package science

import (
	"context"
	"errors"
	"fmt"
	"sort"

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

// Competition groups rival explanations of the same pressure.
//
// FluidAPIStandards.md section 19 requires this and section 65 of the same
// document asks the Daimon to preserve uncertainty where evidence is
// insufficient. A framework that forced one explanation forward would make
// premature commitment the default and hide the alternatives that a later
// reader might have preferred.
type Competition struct {
	GroupID   string                  `json:"group_id"`
	Members   []contract.HypothesisID `json:"members"`
	Resolved  bool                    `json:"resolved"`
	Preferred contract.HypothesisID   `json:"preferred,omitempty"`
}

// ErrNotCompeting reports hypotheses that are not in the same group.
var ErrNotCompeting = errors.New("hypotheses are not in the same competition group")

// Compete places hypotheses into a competition group.
//
// Membership is symmetric: every member lists every other. A one-way link
// would let a reader looking at one hypothesis miss that a rival exists, which
// is precisely the mistake the group is meant to prevent.
func (s *HypothesisStore) Compete(ctx context.Context, groupID string, ids []contract.HypothesisID, actor contract.Actor) (Competition, error) {
	if groupID == "" {
		return Competition{}, errors.New("a competition group needs an id")
	}
	if len(ids) < 2 {
		return Competition{}, errors.New("a competition group needs at least two hypotheses")
	}

	seen := map[contract.HypothesisID]struct{}{}
	members := make([]contract.HypothesisID, 0, len(ids))
	for _, id := range ids {
		if _, dup := seen[id]; dup {
			continue
		}
		if _, err := s.Get(ctx, id); err != nil {
			return Competition{}, fmt.Errorf("cannot add %s to %s: %w", id, groupID, err)
		}
		seen[id] = struct{}{}
		members = append(members, id)
	}
	sort.Slice(members, func(i, j int) bool { return members[i] < members[j] })

	for _, id := range members {
		h, err := s.Get(ctx, id)
		if err != nil {
			return Competition{}, err
		}

		alternatives := make([]contract.HypothesisID, 0, len(members)-1)
		for _, other := range members {
			if other != id {
				alternatives = append(alternatives, other)
			}
		}

		h.Competition = &contract.FluidHypothesisCompetition{
			GroupID:      groupID,
			Alternatives: alternatives,
		}
		if err := s.put(ctx, h); err != nil {
			return Competition{}, err
		}
		if err := s.event(ctx, h, "HYPOTHESIS_COMPETING", actor,
			fmt.Sprintf("joined competition group %s against %v", groupID, alternatives), nil); err != nil {
			return Competition{}, err
		}
	}

	return Competition{GroupID: groupID, Members: members}, nil
}

// Group returns the members of a competition group.
func (s *HypothesisStore) Group(ctx context.Context, groupID string) (Competition, error) {
	all, err := s.List(ctx, "")
	if err != nil {
		return Competition{}, err
	}

	c := Competition{GroupID: groupID}
	for _, h := range all {
		if h.Competition == nil || h.Competition.GroupID != groupID {
			continue
		}
		c.Members = append(c.Members, h.ID)
		if h.State == contract.FluidHypothesisStateACCEPTED {
			c.Resolved = true
			c.Preferred = h.ID
		}
	}
	sort.Slice(c.Members, func(i, j int) bool { return c.Members[i] < c.Members[j] })

	if len(c.Members) == 0 {
		return c, fmt.Errorf("no hypotheses in competition group %q", groupID)
	}
	return c, nil
}

// Resolve accepts one member of a competition and supersedes the rest.
//
// The losers become SUPERSEDED rather than REJECTED. Rejection says the
// explanation was wrong; superseded says a better one won. The distinction
// matters when the winner is later refuted and someone goes looking for what
// else had been considered.
func (s *HypothesisStore) Resolve(ctx context.Context, groupID string, winner contract.HypothesisID, actor contract.Actor, reason string) (Competition, error) {
	if reason == "" {
		return Competition{}, errors.New("resolving a competition requires a reason")
	}

	group, err := s.Group(ctx, groupID)
	if err != nil {
		return group, err
	}

	found := false
	for _, id := range group.Members {
		if id == winner {
			found = true
		}
	}
	if !found {
		return group, fmt.Errorf("%w: %s is not in %s", ErrNotCompeting, winner, groupID)
	}

	for _, id := range group.Members {
		if id == winner {
			continue
		}
		h, err := s.Get(ctx, id)
		if err != nil {
			return group, err
		}
		if h.State == contract.FluidHypothesisStateSUPERSEDED ||
			h.State == contract.FluidHypothesisStateREJECTED {
			continue
		}
		if _, err := s.Transition(ctx, id, contract.FluidHypothesisStateSUPERSEDED, actor,
			fmt.Sprintf("superseded by %s in competition %s: %s", winner, groupID, reason)); err != nil {
			return group, err
		}
	}

	group.Resolved = true
	group.Preferred = winner
	return group, nil
}