package engine

import (
	"fmt"

	"makarna/pkg/backend/cpu"
	"makarna/pkg/graph"
	"makarna/pkg/model"
	"makarna/pkg/tensor"
)

// Scheduler orchestrates prefill and decode while reusing a static graph plan.
// It supports multi-token steps (len(tokens) > 1) for continuous batching.
type Scheduler struct {
	engine *Engine
	plan   graph.ExecutionPlan
	cache  model.KVCache
}

// NewScheduler binds a model engine with a graph plan and KV cache.
func (e *Engine) NewScheduler(plan graph.ExecutionPlan, cache model.KVCache) *Scheduler {
	return &Scheduler{
		engine: e,
		plan:   plan,
		cache:  cache,
	}
}

// RemainingContext returns how many tokens can still fit in the reserved plan.
func (s *Scheduler) RemainingContext() int {
	return s.plan.MaxContext - s.cache.SeqLen()
}

// Prefill runs a single step prefill (full prompt) and advances the cache.
func (s *Scheduler) Prefill(tokens []int) (tensor.Tensor, error) {
	return s.run(tokens)
}

// Decode runs multi-token decode in one step. The caller is responsible for
// picking the next tokens from the returned logits.
func (s *Scheduler) Decode(tokens []int) (tensor.Tensor, error) {
	return s.run(tokens)
}

func (s *Scheduler) run(tokens []int) (tensor.Tensor, error) {
	if len(tokens) == 0 {
		return nil, fmt.Errorf("no tokens to run")
	}
	if s.cache.SeqLen()+len(tokens) > s.plan.MaxContext {
		return nil, fmt.Errorf("context limit exceeded: need %d, max %d", s.cache.SeqLen()+len(tokens), s.plan.MaxContext)
	}

	input := cpu.NewTensor(tensor.Shape{len(tokens)}, nil)
	for i, id := range tokens {
		input.DataFloat32()[i] = float32(id)
	}

	pos := cpu.NewTensor(tensor.Shape{len(tokens)}, nil)
	base := s.cache.SeqLen()
	for i := range tokens {
		pos.DataFloat32()[i] = float32(base + i)
	}

	before := s.cache.SeqLen()
	logits, err := s.engine.Forward(nil, input, pos, s.cache)
	if err != nil {
		return nil, err
	}
	// Some model implementations advance the cache internally.
	// If the cache didn't advance, commit here.
	if s.cache.SeqLen() == before {
		s.cache.Commit(len(tokens))
	}

	return logits, nil
}