// Package metrics implements Prometheus-compatible metrics for applications.
//
// This package is lightweight alternative to https://github.com/prometheus/client_golang
// with simpler API and smaller dependencies.
//
// Usage:
//
//  1. Register the required metrics via New* functions.
//  2. Expose them to `/metrics` page via WritePrometheus.
//  3. Update the registered metrics during application lifetime.
//
// The package has been extracted from https://victoriametrics.com/
package metrics

import (
	"fmt"
	"io"
	"sort"
	"strings"
	"sync"
	"sync/atomic"
	"unsafe"
)

type namedMetric struct {
	name   string
	metric metric
	isAux  bool
}

type metric interface {
	marshalTo(prefix string, w io.Writer)
	metricType() string
}

var defaultSet = NewSet()

func init() {
	RegisterSet(defaultSet)
}

var (
	registeredSets     = make(map[*Set]struct{})
	registeredSetsLock sync.Mutex
)

// RegisterSet registers the given set s for metrics export via global WritePrometheus() call.
//
// See also UnregisterSet.
func RegisterSet(s *Set) {
	registeredSetsLock.Lock()
	registeredSets[s] = struct{}{}
	registeredSetsLock.Unlock()
}

// UnregisterSet stops exporting metrics for the given s via global WritePrometheus() call.
//
// If destroySet is set to true, then s.UnregisterAllMetrics() is called on s after unregistering it,
// so s becomes destroyed. Otherwise the s can be registered again in the set by passing it to RegisterSet().
func UnregisterSet(s *Set, destroySet bool) {
	registeredSetsLock.Lock()
	delete(registeredSets, s)
	registeredSetsLock.Unlock()

	if destroySet {
		s.UnregisterAllMetrics()
	}
}

// RegisterMetricsWriter registers writeMetrics callback for including metrics in the output generated by WritePrometheus.
//
// The writeMetrics callback must write metrics to w in Prometheus text exposition format without timestamps and trailing comments.
// The last line generated by writeMetrics must end with \n.
// See https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#text-based-format
//
// It is OK to register multiple writeMetrics callbacks - all of them will be called sequentially for gererating the output at WritePrometheus.
func RegisterMetricsWriter(writeMetrics func(w io.Writer)) {
	defaultSet.RegisterMetricsWriter(writeMetrics)
}

// WritePrometheus writes all the metrics in Prometheus format from the default set, all the added sets and metrics writers to w.
//
// Additional sets can be registered via RegisterSet() call.
// Additional metric writers can be registered via RegisterMetricsWriter() call.
//
// If exposeProcessMetrics is true, then various `go_*` and `process_*` metrics
// are exposed for the current process.
//
// The WritePrometheus func is usually called inside "/metrics" handler:
//
//	http.HandleFunc("/metrics", func(w http.ResponseWriter, req *http.Request) {
//	    metrics.WritePrometheus(w, true)
//	})
func WritePrometheus(w io.Writer, exposeProcessMetrics bool) {
	registeredSetsLock.Lock()
	sets := make([]*Set, 0, len(registeredSets))
	for s := range registeredSets {
		sets = append(sets, s)
	}
	registeredSetsLock.Unlock()

	sort.Slice(sets, func(i, j int) bool {
		return uintptr(unsafe.Pointer(sets[i])) < uintptr(unsafe.Pointer(sets[j]))
	})
	for _, s := range sets {
		s.WritePrometheus(w)
	}
	if exposeProcessMetrics {
		WriteProcessMetrics(w)
	}
}

// WriteProcessMetrics writes additional process metrics in Prometheus format to w.
//
// The following `go_*` and `process_*` metrics are exposed for the currently
// running process. Below is a short description for the exposed `process_*` metrics:
//
//   - process_cpu_seconds_system_total - CPU time spent in syscalls
//
//   - process_cpu_seconds_user_total - CPU time spent in userspace
//
//   - process_cpu_seconds_total - CPU time spent by the process
//
//   - process_major_pagefaults_total - page faults resulted in disk IO
//
//   - process_minor_pagefaults_total - page faults resolved without disk IO
//
//   - process_resident_memory_bytes - recently accessed memory (aka RSS or resident memory)
//
//   - process_resident_memory_peak_bytes - the maximum RSS memory usage
//
//   - process_resident_memory_anon_bytes - RSS for memory-mapped files
//
//   - process_resident_memory_file_bytes - RSS for memory allocated by the process
//
//   - process_resident_memory_shared_bytes - RSS for memory shared between multiple processes
//
//   - process_virtual_memory_bytes - virtual memory usage
//
//   - process_virtual_memory_peak_bytes - the maximum virtual memory usage
//
//   - process_num_threads - the number of threads
//
//   - process_start_time_seconds - process start time as unix timestamp
//
//   - process_io_read_bytes_total - the number of bytes read via syscalls
//
//   - process_io_written_bytes_total - the number of bytes written via syscalls
//
//   - process_io_read_syscalls_total - the number of read syscalls
//
//   - process_io_write_syscalls_total - the number of write syscalls
//
//   - process_io_storage_read_bytes_total - the number of bytes actually read from disk
//
//   - process_io_storage_written_bytes_total - the number of bytes actually written to disk
//
//   - go_sched_latencies_seconds - time spent by goroutines in ready state before they start execution
//
//   - go_mutex_wait_seconds_total - summary time spent by all the goroutines while waiting for locked mutex
//
//   - go_gc_mark_assist_cpu_seconds_total - summary CPU time spent by goroutines in GC mark assist state
//
//   - go_gc_cpu_seconds_total - summary time spent in GC
//
//   - go_gc_pauses_seconds - duration of GC pauses
//
//   - go_scavenge_cpu_seconds_total - CPU time spent on returning the memory to OS
//
//   - go_memlimit_bytes - the GOMEMLIMIT env var value
//
//   - go_memstats_alloc_bytes - memory usage for Go objects in the heap
//
//   - go_memstats_alloc_bytes_total - the cumulative counter for total size of allocated Go objects
//
//   - go_memstats_buck_hash_sys_bytes - bytes of memory in profiling bucket hash tables
//
//   - go_memstats_frees_total - the cumulative counter for number of freed Go objects
//
//   - go_memstats_gc_cpu_fraction - the fraction of CPU spent in Go garbage collector
//
//   - go_memstats_gc_sys_bytes - the size of Go garbage collector metadata
//
//   - go_memstats_heap_alloc_bytes - the same as go_memstats_alloc_bytes
//
//   - go_memstats_heap_idle_bytes - idle memory ready for new Go object allocations
//
//   - go_memstats_heap_inuse_bytes - bytes in in-use spans
//
//   - go_memstats_heap_objects - the number of Go objects in the heap
//
//   - go_memstats_heap_released_bytes - bytes of physical memory returned to the OS
//
//   - go_memstats_heap_sys_bytes - memory requested for Go objects from the OS
//
//   - go_memstats_last_gc_time_seconds - unix timestamp the last garbage collection finished
//
//   - go_memstats_lookups_total - the number of pointer lookups performed by the runtime
//
//   - go_memstats_mallocs_total - the number of allocations for Go objects
//
//   - go_memstats_mcache_inuse_bytes - bytes of allocated mcache structures
//
//   - go_memstats_mcache_sys_bytes - bytes of memory obtained from the OS for mcache structures
//
//   - go_memstats_mspan_inuse_bytes - bytes of allocated mspan structures
//
//   - go_memstats_mspan_sys_bytes - bytes of memory obtained from the OS for mspan structures
//
//   - go_memstats_next_gc_bytes - the target heap size when the next garbage collection should start
//
//   - go_memstats_other_sys_bytes - bytes of memory in miscellaneous off-heap runtime allocations
//
//   - go_memstats_stack_inuse_bytes - memory used for goroutine stacks
//
//   - go_memstats_stack_sys_bytes - memory requested fromthe OS for goroutine stacks
//
//   - go_memstats_sys_bytes - memory requested by Go runtime from the OS
//
//   - go_cgo_calls_count - the total number of CGO calls
//
//   - go_cpu_count - the number of CPU cores on the host where the app runs
//
// The WriteProcessMetrics func is usually called in combination with writing Set metrics
// inside "/metrics" handler:
//
//	http.HandleFunc("/metrics", func(w http.ResponseWriter, req *http.Request) {
//	    mySet.WritePrometheus(w)
//	    metrics.WriteProcessMetrics(w)
//	})
//
// See also WriteFDMetrics.
func WriteProcessMetrics(w io.Writer) {
	writeGoMetrics(w)
	writeProcessMetrics(w)
	writePushMetrics(w)
}

// WriteFDMetrics writes `process_max_fds` and `process_open_fds` metrics to w.
func WriteFDMetrics(w io.Writer) {
	writeFDMetrics(w)
}

// UnregisterMetric removes metric with the given name from default set.
//
// See also UnregisterAllMetrics.
func UnregisterMetric(name string) bool {
	return defaultSet.UnregisterMetric(name)
}

// UnregisterAllMetrics unregisters all the metrics from default set.
//
// It also unregisters writeMetrics callbacks passed to RegisterMetricsWriter.
func UnregisterAllMetrics() {
	defaultSet.UnregisterAllMetrics()
}

// ListMetricNames returns sorted list of all the metric names from default set.
func ListMetricNames() []string {
	return defaultSet.ListMetricNames()
}

// GetDefaultSet returns the default metrics set.
func GetDefaultSet() *Set {
	return defaultSet
}

// ExposeMetadata allows enabling adding TYPE and HELP metadata to the exposed metrics globally.
//
// It is safe to call this method multiple times. It is allowed to change it in runtime.
// ExposeMetadata is set to false by default.
func ExposeMetadata(v bool) {
	n := 0
	if v {
		n = 1
	}
	atomic.StoreUint32(&exposeMetadata, uint32(n))
}

func isMetadataEnabled() bool {
	n := atomic.LoadUint32(&exposeMetadata)
	return n != 0
}

var exposeMetadata uint32

func isCounterName(name string) bool {
	return strings.HasSuffix(name, "_total")
}

// WriteGaugeUint64 writes gauge metric with the given name and value to w in Prometheus text exposition format.
func WriteGaugeUint64(w io.Writer, name string, value uint64) {
	writeMetricUint64(w, name, "gauge", value)
}

// WriteGaugeFloat64 writes gauge metric with the given name and value to w in Prometheus text exposition format.
func WriteGaugeFloat64(w io.Writer, name string, value float64) {
	writeMetricFloat64(w, name, "gauge", value)
}

// WriteCounterUint64 writes counter metric with the given name and value to w in Prometheus text exposition format.
func WriteCounterUint64(w io.Writer, name string, value uint64) {
	writeMetricUint64(w, name, "counter", value)
}

// WriteCounterFloat64 writes counter metric with the given name and value to w in Prometheus text exposition format.
func WriteCounterFloat64(w io.Writer, name string, value float64) {
	writeMetricFloat64(w, name, "counter", value)
}

func writeMetricUint64(w io.Writer, metricName, metricType string, value uint64) {
	WriteMetadataIfNeeded(w, metricName, metricType)
	fmt.Fprintf(w, "%s %d\n", metricName, value)
}

func writeMetricFloat64(w io.Writer, metricName, metricType string, value float64) {
	WriteMetadataIfNeeded(w, metricName, metricType)
	fmt.Fprintf(w, "%s %g\n", metricName, value)
}

// WriteMetadataIfNeeded writes HELP and TYPE metadata for the given metricName and metricType if this is globally enabled via ExposeMetadata().
//
// If the metadata exposition isn't enabled, then this function is no-op.
func WriteMetadataIfNeeded(w io.Writer, metricName, metricType string) {
	if !isMetadataEnabled() {
		return
	}
	metricFamily := getMetricFamily(metricName)
	fmt.Fprintf(w, "# HELP %s\n", metricFamily)
	fmt.Fprintf(w, "# TYPE %s %s\n", metricFamily, metricType)
}

func getMetricFamily(metricName string) string {
	n := strings.IndexByte(metricName, '{')
	if n < 0 {
		return metricName
	}
	return metricName[:n]
}