// calogTimer.c -- calog timer library (see calogTimer.h). ONE background thread owns a
// mutex-guarded list of timers and, for each one that comes due, retains its callback, drops
// the list lock, and invokes it (which marshals to the callback's owning context thread). The
// list is scheduled on CLOCK_MONOTONIC; the thread sleeps on a condition variable (given the
// MONOTONIC clock via a condattr) until the earliest nextFire, or until it is signalled when a
// timer is added or cancelled.
//
// The registry state is STATIC and its small bookkeeping is never freed: the three natives
// stay reachable (via any context) right up until calogDestroy tears the broker registry down,
// so freeing the state in calogTimerShutdown -- which the contract requires BEFORE calogDestroy
// -- would leave those natives dereferencing freed memory. Shutdown instead stops the thread,
// releases every remaining callback, and empties the list; the mutexes/counters persist for the
// process (no per-cycle leak), so the library re-registers cleanly across runtimes.
//
// Locking discipline: the list mutex is NEVER held across calogFnInvoke (a blocking, cross-
// thread call) -- the callback is retained, the lock dropped, then the callback invoked and
// released. calogFnRelease is only ever held under the list mutex to post a non-blocking
// finalize (as in calogExport); it never re-enters the timer library.

#define _POSIX_C_SOURCE 200809L

#include "calogTimer.h"

#include <pthread.h>
#include <stdint.h>
#include <stdlib.h>
#include <time.h>

#define TIMER_INITIAL_CAP 8
#define TIMER_GROWTH      2

// One scheduled timer. intervalNs == 0 marks a one-shot; a periodic timer keeps a non-zero
// interval. A cancelled timer is tombstoned here and pruned by the thread at the top of its
// loop, which keeps cancellation safe against a fire that is already in flight.
typedef struct TimerEntryT {
    int64_t   id;
    CalogFnT *callback;
    int64_t   nextFireMonoNs;
    int64_t   intervalNs;
    bool      cancelled;
} TimerEntryT;

// The timer list (heap array) plus its background thread, guarded by gTimerMutex/gTimerCond.
// refCount (guarded by gInitMutex) counts registered runtimes so the LAST calogTimerShutdown
// stops the thread and frees the list. gTimerCond is initialised (with CLOCK_MONOTONIC) when
// the thread starts and destroyed when it stops, so every cond operation is guarded by
// gThreadStarted.
static pthread_mutex_t gTimerMutex    = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t  gTimerCond;
static TimerEntryT    *gTimers        = NULL;
static int32_t         gCount         = 0;
static int32_t         gCap           = 0;
static int64_t         gNextId        = 1;
static pthread_t       gThread;
static bool            gThreadStarted = false;
static bool            gShutdown      = false;
static pthread_mutex_t gInitMutex     = PTHREAD_MUTEX_INITIALIZER;
static int32_t         gRefCount      = 0;

static int32_t timerAfterNative(CalogValueT *args, int32_t argCount, CalogValueT *result, void *userData);
static int32_t timerCancelNative(CalogValueT *args, int32_t argCount, CalogValueT *result, void *userData);
static int32_t timerEnsureThreadLocked(void);
static int32_t timerEveryNative(CalogValueT *args, int32_t argCount, CalogValueT *result, void *userData);
static int32_t timerFindByIdLocked(int64_t id);
static int64_t timerNowNs(void);
static void    timerPruneLocked(void);
static int32_t timerSchedule(CalogValueT *args, int32_t argCount, CalogValueT *result, bool periodic);
static void   *timerThreadMain(void *arg);


int32_t calogTimerRegister(CalogT *calog) {
    pthread_mutex_lock(&gInitMutex);
    gRefCount++;
    pthread_mutex_unlock(&gInitMutex);
    calogRegisterInline(calog, "timerAfter", timerAfterNative, NULL);
    calogRegisterInline(calog, "timerEvery", timerEveryNative, NULL);
    calogRegisterInline(calog, "timerCancel", timerCancelNative, NULL);
    return calogOkE;
}


void calogTimerShutdown(void) {
    pthread_t threadHandle;
    bool      joinThread;
    int32_t   index;

    joinThread = false;
    pthread_mutex_lock(&gInitMutex);
    if (gRefCount > 0) {
        gRefCount--;
    }
    if (gRefCount > 0) {
        pthread_mutex_unlock(&gInitMutex);
        return;
    }
    // Last runtime is gone: stop the thread (if it ever started), then release every remaining
    // callback and free the list. Contexts are still alive here, so a callback release routes a
    // finalize to its owner thread just like calogExport.
    pthread_mutex_lock(&gTimerMutex);
    gShutdown    = true;
    threadHandle = gThread;
    if (gThreadStarted) {
        joinThread = true;
        pthread_cond_signal(&gTimerCond);
    }
    pthread_mutex_unlock(&gTimerMutex);
    if (joinThread) {
        pthread_join(threadHandle, NULL);
    }
    pthread_mutex_lock(&gTimerMutex);
    for (index = 0; index < gCount; index++) {
        calogFnRelease(gTimers[index].callback);
    }
    free(gTimers);
    gTimers = NULL;
    gCount  = 0;
    gCap    = 0;
    if (joinThread) {
        pthread_cond_destroy(&gTimerCond);
    }
    gThreadStarted = false;
    gShutdown      = false;
    pthread_mutex_unlock(&gTimerMutex);
    pthread_mutex_unlock(&gInitMutex);
}


static int32_t timerAfterNative(CalogValueT *args, int32_t argCount, CalogValueT *result, void *userData) {
    (void)userData;
    return timerSchedule(args, argCount, result, false);
}


static int32_t timerCancelNative(CalogValueT *args, int32_t argCount, CalogValueT *result, void *userData) {
    int32_t index;

    (void)userData;
    calogValueNil(result);
    if (argCount != 1 || args[0].type != calogIntE) {
        return calogFail(result, calogErrArgE, "timerCancel expects (id)");
    }
    pthread_mutex_lock(&gTimerMutex);
    index = timerFindByIdLocked(args[0].as.i);
    if (index >= 0) {
        // Tombstone it; the thread prunes it (and stops firing it) at the top of its loop.
        gTimers[index].cancelled = true;
        pthread_cond_signal(&gTimerCond);
    }
    pthread_mutex_unlock(&gTimerMutex);
    return calogOkE;
}


static int32_t timerEnsureThreadLocked(void) {
    pthread_condattr_t attr;

    // Caller holds gTimerMutex. Start the single background thread lazily, on the first timer.
    if (gThreadStarted) {
        return calogOkE;
    }
    if (pthread_condattr_init(&attr) != 0) {
        return calogErrUnsupportedE;
    }
    pthread_condattr_setclock(&attr, CLOCK_MONOTONIC);
    if (pthread_cond_init(&gTimerCond, &attr) != 0) {
        pthread_condattr_destroy(&attr);
        return calogErrUnsupportedE;
    }
    pthread_condattr_destroy(&attr);
    gShutdown = false;
    if (pthread_create(&gThread, NULL, timerThreadMain, NULL) != 0) {
        pthread_cond_destroy(&gTimerCond);
        return calogErrUnsupportedE;
    }
    gThreadStarted = true;
    return calogOkE;
}


static int32_t timerEveryNative(CalogValueT *args, int32_t argCount, CalogValueT *result, void *userData) {
    (void)userData;
    return timerSchedule(args, argCount, result, true);
}


static int32_t timerFindByIdLocked(int64_t id) {
    int32_t index;

    for (index = 0; index < gCount; index++) {
        if (gTimers[index].id == id) {
            return index;
        }
    }
    return -1;
}


static int64_t timerNowNs(void) {
    struct timespec ts;

    clock_gettime(CLOCK_MONOTONIC, &ts);
    return (int64_t)ts.tv_sec * 1000000000 + (int64_t)ts.tv_nsec;
}


static void timerPruneLocked(void) {
    int32_t index;

    index = 0;
    while (index < gCount) {
        if (gTimers[index].cancelled) {
            calogFnRelease(gTimers[index].callback);
            gTimers[index] = gTimers[gCount - 1];
            gCount--;
        } else {
            index++;
        }
    }
}


static int32_t timerSchedule(CalogValueT *args, int32_t argCount, CalogValueT *result, bool periodic) {
    const char *label;
    double      ms;
    int64_t     delayNs;
    int64_t     id;
    int32_t     status;

    label = periodic ? "timerEvery expects (ms, function)" : "timerAfter expects (ms, function)";
    calogValueNil(result);
    if (argCount != 2 || (args[0].type != calogIntE && args[0].type != calogRealE) || args[1].type != calogFnE) {
        return calogFail(result, calogErrArgE, label);
    }
    ms = (args[0].type == calogIntE) ? (double)args[0].as.i : args[0].as.r;
    // Bound ms so ms*1e6 stays well within int64 nanoseconds. This also rejects +Inf and NaN
    // (both fail the comparison); the cast below is otherwise undefined once the product
    // exceeds INT64_MAX. 9.0e12 ms is ~285 years -- far beyond any real timer.
    if (!(ms >= 0.0 && ms <= 9.0e12)) {
        return calogFail(result, calogErrRangeE, "timer: milliseconds out of range");
    }
    delayNs = (int64_t)(ms * 1000000.0);
    pthread_mutex_lock(&gTimerMutex);
    if (gCount == gCap) {
        int32_t      newCap;
        TimerEntryT *grown;
        newCap = (gCap == 0) ? TIMER_INITIAL_CAP : gCap * TIMER_GROWTH;
        grown  = (TimerEntryT *)realloc(gTimers, (size_t)newCap * sizeof(TimerEntryT));
        if (grown == NULL) {
            pthread_mutex_unlock(&gTimerMutex);
            return calogFail(result, calogErrOomE, "timer: out of memory");
        }
        gTimers = grown;
        gCap    = newCap;
    }
    status = timerEnsureThreadLocked();
    if (status != calogOkE) {
        pthread_mutex_unlock(&gTimerMutex);
        return calogFail(result, status, "timer: could not start the timer thread");
    }
    id = gNextId++;
    // Retain the callback for as long as the list holds it; released on cancel/fire/shutdown.
    calogFnRetain(args[1].as.fn);
    gTimers[gCount].id             = id;
    gTimers[gCount].callback       = args[1].as.fn;
    gTimers[gCount].nextFireMonoNs = timerNowNs() + delayNs;
    gTimers[gCount].intervalNs     = periodic ? ((delayNs > 0) ? delayNs : 1) : 0;
    gTimers[gCount].cancelled      = false;
    gCount++;
    pthread_cond_signal(&gTimerCond);
    pthread_mutex_unlock(&gTimerMutex);
    calogValueInt(result, id);
    return calogOkE;
}


static void *timerThreadMain(void *arg) {
    (void)arg;
    pthread_mutex_lock(&gTimerMutex);
    for (;;) {
        int64_t nowNs;
        int32_t index;
        int32_t minIndex;

        // Drop tombstoned timers first, so the scan below only sees live entries.
        timerPruneLocked();
        if (gShutdown) {
            break;
        }
        if (gCount == 0) {
            pthread_cond_wait(&gTimerCond, &gTimerMutex);
            continue;
        }
        nowNs    = timerNowNs();
        minIndex = 0;
        for (index = 1; index < gCount; index++) {
            if (gTimers[index].nextFireMonoNs < gTimers[minIndex].nextFireMonoNs) {
                minIndex = index;
            }
        }
        if (gTimers[minIndex].nextFireMonoNs > nowNs) {
            struct timespec target;
            int64_t         fireAt;
            fireAt         = gTimers[minIndex].nextFireMonoNs;
            target.tv_sec  = (time_t)(fireAt / 1000000000);
            target.tv_nsec = (long)(fireAt % 1000000000);
            pthread_cond_timedwait(&gTimerCond, &gTimerMutex, &target);
            continue;
        }
        {
            CalogFnT   *cb;
            CalogValueT res;
            int64_t     id;
            int32_t     status;
            bool        oneShot;

            cb      = gTimers[minIndex].callback;
            id      = gTimers[minIndex].id;
            oneShot = (gTimers[minIndex].intervalNs == 0);
            // Retain across the unlocked invoke so a concurrent cancel/shutdown can't free it.
            calogFnRetain(cb);
            if (oneShot) {
                gTimers[minIndex].cancelled = true;
            } else {
                gTimers[minIndex].nextFireMonoNs += gTimers[minIndex].intervalNs;
                if (gTimers[minIndex].nextFireMonoNs <= nowNs) {
                    // Fell behind (a slow callback): resync rather than fire in a burst.
                    gTimers[minIndex].nextFireMonoNs = nowNs + gTimers[minIndex].intervalNs;
                }
            }
            pthread_mutex_unlock(&gTimerMutex);

            status = calogFnInvoke(cb, NULL, 0, &res);
            calogValueFree(&res);
            if (status == calogErrDeadE) {
                // The owning context is gone: auto-cancel the timer (find it again by id, since
                // the list may have changed while unlocked).
                pthread_mutex_lock(&gTimerMutex);
                index = timerFindByIdLocked(id);
                if (index >= 0) {
                    gTimers[index].cancelled = true;
                }
                pthread_mutex_unlock(&gTimerMutex);
            }
            calogFnRelease(cb);
            pthread_mutex_lock(&gTimerMutex);
        }
    }
    pthread_mutex_unlock(&gTimerMutex);
    return NULL;
}