// Package science implements the FLUID scientific adaptation loop: hypotheses,
// competition between them, bounded experiments, and the promotion decisions
// they inform.
//
// Nothing here is generative. ArchitectureBlueprint.md Phase C is deliberately
// human-driven: the point of this stage, per section 50, is to prove that the
// revision-experiment-fitness loop works cleanly and safely, not to automate
// the reasoning inside it.
//
// The schema document's section 18 sets the discipline this package keeps:
// what we observed, what we think explains it, what we changed, and what
// happened afterwards must not collapse into one narrative. Keeping them apart
// is what makes criticism, competing explanations and rollback possible.
package science

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"sort"
	"time"

	"github.com/tegwick/fluid-core/internal/contract"
	"github.com/tegwick/fluid-core/internal/evidence"
)

// HypothesisStore persists hypotheses and enforces their lifecycle.
type HypothesisStore struct {
	store evidence.Store
	iface contract.InterfaceID
	now   func() time.Time
}

// NewHypothesisStore returns a store backed by the evidence log.
func NewHypothesisStore(store evidence.Store, iface contract.InterfaceID) *HypothesisStore {
	return &HypothesisStore{store: store, iface: iface, now: time.Now}
}

// State is a point in the hypothesis lifecycle.
type State = contract.FluidHypothesisState

// transitions is the permitted lifecycle graph.
//
// It is explicit rather than free-form because the states carry meaning a
// reader relies on. A hypothesis that jumped from DRAFT to ACCEPTED would claim
// evidence it never gathered, and the audit trail would show nothing wrong.
var transitions = map[State][]State{
	contract.FluidHypothesisStateDRAFT: {
		contract.FluidHypothesisStateREADY,
		contract.FluidHypothesisStateREJECTED,
		contract.FluidHypothesisStateDEFERRED,
		// A draft can lose a competition to a better-formed rival before it is
		// ever completed. That is superseding, not rejection: nobody showed the
		// idea was wrong, only that something else explained the pressure first.
		contract.FluidHypothesisStateSUPERSEDED,
	},
	contract.FluidHypothesisStateREADY: {
		contract.FluidHypothesisStatePRIORITIZED,
		contract.FluidHypothesisStateDEFERRED,
		contract.FluidHypothesisStateREJECTED,
		contract.FluidHypothesisStateSUPERSEDED,
	},
	contract.FluidHypothesisStatePRIORITIZED: {
		contract.FluidHypothesisStateDESIGNING,
		contract.FluidHypothesisStateDEFERRED,
		contract.FluidHypothesisStateREJECTED,
		contract.FluidHypothesisStateSUPERSEDED,
	},
	contract.FluidHypothesisStateDESIGNING: {
		contract.FluidHypothesisStateEXPERIMENTING,
		contract.FluidHypothesisStateDEFERRED,
		contract.FluidHypothesisStateREJECTED,
		contract.FluidHypothesisStateSUPERSEDED,
	},
	contract.FluidHypothesisStateEXPERIMENTING: {
		contract.FluidHypothesisStateEVALUATING,
		// An experiment can be stopped without a verdict; that is a normal
		// outcome, not a failure of the hypothesis.
		contract.FluidHypothesisStateDEFERRED,
		contract.FluidHypothesisStateREJECTED,
	},
	contract.FluidHypothesisStateEVALUATING: {
		contract.FluidHypothesisStateACCEPTED,
		contract.FluidHypothesisStateREJECTED,
		contract.FluidHypothesisStateDEFERRED,
		contract.FluidHypothesisStateSUPERSEDED,
	},
	// A deferred hypothesis can come back when circumstances change.
	contract.FluidHypothesisStateDEFERRED: {
		contract.FluidHypothesisStateREADY,
		contract.FluidHypothesisStatePRIORITIZED,
		contract.FluidHypothesisStateREJECTED,
		contract.FluidHypothesisStateSUPERSEDED,
	},
	// Terminal. A rejected hypothesis that turns out to be right becomes a new
	// hypothesis citing the old one, so the reversal is visible.
	contract.FluidHypothesisStateACCEPTED:   {contract.FluidHypothesisStateSUPERSEDED},
	contract.FluidHypothesisStateREJECTED:   {},
	contract.FluidHypothesisStateSUPERSEDED: {},
}

var (
	// ErrInvalidTransition reports a lifecycle move that is not permitted.
	ErrInvalidTransition = errors.New("invalid hypothesis state transition")
	// ErrIncomplete reports a hypothesis missing something its state requires.
	ErrIncomplete = errors.New("hypothesis is missing required content")
	// ErrNotFound reports an unknown hypothesis.
	ErrNotFound = evidence.ErrNotFound
)

// CanTransition reports whether a lifecycle move is permitted.
func CanTransition(from, to State) bool {
	if from == to {
		return true
	}
	for _, allowed := range transitions[from] {
		if allowed == to {
			return true
		}
	}
	return false
}

// Create records a new hypothesis in DRAFT.
//
// A draft is allowed to be incomplete: the point of the state is to hold a
// half-formed idea. Completeness is checked when it moves to READY, which is
// the claim that it is worth someone's time.
func (s *HypothesisStore) Create(ctx context.Context, h contract.FluidHypothesis, actor contract.Actor) (contract.FluidHypothesis, error) {
	if h.ID == "" {
		return h, errors.New("hypothesis needs an id")
	}
	if err := contract.RequireKind(string(h.ID), contract.KindHypothesis); err != nil {
		return h, err
	}
	if h.Title == "" {
		return h, fmt.Errorf("%w: a hypothesis needs a title", ErrIncomplete)
	}

	if _, err := s.Get(ctx, h.ID); err == nil {
		return h, fmt.Errorf("hypothesis %s already exists", h.ID)
	} else if !errors.Is(err, ErrNotFound) {
		return h, err
	}

	h.SchemaVersion = "0.1"
	h.InterfaceID = s.iface
	if h.State == "" {
		h.State = contract.FluidHypothesisStateDRAFT
	}
	if h.CreatedAt == nil {
		now := s.now().UTC()
		h.CreatedAt = &now
	}
	if h.CreatedBy == nil {
		h.CreatedBy = &actor
	}

	if err := s.put(ctx, h); err != nil {
		return h, err
	}
	return h, s.event(ctx, h, "HYPOTHESIS_CREATED", actor, h.Title, nil)
}

// readyRequirements lists what a hypothesis must have before it leaves DRAFT.
//
// These mirror the schema document's section 4 mandatory fields. The check runs
// at READY rather than at creation so that drafting stays cheap, but nothing
// can be prioritized, experimented on or accepted without them.
func readyRequirements(h contract.FluidHypothesis) []string {
	var missing []string

	if h.Observation.Summary == "" {
		missing = append(missing, "observation.summary: what was actually seen")
	}
	if len(h.Observation.EvidenceRefs) == 0 {
		missing = append(missing, "observation.evidence_refs: a claim with no evidence is not falsifiable")
	}
	if len(h.Pressure.Classes) == 0 {
		missing = append(missing, "pressure.classes: which kind of pressure this addresses")
	}
	if h.Explanation.Claim == "" {
		missing = append(missing, "explanation.claim: what you think explains the observation")
	}
	if h.ProposedAdaptation.Summary == "" {
		missing = append(missing, "proposed_adaptation.summary: what you propose to change")
	}
	if len(h.ExpectedOutcomes) == 0 {
		missing = append(missing, "expected_outcomes: a prediction, or the hypothesis cannot be wrong")
	}
	if h.SuccessCriteria.Expression == "" {
		missing = append(missing, "success_criteria.expression: how you will know")
	}
	// Complexity and risk are required structs rather than pointers, so an
	// unset one shows up as a zero value: an empty delta and an empty level.
	if h.Complexity.ExpectedDelta == (contract.ComplexityDelta{}) {
		missing = append(missing, "complexity.expected_delta: complexity is a budget")
	}
	if h.Risk.Level == "" {
		missing = append(missing, "risk.level")
	}
	return missing
}

// Transition moves a hypothesis to a new state.
func (s *HypothesisStore) Transition(ctx context.Context, id contract.HypothesisID, to State, actor contract.Actor, reason string) (contract.FluidHypothesis, error) {
	h, err := s.Get(ctx, id)
	if err != nil {
		return h, err
	}

	if !to.Valid() {
		return h, fmt.Errorf("unknown hypothesis state %q", to)
	}
	if !CanTransition(h.State, to) {
		return h, fmt.Errorf("%w: %s cannot become %s", ErrInvalidTransition, h.State, to)
	}
	if reason == "" {
		return h, errors.New("a lifecycle transition requires a reason")
	}

	// Everything past DRAFT is a claim that the hypothesis is worth acting on.
	if to != contract.FluidHypothesisStateDRAFT && to != contract.FluidHypothesisStateREJECTED &&
		to != contract.FluidHypothesisStateDEFERRED && to != contract.FluidHypothesisStateSUPERSEDED {
		if missing := readyRequirements(h); len(missing) > 0 {
			return h, fmt.Errorf("%w: %s cannot reach %s until it has:\n  - %s",
				ErrIncomplete, id, to, joinLines(missing))
		}
	}

	previous := h.State
	h.State = to

	if err := s.put(ctx, h); err != nil {
		return h, err
	}
	return h, s.event(ctx, h, "HYPOTHESIS_"+string(to), actor,
		fmt.Sprintf("%s -> %s: %s", previous, to, reason), nil)
}

// RecordOutcome closes a hypothesis with its result.
//
// The outcome is written separately from the explanation it tests, and only in
// EVALUATING. A hypothesis that recorded its own result while still running
// would be assuming what it set out to find.
func (s *HypothesisStore) RecordOutcome(ctx context.Context, id contract.HypothesisID, status contract.FluidHypothesisOutcomeStatus, summary string, evidenceRefs []contract.EvidenceRef, actor contract.Actor) (contract.FluidHypothesis, error) {
	h, err := s.Get(ctx, id)
	if err != nil {
		return h, err
	}
	if h.State != contract.FluidHypothesisStateEVALUATING {
		return h, fmt.Errorf("an outcome can only be recorded while EVALUATING, not in %s", h.State)
	}
	if summary == "" {
		return h, errors.New("an outcome needs a summary")
	}

	h.Outcome = &contract.FluidHypothesisOutcome{
		Status:       &status,
		Summary:      &summary,
		EvidenceRefs: evidenceRefs,
	}

	next := contract.FluidHypothesisStateREJECTED
	if status == contract.FluidHypothesisOutcomeStatusCONFIRMED {
		next = contract.FluidHypothesisStateACCEPTED
	}
	if status == contract.FluidHypothesisOutcomeStatusINCONCLUSIVE {
		// Inconclusive is not refuted. Deferring keeps it available for a
		// better-powered experiment instead of burying a possibly-good idea.
		next = contract.FluidHypothesisStateDEFERRED
	}
	h.State = next

	if err := s.put(ctx, h); err != nil {
		return h, err
	}
	return h, s.event(ctx, h, "HYPOTHESIS_OUTCOME_RECORDED", actor,
		fmt.Sprintf("%s: %s", status, summary), evidenceRefs)
}

// Get returns one hypothesis.
func (s *HypothesisStore) Get(ctx context.Context, id contract.HypothesisID) (contract.FluidHypothesis, error) {
	body, err := s.store.Record(ctx, contract.KindHypothesis, string(id))
	if err != nil {
		return contract.FluidHypothesis{}, err
	}
	var doc contract.HypothesisDocument
	if err := json.Unmarshal(body, &doc); err != nil {
		return contract.FluidHypothesis{}, fmt.Errorf("decode hypothesis %s: %w", id, err)
	}
	return doc.FluidHypothesis, nil
}

// List returns hypotheses, optionally filtered by state.
func (s *HypothesisStore) List(ctx context.Context, state State) ([]contract.FluidHypothesis, error) {
	records, err := s.store.Records(ctx, contract.KindHypothesis)
	if err != nil {
		return nil, err
	}

	out := make([]contract.FluidHypothesis, 0, len(records))
	for _, body := range records {
		var doc contract.HypothesisDocument
		if err := json.Unmarshal(body, &doc); err != nil {
			continue
		}
		if state != "" && doc.FluidHypothesis.State != state {
			continue
		}
		out = append(out, doc.FluidHypothesis)
	}

	// Highest priority first; the list doubles as a queue.
	sort.Slice(out, func(i, j int) bool {
		pi, pj := priorityOf(out[i]), priorityOf(out[j])
		if pi != pj {
			return pi > pj
		}
		return out[i].ID < out[j].ID
	})
	return out, nil
}

func priorityOf(h contract.FluidHypothesis) float64 {
	if h.Priority == nil || h.Priority.Score == nil {
		return 0
	}
	return *h.Priority.Score
}

// AttachRevision links a candidate revision to the hypothesis it came from.
func (s *HypothesisStore) AttachRevision(ctx context.Context, id contract.HypothesisID, rev contract.RevisionID, actor contract.Actor) error {
	if err := contract.RequireKind(string(rev), contract.KindRevision); err != nil {
		return err
	}
	h, err := s.Get(ctx, id)
	if err != nil {
		return err
	}
	for _, existing := range h.CandidateRevisionRefs {
		if existing == rev {
			return nil
		}
	}
	h.CandidateRevisionRefs = append(h.CandidateRevisionRefs, rev)

	if err := s.put(ctx, h); err != nil {
		return err
	}
	return s.event(ctx, h, "HYPOTHESIS_REVISION_ATTACHED", actor, fmt.Sprintf("attached %s", rev), nil)
}

// AttachExperiment links an experiment to the hypothesis it tests.
func (s *HypothesisStore) AttachExperiment(ctx context.Context, id contract.HypothesisID, exp contract.ExperimentID, actor contract.Actor) error {
	h, err := s.Get(ctx, id)
	if err != nil {
		return err
	}
	for _, existing := range h.ExperimentRefs {
		if existing == exp {
			return nil
		}
	}
	h.ExperimentRefs = append(h.ExperimentRefs, exp)

	if err := s.put(ctx, h); err != nil {
		return err
	}
	return s.event(ctx, h, "HYPOTHESIS_EXPERIMENT_ATTACHED", actor, fmt.Sprintf("attached %s", exp), nil)
}

func (s *HypothesisStore) put(ctx context.Context, h contract.FluidHypothesis) error {
	body, err := json.Marshal(contract.HypothesisDocument{FluidHypothesis: h})
	if err != nil {
		return err
	}
	return s.store.PutRecord(ctx, contract.KindHypothesis, string(h.ID), body)
}

func (s *HypothesisStore) event(ctx context.Context, h contract.FluidHypothesis, kind string, actor contract.Actor, reason string, refs []contract.EvidenceRef) error {
	return s.store.AppendEvent(ctx, contract.FluidEvent{
		SchemaVersion: "0.1",
		ID:            contract.EventID(fmt.Sprintf("EV-%s-%d", h.ID, s.now().UnixNano())),
		OccurredAt:    s.now().UTC(),
		EntityType:    contract.FluidEventEntityTypeHypothesis,
		EntityID:      string(h.ID),
		EventType:     kind,
		Actor:         actor,
		Reason:        reason,
		EvidenceRefs:  refs,
	})
}

func joinLines(items []string) string {
	out := items[0]
	for _, item := range items[1:] {
		out += "\n  - " + item
	}
	return out
}