package backend

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"strings"
	"sync"
	"time"

	"github.com/google/trillian"
	"github.com/google/trillian/types"
	"google.golang.org/grpc"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/credentials/insecure"
	"google.golang.org/grpc/status"
)

// Trillian is the production backend that talks to a Trillian LogServer
// via gRPC. ADR-0005 §Follow-ups spells out the wiring: DSSE envelope as
// LogLeaf.LeafValue, content_hash (32 raw bytes) as LeafIdentityHash,
// QueueLeaf → poll GetInclusionProofByHash → return the proof against the
// latest signed tree head.
//
// Idempotency: Trillian treats QueueLeaf with an already-known
// LeafIdentityHash as a no-op and signals it via
// QueuedLogLeaf.Status.Code == ALREADY_EXISTS. We translate that into
// fresh=false, so a repeated submit from the admin-api orchestrator still
// gets a 200 OK plus a valid current proof.
type Trillian struct {
	// Endpoint is the gRPC address of the Trillian LogServer, e.g.
	// "trillian-log-server.agent-trust.svc.cluster.local:8090".
	Endpoint string
	// LogID is the integer tree id this service writes to.
	LogID int64

	// PollInterval is the delay between inclusion-proof retries while the
	// LogSigner integrates the queued leaf. Defaults to 100ms.
	PollInterval time.Duration
	// PollTimeout bounds the wait for the leaf to be sequenced. Defaults
	// to 10s, well above the default sequencer interval (200ms) in the
	// deploy/trillian LogSigner.
	PollTimeout time.Duration

	// DialOptions lets callers inject TLS or interceptors. If nil the
	// connection is dialed with insecure.NewCredentials() — fine inside
	// the agent-trust namespace behind cluster mTLS, not fine outside it.
	DialOptions []grpc.DialOption

	// Clock is injectable for tests. Defaults to time.Now().UTC().
	Clock func() time.Time

	connOnce sync.Once
	conn     *grpc.ClientConn
	client   trillian.TrillianLogClient
	dialErr  error
}

// Get is not yet wired against Trillian; the production read path is a
// future subtask. The handler short-circuits to 503 when the client is
// absent, so this stub only has to satisfy the interface.
func (t *Trillian) Get(_ context.Context, _ string) (Entry, error) {
	return Entry{}, ErrTrillianUnavailable
}

// Submit queues the envelope and returns the inclusion proof against the
// current signed tree head. See the Backend interface for the fresh
// semantics.
func (t *Trillian) Submit(ctx context.Context, env Envelope, contentHash string) (Receipt, bool, error) {
	if contentHash == "" {
		return Receipt{}, false, errors.New("transparency-log: empty content_hash")
	}
	// Unconfigured backend — map to the historical "trillian unavailable"
	// signal so the httpapi layer keeps returning 503 instead of 500 when
	// the operator forgot to supply --trillian-log-id or --trillian-addr.
	if t.LogID == 0 || t.Endpoint == "" {
		return Receipt{}, false, ErrTrillianUnavailable
	}
	recomputed, err := EnvelopeContentHash(env)
	if err != nil {
		return Receipt{}, false, err
	}
	if recomputed != contentHash {
		return Receipt{}, false, ErrContentHashMismatch
	}

	canonical, err := canonicalJSON(env)
	if err != nil {
		return Receipt{}, false, err
	}
	identity, err := decodeSHA256HexPrefixed(contentHash)
	if err != nil {
		return Receipt{}, false, err
	}

	if err := t.ensureClient(); err != nil {
		return Receipt{}, false, err
	}

	// QueueLeaf.
	queueResp, err := t.client.QueueLeaf(ctx, &trillian.QueueLeafRequest{
		LogId: t.LogID,
		Leaf: &trillian.LogLeaf{
			LeafValue:        canonical,
			LeafIdentityHash: identity,
		},
	})
	if err != nil {
		return Receipt{}, false, fmt.Errorf("transparency-log: trillian QueueLeaf: %w", err)
	}

	fresh := true
	if queueResp.GetQueuedLeaf() != nil && queueResp.GetQueuedLeaf().GetStatus() != nil {
		if codes.Code(queueResp.GetQueuedLeaf().GetStatus().GetCode()) == codes.AlreadyExists {
			fresh = false
		}
	}

	// Poll for the inclusion proof. The merkle leaf hash is the RFC 6962
	// leaf hash of the canonical envelope — same hashing we use in the
	// in-memory backend, which keeps the two backends interchangeable.
	merkleHash := leafHash(canonical)
	receipt, err := t.waitForInclusionProof(ctx, merkleHash, contentHash)
	if err != nil {
		return Receipt{}, false, err
	}
	return receipt, fresh, nil
}

// waitForInclusionProof polls GetLatestSignedLogRoot + GetInclusionProofByHash
// until the queued leaf has been sequenced into the tree. See PollInterval
// and PollTimeout for tuning.
func (t *Trillian) waitForInclusionProof(ctx context.Context, merkleHash []byte, contentHash string) (Receipt, error) {
	pollInterval := t.PollInterval
	if pollInterval <= 0 {
		pollInterval = 100 * time.Millisecond
	}
	pollTimeout := t.PollTimeout
	if pollTimeout <= 0 {
		pollTimeout = 10 * time.Second
	}

	deadline := time.Now().Add(pollTimeout)
	for {
		slr, treeSize, rootHash, err := t.latestSignedRoot(ctx)
		if err != nil {
			return Receipt{}, err
		}
		if treeSize > 0 {
			proof, err := t.client.GetInclusionProofByHash(ctx, &trillian.GetInclusionProofByHashRequest{
				LogId:    t.LogID,
				LeafHash: merkleHash,
				TreeSize: int64(treeSize),
			})
			switch {
			case err == nil && len(proof.GetProof()) > 0:
				p := proof.GetProof()[0]
				hashes := make([]string, 0, len(p.GetHashes()))
				for _, h := range p.GetHashes() {
					hashes = append(hashes, "sha256:"+hex.EncodeToString(h))
				}
				return Receipt{
					LogIndex:    p.GetLeafIndex(),
					ContentHash: contentHash,
					LoggedAt:    t.clock(),
					InclusionProof: InclusionProof{
						TreeSize:          int64(treeSize),
						TreeRootHash:      "sha256:" + hex.EncodeToString(rootHash),
						SignedTreeHeadRef: signedTreeHeadRefTrillian(slr),
						Hashes:            hashes,
					},
				}, nil
			case err != nil && status.Code(err) != codes.NotFound:
				return Receipt{}, fmt.Errorf("transparency-log: trillian GetInclusionProofByHash: %w", err)
			}
		}

		// Not sequenced yet (NotFound or TreeSize==0). Sleep + retry up to the deadline.
		if time.Now().After(deadline) {
			return Receipt{}, fmt.Errorf("transparency-log: trillian inclusion proof wait exceeded %s", pollTimeout)
		}
		select {
		case <-ctx.Done():
			return Receipt{}, ctx.Err()
		case <-time.After(pollInterval):
		}
	}
}

// latestSignedRoot pulls the current STH and decodes TreeSize + RootHash.
func (t *Trillian) latestSignedRoot(ctx context.Context) (*trillian.SignedLogRoot, uint64, []byte, error) {
	resp, err := t.client.GetLatestSignedLogRoot(ctx, &trillian.GetLatestSignedLogRootRequest{
		LogId: t.LogID,
	})
	if err != nil {
		return nil, 0, nil, fmt.Errorf("transparency-log: trillian GetLatestSignedLogRoot: %w", err)
	}
	slr := resp.GetSignedLogRoot()
	if slr == nil {
		return nil, 0, nil, errors.New("transparency-log: trillian GetLatestSignedLogRoot returned nil root")
	}
	var lr types.LogRootV1
	if err := lr.UnmarshalBinary(slr.GetLogRoot()); err != nil {
		return nil, 0, nil, fmt.Errorf("transparency-log: trillian LogRoot decode: %w", err)
	}
	return slr, lr.TreeSize, lr.RootHash, nil
}

func (t *Trillian) ensureClient() error {
	t.connOnce.Do(func() {
		opts := t.DialOptions
		if len(opts) == 0 {
			opts = []grpc.DialOption{grpc.WithTransportCredentials(insecure.NewCredentials())}
		}
		conn, err := grpc.NewClient(t.Endpoint, opts...)
		if err != nil {
			t.dialErr = fmt.Errorf("transparency-log: trillian dial %q: %w", t.Endpoint, err)
			return
		}
		t.conn = conn
		t.client = trillian.NewTrillianLogClient(conn)
	})
	return t.dialErr
}

// Close releases the gRPC connection. Safe to call multiple times.
func (t *Trillian) Close() error {
	if t.conn != nil {
		return t.conn.Close()
	}
	return nil
}

func (t *Trillian) clock() time.Time {
	if t.Clock != nil {
		return t.Clock()
	}
	return time.Now().UTC()
}

// signedTreeHeadRefTrillian returns a stable reference to the current STH.
// We hash the serialised LogRoot bytes (which include tree size, root hash
// and revision) so the ref is deterministic per head and does not depend
// on the signature algorithm chosen for the Trees row.
func signedTreeHeadRefTrillian(slr *trillian.SignedLogRoot) string {
	if slr == nil {
		return ""
	}
	h := sha256.Sum256(slr.GetLogRoot())
	return "sha256:" + hex.EncodeToString(h[:])
}

// decodeSHA256HexPrefixed decodes "sha256:<64 hex>" into 32 raw bytes.
// Used both for the Trillian LeafIdentityHash and defensive validation
// at the public Submit entry point.
func decodeSHA256HexPrefixed(s string) ([]byte, error) {
	hexPart := strings.TrimPrefix(s, "sha256:")
	if len(hexPart) != 64 {
		return nil, fmt.Errorf("transparency-log: expected sha256:<64 hex>, got %q", s)
	}
	b, err := hex.DecodeString(hexPart)
	if err != nil {
		return nil, fmt.Errorf("transparency-log: content_hash hex decode: %w", err)
	}
	return b, nil
}

// ErrTrillianUnavailable is kept for backward compatibility with existing
// callers / tests. The httpapi layer still maps it to HTTP 503 for the
// "trillian not configured" and "dial failed" cases that happen before a
// request reaches the LogServer. Real server-side errors from Trillian
// surface as regular wrapped errors and map to 500.
var ErrTrillianUnavailable = errors.New("transparency-log: trillian backend not deployed")