/*
 * task.c – Cooperative multitasking: PCB pool, scheduler, yield/exit.
 *
 * Task 0 is the always-present kernel core thread (uses the boot stack).
 * Additional tasks are created with task_create(), which allocates a
 * stack from the PMM and sets up a fake context-switch frame so that
 * context_switch() can "return" into a trampoline that calls the real
 * entry function.
 *
 * Scheduling is purely cooperative: tasks must call task_yield() to
 * let others run.  The scheduler uses round-robin selection among
 * READY tasks.
 */

#include "task.h"
#include "memory.h"

/* Null-safe print helper used throughout the kernel. */
#define SAFE_PRINT(Boot, ...)                           \
    do {                                                \
        if ((Boot) != NULL && (Boot)->print != NULL) {  \
            (Boot)->print(__VA_ARGS__);                 \
        }                                               \
    } while (0)

/* ================================================================
 *  Module state
 * ================================================================ */

static Task     tasks[TASK_MAX];   /* PCB pool (static array)         */
static Task    *current_task = NULL;
static UINT32   next_pid     = 0;
static BootInfo *task_boot   = NULL;
static BOOLEAN  task_ready   = FALSE;

/* Forward declaration */
static void task_trampoline(void);

/* ----------------------------------------------------------------
 *  Helpers
 * ---------------------------------------------------------------- */

/* Copy a wide string with a maximum length (including NUL). */
static void wstrcpy16(CHAR16 *dst, const CHAR16 *src, UINTN max)
{
    UINTN i = 0;
    while (i < max - 1 && src != NULL && src[i] != L'\0') {
        dst[i] = src[i];
        i++;
    }
    dst[i] = L'\0';
}

/* ----------------------------------------------------------------
 *  Initialisation – make the current (kernel) thread task 0
 * ---------------------------------------------------------------- */

/*
 * Initialise the scheduler: clear all PCB slots and register the
 * currently running kernel core thread as task 0.
 */
void task_init(BootInfo *Boot)
{
    UINTN i;

    task_boot = Boot;

    /* Clear all PCB slots */
    for (i = 0; i < TASK_MAX; i++) {
        tasks[i].state       = TASK_STATE_FREE;
        tasks[i].pid         = 0;
        tasks[i].privilege   = TASK_PRIV_USER;
        tasks[i].saved_rsp   = 0;
        tasks[i].stack_base  = 0;
        tasks[i].stack_pages = 0;
        tasks[i].entry       = NULL;
        tasks[i].arg         = NULL;
        tasks[i].switches    = 0;
        tasks[i].name[0]     = L'\0';
    }

    /*
     * Task 0 = the currently running kernel core thread.
     * It already has a stack (the kernel's boot stack), so we don't
     * allocate one.  Its saved_rsp will be filled in during the
     * first context_switch call in task_yield().
     */
    tasks[0].pid       = next_pid++;
    tasks[0].state     = TASK_STATE_RUNNING;
    tasks[0].privilege = TASK_PRIV_KERNEL;
    tasks[0].switches  = 1;
    wstrcpy16(tasks[0].name, L"core", TASK_NAME_LEN);

    current_task = &tasks[0];
    task_ready   = TRUE;

    SAFE_PRINT(Boot, L"  Tasks: scheduler ready (max %d tasks)\n\r",
               (UINTN)TASK_MAX);
}

/* ----------------------------------------------------------------
 *  Trampoline – first code a new task executes after context_switch
 *  returns into it.  It calls the real entry function and then
 *  performs a clean task_exit().
 * ---------------------------------------------------------------- */

static void task_trampoline(void)
{
    Task *t = task_current();
    if (t != NULL && t->entry != NULL) {
        t->entry(t->arg);
    }
    task_exit();
    /* Should never reach here, but just in case: */
    for (;;) {
        __asm__ __volatile__("hlt");
    }
}

/* ----------------------------------------------------------------
 *  Create a new task
 * ---------------------------------------------------------------- */

Task *task_create_with_priv(const CHAR16 *name,
                            TaskEntryFn entry,
                            void *arg,
                            TaskPrivilege privilege)
{
    Task  *t = NULL;
    UINTN  i;
    UINT64 stack_phys;
    UINT64 *sp;

    if (!task_ready || entry == NULL) {
        return NULL;
    }

    /* Subsystem-level privilege enforcement: prevent privilege escalation. */
    {
        Task *caller = task_current();
        if (caller != NULL && privilege > task_get_privilege(caller)) {
            return NULL;
        }
    }

    /* Find a free PCB slot */
    for (i = 0; i < TASK_MAX; i++) {
        if (tasks[i].state == TASK_STATE_FREE) {
            t = &tasks[i];
            break;
        }
    }
    if (t == NULL) {
        return NULL;   /* out of slots */
    }

    /* Allocate stack pages from the physical memory manager */
    stack_phys = pmm_alloc_pages(TASK_STACK_PAGES);
    if (stack_phys == 0) {
        return NULL;   /* out of memory */
    }

    /* Fill in the PCB */
    t->pid         = next_pid++;
    t->state       = TASK_STATE_READY;
    t->privilege   = privilege;
    t->entry       = entry;
    t->arg         = arg;
    t->switches    = 0;
    t->stack_base  = stack_phys;
    t->stack_pages = TASK_STACK_PAGES;
    wstrcpy16(t->name, name != NULL ? name : L"unnamed", TASK_NAME_LEN);

    /*
     * Set up the initial stack frame so that context_switch() can
     * "return" into task_trampoline().
     *
     * context_switch saves/restores (low → high on stack):
     *   flags, r15, r14, r13, r12, rbx, rbp   (pushes)
     *   then `ret` pops the return address     (→ trampoline)
     *
     * Above the return address we place a safety-net address
     * (task_exit) so that if the trampoline or entry function does
     * a bare `ret`, it lands in task_exit().
     */
    sp = (UINT64 *)(stack_phys + TASK_STACK_SIZE);

    /* Align stack top to 16 bytes */
    sp = (UINT64 *)((UINT64)sp & ~0xFULL);

    /* Safety-net return address for the trampoline */
    *(--sp) = (UINT64)(UINTN)task_exit;

    /* Return address for context_switch's `ret` → trampoline */
    *(--sp) = (UINT64)(UINTN)task_trampoline;

    /* Callee-saved registers – all zero for fresh task */
    *(--sp) = 0;      /* rbp */
    *(--sp) = 0;      /* rbx */
    *(--sp) = 0;      /* r12 */
    *(--sp) = 0;      /* r13 */
    *(--sp) = 0;      /* r14 */
    *(--sp) = 0;      /* r15 */

    /* RFLAGS – interrupts enabled (IF = bit 9) */
    *(--sp) = 0x202;  /* flags */

    t->saved_rsp = (UINT64)(UINTN)sp;

    return t;
}

Task *task_create(const CHAR16 *name, TaskEntryFn entry, void *arg)
{
    /* Inherit privilege from the calling task (kernel if no task context). */
    Task *caller = task_current();
    TaskPrivilege priv = (caller != NULL) ? task_get_privilege(caller)
                                          : TASK_PRIV_KERNEL;
    return task_create_with_priv(name, entry, arg, priv);
}

/* ----------------------------------------------------------------
 *  Schedule – pick the next READY task (round-robin)
 * ---------------------------------------------------------------- */

static Task *schedule_next(void)
{
    UINTN start, idx, i;

    if (current_task == NULL) {
        return &tasks[0];
    }

    /* Find current task's index in the array */
    start = (UINTN)(current_task - tasks);

    /* Round-robin: scan from (current+1) wrapping around */
    for (i = 1; i <= TASK_MAX; i++) {
        idx = (start + i) % TASK_MAX;
        if (tasks[idx].state == TASK_STATE_READY) {
            return &tasks[idx];
        }
    }

    /* No other ready task – stay with current if still runnable */
    if (current_task->state == TASK_STATE_RUNNING ||
        current_task->state == TASK_STATE_READY) {
        return current_task;
    }

    /* Fallback to task 0 (kernel / shell) */
    return &tasks[0];
}

/* ----------------------------------------------------------------
 *  Yield – voluntarily give up the CPU to the next ready task
 * ---------------------------------------------------------------- */

void task_yield(void)
{
    Task *prev, *next;

    if (!task_ready) {
        return;
    }

    prev = current_task;
    next = schedule_next();

    if (next == prev) {
        return;   /* nothing else to switch to */
    }

    /* Mark the previous task as READY (still runnable) */
    if (prev->state == TASK_STATE_RUNNING) {
        prev->state = TASK_STATE_READY;
    }

    next->state = TASK_STATE_RUNNING;
    next->switches++;
    current_task = next;

    /*
     * context_switch saves callee-saved regs + flags on prev's stack,
     * stores prev's RSP into prev->saved_rsp, loads next->saved_rsp
     * into RSP, restores regs + flags, and `ret`s into next's code.
     */
    context_switch(&prev->saved_rsp, next->saved_rsp);
}

/* ----------------------------------------------------------------
 *  Exit – terminate the current task and switch away
 * ---------------------------------------------------------------- */

void task_exit(void)
{
    Task *prev, *next;

    if (!task_ready) {
        return;
    }

    prev = current_task;
    prev->state = TASK_STATE_TERMINATED;

    /* Free the stack memory back to the PMM */
    if (prev->stack_base != 0 && prev->stack_pages != 0) {
        pmm_free_pages(prev->stack_base, prev->stack_pages);
        prev->stack_base  = 0;
        prev->stack_pages = 0;
    }

    /* Mark the PCB slot as free for reuse */
    prev->state = TASK_STATE_FREE;

    next = schedule_next();
    if (next == prev) {
        /* Shouldn't happen if task 0 (kernel) is always alive */
        next = &tasks[0];
    }

    next->state = TASK_STATE_RUNNING;
    next->switches++;
    current_task = next;

    /* One-way switch: we never return to the exited task */
    context_switch(&prev->saved_rsp, next->saved_rsp);

    /* Should never reach here */
    for (;;) {
        __asm__ __volatile__("hlt");
    }
}

/* ----------------------------------------------------------------
 *  Accessors
 * ---------------------------------------------------------------- */

Task *task_current(void)
{
    return current_task;
}

TaskPrivilege task_get_privilege(Task *t)
{
    if (t == NULL) {
        return TASK_PRIV_KERNEL;
    }
    return t->privilege;
}

void task_set_privilege(Task *t, TaskPrivilege privilege)
{
    if (t == NULL) {
        return;
    }
    t->privilege = privilege;
}

UINTN task_count(void)
{
    UINTN i, count = 0;
    for (i = 0; i < TASK_MAX; i++) {
        if (tasks[i].state != TASK_STATE_FREE) {
            count++;
        }
    }
    return count;
}

/* ----------------------------------------------------------------
 *  Wait for another task to finish
 * ---------------------------------------------------------------- */

void task_wait(Task *t)
{
    if (!task_ready || t == NULL) {
        return;
    }

    /*
     * Busy-wait cooperatively until the target task's PCB slot has
     * been recycled back to FREE by task_exit().
     */
    while (t->state != TASK_STATE_FREE) {
        task_yield();
    }
}

/* ----------------------------------------------------------------
 *  Print task list  (implements the `ps` command)
 * ---------------------------------------------------------------- */

static const CHAR16 *state_str(TaskState s)
{
    switch (s) {
    case TASK_STATE_FREE:       return L"FREE";
    case TASK_STATE_READY:      return L"READY";
    case TASK_STATE_RUNNING:    return L"RUNNING";
    case TASK_STATE_TERMINATED: return L"ENDED";
    default:                    return L"???";
    }
}

void task_print_list(BootInfo *Boot)
{
    UINTN i;
    Task *caller;

    /* Subsystem-level privilege enforcement: task list requires DRIVER. */
    caller = task_current();
    if (caller != NULL && task_get_privilege(caller) < TASK_PRIV_DRIVER) {
        SAFE_PRINT(Boot, L"Permission denied: task list requires driver privilege.\n\r");
        return;
    }

    SAFE_PRINT(Boot, L"\n\r");
    SAFE_PRINT(Boot, L"  PID  STATE       PRIV  SWITCHES  NAME\n\r");
    SAFE_PRINT(Boot, L"  ---  ----------  ----  --------  ----\n\r");

    for (i = 0; i < TASK_MAX; i++) {
        if (tasks[i].state == TASK_STATE_FREE) {
            continue;
        }

        SAFE_PRINT(Boot, L"  %3d  %-10s  %4d  %8d  %s\n\r",
                   tasks[i].pid,
                   state_str(tasks[i].state),
                   (INT32)tasks[i].privilege,
                   tasks[i].switches,
                   tasks[i].name);
    }

    SAFE_PRINT(Boot, L"\n\r");
    SAFE_PRINT(Boot, L"  Active tasks: %d / %d\n\r",
               task_count(), (UINTN)TASK_MAX);
    SAFE_PRINT(Boot, L"\n\r");
}