package grib

import (
	"context"
	"encoding/binary"
	"math"
	"net/http"
	"os"
	"path/filepath"
	"sync/atomic"
	"time"

	"git.intra.yksa.space/gsn/predictor/internal/pkg/errcodes"
	"github.com/edsrzf/mmap-go"
	"github.com/nilsmagnus/grib/griblib"
)

type RedisIface interface {
	Lock(ctx context.Context, key string, ttl time.Duration) (func(context.Context), error)
	Set(key string, value []byte, ttl time.Duration) error
	Get(key string) ([]byte, error)
}

type ServiceConfig struct {
	Dir      string
	TTL      time.Duration
	CacheTTL time.Duration
	Redis    RedisIface
	Parallel int
	Client   *http.Client
}

type service struct {
	cfg   ServiceConfig
	cache memCache
	data  atomic.Pointer[dataset]
}

func New(cfg ServiceConfig) (*service, error) {
	if cfg.TTL == 0 {
		cfg.TTL = 24 * time.Hour
	}
	if err := os.MkdirAll(cfg.Dir, 0o755); err != nil {
		return nil, err
	}
	s := &service{cfg: cfg, cache: memCache{ttl: cfg.CacheTTL}}
	return s, nil
}

// Update() downloads missing GRIBs, assembles cube into a single mmap‑file.
func (s *service) Update(ctx context.Context) error {
	unlock, err := s.cfg.Redis.Lock(ctx, "grib-dl", 45*time.Minute)
	if err != nil {
		return err
	}
	defer unlock(ctx)

	dl := downloader.Downloader{Dir: s.cfg.Dir, Parallel: s.cfg.Parallel, Client: s.cfg.Client}
	run := nearestRun(time.Now().UTC().Add(-4 * time.Hour))
	if err := dl.Run(ctx, run); err != nil {
		return err
	}

	cubePath := filepath.Join(s.cfg.Dir, run.Format("20060102_15")) + ".cube"
	if _, err := os.Stat(cubePath); err != nil {
		if err := assembleCube(s.cfg.Dir, run, cubePath); err != nil {
			return err
		}
	}
	c, err := openCube(cubePath)
	if err != nil {
		return err
	}
	ds := &dataset{cube: c, runUTC: run.Unix()}
	s.data.Store(ds)
	s.cache = memCache{ttl: s.cfg.CacheTTL}
	return nil
}

func assembleCube(dir string, run time.Time, cubePath string) error {
	const sizePerVar = 17 * 34 * 361 * 720 * 4
	total := int64(sizePerVar * 2)
	f, err := os.Create(cubePath)
	if err != nil {
		return err
	}
	if err := f.Truncate(total); err != nil {
		return err
	}
	mm, err := mmap.MapRegion(f, int(total), mmap.RDWR, 0, 0)
	if err != nil {
		return err
	}
	defer mm.Unmap()
	defer f.Close()

	pIndex := make(map[int]int)
	for i, p := range pressureLevels {
		pIndex[int(math.Round(p))] = i
	}

	for ti, step := range steps {
		fn := filepath.Join(dir, fileName(run, step))
		gf, err := griblib.Read(fn)
		if err != nil {
			return err
		}
		for _, m := range gf.Messages {
			if m.ParameterShortName != "u" && m.ParameterShortName != "v" {
				continue
			}
			if m.TypeOfFirstFixedSurface != 100 {
				continue
			}
			pIdx, ok := pIndex[int(m.PressureLevel)]
			if !ok {
				continue
			}
			varIdx := 0
			if m.ParameterShortName == "v" {
				varIdx = 1
			}
			vals := m.Values
			// GRIB library returns scan north->south, west->east already in row-major order
			raw := make([]byte, len(vals)*4)
			for i, v := range vals {
				binary.LittleEndian.PutUint32(raw[i*4:], math.Float32bits(float32(v)))
			}
			base := int64(varIdx*sizePerVar + (ti*34+pIdx)*361*720*4)
			copy(mm[base:base+int64(len(raw))], raw)
		}
	}
	return mm.Flush()
}

func (s *service) Extract(ctx context.Context, lat, lon, alt float64, ts time.Time) ([2]float64, error) {
	var zero [2]float64
	d := s.data.Load()
	if d == nil {
		return zero, errcodes.ErrNoDataset
	}
	if ts.Before(time.Unix(d.runUTC, 0)) || ts.After(time.Unix(d.runUTC, 0).Add(48*time.Hour)) {
		return zero, errcodes.ErrOutOfBounds
	}
	key := encodeKey(lat, lon, alt, ts)
	if v, ok := s.cache.get(key); ok {
		return [2]float64(v), nil
	}
	td := ts.Sub(time.Unix(d.runUTC, 0)).Hours()
	u, v := d.uv(lat, lon, alt, td)
	out := [2]float64{u, v}
	s.cache.set(key, vec(out))
	return out, nil
}