#include "merkle_sync.h"

#include "../../../src/utun_instance.h"
#include "etcp_api.h"
#include "etcp.h"
#include "../../../lib/debug_config.h"
#include "../../../lib/mem.h"
#include "../../../lib/u_async.h"

#include <string.h>
#include <stdlib.h>
#include <sqlite3.h>

#define MS_ID   "merkle_sync"
#define MS_SYNC_TIMEOUT_MS  10000
#define MS_BG_INTERVAL_MS   100

static struct merkle_sync* g_merkle = NULL;

struct bucket_entry { uint8_t level; uint8_t prefix_bytes; uint64_t prefix; };

struct ms_session {
    struct ms_session* next;
    char               ns[64];
    uint64_t           peer;
    uint8_t            active;
    uint8_t            synced;
    void*              timer;
    uint8_t            retries;
    merkle_sync_done_cb done_cb;
    void*              cb_arg;
};

struct merkle_sync {
    struct UTUN_INSTANCE*          inst;
    uint8_t                       svc_id;
    const struct merkle_sync_data_ops* ops;
    void*                         data_ctx;
    struct ms_session*            sessions;
    uint8_t                       initialized;
    void*                         bg_timer;
};

/* ── DB access ── */

static sqlite3* _db(struct UTUN_INSTANCE* inst) {
    return inst ? inst->topo_sqlite_db : NULL;
}

static int _send_msg(struct merkle_sync* ms, uint64_t peer, const uint8_t* payload, size_t len);

/* ── Prefix arithmetic ── */

uint64_t merkle_sync_level_prefix(uint64_t key, uint8_t level) {
    int shift = 63 - (int)level * 5; if (shift < 0) shift = 0;
    return (key >> shift) << shift;
}

uint8_t merkle_sync_prefix_bytes(uint8_t level) { int bits = level * 5; return (uint8_t)((bits + 7) / 8); }

static void _prefix_write(uint8_t* out, uint64_t prefix, uint8_t pb) {
    for (int i = (int)pb - 1; i >= 0; i--) out[pb - 1 - i] = (uint8_t)(prefix >> (i * 8));
}

static uint64_t _prefix_read(const uint8_t* in, uint8_t pb) {
    uint64_t v = 0;
    for (uint8_t i = 0; i < pb && i < 8; i++) v = (v << 8) | in[i];
    return v;
}

static int _popcount_u32(uint32_t v) { return __builtin_popcount(v); }

/* ── Ensure merkle_tree_hash table ── */

static void _ensure_table(struct merkle_sync* ms) {
    sqlite3* db = _db(ms->inst); if (!db) return;
    sqlite3_exec(db,
        "CREATE TABLE IF NOT EXISTS merkle_tree_hash ("
        "  namespace    TEXT NOT NULL,"
        "  level        INTEGER NOT NULL CHECK(level BETWEEN 1 AND 5),"
        "  prefix64     INTEGER NOT NULL,"
        "  hash         BLOB NOT NULL,"
        "  member_count INTEGER NOT NULL,"
        "  PRIMARY KEY (namespace, level, prefix64))",
        NULL, NULL, NULL);
}

/* ── Tree operations ── */

static int _recompute_bucket(struct merkle_sync* ms, const char* ns,
                              uint8_t level, uint64_t prefix64) {
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: ns=%s L%d/P%016llx", MS_ID, ns, level, (unsigned long long)prefix64);
    sqlite3* db = _db(ms->inst); if (!db) return -1;

    EVP_MD_CTX* ctx = EVP_MD_CTX_new();
    EVP_DigestInit_ex(ctx, EVP_sha256(), NULL);
    int count = ms->ops->update_bucket_hash(ms->data_ctx, ns, level, prefix64, ctx);

    if (count < 0) { DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: → error", MS_ID); EVP_MD_CTX_free(ctx); return -1; }
    if (count == 0) {
        DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: → empty, deleted", MS_ID);
        EVP_MD_CTX_free(ctx);
        sqlite3_stmt* ds = NULL;
        sqlite3_prepare_v2(db,
            "DELETE FROM merkle_tree_hash WHERE namespace=? AND level=? AND prefix64=?",
            -1, &ds, NULL);
        if (ds) { sqlite3_bind_text(ds, 1, ns, -1, SQLITE_STATIC); sqlite3_bind_int(ds, 2, level);
            sqlite3_bind_int64(ds, 3, (sqlite3_int64)prefix64); sqlite3_step(ds); sqlite3_finalize(ds); }
        return 0;
    }

    uint8_t hash[MT_HASH_SIZE];
    EVP_DigestFinal_ex(ctx, hash, NULL);
    EVP_MD_CTX_free(ctx);
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: → saved, count=%d", MS_ID, count);

    sqlite3_stmt* is = NULL;
    sqlite3_prepare_v2(db,
        "INSERT OR REPLACE INTO merkle_tree_hash(namespace, level, prefix64, hash, member_count)"
        " VALUES(?,?,?,?,?)", -1, &is, NULL);
    if (is) {
        sqlite3_bind_text(is, 1, ns, -1, SQLITE_STATIC);
        sqlite3_bind_int(is, 2, level);
        sqlite3_bind_int64(is, 3, (sqlite3_int64)prefix64);
        sqlite3_bind_blob(is, 4, hash, MT_HASH_SIZE, SQLITE_STATIC);
        sqlite3_bind_int(is, 5, count);
        sqlite3_step(is); sqlite3_finalize(is);
    }
    return 0;
}

void merkle_sync_recompute_path(struct UTUN_INSTANCE* inst, const char* ns, uint64_t key) {
    struct merkle_sync* ms = g_merkle;
    if (!ms || !ms->initialized) return;
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: ns=%s key=%016llx", MS_ID, ns, (unsigned long long)key);
    for (uint8_t level = 1; level <= MT_MAX_LEVEL; level++)
        _recompute_bucket(ms, ns, level, merkle_sync_level_prefix(key, level));
}

const uint8_t* merkle_sync_get_hash(struct UTUN_INSTANCE* inst, const char* ns,
                                     uint8_t level, uint64_t prefix64) {
    static uint8_t zero[MT_HASH_SIZE];
    sqlite3* db = _db(inst); if (!db) { memset(zero, 0, MT_HASH_SIZE); return zero; }
    sqlite3_stmt* stmt = NULL;
    sqlite3_prepare_v2(db,
        "SELECT hash FROM merkle_tree_hash WHERE namespace=? AND level=? AND prefix64=?",
        -1, &stmt, NULL);
    if (!stmt) { memset(zero, 0, MT_HASH_SIZE); return zero; }
    sqlite3_bind_text(stmt, 1, ns, -1, SQLITE_STATIC);
    sqlite3_bind_int(stmt, 2, level);
    sqlite3_bind_int64(stmt, 3, (sqlite3_int64)prefix64);
    memset(zero, 0, MT_HASH_SIZE);
    if (sqlite3_step(stmt) == SQLITE_ROW) {
        const void* h = sqlite3_column_blob(stmt, 0);
        if (h) memcpy(zero, h, MT_HASH_SIZE);
    }
    sqlite3_finalize(stmt);
    return zero;
}

static int _get_level_hashes(struct merkle_sync* ms, const char* ns,
                              uint8_t level, uint64_t prefix, uint8_t prefix_bytes,
                              uint32_t* bitmap, uint8_t hashes[MT_BUCKETS][MT_HASH_SIZE]) {
    (void)prefix_bytes;
    memset(hashes, 0, sizeof(uint8_t) * MT_BUCKETS * MT_HASH_SIZE);
    *bitmap = 0;
    if (level >= MT_MAX_LEVEL) return 0;

    sqlite3* db = _db(ms->inst); if (!db) return -1;
    int parent_shift = 63 - (int)level * 5; if (parent_shift < 0) parent_shift = 0;
    int next_shift = 63 - ((int)level + 1) * 5; if (next_shift < 0) next_shift = 0;
    uint64_t range_end = prefix | (0x1FULL << parent_shift) | (0x1FULL << (next_shift > 0 ? next_shift : 0));

    sqlite3_stmt* stmt = NULL;
    if (sqlite3_prepare_v2(db,
            "SELECT prefix64, hash FROM merkle_tree_hash"
            " WHERE namespace=? AND level=? AND prefix64>=? AND prefix64<=?"
            " ORDER BY prefix64", -1, &stmt, NULL) != SQLITE_OK || !stmt) {
        DEBUG_ERROR(DEBUG_CATEGORY_DB_SYNC, "%s: _get_level_hashes SQL error ns=%s L%d — %s", MS_ID, ns, level, sqlite3_errmsg(db));
        return -1;
    }
    sqlite3_bind_text(stmt, 1, ns, -1, SQLITE_STATIC);
    sqlite3_bind_int(stmt, 2, (int)(level + 1));
    sqlite3_bind_int64(stmt, 3, (sqlite3_int64)prefix);
    sqlite3_bind_int64(stmt, 4, (sqlite3_int64)range_end);
    while (sqlite3_step(stmt) == SQLITE_ROW) {
        sqlite3_int64 row_pfx = sqlite3_column_int64(stmt, 0);
        const void* h = sqlite3_column_blob(stmt, 1);
        if (!h || sqlite3_column_bytes(stmt, 1) < MT_HASH_SIZE) continue;
        int bucket = (int)(((uint64_t)row_pfx >> next_shift) & 0x1F);
        *bitmap |= (1u << bucket);
        memcpy(hashes[bucket], h, MT_HASH_SIZE);
    }
    sqlite3_finalize(stmt);
    DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: _get_level_hashes ns=%s L%d/P%016llx bitmap=%08x",
                MS_ID, ns, level, (unsigned long long)prefix, *bitmap);
    return 0;
}

/* ── Send helpers ── */

static struct ETCP_CONN* ms_find_conn_for_node(struct UTUN_INSTANCE* inst, uint64_t node_id) {
    if (!inst->connections) { DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: find_conn — inst->connections is NULL", MS_ID); return NULL; }
    struct ll_entry* e = queue_find_data_by_index(inst->connections, (const uint8_t*)&node_id);
    if (!e) { DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: find_conn — node=%016llx NOT FOUND in connections", MS_ID, (unsigned long long)node_id); return NULL; }
    struct conn_queue_entry* ce = (struct conn_queue_entry*)e->data;
    DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: find_conn — node=%016llx found=%d initialized=%d links_up=%d", MS_ID, (unsigned long long)node_id, 1, ce->conn->initialized, ce->conn->links_up);
    if (ce->conn->initialized && ce->conn->links_up) return ce->conn;
    return NULL;
}

static int _send_msg(struct merkle_sync* ms, uint64_t peer,
                     const uint8_t* payload, size_t len) {
    if (!ms->inst || len < 1) return -1;
    uint8_t* buf = u_malloc(1 + len);
    if (!buf) return -1;
    buf[0] = ms->svc_id;
    memcpy(buf + 1, payload, len);
    struct ll_entry* entry = queue_entry_new(0);
    if (!entry) { u_free(buf); return -1; }
    entry->dgram = buf; entry->len = 1 + len;
    struct ETCP_CONN* conn = ms_find_conn_for_node(ms->inst, peer);
    if (!conn) { DEBUG_ERROR(DEBUG_CATEGORY_DB_SYNC, "%s: no conn for node %016llx", MS_ID, (unsigned long long)peer); u_free(buf); queue_entry_free(entry); return -1; }
    int r = etcp_send(conn, entry);
    if (r != 0) { u_free(buf); queue_entry_free(entry); }
    DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: send OK peer=%016llx len=%zu rc=%d", MS_ID, (unsigned long long)peer, len, r);
    return r;
}

static int _send_hashes(struct merkle_sync* ms, uint64_t peer, const char* ns,
                         uint8_t level, uint64_t prefix, uint8_t prefix_bytes, int is_data) {
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: send_hashes peer=%016llx ns=%s L%d P%016llx is_data=%d", MS_ID, (unsigned long long)peer, ns, level, (unsigned long long)prefix, is_data);
    uint8_t ch_len = (uint8_t)strlen(ns);
    size_t max_sz = 1 + 1 + ch_len + 1 + 1 + prefix_bytes + 1 + 4 + MT_BUCKETS * MT_HASH_SIZE + 2 + 65536;
    uint8_t* buf = u_malloc(max_sz);
    if (!buf) return -1;
    uint8_t* p = buf;
    *p++ = ch_len; memcpy(p, ns, ch_len); p += ch_len;
    *p++ = 0x01; /* MS_MSG_HASHES */
    *p++ = level;
    *p++ = prefix_bytes; _prefix_write(p, prefix, prefix_bytes); p += prefix_bytes;
    *p++ = (uint8_t)(is_data ? 1 : 0);

    if (is_data) {
        size_t mlen = 65536; uint8_t* mbuf = u_malloc(mlen);
        if (mbuf) {
            if (ms->ops->get_items(ms->data_ctx, ns, level, prefix, prefix_bytes, mbuf, &mlen) == 0)
                { memcpy(p, mbuf, mlen); p += mlen; }
            else { uint16_t zero = 0; memcpy(p, &zero, 2); p += 2; }
            u_free(mbuf);
        }
    } else {
        uint32_t bitmap; uint8_t hashes[MT_BUCKETS][MT_HASH_SIZE];
        _get_level_hashes(ms, ns, level, prefix, prefix_bytes, &bitmap, hashes);
        DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: send_hashes peer=%016llx ns=%s level=%d prefix=%016llx bitmap=%08x", MS_ID, (unsigned long long)peer, ns, level, (unsigned long long)prefix, bitmap);
        memcpy(p, &bitmap, 4); p += 4;
        for (int i = 0; i < MT_BUCKETS; i++)
            if (bitmap & (1u << i)) { memcpy(p, hashes[i], MT_HASH_SIZE); p += MT_HASH_SIZE; }
    }
    int r = _send_msg(ms, peer, buf, (size_t)(p - buf));
    u_free(buf);
    return r;
}

static int _send_batch(struct merkle_sync* ms, uint64_t peer, const char* ns,
                        struct bucket_entry* buckets, int count) {
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: send_batch peer=%016llx ns=%s count=%d", MS_ID, (unsigned long long)peer, ns, count);
    uint8_t ch_len = (uint8_t)strlen(ns);
    size_t max_sz = 1 + 1 + ch_len + 1 + (size_t)count * (1 + 1 + 8 + 1 + 4 + MT_BUCKETS * MT_HASH_SIZE + 65536);
    uint8_t* buf = u_malloc(max_sz);
    if (!buf) return -1;
    uint8_t* p = buf;
    *p++ = ch_len; memcpy(p, ns, ch_len); p += ch_len;
    *p++ = 0x03; /* MS_MSG_BATCH */
    *p++ = (uint8_t)count;

    for (int i = 0; i < count; i++) {
        *p++ = buckets[i].level;
        *p++ = buckets[i].prefix_bytes;
        _prefix_write(p, buckets[i].prefix, buckets[i].prefix_bytes); p += buckets[i].prefix_bytes;

        EVP_MD_CTX* tctx = EVP_MD_CTX_new();
        EVP_DigestInit_ex(tctx, EVP_sha256(), NULL);
        int bcount = ms->ops->update_bucket_hash(ms->data_ctx, ns,
            buckets[i].level, buckets[i].prefix, tctx);
        if (bcount < 0) bcount = 0;
        EVP_MD_CTX_free(tctx);

        int is_terminal = (buckets[i].level >= MT_MAX_LEVEL || bcount < 8);
        *p++ = (uint8_t)(is_terminal ? 1 : 0);

        if (is_terminal) {
            size_t mlen = 65536; uint8_t* mbuf = u_malloc(mlen);
            if (mbuf) {
                if (ms->ops->get_items(ms->data_ctx, ns, buckets[i].level,
                        buckets[i].prefix, buckets[i].prefix_bytes, mbuf, &mlen) == 0)
                    { memcpy(p, mbuf, mlen); p += mlen; }
                else { uint16_t z = 0; memcpy(p, &z, 2); p += 2; }
                u_free(mbuf);
            }
        } else {
            uint32_t bm; uint8_t hs[MT_BUCKETS][MT_HASH_SIZE];
            _get_level_hashes(ms, ns, buckets[i].level,
                              buckets[i].prefix, buckets[i].prefix_bytes, &bm, hs);
            memcpy(p, &bm, 4); p += 4;
            for (int j = 0; j < MT_BUCKETS; j++)
                if (bm & (1u << j)) { memcpy(p, hs[j], MT_HASH_SIZE); p += MT_HASH_SIZE; }
        }
    }
    int r = _send_msg(ms, peer, buf, (size_t)(p - buf));
    u_free(buf);
    return r;
}

/* ── Sessions ── */

static void _session_start_timer(struct ms_session* s);

static void _session_timeout_cb(void* arg) {
    struct ms_session* s = (struct ms_session*)arg;
    if (!s || !s->active || !g_merkle || !g_merkle->initialized) return;
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: timeout peer=%016llx ns=%s retry=%d/3", MS_ID, (unsigned long long)s->peer, s->ns, s->retries);
    s->timer = NULL;
    s->retries++;
    if (s->retries > 3) {
        DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: → max retries, done(ERR_TIMEOUT)", MS_ID);
        DEBUG_WARN(DEBUG_CATEGORY_DB_SYNC, "%s: sync timeout peer=%016llx ns=%s", MS_ID,
                   (unsigned long long)s->peer, s->ns);
        s->active = 0;
        if (s->done_cb) s->done_cb(s->peer, s->ns, MT_ERR_TIMEOUT, s->cb_arg);
        return;
    }
    DEBUG_INFO(DEBUG_CATEGORY_DB_SYNC, "%s: retry %d peer=%016llx ns=%s", MS_ID,
               s->retries, (unsigned long long)s->peer, s->ns);
    _send_hashes(g_merkle, s->peer, s->ns, 1, 0, 1, 0);
    _session_start_timer(s);
}

static void _session_start_timer(struct ms_session* s) {
    if (!g_merkle || !g_merkle->inst) return;
    s->timer = uasync_set_timeout(g_merkle->inst->ua, (uint32_t)(MS_SYNC_TIMEOUT_MS * 10),
                                   s, _session_timeout_cb, "ms_sync");
}

static struct ms_session* _session_find(struct merkle_sync* ms, uint64_t peer, const char* ns) {
    for (struct ms_session* s = ms->sessions; s; s = s->next)
        if (s->peer == peer && strcmp(s->ns, ns) == 0) return s;
    return NULL;
}

static void _session_done(struct ms_session* s, int result) {
    s->active = 0;
    if (s->timer) { uasync_cancel_timeout(g_merkle->inst->ua, s->timer); s->timer = NULL; }
    if (result == MT_OK) { s->synced = 1; DEBUG_INFO(DEBUG_CATEGORY_DB_SYNC, "%s: session SYNCED peer=%016llx ns=%s", MS_ID, (unsigned long long)s->peer, s->ns); }
    else { s->synced = 0; DEBUG_WARN(DEBUG_CATEGORY_DB_SYNC, "%s: session FAILED peer=%016llx ns=%s result=%d", MS_ID, (unsigned long long)s->peer, s->ns, result); }
    if (s->done_cb) s->done_cb(s->peer, s->ns, result, s->cb_arg);
}

/* ── Recv handlers ── */

static void _handle_hashes(struct merkle_sync* ms, uint64_t peer, const char* ns,
                            const uint8_t* pl, size_t plen) {
    if (plen < 3) return;
    uint8_t level = pl[0]; uint8_t pb = pl[1];
    uint64_t prefix = _prefix_read(pl + 2, pb);
    uint8_t is_data = pl[2 + pb];
    const uint8_t* payload = pl + 3 + pb;
    size_t paylen = plen - 3 - pb;
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: handle_hashes peer=%016llx ns=%s L%d/P%016llx is_data=%d", MS_ID, (unsigned long long)peer, ns, level, (unsigned long long)prefix, is_data);

    struct ms_session* s = _session_find(ms, peer, ns);
    if (s && s->timer) { uasync_cancel_timeout(g_merkle->inst->ua, s->timer); s->timer = NULL; }

    if (is_data) {
        if (paylen < 2) return;
        ms->ops->apply_items(ms->data_ctx, ns, payload, paylen);
        if (s && s->active) _session_done(s, MT_OK);
        return;
    }

    if (!s) {
        s = u_calloc(1, sizeof(*s)); if (!s) return;
        snprintf(s->ns, sizeof(s->ns), "%s", ns); s->peer = peer;
        s->next = ms->sessions; ms->sessions = s;
    }
    s->active = 1;

    if (paylen < 4) return;
    uint32_t remote_bm; memcpy(&remote_bm, payload, 4);
    const uint8_t* rh = payload + 4;

    uint32_t local_bm; uint8_t lh[MT_BUCKETS][MT_HASH_SIZE];
    _get_level_hashes(ms, ns, level, prefix, pb, &local_bm, lh);

    uint32_t differs = remote_bm ^ local_bm;
    for (int i = 0; i < MT_BUCKETS; i++) {
        if (!(local_bm & (1u << i)) && !(remote_bm & (1u << i))) continue;
        if ((local_bm & (1u << i)) && (remote_bm & (1u << i))) {
            int rh_idx = 0;
            for (int j = 0; j < i; j++) if (remote_bm & (1u << j)) rh_idx++;
            if (memcmp(rh + (size_t)rh_idx * MT_HASH_SIZE, lh[i], MT_HASH_SIZE) == 0)
                differs &= ~(1u << i);
        }
    }

    DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: handle_hashes peer=%016llx ns=%s level=%d remote_bm=%08x local_bm=%08x differs=%08x is_data=%d",
                MS_ID, (unsigned long long)peer, ns, level, remote_bm, local_bm, differs, is_data);

    if (differs == 0) { _session_done(s, MT_OK); return; }

    int next_shift = 63 - ((int)level + 1) * 5;
    struct bucket_entry requests[MT_BUCKETS]; int rcount = 0;
    for (int i = 0; i < MT_BUCKETS && rcount < 32; i++) {
        if (!(differs & (1u << i))) continue;
        uint8_t nl = (uint8_t)(level + 1); if (nl > MT_MAX_LEVEL) nl = MT_MAX_LEVEL;
        requests[rcount].level = nl;
        requests[rcount].prefix_bytes = merkle_sync_prefix_bytes(nl);
        requests[rcount].prefix = prefix | ((uint64_t)i << (next_shift > 0 ? next_shift : 0));
        rcount++;
    }

    if (rcount > 0) {
        uint8_t ch_len = (uint8_t)strlen(ns);
        size_t rs = 1 + 1 + ch_len + 1;
        for (int i = 0; i < rcount; i++) rs += 1 + 1 + requests[i].prefix_bytes + 1;
        uint8_t* rbuf = u_malloc(rs);
        if (rbuf) {
            uint8_t* wr = rbuf;
            *wr++ = ch_len;
            memcpy(wr, ns, ch_len); wr += ch_len;
            *wr++ = 0x02; /* MS_MSG_REQUEST */
            *wr++ = (uint8_t)rcount;
            for (int i = 0; i < rcount; i++) {
                *wr++ = requests[i].level;
                *wr++ = requests[i].prefix_bytes;
                _prefix_write(wr, requests[i].prefix, requests[i].prefix_bytes);
                wr += requests[i].prefix_bytes;
                EVP_MD_CTX* tctx = EVP_MD_CTX_new();
                EVP_DigestInit_ex(tctx, EVP_sha256(), NULL);
                int cnt = ms->ops->update_bucket_hash(ms->data_ctx, ns,
                    requests[i].level, requests[i].prefix, tctx);
                if (cnt < 0) cnt = 0;
                EVP_MD_CTX_free(tctx);
                *wr++ = (uint8_t)(cnt < 8 || requests[i].level >= MT_MAX_LEVEL ? 1 : 0);
            }
            _send_msg(ms, peer, rbuf, (size_t)(wr - rbuf));
            u_free(rbuf);
        }
        if (s) _session_start_timer(s);
    } else {
        if (s) _session_done(s, MT_OK);
    }
}

static void _handle_request(struct merkle_sync* ms, uint64_t peer, const char* ns,
                             const uint8_t* pl, size_t plen) {
    if (plen < 1) return;
    uint8_t count = pl[0]; const uint8_t* bp = pl + 1; size_t off = 0;
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: handle_request peer=%016llx ns=%s count=%d", MS_ID, (unsigned long long)peer, ns, count);

    DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: handle_request peer=%016llx ns=%s count=%d", MS_ID, (unsigned long long)peer, ns, count);

    struct bucket_entry buckets[32]; int bc = 0;
    for (uint8_t i = 0; i < count && bc < 32; i++) {
        if (off + 2 > plen - 1) break;
        uint8_t lvl = bp[off++]; uint8_t pb_i = bp[off++];
        if (off + pb_i > plen - 1) break;
        uint64_t pr = _prefix_read(bp + off, pb_i); off += pb_i;
        buckets[bc].level = lvl; buckets[bc].prefix_bytes = pb_i; buckets[bc].prefix = pr; bc++;
        off++; /* skip is_terminal byte */
    }
    if (bc > 0) {
        _send_batch(ms, peer, ns, buckets, bc);
        struct ms_session* s = _session_find(ms, peer, ns);
        if (s && s->active) {
            if (s->timer) { uasync_cancel_timeout(ms->inst->ua, s->timer); s->timer = NULL; }
            _session_start_timer(s);
        }
    }
}

static void _handle_batch(struct merkle_sync* ms, uint64_t peer, const char* ns,
                           const uint8_t* pl, size_t plen) {
    if (plen < 1) return;
    uint8_t count = pl[0]; const uint8_t* bp = pl + 1; size_t rem = plen - 1;
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: handle_batch peer=%016llx ns=%s count=%d len=%zu", MS_ID, (unsigned long long)peer, ns, count, plen - 1);

    DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: handle_batch peer=%016llx ns=%s count=%d len=%zu", MS_ID, (unsigned long long)peer, ns, count, plen - 1);

    for (uint8_t i = 0; i < count && rem >= 3; i++) {
        uint8_t lvl = bp[0]; uint8_t pb_i = bp[1]; rem -= 2; bp += 2;
        if (rem < pb_i + 1) break;
        uint64_t pr = _prefix_read(bp, pb_i); bp += pb_i; rem -= pb_i;
        uint8_t is_data = *bp++; rem--;

        if (is_data && rem >= 2) {
            uint16_t mc; memcpy(&mc, bp, 2);
            size_t item_len = rem; /* pass entire remainder to apply_items */
            ms->ops->apply_items(ms->data_ctx, ns, bp, item_len);
            bp += rem; rem = 0;
        } else if (!is_data && rem >= 4) {
            uint8_t sub_pl[4096]; size_t sub_len = 0;
            uint8_t next_lvl = (uint8_t)(lvl < MT_MAX_LEVEL ? lvl + 1 : lvl);
            sub_pl[sub_len++] = next_lvl;
            sub_pl[sub_len++] = pb_i;
            _prefix_write(sub_pl + sub_len, pr, pb_i); sub_len += pb_i;
            sub_pl[sub_len++] = 0; /* is_data=0 */
            uint32_t bm; memcpy(&bm, bp, 4); bp += 4; rem -= 4;
            memcpy(sub_pl + sub_len, &bm, 4); sub_len += 4;
            int nh = _popcount_u32(bm);
            if (rem >= (size_t)nh * MT_HASH_SIZE) {
                memcpy(sub_pl + sub_len, bp, (size_t)nh * MT_HASH_SIZE);
                sub_len += (size_t)nh * MT_HASH_SIZE;
                bp += (size_t)nh * MT_HASH_SIZE; rem -= (size_t)nh * MT_HASH_SIZE;
            }
            _handle_hashes(ms, peer, ns, sub_pl, sub_len);
        }
    }
}

static void _recv_cb(struct ETCP_CONN* conn, struct ll_entry* entry) {
    if (!entry || entry->len < 4) {
        if (entry) { if (entry->dgram) u_free(entry->dgram); queue_entry_free(entry); }
        return;
    }
    if (!g_merkle || !g_merkle->initialized) { u_free(entry->dgram); queue_entry_free(entry); return; }
    uint64_t peer = conn ? conn->peer_node_id : 0;
    const uint8_t* d = entry->dgram;
    size_t dlen = entry->len;

    uint8_t ch_len = d[1];
    if (dlen < (size_t)(2 + ch_len + 1)) { u_free(entry->dgram); queue_entry_free(entry); return; }
    char ns[64]; memcpy(ns, d + 2, ch_len); ns[ch_len] = '\0';
    uint8_t type = d[2 + ch_len];
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: recv type=%02x from=%016llx ns=%s len=%zu", MS_ID, type, (unsigned long long)peer, ns, dlen);
    const uint8_t* pl = d + 3 + ch_len;
    size_t plen = dlen - 3 - ch_len;

    switch (type) {
    case 0x01: _handle_hashes(g_merkle, peer, ns, pl, plen); break;
    case 0x02: _handle_request(g_merkle, peer, ns, pl, plen); break;
    case 0x03: _handle_batch(g_merkle, peer, ns, pl, plen); break;
    case 0x04:
        if (plen >= 9 && g_merkle->ops->apply_update) {
            uint64_t key; memcpy(&key, pl, 8);
            uint8_t utype = pl[8];
            DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: recv ITEM_UPDATE key=%016llx type=%02x from=%016llx ns=%s",
                        MS_ID, (unsigned long long)key, utype, (unsigned long long)peer, ns);
            g_merkle->ops->apply_update(g_merkle->data_ctx, ns, key, utype, pl + 9, plen - 9);
        }
        break;
    }

    u_free(entry->dgram); queue_entry_free(entry);
}

/* ── Push lightweight update to synced peers ── */

void merkle_sync_push_update(struct UTUN_INSTANCE* inst, const char* ns,
        uint64_t key, uint8_t type, const uint8_t* data, size_t len) {
    struct merkle_sync* ms = g_merkle;
    if (!ms || !ms->initialized || !ns || !data) return;
    DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: push_update ns=%s key=%016llx type=%02x len=%zu",
                MS_ID, ns, (unsigned long long)key, type, len);

    uint8_t ch_len = (uint8_t)strlen(ns);
    if (ch_len > 63) return;
    size_t pkt = 1 + 1 + ch_len + 1 + 8 + 1 + len;
    uint8_t* buf = u_malloc(pkt);
    if (!buf) return;
    uint8_t* p = buf;
    *p++ = ms->svc_id;
    *p++ = ch_len; memcpy(p, ns, ch_len); p += ch_len;
    *p++ = 0x04; /* MSG_ITEM_UPDATE */
    memcpy(p, &key, 8); p += 8;
    *p++ = type;
    memcpy(p, data, len);

    for (struct ms_session* s = ms->sessions; s; s = s->next) {
        if (!s->synced || strcmp(s->ns, ns) != 0) continue;
        struct ll_entry* entry = queue_entry_new(0);
        if (!entry) continue;
        uint8_t* dcopy = u_malloc(pkt);
        if (!dcopy) { queue_entry_free(entry); continue; }
        memcpy(dcopy, buf, pkt);
        entry->dgram = dcopy; entry->len = pkt;
        struct ETCP_CONN* conn = ms_find_conn_for_node(inst, s->peer);
        if (!conn) { u_free(dcopy); queue_entry_free(entry); continue; }
        int r = etcp_send(conn, entry);
        DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: push_update sent type=%02x to=%016llx rc=%d",
                    MS_ID, type, (unsigned long long)s->peer, r);
        if (r != 0) { u_free(dcopy); queue_entry_free(entry); }
    }
    u_free(buf);
}

/* ── Background consistency check ── */

int merkle_sync_bg_check(struct UTUN_INSTANCE* inst, const char* ns) {
    struct merkle_sync* ms = g_merkle;
    if (!ms || !ms->initialized) return -1;
    sqlite3* db = _db(inst); if (!db || !ns) return -1;

    sqlite3_stmt* stmt = NULL;
    if (sqlite3_prepare_v2(db,
            "SELECT prefix64 FROM merkle_tree_hash WHERE namespace=? AND level=? LIMIT 1",
            -1, &stmt, NULL) != SQLITE_OK) return -1;
    sqlite3_bind_text(stmt, 1, ns, -1, SQLITE_STATIC);
    sqlite3_bind_int(stmt, 2, MT_MAX_LEVEL);

    if (sqlite3_step(stmt) != SQLITE_ROW) { sqlite3_finalize(stmt); return -1; }
    uint64_t pref = (uint64_t)sqlite3_column_int64(stmt, 0);
    sqlite3_finalize(stmt);

    uint8_t old_hash[MT_HASH_SIZE];
    memcpy(old_hash, merkle_sync_get_hash(inst, ns, MT_MAX_LEVEL, pref), MT_HASH_SIZE);

    _recompute_bucket(ms, ns, MT_MAX_LEVEL, pref);

    const uint8_t* new_hash = merkle_sync_get_hash(inst, ns, MT_MAX_LEVEL, pref);
    if (memcmp(old_hash, new_hash, MT_HASH_SIZE) != 0) {
        for (uint8_t lv = (uint8_t)(MT_MAX_LEVEL - 1); lv >= 1; lv--)
            _recompute_bucket(ms, ns, lv, merkle_sync_level_prefix(pref, lv));
        return 1;
    }
    return 0;
}

static void _bg_timer_cb(void* arg) {
    struct merkle_sync* ms = (struct merkle_sync*)arg;
    if (!ms || !ms->initialized || !ms->inst) return;
    sqlite3* db = _db(ms->inst); if (!db) return;

    sqlite3_stmt* cs = NULL;
    sqlite3_prepare_v2(db, "SELECT channel_id FROM channels", -1, &cs, NULL);
    if (!cs) return;
    while (sqlite3_step(cs) == SQLITE_ROW) {
        const char* ch_id = (const char*)sqlite3_column_text(cs, 0);
        if (!ch_id) continue;
        int rc = merkle_sync_bg_check(ms->inst, ch_id);
        if (rc < 0) continue;
        DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: bg_check ns=%s → %s", MS_ID, ch_id, rc == 0 ? "consistent" : "RECALCULATED");
        break;
    }
    sqlite3_finalize(cs);

    ms->bg_timer = uasync_set_timeout(ms->inst->ua, (uint32_t)(MS_BG_INTERVAL_MS * 10),
                                       ms, _bg_timer_cb, "ms_bg");
}

/* ── Public lifecycle ── */

int merkle_sync_init(struct UTUN_INSTANCE* inst, uint8_t svc_id,
                     const struct merkle_sync_data_ops* ops, void* data_ctx) {
    if (!inst || !ops) return -1;
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: init svc=%02x", MS_ID, svc_id);
    struct merkle_sync* ms = u_calloc(1, sizeof(*ms));
    if (!ms) return -1;
    ms->inst = inst; ms->svc_id = svc_id; ms->ops = ops; ms->data_ctx = data_ctx;
    ms->initialized = 1;
    g_merkle = ms;

    _ensure_table(ms);

    etcp_bind(inst, svc_id, _recv_cb);

    ms->bg_timer = uasync_set_timeout(inst->ua, (uint32_t)(MS_BG_INTERVAL_MS * 10),
                                       ms, _bg_timer_cb, "ms_bg");

    DEBUG_INFO(DEBUG_CATEGORY_DB_SYNC, "%s: initialized svc=%02x", MS_ID, svc_id);
    return 0;
}

void merkle_sync_destroy(struct UTUN_INSTANCE* inst) {
    struct merkle_sync* ms = g_merkle;
    if (!ms || !inst) return;
    ms->initialized = 0; g_merkle = NULL;

    etcp_unbind(inst, ms->svc_id);

    if (ms->bg_timer) { uasync_cancel_timeout(inst->ua, ms->bg_timer); ms->bg_timer = NULL; }

    struct ms_session* s = ms->sessions;
    while (s) { struct ms_session* next = s->next;
        if (s->timer) uasync_cancel_timeout(inst->ua, s->timer);
        u_free(s); s = next; }

    u_free(ms);
}

/* ── Public async sync ── */

int merkle_sync_start(struct UTUN_INSTANCE* inst, uint64_t peer,
                       const char* ns, merkle_sync_done_cb done_cb, void* arg) {
    struct merkle_sync* ms = g_merkle;
    if (!inst || !ns || !ms || !ms->initialized) return -1;
    _ensure_table(ms);

    struct ms_session* s = _session_find(ms, peer, ns);
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: start peer=%016llx ns=%s new=%d", MS_ID, (unsigned long long)peer, ns, s ? 0 : 1);
    if (!s) {
        s = u_calloc(1, sizeof(*s));
        if (!s) return -1;
        snprintf(s->ns, sizeof(s->ns), "%s", ns);
        s->peer = peer;
        s->next = ms->sessions;
        ms->sessions = s;
    } else {
        /* replace callback if session already exists */
        if (s->timer) { uasync_cancel_timeout(inst->ua, s->timer); s->timer = NULL; }
        s->synced = 0;
    }
    s->active = 1; s->retries = 0;
    s->done_cb = done_cb; s->cb_arg = arg;

    _send_hashes(ms, peer, ns, 1, 0, 1, 0);
    _session_start_timer(s);
    return 0;
}

void merkle_sync_cancel(struct UTUN_INSTANCE* inst, uint64_t peer, const char* ns) {
    struct merkle_sync* ms = g_merkle;
    if (!ms || !ns) return;
    DEBUG_TRACE(DEBUG_CATEGORY_DB_SYNC, "%s: cancel peer=%016llx ns=%s", MS_ID, (unsigned long long)peer, ns);
    struct ms_session** p = &ms->sessions;
    while (*p) {
        struct ms_session* s = *p;
        if (s->peer == peer && strcmp(s->ns, ns) == 0) {
            if (s->timer) { uasync_cancel_timeout(inst->ua, s->timer); s->timer = NULL; }
            *p = s->next; u_free(s);
            return;
        }
        p = &(*p)->next;
    }
}