package upwatch

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"
)

// ── The upload line, learnt from what the box already does ────────────────────────────────────────────────────────
//
// The box's off-site push (restic, inside this controller's own container) is the only big upload the box makes on
// its own. While it runs, the controller samples its OWN network counter once a second and keeps the best 10-second
// rate. Over nights, the highest such rate is a LOWER BOUND of the household's upload line — the push may not fill
// the line (the family uploads too, the Storage Box may be slow), so the page says „at least".
//
// Measured (spike §2): at a one-minute sample the push looked like 0.3–37 Mbit/s, because most nights it moves a few
// megabytes in under a minute — the one-minute counter shows the data size, not the line. Hence a 1-second sampler,
// and a night only COUNTS when its best 10 seconds moved ≥ MinWindowBytes: a short push says nothing about the line.

// MinWindowBytes is the least a push must move in its best 10 seconds to say anything about the line (≥ 8 Mbit/s).
const MinWindowBytes = 10 << 20

// KeepNights is how many nights' records the line is the maximum of.
const KeepNights = 14

// LineRecord is one push's measurement.
type LineRecord struct {
	At         string  `json:"at"`          // RFC3339, the push's end
	Peak10Mbps float64 `json:"peak10_mbps"` // the best 10-second rate
	Bytes      uint64  `json:"bytes"`       // everything the container sent during the push
	Seconds    int     `json:"seconds"`
	Counts     bool    `json:"counts"` // the best 10 seconds moved ≥ MinWindowBytes
}

// LineFromRecords is the line estimate: the highest counting peak over the kept nights. ok=false while none counts.
func LineFromRecords(recs []LineRecord) (float64, bool) {
	best, ok := 0.0, false
	for _, r := range recs {
		if r.Counts && r.Peak10Mbps > best {
			best, ok = r.Peak10Mbps, true
		}
	}
	return best, ok
}

// AddRecord appends one push and keeps the newest KeepNights.
func AddRecord(recs []LineRecord, r LineRecord) []LineRecord {
	recs = append(recs, r)
	if len(recs) > KeepNights {
		recs = recs[len(recs)-KeepNights:]
	}
	return recs
}

// OwnTxBytes sums the bytes sent on every interface but loopback in THIS process's network namespace
// (/proc/self/net/dev; the 9th field of an interface line is transmit bytes).
func OwnTxBytes() (uint64, error) {
	return txFromNetDev("/proc/self/net/dev")
}

func txFromNetDev(path string) (uint64, error) {
	f, err := os.Open(path)
	if err != nil {
		return 0, err
	}
	defer f.Close()
	var sum uint64
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		line := sc.Text()
		i := strings.Index(line, ":")
		if i < 0 {
			continue
		}
		if strings.TrimSpace(line[:i]) == "lo" {
			continue
		}
		fs := strings.Fields(line[i+1:])
		if len(fs) < 9 {
			continue
		}
		v, err := strconv.ParseUint(fs[8], 10, 64)
		if err != nil {
			return 0, fmt.Errorf("%s: %q: %w", path, line, err)
		}
		sum += v
	}
	return sum, sc.Err()
}

// Point is one reading of the counter.
type Point struct {
	At time.Time
	V  uint64
}

// Summarize turns a push's readings into its record: total bytes, seconds, and the best rate over any window of at
// least 10 seconds (for each start, the first reading ≥ 10 s later). A counter that went down is no measurement.
func Summarize(pts []Point, end time.Time) LineRecord {
	r := LineRecord{At: end.UTC().Format(time.RFC3339)}
	if len(pts) < 2 {
		return r
	}
	first, last := pts[0], pts[len(pts)-1]
	if last.V >= first.V {
		r.Bytes = last.V - first.V
	}
	r.Seconds = int(last.At.Sub(first.At).Seconds() + 0.5)
	var bestBytes uint64
	var bestRate float64
	j := 0
	for i := range pts {
		if j < i {
			j = i
		}
		for j < len(pts) && pts[j].At.Sub(pts[i].At) < 10*time.Second {
			j++
		}
		if j >= len(pts) {
			break
		}
		if pts[j].V < pts[i].V {
			continue
		}
		b := pts[j].V - pts[i].V
		rate := float64(b) * 8 / pts[j].At.Sub(pts[i].At).Seconds() / 1e6
		if rate > bestRate {
			bestRate, bestBytes = rate, b
		}
	}
	r.Peak10Mbps = round2(bestRate)
	r.Counts = bestBytes >= MinWindowBytes
	return r
}

// PushSampler samples a byte counter once a second between Start and the stop function it returns.
type PushSampler struct {
	Read  func() (uint64, error) // default OwnTxBytes
	Every time.Duration          // default 1 s
	Now   func() time.Time
}

// Start begins sampling; the returned stop ends it and returns the push's record.
func (p *PushSampler) Start() (stop func() LineRecord) {
	read := p.Read
	if read == nil {
		read = OwnTxBytes
	}
	every := p.Every
	if every <= 0 {
		every = time.Second
	}
	now := p.Now
	if now == nil {
		now = time.Now
	}
	var mu sync.Mutex
	var pts []Point
	take := func() {
		if v, err := read(); err == nil {
			mu.Lock()
			pts = append(pts, Point{now(), v})
			mu.Unlock()
		}
	}
	take()
	done := make(chan struct{})
	finished := make(chan struct{})
	go func() {
		defer close(finished)
		t := time.NewTicker(every)
		defer t.Stop()
		for {
			select {
			case <-done:
				return
			case <-t.C:
				take()
			}
		}
	}()
	return func() LineRecord {
		close(done)
		<-finished
		take()
		mu.Lock()
		defer mu.Unlock()
		return Summarize(pts, now())
	}
}