package main

import (
	"sync"
)

// SafeCounter is safe to use concurrently.
type SafeCounter struct {
	mu sync.RWMutex
	v  map[string]int
}

// Inc increments the counter for the given key.
func (c *SafeCounter) Write(key string) {
	c.mu.Lock()
	// Lock so only one goroutine at a time can access the map c.v.
	c.v[key]++
	c.mu.Unlock()
}

// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Read(key string) int {
	c.mu.RLock()
	defer c.mu.RUnlock()
	return c.v[key]
}

func main() {
	c := SafeCounter{v: make(map[string]int)}
	go func() {
		for {
			_ = c.Read("test")
		}
	}()
	go func() {
		for {
			c.Write("test")
		}
	}()
	select {}
}

// NewGoCollector returns a collector that exports metrics about the current Go
// process. This includes memory stats. To collect those, runtime.ReadMemStats
// is called. This requires to “stop the world”, which usually only happens for
// garbage collection (GC). Take the following implications into account when
// deciding whether to use the Go collector:
//
//...
func NewGoCollector() Collector {
	return &goCollector{
		goroutinesDesc: NewDesc(
			"go_goroutines",
			"Number of goroutines that currently exist.",
			nil, nil),
		threadsDesc: NewDesc(
			"go_threads",
			"Number of OS threads created.",
			nil, nil),
		gcDesc: NewDesc(
			"go_gc_duration_seconds",
			"A summary of the pause duration of garbage collection cycles.",
			nil, nil),
		goInfoDesc: NewDesc(
			"go_info",
			"Information about the Go environment.",
			nil, Labels{"version": runtime.Version()}),
		msLast:    &runtime.MemStats{},
		msRead:    runtime.ReadMemStats,
		msMaxWait: time.Second,
		msMaxAge:  5 * time.Minute,
		msMetrics: memStatsMetrics{
			{
				desc: NewDesc(
					memstatNamespace("alloc_bytes"),
					"Number of bytes allocated and still in use.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.Alloc) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("alloc_bytes_total"),
					"Total number of bytes allocated, even if freed.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.TotalAlloc) },
				valType: CounterValue,
			}, {
				desc: NewDesc(
					memstatNamespace("sys_bytes"),
					"Number of bytes obtained from system.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.Sys) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("lookups_total"),
					"Total number of pointer lookups.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.Lookups) },
				valType: CounterValue,
			}, {
				desc: NewDesc(
					memstatNamespace("mallocs_total"),
					"Total number of mallocs.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.Mallocs) },
				valType: CounterValue,
			}, {
				desc: NewDesc(
					memstatNamespace("frees_total"),
					"Total number of frees.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.Frees) },
				valType: CounterValue,
			}, {
				desc: NewDesc(
					memstatNamespace("heap_alloc_bytes"),
					"Number of heap bytes allocated and still in use.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.HeapAlloc) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("heap_sys_bytes"),
					"Number of heap bytes obtained from system.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.HeapSys) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("heap_idle_bytes"),
					"Number of heap bytes waiting to be used.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.HeapIdle) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("heap_inuse_bytes"),
					"Number of heap bytes that are in use.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.HeapInuse) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("heap_released_bytes"),
					"Number of heap bytes released to OS.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.HeapReleased) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("heap_objects"),
					"Number of allocated objects.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.HeapObjects) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("stack_inuse_bytes"),
					"Number of bytes in use by the stack allocator.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.StackInuse) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("stack_sys_bytes"),
					"Number of bytes obtained from system for stack allocator.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.StackSys) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("mspan_inuse_bytes"),
					"Number of bytes in use by mspan structures.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.MSpanInuse) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("mspan_sys_bytes"),
					"Number of bytes used for mspan structures obtained from system.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.MSpanSys) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("mcache_inuse_bytes"),
					"Number of bytes in use by mcache structures.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.MCacheInuse) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("mcache_sys_bytes"),
					"Number of bytes used for mcache structures obtained from system.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.MCacheSys) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("buck_hash_sys_bytes"),
					"Number of bytes used by the profiling bucket hash table.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.BuckHashSys) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("gc_sys_bytes"),
					"Number of bytes used for garbage collection system metadata.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.GCSys) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("other_sys_bytes"),
					"Number of bytes used for other system allocations.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.OtherSys) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("next_gc_bytes"),
					"Number of heap bytes when next garbage collection will take place.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.NextGC) },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("last_gc_time_seconds"),
					"Number of seconds since 1970 of last garbage collection.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return float64(ms.LastGC) / 1e9 },
				valType: GaugeValue,
			}, {
				desc: NewDesc(
					memstatNamespace("gc_cpu_fraction"),
					"The fraction of this program's available CPU time used by the GC since the program started.",
					nil, nil,
				),
				eval:    func(ms *runtime.MemStats) float64 { return ms.GCCPUFraction },
				valType: GaugeValue,
			},
		},
	}
}