package chroncore

import (
	"bytes"
	"context"
	"errors"
	"slices"
	"sort"
	"time"
)

var (
	ErrHeadNotSet      = errors.New("HEAD not set")
	ErrEntryIDMismatch = errors.New("entry id mismatch")
	ErrBatchLoadSize   = errors.New("LoadBatch returned unexpected number of entries")
)

type Ledger struct {
	entryStore     EntryStore
	referenceStore ReferenceStore
}

func NewLedger(entryStore EntryStore, referenceStore ReferenceStore) (*Ledger, error) {
	return &Ledger{
		entryStore:     entryStore,
		referenceStore: referenceStore,
	}, nil
}

func (l *Ledger) Add(ctx context.Context, parents []EntryID, payload []byte) (EntryID, error) {
	ps := normalizeParents(parents)

	ts := time.Now()
	id := ComputeEntryID(ps, ts, payload)

	e := Entry{
		EntryID:   id,
		Parents:   ps,
		Timestamp: ts,
		Payload:   payload,
	}

	if err := l.VerifyEntry(e); err != nil {
		return EntryID{}, err
	}

	if err := l.entryStore.Store(ctx, e); err != nil {
		return EntryID{}, err
	}

	return id, nil
}

func (l *Ledger) Append(ctx context.Context, payload []byte) (EntryID, error) {
	head, ok, err := l.referenceStore.Get(ctx, "HEAD")
	if err != nil {
		return EntryID{}, err
	}

	var parents []EntryID
	if ok {
		parents = []EntryID{head}
	} else {
		parents = nil
	}

	id, err := l.Add(ctx, parents, payload)
	if err != nil {
		return EntryID{}, err
	}

	if err := l.referenceStore.Set(ctx, "HEAD", id); err != nil {
		return EntryID{}, err
	}

	return id, nil
}

func (l *Ledger) AppendTo(ctx context.Context, parent EntryID, payload []byte) (EntryID, error) {
	return l.Add(ctx, []EntryID{parent}, payload)
}

func (l *Ledger) Get(ctx context.Context, id EntryID) (Entry, error) {
	return l.entryStore.Load(ctx, id)
}

func (l *Ledger) Exists(ctx context.Context, id EntryID) (bool, error) {
	return l.entryStore.Exists(ctx, id)
}

func (l *Ledger) Verify(ctx context.Context, id EntryID) error {
	e, err := l.entryStore.Load(ctx, id)
	if err != nil {
		return err
	}
	return l.VerifyEntry(e)
}

func (l *Ledger) VerifyEntry(e Entry) error {
	want := ComputeEntryID(e.Parents, e.Timestamp, e.Payload)
	if want != e.EntryID {
		return ErrEntryIDMismatch
	}
	return nil
}

func (l *Ledger) GetRef(ctx context.Context, name string) (EntryID, bool, error) {
	return l.referenceStore.Get(ctx, name)
}

func (l *Ledger) SetRef(ctx context.Context, name string, id EntryID) error {
	return l.referenceStore.Set(ctx, name, id)
}

func (l *Ledger) DeleteRef(ctx context.Context, name string) error {
	return l.referenceStore.Delete(ctx, name)
}

func (l *Ledger) ListRefs(ctx context.Context, prefix string) (map[string]EntryID, error) {
	return l.referenceStore.List(ctx, prefix)
}

func (l *Ledger) SetRefs(ctx context.Context, refs map[string]EntryID) error {
	return l.referenceStore.SetBatch(ctx, refs)
}

func (l *Ledger) GetRefs(ctx context.Context, names []string) (map[string]EntryID, error) {
	return l.referenceStore.GetBatch(ctx, names)
}

func (l *Ledger) GetHead(ctx context.Context) (EntryID, bool, error) {
	return l.referenceStore.Get(ctx, "HEAD")
}

func (l *Ledger) SetHead(ctx context.Context, id EntryID) error {
	return l.referenceStore.Set(ctx, "HEAD", id)
}

func (l *Ledger) GetHeads(ctx context.Context, prefix string) ([]EntryID, error) {
	m, err := l.referenceStore.List(ctx, prefix)
	if err != nil {
		return nil, err
	}

	seen := make(map[EntryID]struct{}, len(m))
	for _, id := range m {
		seen[id] = struct{}{}
	}

	out := make([]EntryID, 0, len(seen))
	for id := range seen {
		out = append(out, id)
	}

	sort.Slice(out, func(i, j int) bool {
		return bytes.Compare(out[i][:], out[j][:]) < 0
	})

	return out, nil
}

func (l *Ledger) WalkAncestors(ctx context.Context, start []EntryID, fn func(Entry) bool) error {
	frontier := dedupeIDs(start)
	visited := make(map[EntryID]struct{}, 1024)

	for len(frontier) > 0 {
		batchIDs := make([]EntryID, 0, len(frontier))
		for _, id := range frontier {
			if _, ok := visited[id]; ok {
				continue
			}
			visited[id] = struct{}{}
			batchIDs = append(batchIDs, id)
		}
		if len(batchIDs) == 0 {
			return nil
		}

		entries, err := l.entryStore.LoadBatch(ctx, batchIDs)
		if err != nil {
			return err
		}
		if len(entries) != len(batchIDs) {
			return ErrBatchLoadSize
		}

		next := make([]EntryID, 0, len(batchIDs)*2)

		for _, e := range entries {
			if err := l.VerifyEntry(e); err != nil {
				return err
			}

			if !fn(e) {
				return nil
			}

			for _, p := range e.Parents {
				next = append(next, p)
			}
		}

		frontier = dedupeIDs(next)
	}

	return nil
}

func (l *Ledger) IsAncestor(ctx context.Context, ancestor, descendant EntryID) (bool, error) {
	if ancestor == descendant {
		return true, nil
	}

	found := false
	err := l.WalkAncestors(ctx, []EntryID{descendant}, func(e Entry) bool {
		if slices.Contains(e.Parents, ancestor) {
			found = true
			return false
		}
		return true
	})
	if err != nil {
		return false, err
	}
	return found, nil
}

func (l *Ledger) CommonAncestors(ctx context.Context, a, b []EntryID, limit int) ([]EntryID, error) {
	aSet := make(map[EntryID]struct{}, 1024)

	if err := l.WalkAncestors(ctx, a, func(e Entry) bool {
		aSet[e.EntryID] = struct{}{}
		return true
	}); err != nil {
		return nil, err
	}

	var out []EntryID
	seen := make(map[EntryID]struct{}, 64)

	err := l.WalkAncestors(ctx, b, func(e Entry) bool {
		if _, ok := aSet[e.EntryID]; ok {
			if _, dup := seen[e.EntryID]; !dup {
				seen[e.EntryID] = struct{}{}
				out = append(out, e.EntryID)
				if limit > 0 && len(out) >= limit {
					return false
				}
			}
		}
		return true
	})
	if err != nil {
		return nil, err
	}

	return out, nil
}

func normalizeParents(parents []EntryID) []EntryID {
	if len(parents) == 0 {
		return nil
	}

	ps := make([]EntryID, len(parents))
	copy(ps, parents)

	sort.Slice(ps, func(i, j int) bool {
		return bytes.Compare(ps[i][:], ps[j][:]) < 0
	})

	out := ps[:0]
	var last EntryID
	for i, p := range ps {
		if i == 0 || p != last {
			out = append(out, p)
			last = p
		}
	}
	return out
}

func dedupeIDs(ids []EntryID) []EntryID {
	if len(ids) == 0 {
		return nil
	}

	tmp := make([]EntryID, len(ids))
	copy(tmp, ids)
	sort.Slice(tmp, func(i, j int) bool {
		return bytes.Compare(tmp[i][:], tmp[j][:]) < 0
	})
	out := tmp[:0]
	var last EntryID
	for i, id := range tmp {
		if i == 0 || id != last {
			out = append(out, id)
			last = id
		}
	}
	return out
}