package packed

import (
	"fmt"

	"codeberg.org/lindenii/furgit/objecttype"
)

// deltaFrame describes one delta payload to apply during reconstruction.
type deltaFrame struct {
	// packName identifies where the delta payload lives.
	packName string
	// dataOffset points to the start of the delta zlib payload in pack.
	dataOffset int
}

// deltaPlan describes how to reconstruct one requested object.
type deltaPlan struct {
	// declaredSize is the target object's declared content size.
	declaredSize int64
	// baseLoc points to the innermost base object.
	baseLoc location
	// baseType is the canonical object type resolved from baseLoc.
	baseType objecttype.Type
	// frames contains deltas from target down toward base.
	frames []deltaFrame
}

// deltaPlanFor walks one object's chain and builds a delta reconstruction plan.
func (store *Store) deltaPlanFor(start location) (deltaPlan, error) {
	visited := make(map[location]struct{})
	current := start

	var plan deltaPlan
	plan.declaredSize = -1

	for {
		if _, ok := visited[current]; ok {
			return deltaPlan{}, fmt.Errorf("objectstore/packed: delta cycle while resolving object")
		}
		visited[current] = struct{}{}

		_, meta, err := store.entryMetaAt(current)
		if err != nil {
			return deltaPlan{}, err
		}
		if plan.declaredSize < 0 {
			plan.declaredSize = meta.size
		}

		if isBaseObjectType(meta.ty) {
			plan.baseLoc = current
			plan.baseType = meta.ty
			return plan, nil
		}

		switch meta.ty {
		case objecttype.TypeRefDelta:
			plan.frames = append(plan.frames, deltaFrame{
				packName:   current.packName,
				dataOffset: meta.dataOffset,
			})
			next, err := store.lookup(meta.baseRefID)
			if err != nil {
				return deltaPlan{}, err
			}
			current = next
		case objecttype.TypeOfsDelta:
			plan.frames = append(plan.frames, deltaFrame{
				packName:   current.packName,
				dataOffset: meta.dataOffset,
			})
			current = location{
				packName: current.packName,
				offset:   meta.baseOfs,
			}
		default:
			return deltaPlan{}, fmt.Errorf("objectstore/packed: unsupported pack type %d", meta.ty)
		}
	}
}