package metrics

import (
	"context"
	"encoding/json"
	"errors"
	"os"
	"time"

	"github.com/carlosframework/platform/internal/release"
)

// Agg is a min/avg/max/count summary of one metric over some window. N
// counts only the samples that actually carried the metric (nil readings —
// a parser that couldn't read a value — are excluded, never treated as
// zero).
type Agg struct {
	Min float64 `json:"min"`
	Avg float64 `json:"avg"`
	Max float64 `json:"max"`
	N   int     `json:"n"`
}

// HourAgg is one hour's rollup: whole-host metrics plus per-unit metrics.
// A metric that no sample in the hour carried is simply absent from the
// map — never a zero-valued Agg standing in for "no data".
type HourAgg struct {
	Host  map[string]Agg            `json:"host"`  // "cpu","mem","disk","load1"
	Units map[string]map[string]Agg `json:"units"` // unit -> "cpu","mem_bytes" -> Agg
}

// DayRollup is a box's rolled-up day, keyed by UTC hour ("00".."23") —
// the object stored under metrics/boxes/<box>/days/<date>.json.
type DayRollup struct {
	V     int                `json:"v"` // 1
	Box   string             `json:"box"`
	Date  string             `json:"date"` // UTC 2006-01-02
	Hours map[string]HourAgg `json:"hours"`
}

// aggOf reduces vals to an Agg, reporting ok=false for an empty slice — the
// "this metric was never carried" case, which callers turn into "key
// absent from the map" rather than a zero-valued Agg.
func aggOf(vals []float64) (Agg, bool) {
	if len(vals) == 0 {
		return Agg{}, false
	}
	min, max, sum := vals[0], vals[0], 0.0
	for _, v := range vals {
		if v < min {
			min = v
		}
		if v > max {
			max = v
		}
		sum += v
	}
	return Agg{Min: min, Avg: sum / float64(len(vals)), Max: max, N: len(vals)}, true
}

// RollupHour aggregates a slice of raw samples (typically an hour's worth)
// into one HourAgg. Nil metric readings are excluded from N and, when a
// metric is nil in every sample, its key is absent from the returned map
// entirely (both at the host level and per-unit). Host metrics are
// "cpu","mem","disk","load1"; unit metrics are "cpu" (from
// UnitSample.CPUFrac) and "mem_bytes" (from MemBytes, cast to float64).
func RollupHour(samples []Sample) HourAgg {
	var hostCPU, hostMem, hostDisk, hostLoad1 []float64
	for _, s := range samples {
		if v := s.Host.CPUFrac; v != nil {
			hostCPU = append(hostCPU, *v)
		}
		if v := s.Host.MemFrac; v != nil {
			hostMem = append(hostMem, *v)
		}
		if v := s.Host.DiskFrac; v != nil {
			hostDisk = append(hostDisk, *v)
		}
		if v := s.Host.Load1; v != nil {
			hostLoad1 = append(hostLoad1, *v)
		}
	}

	host := map[string]Agg{}
	for key, vals := range map[string][]float64{
		"cpu": hostCPU, "mem": hostMem, "disk": hostDisk, "load1": hostLoad1,
	} {
		if agg, ok := aggOf(vals); ok {
			host[key] = agg
		}
	}

	unitCPU := map[string][]float64{}
	unitMem := map[string][]float64{}
	unitNames := map[string]bool{}
	for _, s := range samples {
		for name, u := range s.Units {
			unitNames[name] = true
			if u.CPUFrac != nil {
				unitCPU[name] = append(unitCPU[name], *u.CPUFrac)
			}
			if u.MemBytes != nil {
				unitMem[name] = append(unitMem[name], float64(*u.MemBytes))
			}
		}
	}

	units := map[string]map[string]Agg{}
	for name := range unitNames {
		inner := map[string]Agg{}
		if agg, ok := aggOf(unitCPU[name]); ok {
			inner["cpu"] = agg
		}
		if agg, ok := aggOf(unitMem[name]); ok {
			inner["mem_bytes"] = agg
		}
		if len(inner) > 0 {
			units[name] = inner
		}
	}

	return HourAgg{Host: host, Units: units}
}

// UpsertRollup reads box's day object for hour's UTC date (a missing
// object is not an error — it starts a fresh DayRollup), sets the entry
// for hour's UTC hour bucket ("00".."23") to agg, and writes the day back
// with the same read-etag, Put, retry-once-on-ErrCASConflict pattern
// WriteRaw uses: a second conflict is not retried again and is returned to
// the caller.
func UpsertRollup(ctx context.Context, s release.Store, box string, hour time.Time, agg HourAgg) error {
	date := hour.UTC().Format("2006-01-02")
	hourKey := hour.UTC().Format("15")
	key := DayKey(box, date)

	day, etag, err := currentDayRollup(ctx, s, key, box, date)
	if err != nil {
		return err
	}
	day.Hours[hourKey] = agg
	body, err := json.MarshalIndent(day, "", "  ")
	if err != nil {
		return err
	}
	if _, err := s.Put(ctx, key, body, etag); err != nil {
		if !errors.Is(err, release.ErrCASConflict) {
			return err
		}
		day, etag, err = currentDayRollup(ctx, s, key, box, date)
		if err != nil {
			return err
		}
		day.Hours[hourKey] = agg
		body, err = json.MarshalIndent(day, "", "  ")
		if err != nil {
			return err
		}
		if _, err := s.Put(ctx, key, body, etag); err != nil {
			return err
		}
	}
	return nil
}

// currentDayRollup reads key's current DayRollup and etag for a
// replace-only Put, folding "doesn't exist yet" into a fresh DayRollup at
// ifMatch "" (create-only) — the same absent-means-empty convention
// currentRawEtag uses for raw chunks.
func currentDayRollup(ctx context.Context, s release.Store, key, box, date string) (DayRollup, string, error) {
	body, etag, err := s.Get(ctx, key)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return DayRollup{V: 1, Box: box, Date: date, Hours: map[string]HourAgg{}}, "", nil
		}
		return DayRollup{}, "", err
	}
	var day DayRollup
	if err := json.Unmarshal(body, &day); err != nil {
		return DayRollup{}, "", err
	}
	if day.Hours == nil {
		day.Hours = map[string]HourAgg{}
	}
	return day, etag, nil
}