package grib

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

// URLFunc returns the GRIB URL for one (date, runHour, member, step, levelSet).
// Sources that don't have members (GFS) ignore the member argument.
type URLFunc func(date string, runHour, member, step int, ls wgfs.LevelSet) string

// Downloader is the generic GRIB-cube downloader.
//
// A Source plugs in its variant, URL templating, and member-resolution
// logic; the Downloader runs the parallel idx fetch, byte-range download,
// GRIB decode, and blit loop with manifest-based resume.
type Downloader struct {
	Variant  *wgfs.Variant
	URLs     URLFunc
	Parallel int
	Client   *http.Client
	Log      *zap.Logger
}

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

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

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

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

// Run downloads the dataset for id, member into store. The caller may
// pass member=0 for non-ensemble sources.
func (d *Downloader) Run(ctx context.Context, id datasets.DatasetID, member int, store datasets.Storage, prog datasets.ProgressSink, throttle datasets.Throttle) error {
	if prog == nil {
		prog = noopSink{}
	}

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

	file, err := openOrCreateCube(handle.Path(), d.Variant)
	if err != nil {
		_ = handle.Abort()
		return err
	}

	epoch := id.Epoch.UTC()
	date := epoch.Format("20060102")
	runHour := epoch.Hour()

	steps := d.Variant.Hours()
	if hr := id.Subset.HourRange; hr != nil {
		filtered := steps[:0]
		for _, step := range steps {
			if step >= hr.MinHour && step <= hr.MaxHour {
				filtered = append(filtered, step)
			}
		}
		steps = filtered
	}
	prog.SetTotal(len(steps) * 2)
	for range manifest.Units() {
		prog.StepComplete()
	}

	start := time.Now()
	g, ctx := errgroup.WithContext(ctx)
	g.SetLimit(d.parallel())
	var fileMu sync.Mutex

	for _, step := range steps {
		hourIdx := d.Variant.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 {
				url := d.URLs(date, runHour, member, step, ls)
				if err := d.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()
		if errors.Is(err, context.Canceled) {
			return err
		}
		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)
	}

	d.log().Info("download complete",
		zap.String("variant", d.Variant.ID),
		zap.Time("epoch", epoch),
		zap.Duration("elapsed", time.Since(start)))
	return nil
}

// openOrCreateCube opens an existing cube at path if it matches variant's
// expected size, else truncate-creates a new one.
func openOrCreateCube(path string, variant *wgfs.Variant) (*wgfs.File, error) {
	info, err := os.Stat(path)
	if err == nil && info.Size() == variant.DatasetSize() {
		return wgfs.OpenWritable(path, variant)
	}
	if err != nil && !errors.Is(err, os.ErrNotExist) {
		return nil, fmt.Errorf("stat cube: %w", err)
	}
	return wgfs.Create(path, variant)
}

// downloadAndBlit fetches and decodes one (URL, level-set) chunk.
func (d *Downloader) 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 := d.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 := d.Variant.PressureLevelSet(e.LevelMB)
		if ok && set == ls {
			relevant = append(relevant, e)
		}
	}
	if len(relevant) == 0 {
		return nil
	}
	ranges := EntriesToRanges(relevant)

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

	for _, r := range ranges {
		body, err := d.httpGetRange(ctx, baseURL, r.Start, r.End, throttle, prog)
		if err != nil {
			tmp.Close()
			return fmt.Errorf("range: %w", 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 := d.Variant.VariableIndex(int(p.ParameterCategory), int(p.ParameterNumber))
		if varIdx < 0 {
			continue
		}
		if p.FirstSurface.Type != 100 {
			continue
		}
		pressureMB := int(math.Round(float64(p.FirstSurface.Value) / 100.0))
		levelIdx := d.Variant.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
}

func (d *Downloader) 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 := d.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)
}

func (d *Downloader) 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 := d.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", resp.StatusCode)
			continue
		}
		if err != nil {
			lastErr = err
			continue
		}
		return body, nil
	}
	return nil, fmt.Errorf("after 3 attempts: %w", lastErr)
}

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"
}

type noopSink struct{}

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