package backend

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

// Memory is an in-memory append-only Merkle log suitable for Dev + CI. It
// stores every leaf (envelope) in memory and recomputes inclusion proofs on
// each submit, which is fine at the entry counts expected in tests and
// development. Production uses Trillian via the trillian.go wrapper stub.
//
// Idempotency: a repeated submit with the same contentHash returns the
// originally assigned log_index with a fresh inclusion proof against the
// current tree size.
type Memory struct {
	mu              sync.Mutex
	leaves          [][]byte             // ordered list of leaf hashes
	indexByHash     map[string]int64     // contentHash -> log_index (zero-based)
	loggedAtByIndex map[int64]time.Time  // original append time per index
	now             func() time.Time
}

// NewMemory returns an empty in-memory backend.
func NewMemory() *Memory {
	return &Memory{
		indexByHash:     map[string]int64{},
		loggedAtByIndex: map[int64]time.Time{},
		now:             func() time.Time { return time.Now().UTC() },
	}
}

// WithClock replaces the time source. For tests.
func (m *Memory) WithClock(now func() time.Time) *Memory {
	m.now = now
	return m
}

// Submit appends (env, contentHash) if the hash is new, otherwise returns
// the existing entry. See Backend.Submit for the fresh semantics.
func (m *Memory) Submit(_ context.Context, env Envelope, contentHash string) (Receipt, bool, error) {
	if contentHash == "" {
		return Receipt{}, false, errors.New("transparency-log: empty content_hash")
	}
	// Recompute the hash to verify what the client sent matches the envelope.
	recomputed, err := EnvelopeContentHash(env)
	if err != nil {
		return Receipt{}, false, err
	}
	if recomputed != contentHash {
		return Receipt{}, false, ErrContentHashMismatch
	}

	// Encode the envelope the same way we hashed it, and use those bytes as
	// the leaf material. This way the leaf hash is a cryptographic commitment
	// to the exact envelope the client stored.
	canonical, err := canonicalJSON(env)
	if err != nil {
		return Receipt{}, false, err
	}

	m.mu.Lock()
	defer m.mu.Unlock()

	fresh := false
	idx, exists := m.indexByHash[contentHash]
	if !exists {
		idx = int64(len(m.leaves))
		m.leaves = append(m.leaves, leafHash(canonical))
		m.indexByHash[contentHash] = idx
		m.loggedAtByIndex[idx] = m.now()
		fresh = true
	}

	root := merkleTreeHash(m.leaves)
	proof := auditPath(m.leaves, int(idx))
	rootHex := sha256Hex(root)
	headRef := signedTreeHeadRef(int64(len(m.leaves)), root)

	hashes := make([]string, 0, len(proof))
	for _, h := range proof {
		hashes = append(hashes, sha256Hex(h))
	}

	return Receipt{
		LogIndex:    idx,
		ContentHash: contentHash,
		LoggedAt:    m.loggedAtByIndex[idx],
		InclusionProof: InclusionProof{
			TreeSize:          int64(len(m.leaves)),
			TreeRootHash:      rootHex,
			SignedTreeHeadRef: headRef,
			Hashes:            hashes,
		},
	}, fresh, nil
}

// signedTreeHeadRef is a stand-in for the real STH reference. In the
// in-memory backend we deterministically derive it from (tree_size, root),
// so callers get a stable per-head identifier without needing a signer.
// The Trillian wrapper will return the actual signed head ref.
func signedTreeHeadRef(size int64, root []byte) string {
	h := sha256.New()
	// Little-endian uint64 encoding of size, then root bytes.
	for i := 0; i < 8; i++ {
		h.Write([]byte{byte(size >> (8 * i))})
	}
	h.Write(root)
	return "sha256:" + hex.EncodeToString(h.Sum(nil))
}

// ErrContentHashMismatch is returned when the client-supplied content_hash
// does not match the SHA-256 of the serialised envelope. ADR-0005 says the
// transparency log MUST reject this with 409.
var ErrContentHashMismatch = errors.New("transparency-log: content_hash does not match envelope")