package health

import (
	"context"
	"encoding/json"
	"fmt"
	"net/http"
	"runtime"
	"sync"
	"time"

	"github.com/fiskerinc/cloud-services/pkg/logger"
	"github.com/gomodule/redigo/redis"
	"github.com/pkg/errors"
)

// Based on https://github.com/hellofresh/health-go

// Status type represents health status
type Status string

// Possible health statuses
const (
	StatusOK                 Status = "OK"
	StatusPartiallyAvailable Status = "partially available"
	StatusUnavailable        Status = "unavailable"
	StatusTimeout            Status = "timeout"
)

type (
	// CheckFunc is the func which executes the check.
	CheckFunc func(context.Context) error

	// InfoFunc is the func which executes to return check info
	InfoFunc func(system *System)

	// Config carries the parameters to run the check.
	Config struct {
		// Name is the name of the resource to be checked.
		Name string
		// Timeout is the timeout defined for every check.
		Timeout time.Duration
		// SkipOnErr if set to true, it will retrieve StatusOK providing the error message from the failed resource.
		SkipOnErr bool
		// Check is the func which executes the check.
		Check CheckFunc
		// Info func sets System information
		Info InfoFunc
		// If Vital is set to true, it means that the service won't work without this resource.
		Vital bool
	}

	// Check represents the health check response.
	Check struct {
		// Status is the check status.
		Status Status `json:"status"`
		// Timestamp is the time in which the check occurred.
		Timestamp time.Time `json:"timestamp"`
		// Failures holds the failed checks along with their messages.
		Failures map[string]string `json:"failures,omitempty"`
		// System holds information of the go process.
		System *System `json:"system"`
	}

	// System runtime variables about the go process.
	System struct {
		// Version is the go version.
		Version string `json:"version"`
		// GoroutinesCount is the number of the current goroutines.
		GoroutinesCount int `json:"goroutines_count"`
		// TotalAllocBytes is the total bytes allocated.
		TotalAllocBytes int `json:"total_alloc_bytes"`
		// HeapObjectsCount is the number of objects in the go heap.
		HeapObjectsCount int `json:"heap_objects_count"`
		// TotalAllocBytes is the bytes allocated and not yet freed.
		AllocBytes int `json:"alloc_bytes"`
		// RedisPoolCount is the current Redis connection pool count
		RedisStats *redis.PoolStats `json:"redis_stats,omitempty"`
	}

	// Health is the health-checks container
	Health struct {
		mu     sync.Mutex
		checks map[string]Config
	}

	checkResponse struct {
		config Config
		err    error
	}

	filterChecks func(checks map[string]Config) map[string]Config
)

// New instantiates and build new health check container
func New(opts ...Option) (*Health, error) {
	h := &Health{
		checks: make(map[string]Config),
	}

	for _, o := range opts {
		if err := o(h); err != nil {
			return nil, err
		}
	}

	return h, nil
}

// Register registers a check config to be performed.
func (h *Health) Register(c Config) error {
	if c.Timeout == 0 {
		c.Timeout = time.Second * 1
	}

	if c.Name == "" {
		return errors.New("health check must have a name to be registered")
	}

	h.mu.Lock()
	defer h.mu.Unlock()

	if _, ok := h.checks[c.Name]; ok {
		return fmt.Errorf("health check %q is already registered", c.Name)
	}

	h.checks[c.Name] = c

	return nil
}

// ReadinessHandler returns an HTTP handler (http.HandlerFunc).
func (h *Health) ReadinessHandler() http.Handler {
	return http.HandlerFunc(h.ReadinessFunc)
}

// LivenessHandler returns an HTTP handler (http.HandlerFunc).
func (h *Health) LivenessHandler() http.Handler {
	return http.HandlerFunc(h.LivenessFunc)
}

// LivenessFunc is the HTTP handler function.
func (h *Health) LivenessFunc(w http.ResponseWriter, r *http.Request) {
	c := h.Measure(r.Context(), getLivenessCheck)

	w.Header().Set("Content-Type", "application/json")
	data, err := json.Marshal(c)
	if err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	code := http.StatusOK
	if c.Status == StatusUnavailable {
		code = http.StatusServiceUnavailable
	}
	w.WriteHeader(code)
	w.Write(data)
}

// ReadinessFunc is the HTTP handler function.
func (h *Health) ReadinessFunc(w http.ResponseWriter, r *http.Request) {
	c := h.Measure(r.Context(), getReadinessCheck)

	w.Header().Set("Content-Type", "application/json")
	data, err := json.Marshal(c)
	if err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	code := http.StatusOK
	if c.Status == StatusUnavailable {
		code = http.StatusServiceUnavailable
	}
	w.WriteHeader(code)
	w.Write(data)
}

func (h *Health) info(system *System) {
	for _, c := range h.checks {
		if c.Info != nil {
			c.Info(system)
		}
	}
}

// Measure runs all the registered health checks and returns summary status
func (h *Health) Measure(ctx context.Context, getChecks filterChecks) Check {
	errTimeout := errors.New("timeout error")

	h.mu.Lock()
	defer h.mu.Unlock()

	checksList := getChecks(h.checks)
	total := len(checksList)

	checkRespChan := make(chan checkResponse, total)

	var wgRes sync.WaitGroup
	wgRes.Add(total)

	go func() {
		wgRes.Wait()
		close(checkRespChan)
	}()

	for _, c := range checksList {
		go func(ctx context.Context, c Config, respChan chan<- checkResponse) {
			defer wgRes.Done()

			locResp := make(chan error)
			go func(ctx context.Context, locResp chan<- error) {
				defer close(locResp)
				locResp <- c.Check(ctx)
			}(ctx, locResp)

			select {
			case <-time.After(c.Timeout):
				respChan <- checkResponse{config: c, err: errTimeout}
			case err := <-locResp:
				respChan <- checkResponse{config: c, err: err}
			}
		}(ctx, c, checkRespChan)
	}

	status := StatusOK
	checks := make(map[string]string)
	for resp := range checkRespChan {
		if resp.err == errTimeout {
			checks[resp.config.Name] = string(StatusTimeout)
			status = getAvailability(status, resp.config)

			continue
		}

		if resp.err != nil {
			checks[resp.config.Name] = resp.err.Error()
			status = getAvailability(status, resp.config)
			logger.Error().Err(errors.WithMessage(resp.err, resp.config.Name)).Send()

			continue
		}

		checks[resp.config.Name] = string(StatusOK)
	}

	system := newSystemMetrics()
	h.info(&system)

	return newCheck(status, checks, &system)
}

func getReadinessCheck(checks map[string]Config) map[string]Config {
	return checks
}

func getLivenessCheck(checks map[string]Config) map[string]Config {
	rez := make(map[string]Config)
	for key, conf := range checks {
		if conf.Vital {
			rez[key] = conf
		}
	}

	return rez
}

func newCheck(s Status, failures map[string]string, system *System) Check {
	return Check{
		Status:    s,
		Timestamp: time.Now(),
		Failures:  failures,
		System:    system,
	}
}

func newSystemMetrics() System {
	s := runtime.MemStats{}
	runtime.ReadMemStats(&s)
	return System{
		Version:          runtime.Version(),
		GoroutinesCount:  runtime.NumGoroutine(),
		TotalAllocBytes:  int(s.TotalAlloc),
		HeapObjectsCount: int(s.HeapObjects),
		AllocBytes:       int(s.Alloc),
	}
}

func getAvailability(s Status, c Config) Status {
	if c.SkipOnErr && s != StatusUnavailable {
		return StatusPartiallyAvailable
	}

	return StatusUnavailable
}