// Package chain implements the Phase-1 two-stage (approval -> transform)
// verification chain described in DEV-612. Rules are hard-coded here;
// OPA for verification arrives in Phase 2.
//
// The six checks (see DEV-612 scope) are, in order:
//
//  1. Both DSSE signatures verify against their signer public keys
//     (approval: `trust-api/approval-signer-2026-q4`; transform:
//     `trust-api/agent-testagent-transform-2026-q4` via the Agent
//     Registry).
//  2. TLog inclusion proofs for both refs exist and reproduce the
//     signed tree root (RFC 6962 §2.1.1).
//  3. Content-binding: approval.subject.digest.sha256 ==
//     strip("sha256:", approval.predicate.content_hash), and
//     transform.subject.digest.sha256 == strip("sha256:", payload_hash).
//     This closes the chain from the signed statement to the raw bytes
//     the sink persists.
//  4. approval.predicate.allowed_transformations covers
//     transform.predicate.operation.
//  5. approval.predicate.allowed_sinks contains sink_target.
//  6. Replay guard: Store.Reserve succeeds for decision_id.
//
// Any failure returns valid=false with a reason code in Verdict.Reasons;
// the HTTP layer maps those codes to a stable JSON response.
package chain

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

	"basicstack.de/proofedmesh/services/verification-service/internal/dsse"
	"basicstack.de/proofedmesh/services/verification-service/internal/registry"
	"basicstack.de/proofedmesh/services/verification-service/internal/store"
	"basicstack.de/proofedmesh/services/verification-service/internal/tlog"
)

// Request is the input to Verify.
type Request struct {
	DecisionID   string
	ApprovalRef  string
	TransformRef string
	PayloadHash  string
	SinkTarget   string
}

// Verdict is the output of Verify.
type Verdict struct {
	Valid        bool     `json:"valid"`
	Replay       bool     `json:"replay"`
	DecisionID   string   `json:"decision_id"`
	Actors       []string `json:"actors,omitempty"`
	ApprovalRef  string   `json:"approval_ref,omitempty"`
	TransformRef string   `json:"transform_ref,omitempty"`
	Reasons      []string `json:"reasons,omitempty"`
}

// Reason codes. Stable strings; dashboards grep them. Keep in sync with
// the HTTP response contract and the dummy-sink's reaction (403 vs 409).
const (
	ReasonBadRequest                 = "bad_request"
	ReasonApprovalFetch              = "approval_fetch_failed"
	ReasonTransformFetch             = "transform_fetch_failed"
	ReasonApprovalSignatureUnknown   = "approval_signature_unknown_key"
	ReasonApprovalSignatureInvalid   = "approval_signature_invalid"
	ReasonTransformSignatureUnknown  = "transform_signature_unknown_key"
	ReasonTransformSignatureInvalid  = "transform_signature_invalid"
	ReasonApprovalInclusionInvalid   = "approval_inclusion_proof_invalid"
	ReasonTransformInclusionInvalid  = "transform_inclusion_proof_invalid"
	ReasonApprovalPredicateMalformed = "approval_predicate_malformed"
	ReasonTransformPredicateMalformed = "transform_predicate_malformed"
	ReasonApprovalHashDrift          = "approval_hash_drift"
	ReasonTransformHashDrift         = "transform_hash_drift"
	ReasonSinkNotAllowed             = "sink_not_in_allowed_sinks"
	ReasonTransformationNotAllowed   = "operation_not_in_allowed_transformations"
	ReasonApprovalExpired            = "approval_expired"
	ReasonReplay                     = "replay_decision_id"
)

// Verifier wires the chain-check dependencies.
type Verifier struct {
	TLog     tlog.Fetcher
	Keyring  registry.Keyring
	Store    store.Store
	Now      func() time.Time
	Approval ApprovalConfig
}

// ApprovalConfig constrains which keys and schema the approval predicate
// is expected to have. The key_id prefix pins the DSSE keyid to the
// configured signer period; a mismatched keyid is rejected before any
// signature math runs.
type ApprovalConfig struct {
	// ExpectedKeyID is the full DSSE keyid for the active approval signer
	// (e.g. "trust-api/approval-signer-2026-q4"). The verifier accepts
	// signatures whose keyid matches this value exactly.
	ExpectedKeyID string
	// TransformKeyID is the full DSSE keyid for the active transform
	// signer (e.g. "trust-api/agent-testagent-transform-2026-q4"). Phase 1
	// has one transform agent; Phase 2 will replace this with a lookup
	// against the agent registry.
	TransformKeyID string
}

// Verify runs the full chain check. It never returns a Go error for a
// chain rejection: the Verdict carries the reasons. A Go error is only
// returned for transport-level problems that prevent us from deciding
// (TLog down, store panics, etc.).
func (v *Verifier) Verify(ctx context.Context, req Request) (Verdict, error) {
	verdict := Verdict{
		DecisionID:   req.DecisionID,
		ApprovalRef:  req.ApprovalRef,
		TransformRef: req.TransformRef,
	}
	if err := validateRequest(req); err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonBadRequest+": "+err.Error())
		return verdict, nil
	}

	// 1a. Fetch approval + verify DSSE signature.
	apEntry, err := v.TLog.Fetch(ctx, req.ApprovalRef)
	if err != nil {
		if errors.Is(err, tlog.ErrNotFound) {
			verdict.Reasons = append(verdict.Reasons, ReasonApprovalFetch+": not_found")
			return verdict, nil
		}
		return verdict, fmt.Errorf("approval fetch: %w", err)
	}
	trEntry, err := v.TLog.Fetch(ctx, req.TransformRef)
	if err != nil {
		if errors.Is(err, tlog.ErrNotFound) {
			verdict.Reasons = append(verdict.Reasons, ReasonTransformFetch+": not_found")
			return verdict, nil
		}
		return verdict, fmt.Errorf("transform fetch: %w", err)
	}

	// 1b. Signature verification for approval.
	apKr, err := v.keyringFor(ctx, v.Approval.ExpectedKeyID)
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonApprovalSignatureUnknown+": "+err.Error())
		return verdict, nil
	}
	apPayload, apKey, err := dsse.Verify(apEntry.Envelope, apKr)
	if err != nil {
		switch {
		case errors.Is(err, dsse.ErrUnknownKey):
			verdict.Reasons = append(verdict.Reasons, ReasonApprovalSignatureUnknown)
		case errors.Is(err, dsse.ErrSignatureInvalid):
			verdict.Reasons = append(verdict.Reasons, ReasonApprovalSignatureInvalid)
		default:
			verdict.Reasons = append(verdict.Reasons, ReasonApprovalSignatureInvalid+": "+err.Error())
		}
		return verdict, nil
	}

	// 1c. Signature verification for transform.
	trKr, err := v.keyringFor(ctx, v.Approval.TransformKeyID)
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonTransformSignatureUnknown+": "+err.Error())
		return verdict, nil
	}
	trPayload, trKey, err := dsse.Verify(trEntry.Envelope, trKr)
	if err != nil {
		switch {
		case errors.Is(err, dsse.ErrUnknownKey):
			verdict.Reasons = append(verdict.Reasons, ReasonTransformSignatureUnknown)
		case errors.Is(err, dsse.ErrSignatureInvalid):
			verdict.Reasons = append(verdict.Reasons, ReasonTransformSignatureInvalid)
		default:
			verdict.Reasons = append(verdict.Reasons, ReasonTransformSignatureInvalid+": "+err.Error())
		}
		return verdict, nil
	}

	// 2. Inclusion proofs.
	if err := tlog.VerifyEntry(apEntry); err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonApprovalInclusionInvalid+": "+err.Error())
		return verdict, nil
	}
	if err := tlog.VerifyEntry(trEntry); err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonTransformInclusionInvalid+": "+err.Error())
		return verdict, nil
	}

	// 3. Parse predicates (needed for checks 3-5) and verify content binding.
	ap, err := parseStatement(apPayload)
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonApprovalPredicateMalformed+": "+err.Error())
		return verdict, nil
	}
	tr, err := parseStatement(trPayload)
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonTransformPredicateMalformed+": "+err.Error())
		return verdict, nil
	}

	apContentHash, err := ap.ContentHash()
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonApprovalPredicateMalformed+": "+err.Error())
		return verdict, nil
	}
	if ap.SubjectDigest() != stripSha256Prefix(apContentHash) {
		verdict.Reasons = append(verdict.Reasons, ReasonApprovalHashDrift+": subject.digest.sha256 != predicate.content_hash")
		return verdict, nil
	}
	if tr.SubjectDigest() != stripSha256Prefix(req.PayloadHash) {
		verdict.Reasons = append(verdict.Reasons, ReasonTransformHashDrift+": subject.digest.sha256 != request.payload_hash")
		return verdict, nil
	}

	// 4. allowed_transformations covers operation.
	op, err := tr.Operation()
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonTransformPredicateMalformed+": "+err.Error())
		return verdict, nil
	}
	allowedOps, err := ap.AllowedTransformations()
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonApprovalPredicateMalformed+": "+err.Error())
		return verdict, nil
	}
	if !containsString(allowedOps, op) {
		verdict.Reasons = append(verdict.Reasons, fmt.Sprintf("%s: %q not in %v", ReasonTransformationNotAllowed, op, allowedOps))
		return verdict, nil
	}

	// 5. allowed_sinks contains sink_target.
	allowedSinks, err := ap.AllowedSinks()
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonApprovalPredicateMalformed+": "+err.Error())
		return verdict, nil
	}
	if !containsString(allowedSinks, req.SinkTarget) {
		verdict.Reasons = append(verdict.Reasons, fmt.Sprintf("%s: %q not in %v", ReasonSinkNotAllowed, req.SinkTarget, allowedSinks))
		return verdict, nil
	}

	// Expiry check belongs with the approval predicate.
	expiresAt, err := ap.ExpiresAt()
	if err != nil {
		verdict.Reasons = append(verdict.Reasons, ReasonApprovalPredicateMalformed+": "+err.Error())
		return verdict, nil
	}
	if !v.now().Before(expiresAt) {
		verdict.Reasons = append(verdict.Reasons, fmt.Sprintf("%s: expires_at=%s now=%s", ReasonApprovalExpired, expiresAt.Format(time.RFC3339), v.now().Format(time.RFC3339)))
		return verdict, nil
	}

	// 6. Replay guard. Reserve atomically; a repeated decision_id lands
	// here only after passing checks 1-5 on the first call, so we can
	// safely treat it as replay without recomputing anything.
	decision := store.Decision{
		DecisionID:   req.DecisionID,
		ApprovalRef:  req.ApprovalRef,
		TransformRef: req.TransformRef,
		PayloadHash:  req.PayloadHash,
		SinkTarget:   req.SinkTarget,
		ApprovalKey:  apKey,
		TransformKey: trKey,
		CreatedAt:    v.now().UTC(),
	}
	if err := v.Store.Reserve(ctx, decision); err != nil {
		if errors.Is(err, store.ErrReplay) {
			verdict.Replay = true
			verdict.Reasons = append(verdict.Reasons, ReasonReplay)
			return verdict, nil
		}
		return verdict, fmt.Errorf("reserve decision: %w", err)
	}

	verdict.Valid = true
	verdict.Actors = []string{apKey, trKey}
	return verdict, nil
}

func validateRequest(req Request) error {
	if strings.TrimSpace(req.DecisionID) == "" {
		return errors.New("decision_id is required")
	}
	if !isSha256Ref(req.ApprovalRef) {
		return errors.New("approval_ref must be sha256:<64 hex>")
	}
	if !isSha256Ref(req.TransformRef) {
		return errors.New("transform_ref must be sha256:<64 hex>")
	}
	if !isSha256Ref(req.PayloadHash) {
		return errors.New("payload_hash must be sha256:<64 hex>")
	}
	if strings.TrimSpace(req.SinkTarget) == "" {
		return errors.New("sink_target is required")
	}
	return nil
}

func isSha256Ref(s string) bool {
	if !strings.HasPrefix(s, "sha256:") {
		return false
	}
	hex := s[len("sha256:"):]
	if len(hex) != 64 {
		return false
	}
	for i := 0; i < len(hex); i++ {
		c := hex[i]
		if !('0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F') {
			return false
		}
	}
	return true
}

func stripSha256Prefix(s string) string {
	return strings.TrimPrefix(s, "sha256:")
}

func containsString(xs []string, want string) bool {
	for _, x := range xs {
		if x == want {
			return true
		}
	}
	return false
}

// keyringFor returns a dsse.Keyring with a single mapping (expectedKeyID
// -> public key fetched from the registry). Restricting the keyring to
// the configured signer id makes DSSE verification refuse a signature
// that happens to be valid against some *other* key the registry knows
// — a defensive measure against key confusion in a shared keyring.
func (v *Verifier) keyringFor(ctx context.Context, expectedKeyID string) (dsse.Keyring, error) {
	if expectedKeyID == "" {
		return nil, errors.New("expected key_id is unset in verifier config")
	}
	pub, err := v.Keyring.Lookup(ctx, expectedKeyID)
	if err != nil {
		return nil, err
	}
	return dsse.Keyring{expectedKeyID: pub}, nil
}

func (v *Verifier) now() time.Time {
	if v.Now != nil {
		return v.Now()
	}
	return time.Now()
}

// Statement is the parsed in-toto v1 Statement with helpers for the
// specific predicate fields the chain check needs. Loosely typed so the
// verifier is forward-compatible with additional predicate fields.
type Statement struct {
	Raw map[string]any
}

func parseStatement(payload []byte) (Statement, error) {
	var m map[string]any
	dec := json.NewDecoder(strings.NewReader(string(payload)))
	dec.UseNumber()
	if err := dec.Decode(&m); err != nil {
		return Statement{}, fmt.Errorf("decode statement: %w", err)
	}
	if _, ok := m["predicate"].(map[string]any); !ok {
		return Statement{}, errors.New("statement has no predicate object")
	}
	return Statement{Raw: m}, nil
}

// SubjectDigest returns the sha256 digest hex (no prefix) of the first
// subject on the statement. The approval and transform statements both
// have a single subject per Phase-1 convention.
func (s Statement) SubjectDigest() string {
	subs, ok := s.Raw["subject"].([]any)
	if !ok || len(subs) == 0 {
		return ""
	}
	first, ok := subs[0].(map[string]any)
	if !ok {
		return ""
	}
	digest, ok := first["digest"].(map[string]any)
	if !ok {
		return ""
	}
	sha, _ := digest["sha256"].(string)
	return sha
}

func (s Statement) predicate() map[string]any {
	m, _ := s.Raw["predicate"].(map[string]any)
	return m
}

// ContentHash returns predicate.content_hash as "sha256:<hex>". Required
// on approval statements; the chain verifier uses it to cross-check the
// subject digest and so detect a mismatched preview-vs-approval row.
func (s Statement) ContentHash() (string, error) {
	v, ok := s.predicate()["content_hash"].(string)
	if !ok || v == "" {
		return "", errors.New("predicate.content_hash is missing")
	}
	return v, nil
}

// Operation returns predicate.operation (transform statements).
func (s Statement) Operation() (string, error) {
	v, ok := s.predicate()["operation"].(string)
	if !ok || v == "" {
		return "", errors.New("predicate.operation is missing")
	}
	return v, nil
}

// AllowedTransformations returns predicate.allowed_transformations
// (approval statements). The predicate MUST carry this field; a missing
// or empty list is a malformed approval, not an implicit allow-all.
func (s Statement) AllowedTransformations() ([]string, error) {
	return s.stringArray("allowed_transformations")
}

// AllowedSinks returns predicate.allowed_sinks (approval statements).
// Same discipline as AllowedTransformations.
func (s Statement) AllowedSinks() ([]string, error) {
	return s.stringArray("allowed_sinks")
}

// ExpiresAt returns predicate.expires_at (approval statements) as a
// time.Time. The predicate MUST carry an RFC 3339 timestamp; the chain
// verifier refuses to approve a sink write past that instant.
func (s Statement) ExpiresAt() (time.Time, error) {
	v, ok := s.predicate()["expires_at"].(string)
	if !ok || v == "" {
		return time.Time{}, errors.New("predicate.expires_at is missing")
	}
	return time.Parse(time.RFC3339, v)
}

func (s Statement) stringArray(key string) ([]string, error) {
	raw, ok := s.predicate()[key].([]any)
	if !ok {
		return nil, fmt.Errorf("predicate.%s is missing or not an array", key)
	}
	out := make([]string, 0, len(raw))
	for i, v := range raw {
		str, ok := v.(string)
		if !ok {
			return nil, fmt.Errorf("predicate.%s[%d] is not a string", key, i)
		}
		out = append(out, str)
	}
	return out, nil
}