// Package gfs implements datasets.Source for NOAA GFS 0.5-degree forecasts.
package gfs

import (
	"context"
	"errors"
	"fmt"
	"io"
	"math"
	"net/http"
	"os"
	"sync"
	"time"

	"github.com/nilsmagnus/grib/griblib"
	"go.uber.org/zap"
	"golang.org/x/sync/errgroup"

	"predictor-refactored/internal/datasets"
	"predictor-refactored/internal/weather"
	wgfs "predictor-refactored/internal/weather/gfs"
)

// Source is the GFS implementation of datasets.Source.
type Source struct {
	Parallel int           // max concurrent step downloads
	Client   *http.Client  // optional; defaults to a 2-minute-timeout client
	Log      *zap.Logger
}

// NewSource returns a default Source.
func NewSource(log *zap.Logger) *Source {
	return &Source{
		Parallel: 8,
		Client:   &http.Client{Timeout: 2 * time.Minute},
		Log:      log,
	}
}

// ID returns the source identifier.
func (s *Source) ID() string { return "noaa-gfs-0p50" }

func (s *Source) log() *zap.Logger {
	if s.Log == nil {
		return zap.NewNop()
	}
	return s.Log
}

func (s *Source) client() *http.Client {
	if s.Client == nil {
		return &http.Client{Timeout: 2 * time.Minute}
	}
	return s.Client
}

func (s *Source) parallel() int {
	if s.Parallel <= 0 {
		return 8
	}
	return s.Parallel
}

// LatestEpoch returns the most recent run NOAA has finished publishing,
// determined by HEAD-ing the .idx for the final forecast hour. Walks back
// up to 8 runs (48 hours) before giving up.
func (s *Source) LatestEpoch(ctx context.Context) (time.Time, error) {
	now := time.Now().UTC()
	hour := now.Hour() - (now.Hour() % 6)
	current := time.Date(now.Year(), now.Month(), now.Day(), hour, 0, 0, 0, time.UTC)

	for range 8 {
		date := current.Format("20060102")
		url := wgfs.GribURL(date, current.Hour(), wgfs.MaxHour) + ".idx"

		req, err := http.NewRequestWithContext(ctx, http.MethodHead, url, nil)
		if err == nil {
			resp, err := s.client().Do(req)
			if err == nil {
				resp.Body.Close()
				if resp.StatusCode == http.StatusOK {
					s.log().Info("latest GFS run discovered",
						zap.Time("run", current),
						zap.String("verified_url", url))
					return current, nil
				}
			}
		}
		current = current.Add(-6 * time.Hour)
	}
	return time.Time{}, fmt.Errorf("no recent GFS run found (checked 8 runs)")
}

// Open loads a stored dataset as a WindField.
func (s *Source) Open(_ context.Context, epoch time.Time, store datasets.Storage) (weather.WindField, error) {
	if !store.Exists(epoch) {
		return nil, fmt.Errorf("epoch %s not found", epoch.Format(time.RFC3339))
	}
	file, err := wgfs.Open(store.Path(epoch), epoch.UTC())
	if err != nil {
		return nil, err
	}
	return wgfs.NewWind(file), nil
}

// neededVariables is the GRIB variable set we extract.
var neededVariables = map[string]bool{"HGT": true, "UGRD": true, "VGRD": true}

// Download fetches the full dataset for epoch in parallel, resuming any
// previously-completed work units. Honours ctx cancellation and prog
// (which may be nil).
func (s *Source) Download(ctx context.Context, epoch time.Time, store datasets.Storage, prog datasets.ProgressSink, throttle datasets.Throttle) error {
	if prog == nil {
		prog = noopSink{}
	}

	handle, err := store.BeginWrite(epoch)
	if err != nil {
		return fmt.Errorf("begin write: %w", err)
	}
	manifest := handle.Manifest()

	// Open or create the temp file. If a previous attempt left a partial
	// file of the right size, reuse it (resume); otherwise Create.
	file, err := openOrCreateCube(handle.Path())
	if err != nil {
		_ = handle.Abort()
		return err
	}

	date := epoch.UTC().Format("20060102")
	runHour := epoch.UTC().Hour()
	steps := wgfs.Hours()
	totalUnits := len(steps) * 2

	prog.SetTotal(totalUnits)
	// Pre-count already-done units so progress is accurate on resume.
	for _, u := range manifest.Units() {
		_ = u
		prog.StepComplete()
	}

	start := time.Now()
	g, ctx := errgroup.WithContext(ctx)
	g.SetLimit(s.parallel())

	// fileMu serialises concurrent BlitGribData calls because the underlying
	// mmap is shared and SetVal isn't atomic.
	var fileMu sync.Mutex

	for _, step := range steps {
		hourIdx := wgfs.HourIndex(step)
		if hourIdx < 0 {
			continue
		}

		for _, ls := range []wgfs.LevelSet{wgfs.LevelSetA, wgfs.LevelSetB} {
			unit := unitKey(step, ls)
			if manifest.Has(unit) {
				continue
			}

			g.Go(func() error {
				var url string
				switch ls {
				case wgfs.LevelSetA:
					url = wgfs.GribURL(date, runHour, step)
				case wgfs.LevelSetB:
					url = wgfs.GribURLB(date, runHour, step)
				}
				if err := s.downloadAndBlit(ctx, file, &fileMu, url, hourIdx, ls, prog, throttle); err != nil {
					return fmt.Errorf("step %d %s: %w", step, levelSetLabel(ls), err)
				}
				if err := manifest.Mark(unit); err != nil {
					return fmt.Errorf("mark unit: %w", err)
				}
				prog.StepComplete()
				return nil
			})
		}
	}

	if err := g.Wait(); err != nil {
		_ = file.Close()
		// Don't Abort on context cancellation — preserve progress for resume.
		if errors.Is(err, context.Canceled) {
			return err
		}
		// Other errors: abort if no progress was made; otherwise leave for resume.
		if len(manifest.Units()) == 0 {
			_ = handle.Abort()
		}
		return err
	}

	if err := file.Flush(); err != nil {
		_ = file.Close()
		return fmt.Errorf("flush: %w", err)
	}
	if err := file.Close(); err != nil {
		return fmt.Errorf("close: %w", err)
	}
	if err := handle.Commit(); err != nil {
		return fmt.Errorf("commit: %w", err)
	}

	s.log().Info("download complete",
		zap.Time("epoch", epoch),
		zap.Duration("elapsed", time.Since(start)))
	return nil
}

// openOrCreateCube returns a writable cube file at path, creating it if the
// file does not exist or has the wrong size.
func openOrCreateCube(path string) (*wgfs.File, error) {
	info, err := os.Stat(path)
	if err == nil && info.Size() == wgfs.DatasetSize {
		return wgfs.OpenWritable(path)
	}
	if err != nil && !errors.Is(err, os.ErrNotExist) {
		return nil, fmt.Errorf("stat cube: %w", err)
	}
	// Wrong-size or missing — truncate-create.
	return wgfs.Create(path)
}

// downloadAndBlit fetches and decodes one (URL, level-set) chunk and writes
// it into the dataset.
func (s *Source) downloadAndBlit(
	ctx context.Context,
	file *wgfs.File,
	fileMu *sync.Mutex,
	baseURL string,
	hourIdx int,
	ls wgfs.LevelSet,
	prog datasets.ProgressSink,
	throttle datasets.Throttle,
) error {
	idxBody, err := s.httpGet(ctx, baseURL+".idx", throttle, prog)
	if err != nil {
		return fmt.Errorf("idx: %w", err)
	}
	entries := ParseIdx(idxBody)
	filtered := FilterIdx(entries, neededVariables)

	var relevant []IdxEntry
	for _, e := range filtered {
		set, ok := wgfs.PressureLevelSet(e.LevelMB)
		if ok && set == ls {
			relevant = append(relevant, e)
		}
	}
	if len(relevant) == 0 {
		return nil
	}

	ranges := EntriesToRanges(relevant)
	tmp, err := os.CreateTemp("", "gfs-msg-*.tmp")
	if err != nil {
		return fmt.Errorf("temp: %w", err)
	}
	tmpPath := tmp.Name()
	defer os.Remove(tmpPath)

	for _, r := range ranges {
		body, err := s.httpGetRange(ctx, baseURL, r.Start, r.End, throttle, prog)
		if err != nil {
			tmp.Close()
			return fmt.Errorf("range %d-%d: %w", r.Start, r.End, err)
		}
		if _, err := tmp.Write(body); err != nil {
			tmp.Close()
			return fmt.Errorf("write tmp: %w", err)
		}
	}
	if err := tmp.Close(); err != nil {
		return err
	}

	f, err := os.Open(tmpPath)
	if err != nil {
		return err
	}
	messages, err := griblib.ReadMessages(f)
	f.Close()
	if err != nil {
		return fmt.Errorf("read grib: %w", err)
	}

	for _, msg := range messages {
		if msg.Section4.ProductDefinitionTemplateNumber != 0 {
			continue
		}
		p := msg.Section4.ProductDefinitionTemplate
		varIdx := wgfs.VariableIndex(int(p.ParameterCategory), int(p.ParameterNumber))
		if varIdx < 0 {
			continue
		}
		if p.FirstSurface.Type != 100 { // isobaric only
			continue
		}
		pressureMB := int(math.Round(float64(p.FirstSurface.Value) / 100.0))
		levelIdx := wgfs.PressureIndex(pressureMB)
		if levelIdx < 0 {
			continue
		}
		data := msg.Data()
		fileMu.Lock()
		err := file.BlitGribData(hourIdx, levelIdx, varIdx, data)
		fileMu.Unlock()
		if err != nil {
			return fmt.Errorf("blit: %w", err)
		}
	}
	return nil
}

// httpGet downloads a URL body with 3 retries and optional throttling.
func (s *Source) httpGet(ctx context.Context, url string, throttle datasets.Throttle, prog datasets.ProgressSink) ([]byte, error) {
	var lastErr error
	for attempt := range 3 {
		if attempt > 0 {
			select {
			case <-time.After(time.Duration(attempt*2) * time.Second):
			case <-ctx.Done():
				return nil, ctx.Err()
			}
		}
		req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
		if err != nil {
			return nil, err
		}
		resp, err := s.client().Do(req)
		if err != nil {
			lastErr = err
			continue
		}
		body, err := readThrottled(ctx, resp.Body, throttle, prog)
		resp.Body.Close()
		if resp.StatusCode != http.StatusOK {
			lastErr = fmt.Errorf("HTTP %d for %s", resp.StatusCode, url)
			continue
		}
		if err != nil {
			lastErr = err
			continue
		}
		return body, nil
	}
	return nil, fmt.Errorf("after 3 attempts: %w", lastErr)
}

// httpGetRange downloads an inclusive byte range with 3 retries and throttling.
func (s *Source) httpGetRange(ctx context.Context, url string, start, end int64, throttle datasets.Throttle, prog datasets.ProgressSink) ([]byte, error) {
	var lastErr error
	for attempt := range 3 {
		if attempt > 0 {
			select {
			case <-time.After(time.Duration(attempt*2) * time.Second):
			case <-ctx.Done():
				return nil, ctx.Err()
			}
		}
		req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
		if err != nil {
			return nil, err
		}
		req.Header.Set("Range", fmt.Sprintf("bytes=%d-%d", start, end))
		resp, err := s.client().Do(req)
		if err != nil {
			lastErr = err
			continue
		}
		body, err := readThrottled(ctx, resp.Body, throttle, prog)
		resp.Body.Close()
		if resp.StatusCode != http.StatusPartialContent && resp.StatusCode != http.StatusOK {
			lastErr = fmt.Errorf("HTTP %d for range %d-%d of %s", resp.StatusCode, start, end, url)
			continue
		}
		if err != nil {
			lastErr = err
			continue
		}
		return body, nil
	}
	return nil, fmt.Errorf("after 3 attempts: %w", lastErr)
}

// readThrottled reads r into memory, consulting throttle (if non-nil) before
// each chunk and reporting bytes to prog.
func readThrottled(ctx context.Context, r io.Reader, throttle datasets.Throttle, prog datasets.ProgressSink) ([]byte, error) {
	buf := make([]byte, 0, 64*1024)
	chunk := make([]byte, 32*1024)
	for {
		if throttle != nil {
			if err := throttle.Wait(ctx, len(chunk)); err != nil {
				return nil, err
			}
		}
		n, err := r.Read(chunk)
		if n > 0 {
			buf = append(buf, chunk[:n]...)
			prog.Bytes(int64(n))
		}
		if errors.Is(err, io.EOF) {
			return buf, nil
		}
		if err != nil {
			return nil, err
		}
	}
}

func unitKey(step int, ls wgfs.LevelSet) string {
	return fmt.Sprintf("step%03d-%s", step, levelSetLabel(ls))
}

func levelSetLabel(ls wgfs.LevelSet) string {
	if ls == wgfs.LevelSetB {
		return "B"
	}
	return "A"
}

// noopSink discards progress events.
type noopSink struct{}

func (noopSink) SetTotal(int)  {}
func (noopSink) StepComplete() {}
func (noopSink) Bytes(int64)   {}