// Package progress renders a live, single-line status display. One goroutine ticks
// once a second, pulls a snapshot of the running downloads, and derives speeds
// from the change since the previous tick. There are no locks here: the data
// arrives by value through the snapshot function.
package progress

import (
	"context"
	"fmt"
	"io"
	"os"
	"strings"
	"time"
	"unicode/utf8"

	"github.com/hanbok/got/download"
	"golang.org/x/term"
)

// Reporter draws the progress line until its context is cancelled.
type Reporter struct {
	snap     func() []download.Stat
	human    bool
	interval time.Duration
	w        io.Writer
	tty      bool
	width    int
	height   int

	win       map[string]*speedWindow // keyed by Stat.ID
	prevLines int                     // lines drawn in the previous TTY frame, redrawn in place
	lastLog   time.Time               // last non-TTY line emitted
}

// logEvery bounds how often progress is printed when output is not a TTY, so a
// redirected or piped run does not get a line every second.
const logEvery = 30 * time.Second

// windowTime is the span of the sliding speed window (10s): the rate is the
// byte delta across this window divided by its real duration, which smooths the
// per-tick jitter.
const windowTime = 10 * time.Second

// sample is one (time, cumulative-counters) observation in a speedWindow.
type sample struct {
	t                   time.Time
	completed, uploaded int64
}

// speedWindow keeps the last ~windowTime of samples for one download so we can
// derive down/up speed from the change across the window rather than the last
// tick alone. Samples are appended in time order, so stale ones live at the
// front.
type speedWindow struct {
	samples []sample
}

// New returns a Reporter that reads live stats from snap.
func New(snap func() []download.Stat, human bool) *Reporter {
	return &Reporter{
		snap:     snap,
		human:    human,
		interval: time.Second,
		w:        os.Stdout,
		tty:      term.IsTerminal(int(os.Stdout.Fd())),
		width:    80,
		height:   24,
		win:      map[string]*speedWindow{},
	}
}

// Run draws on every tick and clears the line when ctx is cancelled.
func (r *Reporter) Run(ctx context.Context) {
	t := time.NewTicker(r.interval)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			r.render(true)
			return
		case <-t.C:
			r.render(false)
		}
	}
}

func (r *Reporter) render(final bool) {
	stats := r.snap()
	now := time.Now()

	// Record one sample per download before deriving rates, and forget windows
	// for downloads that have dropped out of the snapshot.
	seen := make(map[string]bool, len(stats))
	for _, s := range stats {
		seen[s.ID] = true
		w := r.win[s.ID]
		if w == nil {
			w = &speedWindow{}
			r.win[s.ID] = w
		}
		w.add(now, s.Completed, s.Uploaded)
	}
	for id := range r.win {
		if !seen[id] {
			delete(r.win, id)
		}
	}

	if r.tty {
		r.width = termWidth(r.width)
		r.height = termHeight(r.height)
		r.redraw(r.block(stats), final)
		return
	}
	// Not a TTY: no cursor control, so emit a single line sparingly (and always
	// the final one) to keep redirected output and logs readable.
	var line string
	switch {
	case len(stats) == 1:
		line = r.lineOne(stats[0])
	case len(stats) > 1:
		line = r.summary(stats)
	}
	if line != "" && (final || now.Sub(r.lastLog) >= logEvery) {
		fmt.Fprintln(r.w, line)
		r.lastLog = now
	}
}

func (r *Reporter) lineOne(s download.Stat) string {
	dl, ul := r.rates(s)
	seeding := s.Status == download.Seeding
	var b strings.Builder
	fmt.Fprintf(&b, "[%s ", shortName(s.Name))
	// While seeding, report the share ratio in place of the size block and drop
	// the DL: field; otherwise show completed/total (or bare completed).
	if seeding {
		fmt.Fprintf(&b, "SEED(%.1f)", ratio(s.Uploaded, s.Completed))
	} else if s.Total > 0 {
		fmt.Fprintf(&b, "%s/%s(%d%%)", humanSize(s.Completed, r.human), humanSize(s.Total, r.human), percent(s.Completed, s.Total))
	} else {
		b.WriteString(humanSize(s.Completed, r.human))
	}
	// CN is always shown (including HTTP); SD is shown for any torrent.
	fmt.Fprintf(&b, " CN:%d", s.Conns)
	if s.IsBT {
		fmt.Fprintf(&b, " SD:%d", s.Seeders)
	}
	if !seeding {
		fmt.Fprintf(&b, " DL:%s", humanSize(dl, r.human))
	}
	if seeding || ul > 0 {
		fmt.Fprintf(&b, " UL:%s", humanSize(ul, r.human))
	}
	if !seeding && s.Total > 0 && dl > 0 {
		fmt.Fprintf(&b, " ETA:%s", secfmt(etaDuration(s.Total-s.Completed, dl)))
	}
	b.WriteByte(']')
	return b.String()
}

// nameW is the fixed rune width of the name column in the table, matching
// shortName's cap so a padded name lines the columns up.
const nameW = 20

// block builds the lines of the live display: the single-download dashboard for
// one download, a name/bar/speed/ETA table for several, or a one-line summary
// when the table would not fit the window.
func (r *Reporter) block(stats []download.Stat) []string {
	switch {
	case len(stats) == 0:
		return nil
	case len(stats) == 1:
		return []string{r.lineOne(stats[0])}
	case len(stats)+1 > r.height: // header + one row each would overflow the screen
		return []string{r.summary(stats)}
	default:
		out := make([]string, 0, len(stats)+1)
		out = append(out, r.summary(stats))
		for _, s := range stats {
			out = append(out, r.tableRow(s))
		}
		return out
	}
}

// redraw repaints the block in place: move to the top of the previous frame,
// clear it, and print the new lines clamped to the terminal width. A stray line
// from stderr that lands in the block is painted over on the next tick.
func (r *Reporter) redraw(lines []string, final bool) {
	if r.prevLines > 0 {
		if r.prevLines > 1 {
			fmt.Fprintf(r.w, "\x1b[%dA", r.prevLines-1) // up to the first line
		}
		fmt.Fprint(r.w, "\r\x1b[J") // column 0, clear to end of screen
	}
	for i, ln := range lines {
		if i > 0 {
			fmt.Fprint(r.w, "\n")
		}
		fmt.Fprint(r.w, clampRunes(ln, r.width))
	}
	r.prevLines = len(lines)
	if final && len(lines) > 0 {
		fmt.Fprintln(r.w) // leave the cursor below the block for the OK/FAIL lines
		r.prevLines = 0
	}
}

// summary is the header/aggregate line: the active count and total down/up
// speed, plus a stalled count when any download has gone quiet.
func (r *Reporter) summary(stats []download.Stat) string {
	if len(stats) == 0 {
		return ""
	}
	var totDL, totUL int64
	stalled, waiting := 0, 0
	for _, s := range stats {
		dl, ul := r.rates(s)
		totDL += dl
		totUL += ul
		if s.Status == download.Waiting {
			waiting++
			continue
		}
		if r.isStalled(s, dl) {
			stalled++
		}
	}
	line := fmt.Sprintf("%d active   DL:%s UL:%s", len(stats)-waiting, humanSize(totDL, r.human), humanSize(totUL, r.human))
	if waiting > 0 {
		line += fmt.Sprintf("  (%d waiting)", waiting)
	}
	if stalled > 0 {
		line += fmt.Sprintf("  (%d stalled)", stalled)
	}
	return line
}

// tableRow renders one download as "name |bar| pct metric tail": the bar and
// percent for progress, then the field that answers "stalled or fast" (DL speed
// or seeding) and a tail (ETA, upload rate, or a stalled flag).
func (r *Reporter) tableRow(s download.Stat) string {
	dl, ul := r.rates(s)
	metric, tail := r.rowMetric(s, dl, ul)
	return fmt.Sprintf("  %s  %s %s  %-9s %s", padRune(shortName(s.Name), nameW), barOf(s), pctField(s), metric, tail)
}

// rowMetric returns a table row's two trailing fields for the download's state:
// the primary metric (DL speed, "seeding", "done", "failed") and a tail (ETA,
// upload rate, or a "stalled" flag).
func (r *Reporter) rowMetric(s download.Stat, dl, ul int64) (metric, tail string) {
	switch s.Status {
	case download.Waiting:
		return "queued", ""
	case download.Seeding:
		return "seeding", "UL:" + humanSize(ul, r.human)
	case download.Complete:
		return "done", ""
	case download.Errored:
		return "failed", ""
	default:
		metric = "DL:" + humanSize(dl, r.human)
		switch {
		case dl > 0 && s.Total > 0:
			tail = secfmt(etaDuration(s.Total-s.Completed, dl))
		case r.isStalled(s, dl):
			tail = "stalled"
		}
		return metric, tail
	}
}

// maxETASeconds caps an ETA before it becomes a Duration: a near-stalled download
// yields an astronomically large seconds value, and time.Duration(secs) *
// time.Second would overflow int64 and wrap to a bogus tiny ETA. secfmt renders
// anything past 99h as "--", so clamping to 100h loses nothing.
const maxETASeconds = 100 * 3600

func etaDuration(remaining, dl int64) time.Duration {
	secs := float64(remaining) / float64(dl)
	if secs > maxETASeconds {
		secs = maxETASeconds
	}
	return time.Duration(secs) * time.Second
}

// isStalled reports an active download that has gone a while with no new bytes,
// so the table can flag it rather than leave the eye to read DL:0B. A download
// too young to have a rate yet is not stalled.
func (r *Reporter) isStalled(s download.Stat, dl int64) bool {
	if s.Status != download.Active || dl > 0 {
		return false
	}
	w := r.win[s.ID]
	if w == nil || len(w.samples) < 2 {
		return false
	}
	return w.samples[len(w.samples)-1].t.Sub(w.samples[0].t) >= 4*time.Second
}

// barOf renders a fixed-width progress bar. A seeding or complete download is
// full; an unknown total (no Content-Length, or pre-metadata) shows empty.
func barOf(s download.Stat) string {
	const cells = 12
	k := 0
	switch {
	case s.Status == download.Seeding || s.Status == download.Complete:
		k = cells
	case s.Total > 0:
		k = int(int64(cells) * s.Completed / s.Total)
		if k > cells {
			k = cells
		}
	}
	return "|" + strings.Repeat("#", k) + strings.Repeat("-", cells-k) + "|"
}

// pctField is the percent column: a number, full for seeding/complete, or "--"
// when the total is unknown.
func pctField(s download.Stat) string {
	switch {
	case s.Status == download.Seeding || s.Status == download.Complete:
		return "100%"
	case s.Total > 0:
		return fmt.Sprintf("%3d%%", percent(s.Completed, s.Total))
	default:
		return "  --"
	}
}

// padRune right-pads s with spaces to w runes (it is never longer, having been
// through shortName) so table columns line up on rune count.
func padRune(s string, w int) string {
	if n := utf8.RuneCountInString(s); n < w {
		return s + strings.Repeat(" ", w-n)
	}
	return s
}

// add records a sample and drops any that have fallen out of the window. A
// sample is only appended once per second, but the latest counters are folded
// into the newest slot so the final cancel-render, which typically fires in the
// same second as the preceding tick, still sees current totals.
func (w *speedWindow) add(now time.Time, completed, uploaded int64) {
	// Drop samples older than the window, keeping the first one that still falls
	// inside it as the baseline for the delta.
	cut := now.Add(-windowTime)
	i := 0
	for i < len(w.samples) && w.samples[i].t.Before(cut) {
		i++
	}
	w.samples = w.samples[i:]

	if n := len(w.samples); n > 0 && now.Sub(w.samples[n-1].t) < time.Second {
		w.samples[n-1].completed = completed
		w.samples[n-1].uploaded = uploaded
		return
	}
	w.samples = append(w.samples, sample{now, completed, uploaded})
}

// rates derives down/up speed in bytes/s from the change across the speed
// window (windowBytes / windowSeconds). With a single sample we have no span
// yet, so we report 0 rather than mistaking the cumulative bytes (e.g. resumed
// data) for one window's worth.
func (r *Reporter) rates(s download.Stat) (dl, ul int64) {
	w := r.win[s.ID]
	if w == nil || len(w.samples) < 2 {
		return 0, 0
	}
	first, last := w.samples[0], w.samples[len(w.samples)-1]
	secs := last.t.Sub(first.t).Seconds()
	if secs <= 0 {
		return 0, 0
	}
	dl = int64(float64(last.completed-first.completed) / secs)
	ul = int64(float64(last.uploaded-first.uploaded) / secs)
	if dl < 0 {
		dl = 0
	}
	if ul < 0 {
		ul = 0
	}
	return dl, ul
}

// ratio is uploaded/completed for the seeding readout; 0 when nothing has been
// downloaded yet to avoid dividing by zero.
func ratio(uploaded, completed int64) float64 {
	if completed <= 0 {
		return 0
	}
	return float64(uploaded) / float64(completed)
}

func shortName(s string) string {
	const max = 20
	// Truncate on runes, not bytes, so a multibyte name (CJK, emoji) is never
	// cut mid-rune.
	if utf8.RuneCountInString(s) > max {
		return string([]rune(s)[:max-1]) + "~"
	}
	return s
}

// clampRunes limits a line to width runes (not bytes) so multibyte glyphs are
// never split when the terminal is narrow. A non-positive width leaves it whole.
func clampRunes(s string, width int) string {
	if width <= 0 || utf8.RuneCountInString(s) <= width {
		return s
	}
	return string([]rune(s)[:width])
}

func termWidth(prev int) int {
	if w, _, err := term.GetSize(int(os.Stdout.Fd())); err == nil && w > 0 {
		return w
	}
	if prev > 0 {
		return prev
	}
	return 80
}

func termHeight(prev int) int {
	if _, h, err := term.GetSize(int(os.Stdout.Fd())); err == nil && h > 0 {
		return h
	}
	if prev > 0 {
		return prev
	}
	return 24
}