsafe { memory::init(phys_mem_offset) };
    let mut frame_allocator = unsafe { memory::BootInfoFrameAllocator::init(&boot_info.memory_map) };

    // Translate some virtual addresses to physical ones by walking the tables.
    let addresses = [
        0xb8000,                         // VGA buffer: identity-mapped
        0x201008,                        // somewhere in our kernel's code
        0x0100_0020_1a10,                // somewhere in our stack
        boot_info.physical_memory_offset, // -> physical address 0
    ];
    for &address in &addresses {
        let virt = VirtAddr::new(address);
        let phys = mapper.translate_addr(virt);
        serial_println!("[paging] translate {:?} -> {:?}", virt, phys);
    }

    // Map a brand-new page (0xdeadbeaf000, previously unmapped) to the VGA
    // buffer frame, then write "New!" (white on red) through it.
    let page = Page::containing_address(VirtAddr::new(0xdeadbeaf000));
    memory::create_example_mapping(page, &mut mapper, &mut frame_allocator);
    let page_ptr: *mut u64 = page.start_address().as_mut_ptr();
    // Offset 400 u64s = 3200 bytes = row 20 of the screen (160 bytes/row).
    unsafe { page_ptr.offset(400).write_volatile(0x_f021_f077_f065_f04e) };
    kprintln!("[paging] mapped page 0xdeadbeaf000 -> VGA frame, wrote \"New!\" through it");

    // --- Heap Allocation --------------------------------------------------
    allocator::init_heap(&mut mapper, &mut frame_allocator).expect("heap initialization failed");
    kprintln!(
        "[heap] {} KiB heap at {:#x} (fixed-size-block allocator)",
        allocator::HEAP_SIZE / 1024,
        allocator::HEAP_START
    );

    let heap_value = Box::new(41);
    kprintln!("[heap] Box::new(41) = {} at {:p}", heap_value, heap_value);

    let mut vec = Vec::new();
    for i in 0..500 {
        vec.push(i);
    }
    let sum: u64 = vec.iter().sum();
    kprintln!("[heap] Vec of 500 numbers at {:p}, sum = {}", vec.as_ptr(), sum);

    let reference_counted = Rc::new(vec![1, 2, 3]);
    let cloned_reference = reference_counted.clone();
    kprintln!("[heap] Rc count after clone = {}", Rc::strong_count(&cloned_reference));
    core::mem::drop(reference_counted);
    kprintln!("[heap] Rc count after drop  = {}", Rc::strong_count(&cloned_reference));
}

/// Recurse forever until we run off the end of the stack into the guard page.
#[cfg(feature = "stack-overflow")]
#[allow(unconditional_recursion)]
fn stack_overflow() {
    stack_overflow();
    // Stop the compiler turning the recursion into a loop (tail call).
    unsafe { core::ptr::read_volatile(&0u8) };
}

/// Called on `panic!`. Print the message to both outputs, then stop.
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    println!("{}", info);
    serial_println!("KERNEL PANIC: {}", info);
    #[cfg(feature = "headless")]
    rust_kernel::exit_qemu(rust_kernel::QemuExitCode::Failed);
    rust_kernel::hlt_loop();
}