// Package handover builds the in-toto v1 Statement for the Phase-1
// handover attestation (predicateType =
// https://basicstack.de/attestations/handover/v1). The attestation
// records that `sink:dummy-sink` persisted `payload` (identified by
// `payload_hash`) on behalf of the (approval_ref, transform_ref) chain,
// per §7.0 of the architecture.
package handover

import (
	"errors"
	"time"

	"basicstack.de/proofedmesh/services/dummy-sink/internal/signer"
)

// PredicateType is the stable URL the signing-service, verification
// chain, and transparency-log key the handover statement off of.
const PredicateType = "https://basicstack.de/attestations/handover/v1"

// Input captures everything the sink knows about a successfully
// persisted delivery.
type Input struct {
	DecisionID   string
	PayloadHash  string // sha256:<hex>
	ApprovalRef  string
	TransformRef string

	SinkID        string // e.g. "sink:dummy-sink"
	SinkRecordRef string // storage locator (e.g. "file:///data/dummy-sink/<decision_id>.json")
	Actor         string // agent registry id of the dummy-sink identity

	IssuedAt time.Time
}

// Build returns the in-toto Statement handed to the signing-service.
// The subject binds the attestation to the payload hash so a downstream
// verifier can detect any tampering between what the agent sent and what
// the sink persisted.
func Build(in Input) (signer.Statement, error) {
	if in.DecisionID == "" || in.PayloadHash == "" ||
		in.ApprovalRef == "" || in.TransformRef == "" ||
		in.SinkID == "" || in.SinkRecordRef == "" || in.Actor == "" {
		return nil, errors.New("handover.Build: Input missing required field")
	}
	hashHex, err := StripSha256Prefix(in.PayloadHash)
	if err != nil {
		return nil, err
	}
	issuedAt := in.IssuedAt
	if issuedAt.IsZero() {
		issuedAt = time.Now()
	}
	return signer.Statement{
		"_type": "https://in-toto.io/Statement/v1",
		"subject": []any{
			map[string]any{
				"name": "handover/" + in.DecisionID,
				"digest": map[string]any{
					"sha256": hashHex,
				},
			},
		},
		"predicateType": PredicateType,
		"predicate": map[string]any{
			"decision_id":     in.DecisionID,
			"payload_hash":    in.PayloadHash,
			"approval_ref":    in.ApprovalRef,
			"transform_ref":   in.TransformRef,
			"sink_id":         in.SinkID,
			"sink_record_ref": in.SinkRecordRef,
			"materials": []any{
				map[string]any{
					"name":            "approval/" + in.ApprovalRef,
					"attestation_ref": in.ApprovalRef,
				},
				map[string]any{
					"name":            "transform/" + in.TransformRef,
					"attestation_ref": in.TransformRef,
				},
			},
			"actor":     in.Actor,
			"issued_at": issuedAt.UTC().Format(time.RFC3339),
		},
	}, nil
}

// StripSha256Prefix normalises a `sha256:<hex>` hash to the bare hex
// subject-digest form in-toto uses.
func StripSha256Prefix(hash string) (string, error) {
	const prefix = "sha256:"
	if len(hash) <= len(prefix) || hash[:len(prefix)] != prefix {
		return "", errors.New("handover: payload_hash must be sha256:<hex>")
	}
	return hash[len(prefix):], nil
}