package engine

import (
	"context"
	"fmt"
	"sort"
	"sync"
	"time"
	"unsafe"

	"makarna/pkg/backend/cpu"
	"makarna/pkg/backend/cuda"
	"makarna/pkg/backend/device"
	"makarna/pkg/compute"
	"makarna/pkg/kvcache"
	"makarna/pkg/model"
	"makarna/pkg/sample"
	"makarna/pkg/tensor"
)

type DecodeEvent struct {
	Token int
	Done  bool
	Err   error
}

type DecodeSequence struct {
	RequestID string
	Ctx       context.Context
	Cache     kvcache.KVCacheInterface

	// History includes prompt + generated tokens so far.
	History []int

	NextInputToken int
	Remaining      int
	EosID          int
	Sampler        *sample.Sampler
}

type Batcher struct {
	eng *Engine

	cmdCh chan any

	onceStart sync.Once
}

type registerCmd struct {
	seq   *DecodeSequence
	event chan DecodeEvent
	resp  chan error
}

type stopCmd struct {
	reqID string
}

type seqState struct {
	seq   *DecodeSequence
	event chan DecodeEvent
}

func NewBatcher(eng *Engine) *Batcher {
	return &Batcher{
		eng:   eng,
		cmdCh: make(chan any, 1024),
	}
}

func (b *Batcher) Start() {
	b.onceStart.Do(func() {
		go b.loop()
	})
}

func (b *Batcher) RegisterDecode(seq *DecodeSequence) (<-chan DecodeEvent, error) {
	if seq == nil {
		return nil, fmt.Errorf("nil sequence")
	}
	if seq.Cache == nil {
		return nil, fmt.Errorf("nil cache")
	}
	if seq.Sampler == nil {
		return nil, fmt.Errorf("nil sampler")
	}
	if seq.Ctx == nil {
		seq.Ctx = context.Background()
	}
	if seq.Remaining <= 0 {
		ch := make(chan DecodeEvent)
		close(ch)
		return ch, nil
	}

	b.Start()

	event := make(chan DecodeEvent, 16)
	resp := make(chan error, 1)
	b.cmdCh <- registerCmd{seq: seq, event: event, resp: resp}
	return event, <-resp
}

func (b *Batcher) Stop(reqID string) {
	if reqID == "" {
		return
	}
	b.Start()
	b.cmdCh <- stopCmd{reqID: reqID}
}

func (b *Batcher) loop() {
	seqs := make(map[string]*seqState)

	// Scratch set reused for all batch steps (supports multi-GPU).
	var scratchSet *compute.ScratchSet
	var baseScratch *compute.ScratchSpace
	if b.eng != nil && b.eng.Dispatcher() != nil && cuda.Available() {
		cfg := b.eng.Model().Config()
		gpus := collectDispatcherGPUs(b.eng.Dispatcher(), cfg.NumLayers)
		if len(gpus) > 0 {
			if ss, err := compute.NewScratchSet(gpus, compute.DefaultScratchBytes); err == nil {
				scratchSet = ss
				defer scratchSet.Free()
				baseScratch = scratchSet.Scratch(gpus[0])
			}
		}
	}

	for {
		// Block when idle.
		if len(seqs) == 0 {
			cmd := <-b.cmdCh
			switch c := cmd.(type) {
			case registerCmd:
				if c.seq.RequestID == "" {
					c.resp <- fmt.Errorf("missing RequestID")
					close(c.event)
					continue
				}
				seqs[c.seq.RequestID] = &seqState{seq: c.seq, event: c.event}
				c.resp <- nil
			case stopCmd:
				if st, ok := seqs[c.reqID]; ok {
					st.event <- DecodeEvent{Done: true, Err: context.Canceled}
					close(st.event)
					delete(seqs, c.reqID)
				}
			}
			continue
		}

		// Drain control commands without blocking.
		for {
			select {
			case cmd := <-b.cmdCh:
				switch c := cmd.(type) {
				case registerCmd:
					if c.seq.RequestID == "" {
						c.resp <- fmt.Errorf("missing RequestID")
						close(c.event)
						continue
					}
					seqs[c.seq.RequestID] = &seqState{seq: c.seq, event: c.event}
					c.resp <- nil
				case stopCmd:
					if st, ok := seqs[c.reqID]; ok {
						st.event <- DecodeEvent{Done: true, Err: context.Canceled}
						close(st.event)
						delete(seqs, c.reqID)
					}
				}
			default:
				goto drained
			}
		}

	drained:

		// Collect ready sequences
		batch := make([]*seqState, 0, len(seqs))
		for id, st := range seqs {
			if st == nil || st.seq == nil {
				delete(seqs, id)
				continue
			}
			seq := st.seq
			if seq.Ctx != nil {
				select {
				case <-seq.Ctx.Done():
					st.event <- DecodeEvent{Done: true, Err: seq.Ctx.Err()}
					close(st.event)
					delete(seqs, id)
					continue
				default:
				}
			}
			if seq.Remaining <= 0 {
				st.event <- DecodeEvent{Done: true}
				close(st.event)
				delete(seqs, id)
				continue
			}
			if seq.NextInputToken == seq.EosID {
				st.event <- DecodeEvent{Done: true}
				close(st.event)
				delete(seqs, id)
				continue
			}
			batch = append(batch, st)
		}

		if len(batch) == 0 {
			time.Sleep(50 * time.Microsecond)
			continue
		}

		// Build input and positions
		input := cpu.NewTensor(tensor.Shape{len(batch)}, nil)
		pos := cpu.NewTensor(tensor.Shape{len(batch)}, nil)
		kvCaches := make([]model.KVCache, len(batch))
		for i, st := range batch {
			seq := st.seq
			input.DataFloat32()[i] = float32(seq.NextInputToken)
			pos.DataFloat32()[i] = float32(seq.Cache.SeqLen())
			kvCaches[i] = seq.Cache
		}

		ctx := context.Background()
		if scratchSet != nil {
			scratchSet.Reset()
			ctx = compute.WithScratchSet(ctx, scratchSet)
		}
		if baseScratch != nil {
			baseScratch.Reset()
			ctx = compute.WithScratch(ctx, baseScratch)
		}

		logits, err := b.eng.ForwardBatch(ctx, input, pos, kvCaches)
		if err != nil {
			for _, st := range batch {
				st.event <- DecodeEvent{Done: true, Err: err}
				close(st.event)
				delete(seqs, st.seq.RequestID)
			}
			continue
		}

		vocab := logits.Shape()[1]
		for i, st := range batch {
			seq := st.seq
			recent := seq.History
			if len(recent) > 64 {
				recent = recent[len(recent)-64:]
			}
			next, rowErr := sampleNextTokenFromLogits(logits, i, vocab, seq.Sampler, recent)
			if rowErr != nil {
				st.event <- DecodeEvent{Done: true, Err: rowErr}
				close(st.event)
				delete(seqs, seq.RequestID)
				continue
			}

			seq.History = append(seq.History, next)
			if pc, ok := seq.Cache.(*kvcache.PagedKVCache); ok {
				pc.AppendToken(next)
			}
			seq.NextInputToken = next
			seq.Remaining--

			st.event <- DecodeEvent{Token: next}
			if next == seq.EosID || seq.Remaining <= 0 {
				st.event <- DecodeEvent{Done: true}
				close(st.event)
				delete(seqs, seq.RequestID)
			}
		}
	}
}

func collectDispatcherGPUs(d *device.DeviceDispatcher, numLayers int) []int {
	if d == nil || numLayers <= 0 {
		return nil
	}
	seen := make(map[int]struct{})
	out := make([]int, 0, 4)
	for i := 0; i < numLayers; i++ {
		p := d.LayerPlacement(i).Normalize()
		if p.Type != tensor.CUDA || p.GPU < 0 {
			continue
		}
		if _, ok := seen[p.GPU]; ok {
			continue
		}
		seen[p.GPU] = struct{}{}
		out = append(out, p.GPU)
	}
	sort.Ints(out)
	return out
}

func logitsRowCPU(logits tensor.Tensor, row int, vocab int) ([]float32, error) {
	if logits == nil {
		return nil, fmt.Errorf("nil logits")
	}
	shape := logits.Shape()
	if len(shape) != 2 {
		return nil, fmt.Errorf("expected 2D logits, got shape %v", shape)
	}
	if row < 0 || row >= shape[0] {
		return nil, fmt.Errorf("row %d out of range", row)
	}
	if vocab <= 0 || vocab != shape[1] {
		return nil, fmt.Errorf("vocab mismatch: %d vs %d", vocab, shape[1])
	}

	if cpuT, ok := logits.(*cpu.Tensor); ok {
		start := row * vocab
		end := start + vocab
		data := cpuT.DataFloat32()
		if end > len(data) {
			return nil, fmt.Errorf("cpu logits out of range")
		}
		out := make([]float32, vocab)
		copy(out, data[start:end])
		return out, nil
	}

	if cudaT, ok := logits.(*cuda.Tensor); ok {
		view, err := cudaT.ViewAt(tensor.Shape{vocab}, uintptr(row*vocab*4))
		if err != nil {
			return nil, err
		}
		host := make([]float32, vocab)
		if err := view.CopyToHost(host); err != nil {
			return nil, err
		}
		return host, nil
	}

	if p, ok := logits.Data().(unsafe.Pointer); ok && p != nil {
		_ = p
	}
	return nil, fmt.Errorf("unsupported logits tensor type %T", logits)
}

func sampleNextTokenFromLogits(logits tensor.Tensor, row int, vocab int, sampler *sample.Sampler, recent []int) (int, error) {
	if logits == nil {
		return 0, fmt.Errorf("nil logits")
	}
	if sampler == nil {
		return 0, fmt.Errorf("nil sampler")
	}
	if row < 0 {
		return 0, fmt.Errorf("row %d out of range", row)
	}

	cfg := sampler.Config()
	k := cfg.TopK
	if cfg.Temperature == 0 {
		k = 1
	}

	// CPU logits: zero-copy row slice.
	if cpuT, ok := logits.(*cpu.Tensor); ok {
		shape := cpuT.Shape()
		if len(shape) != 2 || shape[1] != vocab {
			return 0, fmt.Errorf("expected logits shape [*,%d], got %v", vocab, shape)
		}
		if row >= shape[0] {
			return 0, fmt.Errorf("row %d out of range", row)
		}
		start := row * vocab
		end := start + vocab
		data := cpuT.DataFloat32()
		if end > len(data) {
			return 0, fmt.Errorf("cpu logits out of range")
		}
		return sampler.Sample(data[start:end], recent), nil
	}

	// CUDA logits: prefer GPU top-k path when enabled/greedy and supported by kernel.
	if cudaT, ok := logits.(*cuda.Tensor); ok {
		shape := cudaT.Shape()
		if len(shape) != 2 || shape[1] != vocab {
			return 0, fmt.Errorf("expected logits shape [*,%d], got %v", vocab, shape)
		}
		if row >= shape[0] {
			return 0, fmt.Errorf("row %d out of range", row)
		}
		view, err := cudaT.ViewAt(tensor.Shape{vocab}, uintptr(row*vocab*4))
		if err != nil {
			return 0, err
		}

		// CUDA TopK kernel supports k<=64; fall back when disabled or too large.
		if k <= 0 || k > 64 {
			host := make([]float32, vocab)
			if err := view.CopyToHost(host); err != nil {
				return 0, err
			}
			return sampler.Sample(host, recent), nil
		}

		repPenalty := cfg.RepetitionPenalty
		if repPenalty <= 0 {
			repPenalty = 1.0
		}
		repIDs := make([]int32, len(recent))
		for i, t := range recent {
			repIDs[i] = int32(t)
		}

		allIDs, allScores, blocks, err := cuda.TopKLogitsF32(view.Data().(unsafe.Pointer), vocab, repIDs, repPenalty, k, cudaT.GPU())
		if err != nil {
			return 0, err
		}
		cands := make([]struct {
			id    int32
			score float32
		}, 0, blocks*k)
		for i := 0; i < blocks*k; i++ {
			if allIDs[i] < 0 {
				continue
			}
			cands = append(cands, struct {
				id    int32
				score float32
			}{id: allIDs[i], score: allScores[i]})
		}
		if len(cands) == 0 {
			host := make([]float32, vocab)
			if err := view.CopyToHost(host); err != nil {
				return 0, err
			}
			return sampler.Sample(host, recent), nil
		}

		sort.Slice(cands, func(i, j int) bool { return cands[i].score > cands[j].score })
		if len(cands) > k {
			cands = cands[:k]
		}
		finalIDs := make([]int32, len(cands))
		finalScores := make([]float32, len(cands))
		for i := range cands {
			finalIDs[i] = cands[i].id
			finalScores[i] = cands[i].score
		}
		return sampler.SampleFromTopK(finalIDs, finalScores), nil
	}

	return 0, fmt.Errorf("unsupported logits type: %T", logits)
}