package capacity

import (
	"context"
	"errors"
	"strings"
	"sync"
	"time"
)

// IntentClassifier is the model-assisted intent classifier behind the L3 layer
// (design/14 §2.2 L3; 裁决 14.4.4). It is an interface so the decision engine
// stays testable without a model: the wiring supplies the LLM-backed
// implementation (NewLLMClassifier), a test supplies a stub.
//
// Classify returns the raw judgment for one turn's text — {isTask, taskKind,
// confidence} plus nothing else. An error means "no verdict": the caller keeps
// the conservative default rather than guessing.
type IntentClassifier interface {
	Classify(ctx context.Context, text string) (Judgment, error)
}

// maxL3CacheEntries bounds the L3 verdict cache so a long-lived process cannot
// grow it without limit. Past the bound, expired entries are dropped first and
// the cache is cleared if every entry is still fresh.
const maxL3CacheEntries = 256

// ErrEmptyTurn is returned when the L3 classifier is asked to judge empty turn
// text. It is a sentinel so callers (and tests) can test the category without
// matching on the message text.
var ErrEmptyTurn = errors.New("capacity: empty turn text for L3 classification")

// DefaultL3CacheTTL is the default reuse window for an L3 verdict — the landing
// suggestion of 裁决 14.4.4 (5m): long enough that a repeated phrase inside one
// conversation costs one model call, short enough that a changed habit re-learns.
const DefaultL3CacheTTL = 5 * time.Minute

// l3Cache memoizes classifier verdicts per turn text for a bounded TTL, so the
// same phrase does not cost a model call on every round. A zero TTL disables
// reuse. The classifier call happens outside the lock: two concurrent turns must
// not serialize on each other's model call.
type l3Cache struct {
	classifier IntentClassifier
	ttl        time.Duration

	mu      sync.Mutex
	entries map[string]l3Entry
}

// l3Entry is one cached verdict with the time it was produced.
type l3Entry struct {
	judgment Judgment
	at       time.Time
}

// newL3Cache builds the cache. A nil classifier yields nil, which the decider
// treats as "L3 not available".
func newL3Cache(classifier IntentClassifier, ttl time.Duration) *l3Cache {
	if classifier == nil {
		return nil
	}
	return &l3Cache{classifier: classifier, ttl: ttl, entries: map[string]l3Entry{}}
}

// classify returns the verdict for text, reusing a fresh cached one when
// possible. The bool reports whether a verdict was produced; an error is returned
// to the caller for reporting instead of being swallowed here.
func (c *l3Cache) classify(ctx context.Context, text string) (Judgment, error) {
	key := l3CacheKey(text)
	if key == "" {
		return Judgment{}, ErrEmptyTurn
	}
	if j, ok := c.lookup(key, time.Now()); ok {
		return j, nil
	}
	j, err := c.classifier.Classify(ctx, text)
	if err != nil {
		return Judgment{}, err
	}
	c.store(key, j, time.Now())
	return j, nil
}

// lookup returns a fresh cached verdict, if any.
func (c *l3Cache) lookup(key string, now time.Time) (Judgment, bool) {
	if c.ttl <= 0 {
		return Judgment{}, false
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	e, ok := c.entries[key]
	if !ok {
		return Judgment{}, false
	}
	if now.Sub(e.at) > c.ttl {
		return Judgment{}, false
	}
	return e.judgment, true
}

// store records a verdict, evicting expired entries (and, if the bound is still
// exceeded, the whole map) so the cache cannot grow without limit.
func (c *l3Cache) store(key string, j Judgment, now time.Time) {
	if c.ttl <= 0 {
		return
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	if len(c.entries) >= maxL3CacheEntries {
		c.evictExpired(now)
	}
	c.entries[key] = l3Entry{judgment: j, at: now}
}

// evictExpired drops entries past their TTL; when none expired, it clears the
// map so a burst of distinct turns cannot pin the memory.
func (c *l3Cache) evictExpired(now time.Time) {
	dropped := 0
	for k, e := range c.entries {
		if now.Sub(e.at) > c.ttl {
			delete(c.entries, k)
			dropped++
		}
	}
	if dropped == 0 {
		c.entries = map[string]l3Entry{}
	}
}

// l3CacheKey normalizes a turn's text into the cache key. Empty text (nothing to
// classify) yields "".
func l3CacheKey(text string) string {
	return strings.TrimSpace(text)
}