#include "kernel.h"
#include "common.h"

extern char __bss[], __bss_end[], __stack_top[];
extern char __free_ram[], __free_ram_end[];

struct sbiret sbi_call(long arg0, long arg1, long arg2, long arg3, long arg4,
                       long arg5, long fid, long eid) {
    register long a0 __asm__("a0") = arg0;
    register long a1 __asm__("a1") = arg1;
    register long a2 __asm__("a2") = arg2;
    register long a3 __asm__("a3") = arg3;
    register long a4 __asm__("a4") = arg4;
    register long a5 __asm__("a5") = arg5;
    register long a6 __asm__("a6") = fid;
    register long a7 __asm__("a7") = eid;

    __asm__ __volatile__("ecall"
                         : "=r"(a0), "=r"(a1)
                         : "r"(a0), "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5),
                           "r"(a6), "r"(a7)
                         : "memory");
    return (struct sbiret){.error = a0, .value = a1};
}

paddr_t alloc_pages(uint32_t n) {
    static paddr_t next_paddr = (paddr_t) __free_ram;
    paddr_t paddr = next_paddr;
    next_paddr += n * PAGE_SIZE;
    
    if (next_paddr > (paddr_t) __free_ram_end)
        PANIC("Out ow memowy~ :((");

    memset((void *) paddr, 0, n * PAGE_SIZE);
    return paddr;  
}

void putchar(char ch) {
    sbi_call(ch, 0, 0, 0, 0, 0, 0, 1 /* Console Putchar */);
}

__attribute__((naked))
__attribute__((aligned(4)))
void kernel_entry(void) {
    __asm__ __volatile__(
        // Retrieve the kernel stack of the running process from sscratch
        "csrrw sp, sscratch, sp\n"

        "addi sp, sp, -4 * 31\n"
        "sw ra, 4 * 0(sp)\n"
        "sw gp, 4 * 1(sp)\n"
        "sw tp, 4 * 2(sp)\n"
        "sw t0, 4 * 3(sp)\n"
        "sw t1, 4 * 4(sp)\n"
        "sw t2, 4 * 5(sp)\n"
        "sw t3, 4 * 6(sp)\n"
        "sw t4, 4 * 7(sp)\n"
        "sw t5, 4 * 8(sp)\n"
        "sw t6, 4 * 9(sp)\n"
        "sw a0, 4 * 10(sp)\n"
        "sw a1, 4 * 11(sp)\n"
        "sw a2, 4 * 12(sp)\n"
        "sw a3, 4 * 13(sp)\n"
        "sw a4, 4 * 14(sp)\n"
        "sw a5, 4 * 15(sp)\n"
        "sw a6, 4 * 16(sp)\n"
        "sw a7, 4 * 17(sp)\n"
        "sw s0, 4 * 18(sp)\n"
        "sw s1, 4 * 19(sp)\n"
        "sw s2, 4 * 20(sp)\n"
        "sw s3, 4 * 21(sp)\n"
        "sw s4, 4 * 22(sp)\n"
        "sw s5, 4 * 23(sp)\n"
        "sw s6, 4 * 24(sp)\n"
        "sw s7, 4 * 25(sp)\n"
        "sw s8, 4 * 26(sp)\n"
        "sw s9, 4 * 27(sp)\n"
        "sw s10, 4 * 28(sp)\n"
        "sw s11, 4 * 29(sp)\n"

        // Retrieve and save the sp at the time of exception
        "csrr a0, sscratch\n"
        "sw a0, 4 * 30(sp)\n"

        // Reset the kernel stack
        "mv a0, sp\n"
        "call handle_trap\n"

        "lw ra, 4 * 0(sp)\n"
        "lw gp, 4 * 1(sp)\n"
        "lw tp, 4 * 2(sp)\n"
        "lw t0, 4 * 3(sp)\n"
        "lw t1, 4 * 4(sp)\n"
        "lw t2, 4 * 5(sp)\n"
        "lw t3, 4 * 6(sp)\n"
        "lw t4, 4 * 7(sp)\n"
        "lw t5, 4 * 8(sp)\n"
        "lw t6, 4 * 9(sp)\n"
        "lw a0, 4 * 10(sp)\n"
        "lw a1, 4 * 11(sp)\n"
        "lw a2, 4 * 12(sp)\n"
        "lw a3, 4 * 13(sp)\n"
        "lw a4, 4 * 14(sp)\n"
        "lw a5, 4 * 15(sp)\n"
        "lw a6, 4 * 16(sp)\n"
        "lw a7, 4 * 17(sp)\n"
        "lw s0, 4 * 18(sp)\n"
        "lw s1, 4 * 19(sp)\n"
        "lw s2, 4 * 20(sp)\n"
        "lw s3, 4 * 21(sp)\n"
        "lw s4, 4 * 22(sp)\n"
        "lw s5, 4 * 23(sp)\n"
        "lw s6, 4 * 24(sp)\n"
        "lw s7, 4 * 25(sp)\n"
        "lw s8, 4 * 26(sp)\n"
        "lw s9, 4 * 27(sp)\n"
        "lw s10, 4 * 28(sp)\n"
        "lw s11, 4 * 29(sp)\n"
        "lw sp, 4 * 30(sp)\n"
        "sret\n"
    );
}

void handle_trap(struct trap_frame *f) {
    uint32_t scause = READ_CSR(scause);
    uint32_t stval = READ_CSR(stval);
    uint32_t user_pc = READ_CSR(sepc);

    PANIC("Unexpected trap: scause=%x, stval=%x, sepc=%x\n", scause, stval, user_pc);
}

__attribute__((naked)) void switch_context(uint32_t *prev_sp,
                                           uint32_t *next_sp) {
    __asm__ __volatile__(
        // Save calle-saved registers onto the stack of current proccess
        "addi sp, sp, -13 * 4\n" // Allocate stack space for 13 4-byte registers
        "sw ra,  0  * 4(sp)\n"   // Save calle-saved registers only
        "sw s0,  1  * 4(sp)\n"
        "sw s1,  2  * 4(sp)\n"
        "sw s2,  3  * 4(sp)\n"
        "sw s3,  4  * 4(sp)\n"
        "sw s4,  5  * 4(sp)\n"
        "sw s5,  6  * 4(sp)\n"
        "sw s6,  7  * 4(sp)\n"
        "sw s7,  8  * 4(sp)\n"
        "sw s8,  9  * 4(sp)\n"
        "sw s9,  10 * 4(sp)\n"
        "sw s10, 11 * 4(sp)\n"
        "sw s11, 12 * 4(sp)\n"

        // Switch stack pointer
        "sw sp, (a0)\n" // *prev_sp = a0; 
        "lw sp, (a1)\n" // Switch stack pointer (sp) here
        
        // Restore calle-saved registers from the next process's stack
        "lw ra,  0  * 4(sp)\n"   // Save calle-saved registers only
        "lw s0,  1  * 4(sp)\n"
        "lw s1,  2  * 4(sp)\n"
        "lw s2,  3  * 4(sp)\n"
        "lw s3,  4  * 4(sp)\n"
        "lw s4,  5  * 4(sp)\n"
        "lw s5,  6  * 4(sp)\n"
        "lw s6,  7  * 4(sp)\n"
        "lw s7,  8  * 4(sp)\n"
        "lw s8,  9  * 4(sp)\n"
        "lw s9,  10 * 4(sp)\n"
        "lw s10, 11 * 4(sp)\n"
        "lw s11, 12 * 4(sp)\n"
        "addi sp, sp, 13 * 4\n" // We've popped 13 4-byte registers from the stack
        "ret\n"
    );
}

struct process procs[PROCS_MAX];

struct process *create_process(uint32_t pc) {
    // Find an unused proccess control structure
    struct process *proc = NULL;
    int i;
    for (i = 0; i <= PROCS_MAX; i++) {
        if (procs[i].state == PROC_UNUSED) {
            proc = &procs[i];
            break;
        }
    }
    
    if (!proc)
        PANIC("No free process slots found! Fuck off!");

    // Stack calle-saved registers 
    uint32_t *sp = (uint32_t *) &proc->stack[sizeof(proc->stack)];
    *--sp = 0;                    // s11
    *--sp = 0;                    // s10
    *--sp = 0;                    // s9
    *--sp = 0;                    // s8
    *--sp = 0;                    // s7
    *--sp = 0;                    // s6
    *--sp = 0;                    // s5
    *--sp = 0;                    // s4
    *--sp = 0;                    // s3
    *--sp = 0;                    // s2
    *--sp = 0;                    // s1
    *--sp = 0;                    // s0
    *--sp = (uint32_t) pc;        // ra
    

    // Initialize fields
    proc->pid = i + 1;
    proc->state = PROC_RUNNABLE;
    proc->sp = (uint32_t) sp;
    return proc;
}

void delay(void) {
    for (int i = 0; i < 69696969; i++)
        __asm__ __volatile__("nop"); // do nothing
}

struct process *current_proc;
struct process *idle_proc;

void yield(void) {
    // Search for a runnable process
    struct process *next = idle_proc;
    for (int i = 0; i < PROCS_MAX; i++) {
        struct process *proc = &procs[(current_proc->pid + i) % PROCS_MAX];
        if (proc->state == PROC_RUNNABLE && proc->pid > 0) {
            next = proc;
            break;
        }
    }

    // Return and continue if there's no other processes running than current one
    if (next == current_proc)
        return;

    __asm__ __volatile__(
        "csrw sscratch, %[sscratch]\n"
        :
        : [sscratch] "r" ((uint32_t) &next->stack[sizeof(next->stack)])
    );

    // Context switch
    struct process *prev = current_proc;
    current_proc = next;
    switch_context(&prev->sp, &next->sp);
}

struct process *proc_a;
struct process *proc_b;

void proc_a_entry(void) {
    printf("Start process A\n");
    while (1) {
        putchar('6');
        yield();
    }
}

void proc_b_entry(void) {
    printf("Start process B\n");
    while (1) {
        putchar('9');
        yield();
    }
}

void kernel_main(void) {
    // printf("\n\nHello %s\n", "friend :3");
    // printf("60 + 9 = %d, %x\n", 60 + 9, 0x1234abcd);

    // for (;;) {
    //     __asm__ __volatile__("wfi");
    // }

    memset(__bss, 0, (size_t) __bss_end - (size_t) __bss);
    
    printf("\n\n");

    WRITE_CSR(stvec, (uint32_t) kernel_entry);

    idle_proc = create_process((uint32_t) NULL);
    idle_proc->pid = 0; // idle
    current_proc = idle_proc;

    // paddr_t paddr0 = alloc_pages(2);
    // paddr_t paddr1 = alloc_pages(1);
    // printf("alloc_pages test paddr0=%x\n", paddr0);
    // printf("alloc_pages test paddr1=%x\n", paddr1);

    proc_a = create_process((uint32_t) proc_a_entry);
    proc_b = create_process((uint32_t) proc_b_entry);

    yield();
    PANIC("Switched to idle process");

    // __asm__ __volatile__("unimp"); // calls a unimp which triggers kernel panic
    
    //
    // PANIC("BOOOO!!! GET INFECTED WITH BOOTKID YOU SKID! 0x1337h@x0r");
    // printf("you lucky bitch got away\n");
}

__attribute__((section(".text.boot")))
__attribute__((naked))
void boot(void) {
    __asm__ __volatile__(
        "mv sp, %[stack_top]\n" // Set the stack pointer
        "j kernel_main\n"       // Jump to the kernel main function
        :
        : [stack_top] "r" (__stack_top) // Pass the stack top address as %[stack_top]
    );
}