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package nowish
import (
"sync"
"sync/atomic"
"time"
"unsafe"
)
// Start returns a new Clock instance initialized and
// started with the provided precision, along with the
// stop function for it's underlying timer
func Start(precision time.Duration) (*Clock, func()) {
c := Clock{}
return &c, c.Start(precision)
}
type Clock struct {
// format stores the time formatting style string
format string
// valid indicates whether the current value stored in .Format is valid
valid uint32
// mutex protects writes to .Format, not because it would be unsafe, but
// because we want to minimize unnnecessary allocations
mutex sync.Mutex
// nowfmt is an unsafe pointer to the last-updated time format string
nowfmt unsafe.Pointer
// now is an unsafe pointer to the last-updated time.Time object
now unsafe.Pointer
}
// Start starts the clock with the provided precision, the returned
// function is the stop function for the underlying timer. For >= 2ms,
// actual precision is usually within AT LEAST 10% of requested precision,
// less than this and the actual precision very quickly deteriorates.
func (c *Clock) Start(precision time.Duration) func() {
// Create ticker from duration
tick := time.NewTicker(precision / 10)
// Set initial time
t := time.Now()
atomic.StorePointer(&c.now, unsafe.Pointer(&t))
// Set initial format
s := ""
atomic.StorePointer(&c.nowfmt, unsafe.Pointer(&s))
// If formatting string unset, set default
c.mutex.Lock()
if c.format == "" {
c.format = time.RFC822
}
c.mutex.Unlock()
// Start main routine
go c.run(tick)
// Return stop fn
return tick.Stop
}
// run is the internal clock ticking loop.
func (c *Clock) run(tick *time.Ticker) {
for {
// Wait on tick
_, ok := <-tick.C
// Channel closed
if !ok {
break
}
// Update time
t := time.Now()
atomic.StorePointer(&c.now, unsafe.Pointer(&t))
// Invalidate format string
atomic.StoreUint32(&c.valid, 0)
}
}
// Now returns a good (ish) estimate of the current 'now' time.
func (c *Clock) Now() time.Time {
return *(*time.Time)(atomic.LoadPointer(&c.now))
}
// NowFormat returns the formatted "now" time, cached until next tick and "now" updates.
func (c *Clock) NowFormat() string {
// If format still valid, return this
if atomic.LoadUint32(&c.valid) == 1 {
return *(*string)(atomic.LoadPointer(&c.nowfmt))
}
// Get mutex lock
c.mutex.Lock()
// Double check still invalid
if atomic.LoadUint32(&c.valid) == 1 {
c.mutex.Unlock()
return *(*string)(atomic.LoadPointer(&c.nowfmt))
}
// Calculate time format
nowfmt := c.Now().Format(c.format)
// Update the stored value and set valid!
atomic.StorePointer(&c.nowfmt, unsafe.Pointer(&nowfmt))
atomic.StoreUint32(&c.valid, 1)
// Unlock and return
c.mutex.Unlock()
return nowfmt
}
// SetFormat sets the time format string used by .NowFormat().
func (c *Clock) SetFormat(format string) {
// Get mutex lock
c.mutex.Lock()
// Update time format
c.format = format
// Invalidate current format string
atomic.StoreUint32(&c.valid, 0)
// Unlock
c.mutex.Unlock()
}
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