// Package emotion implements the persona emotion state machine and affection
// tracking. Emotion and affection are always keyed by persona id so that each
// persona's internal state stays isolated — they follow the persona/character,
// never a behavior mode (see DevDocs/design/01).
package emotion

import (
	"fmt"
	"sync"
)

// EmotionState is one state in the emotion state machine.
type EmotionState string

// Standard emotion states. More states can be added at any time by registering
// transitions via AddTransition, so the machine is extensible rather than a
// closed enum.
const (
	Calm    EmotionState = "calm"
	Happy   EmotionState = "happy"
	Excited EmotionState = "excited"
	Sad     EmotionState = "sad"
	Angry   EmotionState = "angry"
)

// Params configures the state machine for one persona (情绪状态机参数). It is the
// static, loadable description of how that persona's emotion may move.
type Params struct {
	// InitialState is the emotion a freshly created machine starts in. When it
	// is empty the machine keeps it empty until Set is called explicitly.
	InitialState EmotionState `yaml:"initialState"`
	// DecayRate is the per-Decay step magnitude. A value greater than zero
	// enables the drift toward calm; zero or negative disables it. The actual
	// step is one position on the toward-calm ladder (see closerToCalm).
	DecayRate float64 `yaml:"decayRate"`
	// Transitions maps a source state to the set of states it may transition
	// to. When empty, DefaultTransitions is used.
	Transitions map[EmotionState][]EmotionState `yaml:"transitions"`
}

// Machine is a persona's live emotion state machine. All methods are safe for
// concurrent use.
type Machine struct {
	mu          sync.RWMutex
	params      Params
	current     EmotionState
	transitions map[EmotionState]map[EmotionState]struct{}
}

// DefaultTransitions returns the built-in transition table used when a Params
// instance carries no explicit transitions.
func DefaultTransitions() map[EmotionState][]EmotionState {
	return map[EmotionState][]EmotionState{
		Calm:    {Happy, Sad},
		Happy:   {Excited, Calm, Sad},
		Excited: {Happy, Calm, Angry},
		Sad:     {Calm, Happy, Angry},
		Angry:   {Calm, Excited},
	}
}

// NewMachine builds a Machine from params. When params.Transitions is empty the
// default transition table is installed. An empty InitialState is allowed, but
// callers should normally Set it before use.
func NewMachine(params Params) (*Machine, error) {
	if params.DecayRate < 0 {
		return nil, fmt.Errorf("emotion: negative decay rate %v", params.DecayRate)
	}
	base := params.Transitions
	if len(base) == 0 {
		base = DefaultTransitions()
	}
	return &Machine{
		params:      params,
		current:     params.InitialState,
		transitions: normalizeTransitions(base),
	}, nil
}

// normalizeTransitions flattens the slice-based table into a membership set so
// lookups are O(1) and symmetric.
func normalizeTransitions(src map[EmotionState][]EmotionState) map[EmotionState]map[EmotionState]struct{} {
	out := make(map[EmotionState]map[EmotionState]struct{}, len(src))
	for from, tos := range src {
		set := make(map[EmotionState]struct{}, len(tos))
		for _, to := range tos {
			set[to] = struct{}{}
		}
		out[from] = set
	}
	return out
}

// Current returns the machine's present emotion state.
func (m *Machine) Current() EmotionState {
	m.mu.RLock()
	defer m.mu.RUnlock()
	return m.current
}

// Params returns a copy of the machine's configuration.
func (m *Machine) Params() Params {
	m.mu.RLock()
	defer m.mu.RUnlock()
	return m.params
}

// CanTransition reports whether the machine may move to to from its current
// state.
func (m *Machine) CanTransition(to EmotionState) bool {
	m.mu.RLock()
	defer m.mu.RUnlock()
	return m.allowed(m.current, to)
}

// allowed reports whether from may transition to to under the installed table.
func (m *Machine) allowed(from, to EmotionState) bool {
	_, ok := m.transitions[from][to]
	return ok
}

// Transition moves the machine to to when the installed table permits it. It
// returns an error when the move is not allowed.
func (m *Machine) Transition(to EmotionState) error {
	m.mu.Lock()
	defer m.mu.Unlock()
	if !m.allowed(m.current, to) {
		return fmt.Errorf("emotion: cannot transition from %q to %q", m.current, to)
	}
	m.current = to
	return nil
}

// Set forces the machine to state regardless of the transition table. It is
// used for initialization, restore, and resetting a persona's emotion.
func (m *Machine) Set(state EmotionState) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.current = state
}

// AddTransition registers a new allowed edge, extending the table.
func (m *Machine) AddTransition(from, to EmotionState) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.ensure(from)
	m.transitions[from][to] = struct{}{}
}

// ensure returns the membership set for from, creating it when absent.
func (m *Machine) ensure(from EmotionState) map[EmotionState]struct{} {
	if m.transitions[from] == nil {
		m.transitions[from] = make(map[EmotionState]struct{})
	}
	return m.transitions[from]
}

// Move aims the machine at target, walking the persona's installed transition
// table. A direct edge is taken when present; otherwise the first hop of a
// shortest legal path is taken, so a caller can aim at an emotion without
// knowing the intermediate states. A target that is unreachable from the current
// state, an empty target, or the current state itself leaves the machine
// unchanged rather than forcing an illegal move. The resulting state is
// returned.
func (m *Machine) Move(target EmotionState) EmotionState {
	m.mu.Lock()
	defer m.mu.Unlock()
	if target == "" || target == m.current {
		return m.current
	}
	if m.allowed(m.current, target) {
		m.current = target
		return m.current
	}
	if hop, ok := m.firstHopTo(target); ok {
		m.current = hop
	}
	return m.current
}

// firstHopTo returns the first state on a shortest transition path from the
// current state to target, and whether such a path exists. Ties between equally
// short paths are broken arbitrarily, which is invisible to callers: any first
// hop on a shortest legal path moves the machine in the wanted direction.
func (m *Machine) firstHopTo(target EmotionState) (EmotionState, bool) {
	type node struct {
		state EmotionState
		first EmotionState
	}
	queue := make([]node, 0, len(m.transitions[m.current]))
	for next := range m.transitions[m.current] {
		if next == target {
			return next, true
		}
		queue = append(queue, node{state: next, first: next})
	}
	seen := map[EmotionState]struct{}{m.current: {}}
	for len(queue) > 0 {
		cur := queue[0]
		queue = queue[1:]
		if _, ok := seen[cur.state]; ok {
			continue
		}
		seen[cur.state] = struct{}{}
		for next := range m.transitions[cur.state] {
			if next == target {
				return cur.first, true
			}
			queue = append(queue, node{state: next, first: cur.first})
		}
	}
	return "", false
}

// Decay drifts the emotion one position toward calm whenever DecayRate is
// positive; otherwise it leaves the current state unchanged. The new state is
// returned.
func (m *Machine) Decay() EmotionState {
	m.mu.Lock()
	defer m.mu.Unlock()
	if m.params.DecayRate <= 0 {
		return m.current
	}
	m.current = closerToCalm(m.current)
	return m.current
}

// closerToCalm returns the single relaxed step toward calm for a state. States
// at or below calm are returned unchanged.
func closerToCalm(s EmotionState) EmotionState {
	switch s {
	case Excited:
		return Happy
	case Happy, Sad, Angry:
		return Calm
	default:
		return s
	}
}