diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2d1cf9b..e97d020 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -28,6 +28,15 @@ if(STRICT_WARNINGS)
 	add_compile_options(-Wextra -Wpedantic -Werror)
 endif()
 
+include(FetchContent)
+FetchContent_Declare(
+  xxhash
+  GIT_REPOSITORY https://github.com/Cyan4973/xxHash
+  GIT_TAG v0.8.3
+  SOURCE_SUBDIR cmake_unofficial
+)
+FetchContent_MakeAvailable(xxhash)
+
 set(THREADS_PREFER_PTHREAD_FLAG ON)
 find_package(Threads REQUIRED)
 
@@ -45,13 +54,13 @@ file(GLOB TEST_SRCS "tests/*.c")
 
 add_executable(server ${SERVER_SRCS} ${SHARED_SRCS})
 target_include_directories(server PRIVATE src/shared src/server src/client)
-target_link_libraries(server PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto)
+target_link_libraries(server PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto xxhash)
 
 add_executable(client ${CLIENT_SRCS} ${SHARED_SRCS})
 target_include_directories(client PRIVATE src/shared src/server src/client)
-target_link_libraries(client PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto)
+target_link_libraries(client PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto xxhash)
 
 add_executable(tests ${TEST_SRCS} ${SHARED_SRCS} src/client/scanner.c)
 target_include_directories(tests PRIVATE tests src/shared src/server src/client)
-target_link_libraries(tests PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto)
+target_link_libraries(tests PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto xxhash)
 
diff --git a/src/client/client_cli.c b/src/client/client_cli.c
index 55b510e..ecf152b 100644
--- a/src/client/client_cli.c
+++ b/src/client/client_cli.c
@@ -1,5 +1,6 @@
 #include "client_send.h"
 #include "config.h"
+#include "delta.h"
 #include "log.h"
 #include "protocol.h"
 #include "transport_tls.h"
@@ -36,6 +37,9 @@ static void print_usage(void) {
   printf("  --max-size <n>      Skip files larger than n bytes\n");
   printf("  --min-size <n>      Skip files smaller than n bytes\n");
   printf("  --incremental       Skip files unchanged since last transfer\n");
+  printf("  --delta             Delta transfer for changed files (requires --incremental)\n");
+  printf("  --delta-block <n>   Delta block size in bytes (default: %d)\n", DELTA_BLOCK_SIZE_DEFAULT);
+  printf("  --delta-max <n>     Max file size for delta transfer (default: %llu)\n", DELTA_MAX_FILE_SIZE);
   printf("  -m                  Enable multithreading\n");
   printf("  -s                  Enable chunk serialization\n");
   printf("  -f                  Enable sendfile (TCP only, not with -c or -s)\n");
@@ -102,6 +106,20 @@ int main(int argc, char* argv[]) {
       config->min_size = strtoull(argv[++i], NULL, 10);
     } else if (strcmp(argv[i], "--incremental") == 0) {
       config->use_incremental = true;
+    } else if (strcmp(argv[i], "--delta") == 0) {
+      config->use_delta = true;
+    } else if (strcmp(argv[i], "--delta-block") == 0 && i + 1 < argc) {
+      unsigned long long val = strtoull(argv[++i], NULL, 10);
+      if (val >= DELTA_BLOCK_SIZE_MIN && val <= DELTA_BLOCK_SIZE_MAX)
+        config->delta_block_size = (uint32_t)val;
+      else
+        fprintf(stderr, "Warning: --delta-block value %llu out of range, using default\n", val);
+    } else if (strcmp(argv[i], "--delta-max") == 0 && i + 1 < argc) {
+      unsigned long long val = strtoull(argv[++i], NULL, 10);
+      if (val >= DELTA_MIN_FILE_SIZE)
+        config->delta_max_file_size = val;
+      else
+        fprintf(stderr, "Warning: --delta-max value %llu too small, using default\n", val);
     } else if (strcmp(argv[i], "-c") == 0 || strcmp(argv[i], "-z") == 0) {
       config->use_compression = true;
       log_message(LOG_LEVEL_INFO, "Enabled Compression");
@@ -232,6 +250,23 @@ int main(int argc, char* argv[]) {
     config->use_metadata = true;
   }
 
+  if (config->use_delta && !config->use_incremental) {
+    fprintf(stderr, "Error: --delta requires --incremental\n");
+    return 1;
+  }
+  if (config->use_delta && config->use_chunk_serialization) {
+    fprintf(stderr, "Error: --delta cannot be combined with -s (chunk serialization)\n");
+    return 1;
+  }
+  if (config->use_delta && config->use_sendfile) {
+    fprintf(stderr, "Error: --delta cannot be combined with -f (sendfile)\n");
+    return 1;
+  }
+  if (config->use_delta && !config->use_metadata) {
+    log_message(LOG_LEVEL_INFO, "Enabling metadata preservation for --delta");
+    config->use_metadata = true;
+  }
+
   if (config->use_tls) {
     if (!config->tls_cert || !config->tls_key) {
       fprintf(stderr, "Error: --tls requires --cert and --key\n");
diff --git a/src/client/client_send.c b/src/client/client_send.c
index a832f51..b682828 100644
--- a/src/client/client_send.c
+++ b/src/client/client_send.c
@@ -1,9 +1,12 @@
 #include "client_send.h"
 #include "array_list.h"
 #include "chunk.h"
+#include "compression.h"
 #include "config.h"
 #include "data.h"
+#include "delta.h"
 #include "file.h"
+#include "metadata.h"
 #include "log.h"
 #include "multiprocessing.h"
 #include "protocol.h"
@@ -19,11 +22,11 @@
 #include <threads.h>
 #include <time.h>
 
-static int incremental_check(Client* client, File* file) {
-  if (!send_status(client->file_descriptor, STATUS_CHECK))
-    return -1;
-  if (!send_str(client->file_descriptor, file->path))
-    return -1;
+static int incremental_check(Client *client, File *file,
+                              DeltaSignature **out_sig) {
+  *out_sig = NULL;
+  if (!send_status(client->file_descriptor, STATUS_CHECK)) return -1;
+  if (!send_str(client->file_descriptor, file->path)) return -1;
   unsigned long long fsize = file->data->size;
   long long mtime = file->metadata ? file->metadata->mtime_sec : 0;
   if (!send_n_data(client->file_descriptor, &fsize, sizeof(fsize)))
@@ -37,8 +40,16 @@ static int incremental_check(Client* client, File* file) {
     log_message(LOG_LEVEL_ERROR, "Server reported error for file");
     return -1;
   }
-  if (s == STATUS_OK)
-    return 1;
+  if (s == STATUS_OK) return 1;
+  if (s == STATUS_DELTA_SIGNATURE) {
+    Data *sig_data = receive_data(client->file_descriptor);
+    if (!sig_data) return -1;
+    DeltaSignature *sig = delta_signature_deserialize(sig_data);
+    data_destroy(sig_data);
+    if (!sig) return -1;
+    *out_sig = sig;
+    return 2;
+  }
   if (s != STATUS_NEXT) {
     log_message(LOG_LEVEL_ERROR, "Unexpected server status");
     return -1;
@@ -46,7 +57,62 @@ static int incremental_check(Client* client, File* file) {
   return 0;
 }
 
-int send_chunk(Client* client, Chunk* chunk, Config* config) {
+static int send_delta(Client *client, File *file, DeltaSignature *sig,
+                       Config *config) {
+  Delta *delta = delta_compute(file->data->data, file->data->size,
+                                sig, config->delta_block_size);
+  if (!delta) return 1;
+
+  if (!delta_is_worthwhile(delta, file->data->size)) {
+    delta_destroy(delta);
+    if (!send_status(client->file_descriptor, STATUS_NEXT)) return -1;
+    return 1;
+  }
+
+  Data *delta_data = delta_serialize(delta);
+  delta_destroy(delta);
+  if (!delta_data) return -1;
+
+  Data *to_send = delta_data;
+  if (config->use_compression) {
+    to_send = data_compress(delta_data, config->compression_level);
+    data_destroy(delta_data);
+    if (!to_send) return -1;
+  }
+
+  bool ok = send_status(client->file_descriptor, STATUS_DELTA_DATA) &&
+            send_data(client->file_descriptor, to_send);
+
+  if (ok && config->use_metadata)
+    ok = metadata_send(client->file_descriptor, file->metadata);
+
+  data_destroy(to_send);
+  return ok ? 0 : -1;
+}
+
+typedef bool (*file_send_fn)(File *, int, bool, int, bool);
+
+static int send_file_incremental(Client *client, File *file, Config *config,
+                                  file_send_fn send_fn) {
+  DeltaSignature *sig = NULL;
+  int rc = incremental_check(client, file, &sig);
+  if (rc < 0) { delta_signature_destroy(sig); return -1; }
+  if (rc == 1) { delta_signature_destroy(sig); return 1; }
+  if (rc == 2 && config->use_delta) {
+    int drc = send_delta(client, file, sig, config);
+    delta_signature_destroy(sig);
+    if (drc == 0) return 0;
+    if (drc < 0) return -1;
+  } else {
+    delta_signature_destroy(sig);
+  }
+  if (!send_fn(file, client->file_descriptor, config->use_metadata,
+               config->use_compression ? config->compression_level : 0, false))
+    return -1;
+  return 0;
+}
+
+int send_chunk(Client *client, Chunk *chunk, Config *config) {
   if (config->use_chunk_serialization) {
     if (!send_status(client->file_descriptor, STATUS_CHUNK))
       return -1;
@@ -66,33 +132,23 @@ int send_chunk(Client* client, Chunk* chunk, Config* config) {
   } else if (config->use_sendfile && !config->use_compression) {
     for (int i = 0; i < chunk->element_count; i++) {
       if (config->use_incremental) {
-        int rc = incremental_check(client, chunk->items[i]);
-        if (rc < 0)
-          return -1;
-        if (rc > 0)
-          continue;
-        if (!file_send_sendfile(chunk->items[i], client->file_descriptor, config->use_metadata,
-                                false))
-          return -1;
+        int rc = send_file_incremental(client, chunk->items[i], config,
+                                        (file_send_fn)file_send_sendfile);
+        if (rc == 1) continue;
+        if (rc < 0) return -1;
       } else {
-        if (!send_status(client->file_descriptor, STATUS_NEXT))
-          return -1;
-        if (!file_send_sendfile(chunk->items[i], client->file_descriptor, config->use_metadata,
-                                true))
+        if (!send_status(client->file_descriptor, STATUS_NEXT)) return -1;
+        if (!file_send_sendfile(chunk->items[i], client->file_descriptor, config->use_metadata, 0, true))
           return -1;
       }
     }
   } else {
     for (int i = 0; i < chunk->element_count; i++) {
       if (config->use_incremental) {
-        int rc = incremental_check(client, chunk->items[i]);
-        if (rc < 0)
-          return -1;
-        if (rc > 0)
-          continue;
-        if (!file_send_single_calls(chunk->items[i], client->file_descriptor, config->use_metadata,
-                                    config->use_compression ? config->compression_level : 0, false))
-          return -1;
+        int rc = send_file_incremental(client, chunk->items[i], config,
+                                        (file_send_fn)file_send_single_calls);
+        if (rc == 1) continue;
+        if (rc < 0) return -1;
       } else {
         if (!send_status(client->file_descriptor, STATUS_NEXT))
           return -1;
diff --git a/src/shared/config.c b/src/shared/config.c
index b7b143c..42aaa41 100644
--- a/src/shared/config.c
+++ b/src/shared/config.c
@@ -1,4 +1,5 @@
 #include "config.h"
+#include "delta.h"
 #include "log.h"
 #include "protocol.h"
 #include "utils.h"
@@ -37,6 +38,9 @@ Config* config_create(char* version, char* send_directory, char* receive_directo
   config->max_size = 0;
   config->min_size = 0;
   config->use_incremental = false;
+  config->use_delta = false;
+  config->delta_block_size = DELTA_BLOCK_SIZE_DEFAULT;
+  config->delta_max_file_size = DELTA_MAX_FILE_SIZE;
   config->use_tls = false;
   config->tls_cert = NULL;
   config->tls_key = NULL;
@@ -88,32 +92,22 @@ void config_delete(Config* config) {
 }
 
 bool config_send(int file_descriptor, const Config* config) {
-  if (!send_str(file_descriptor, config->version))
-    return false;
-  if (!send_str(file_descriptor, config->send_directory))
-    return false;
-  if (!send_str(file_descriptor, config->receive_root_directory))
-    return false;
-  if (!send_int(file_descriptor, config->save_to_disk))
-    return false;
-  if (!send_int(file_descriptor, config->use_multithreading))
-    return false;
-  if (!send_int(file_descriptor, config->use_chunk_serialization))
-    return false;
-  if (!send_int(file_descriptor, config->use_compression))
-    return false;
-  if (!send_int(file_descriptor, config->use_metadata))
-    return false;
-  if (!send_int(file_descriptor, config->compression_level))
-    return false;
-  if (!send_int(file_descriptor, (int)config->chunk_size))
-    return false;
-  if (!send_int(file_descriptor, config->use_sendfile))
-    return false;
-  if (!send_int(file_descriptor, config->use_delete))
-    return false;
-  if (!send_int(file_descriptor, config->use_incremental))
-    return false;
+  if (!send_str(file_descriptor, config->version)) return false;
+  if (!send_str(file_descriptor, config->send_directory)) return false;
+  if (!send_str(file_descriptor, config->receive_root_directory)) return false;
+  if (!send_int(file_descriptor, config->save_to_disk)) return false;
+  if (!send_int(file_descriptor, config->use_multithreading)) return false;
+  if (!send_int(file_descriptor, config->use_chunk_serialization)) return false;
+  if (!send_int(file_descriptor, config->use_compression)) return false;
+  if (!send_int(file_descriptor, config->use_metadata)) return false;
+  if (!send_int(file_descriptor, config->compression_level)) return false;
+  if (!send_int(file_descriptor, (int)config->chunk_size)) return false;
+  if (!send_int(file_descriptor, config->use_sendfile)) return false;
+  if (!send_int(file_descriptor, config->use_delete)) return false;
+  if (!send_int(file_descriptor, config->use_incremental)) return false;
+  if (!send_int(file_descriptor, config->use_delta)) return false;
+  if (!send_int(file_descriptor, (int)config->delta_block_size)) return false;
+  if (!send_n_data(file_descriptor, &config->delta_max_file_size, sizeof(unsigned long long))) return false;
   Status status;
   if (!receive_status(file_descriptor, &status))
     return false;
@@ -185,6 +179,11 @@ Config* config_receive(int file_descriptor) {
   if (!receive_int(file_descriptor, &tmp))
     goto error;
   config->use_incremental = tmp;
+  if (!receive_int(file_descriptor, &tmp)) goto error;
+  config->use_delta = tmp;
+  if (!receive_int(file_descriptor, &tmp)) goto error;
+  config->delta_block_size = (uint32_t)tmp;
+  if (!receive_n_data(file_descriptor, &config->delta_max_file_size, sizeof(unsigned long long))) goto error;
   config->show_progress = false;
   config->dry_run = false;
   config->ssh_port = 22;
diff --git a/src/shared/config.h b/src/shared/config.h
index eb646cf..c1f025b 100644
--- a/src/shared/config.h
+++ b/src/shared/config.h
@@ -2,6 +2,7 @@
 #define CONFIG_H
 
 #include <stdbool.h>
+#include <stdint.h>
 
 typedef enum { TRANSPORT_TCP, TRANSPORT_SSH } TransportType;
 
@@ -30,13 +31,16 @@ typedef struct Config {
   unsigned long long max_size;
   unsigned long long min_size;
   bool use_incremental;
+  bool use_delta;
+  uint32_t delta_block_size;
+  unsigned long long delta_max_file_size;
   bool use_tls;
   char* tls_cert;
   char* tls_key;
   char* tls_ca;
 } Config;
 
-#define PROTOCOL_VERSION "1.1.0"
+#define PROTOCOL_VERSION "1.2.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 
 Config* config_create(char* version, char* send_directory, char* receive_directory,
diff --git a/src/shared/delta.c b/src/shared/delta.c
new file mode 100644
index 0000000..fc13a1e
--- /dev/null
+++ b/src/shared/delta.c
@@ -0,0 +1,486 @@
+#include "delta.h"
+#include "log.h"
+#include <stdlib.h>
+#include <string.h>
+
+#define XXH_STATIC_LINKING_ONLY
+#define XXH_IMPLEMENTATION
+#include <xxhash.h>
+
+uint32_t delta_adler32(const void *data, uint32_t len) {
+  const uint8_t *p = (const uint8_t *)data;
+  uint32_t s1 = 1;
+  uint32_t s2 = 0;
+  for (uint32_t i = 0; i < len; i++) {
+    s1 = (s1 + p[i]) % DELTA_ADLER32_MODULUS;
+    s2 = (s2 + s1) % DELTA_ADLER32_MODULUS;
+  }
+  return (s2 << 16) | s1;
+}
+
+uint32_t delta_xxhash32(const void *data, uint32_t len) {
+  return XXH32(data, len, 0);
+}
+
+DeltaSignature *delta_signature_create(const void *old_file_data,
+                                        uint64_t old_file_size,
+                                        uint32_t block_size) {
+  if (old_file_data == NULL || old_file_size == 0 || block_size == 0)
+    return NULL;
+
+  uint32_t block_count = (uint32_t)((old_file_size + block_size - 1) / block_size);
+
+  DeltaSignature *sig = malloc(sizeof(DeltaSignature));
+  if (!sig) return NULL;
+
+  sig->file_size = old_file_size;
+  sig->block_size = block_size;
+  sig->block_count = block_count;
+  sig->blocks = malloc(block_count * sizeof(DeltaBlockSig));
+  if (!sig->blocks) {
+    free(sig);
+    return NULL;
+  }
+
+  const uint8_t *data = (const uint8_t *)old_file_data;
+  for (uint32_t i = 0; i < block_count; i++) {
+    uint64_t offset = (uint64_t)i * block_size;
+    uint32_t len = (uint32_t)((old_file_size - offset < block_size)
+                                  ? (old_file_size - offset)
+                                  : block_size);
+    sig->blocks[i].adler32 = delta_adler32(data + offset, len);
+    sig->blocks[i].xxhash = delta_xxhash32(data + offset, len);
+  }
+
+  return sig;
+}
+
+Data *delta_signature_serialize(const DeltaSignature *sig) {
+  if (!sig) return NULL;
+
+  uint64_t total = sizeof(uint64_t) + sizeof(uint32_t) + sizeof(uint32_t) +
+                   (uint64_t)sig->block_count * (sizeof(uint32_t) + sizeof(uint32_t));
+
+  uint8_t *buf = malloc((size_t)total);
+  if (!buf) return NULL;
+
+  size_t pos = 0;
+  memcpy(buf + pos, &sig->file_size, sizeof(uint64_t));
+  pos += sizeof(uint64_t);
+  memcpy(buf + pos, &sig->block_size, sizeof(uint32_t));
+  pos += sizeof(uint32_t);
+  memcpy(buf + pos, &sig->block_count, sizeof(uint32_t));
+  pos += sizeof(uint32_t);
+
+  for (uint32_t i = 0; i < sig->block_count; i++) {
+    memcpy(buf + pos, &sig->blocks[i].adler32, sizeof(uint32_t));
+    pos += sizeof(uint32_t);
+    memcpy(buf + pos, &sig->blocks[i].xxhash, sizeof(uint32_t));
+    pos += sizeof(uint32_t);
+  }
+
+  return data_create(buf, (size_t)total);
+}
+
+DeltaSignature *delta_signature_deserialize(const Data *data) {
+  if (!data || data->size < sizeof(uint64_t) + sizeof(uint32_t) + sizeof(uint32_t))
+    return NULL;
+
+  const uint8_t *buf = (const uint8_t *)data->data;
+  size_t pos = 0;
+
+  DeltaSignature *sig = malloc(sizeof(DeltaSignature));
+  if (!sig) return NULL;
+
+  memcpy(&sig->file_size, buf + pos, sizeof(uint64_t));
+  pos += sizeof(uint64_t);
+  memcpy(&sig->block_size, buf + pos, sizeof(uint32_t));
+  pos += sizeof(uint32_t);
+  memcpy(&sig->block_count, buf + pos, sizeof(uint32_t));
+  pos += sizeof(uint32_t);
+
+  uint64_t expected = sizeof(uint64_t) + sizeof(uint32_t) + sizeof(uint32_t) +
+                      (uint64_t)sig->block_count * (sizeof(uint32_t) + sizeof(uint32_t));
+  if (data->size < expected) {
+    free(sig);
+    return NULL;
+  }
+
+  sig->blocks = malloc(sig->block_count * sizeof(DeltaBlockSig));
+  if (!sig->blocks) {
+    free(sig);
+    return NULL;
+  }
+
+  for (uint32_t i = 0; i < sig->block_count; i++) {
+    memcpy(&sig->blocks[i].adler32, buf + pos, sizeof(uint32_t));
+    pos += sizeof(uint32_t);
+    memcpy(&sig->blocks[i].xxhash, buf + pos, sizeof(uint32_t));
+    pos += sizeof(uint32_t);
+  }
+
+  return sig;
+}
+
+void delta_signature_destroy(DeltaSignature *sig) {
+  if (!sig) return;
+  free(sig->blocks);
+  free(sig);
+}
+
+static bool ensure_capacity(DeltaInstruction **instrs, uint32_t *capacity,
+                             uint32_t count) {
+  if (count < *capacity) return true;
+  uint32_t new_cap = *capacity * 2;
+  DeltaInstruction *tmp = realloc(*instrs, new_cap * sizeof(DeltaInstruction));
+  if (!tmp) return false;
+  *instrs = tmp;
+  *capacity = new_cap;
+  return true;
+}
+
+static bool flush_literal(DeltaInstruction **instrs, uint32_t *capacity,
+                           uint32_t *count, const uint8_t *data,
+                           uint64_t start, uint64_t end) {
+  if (start >= end) return true;
+  uint32_t lit_len = (uint32_t)(end - start);
+  if (!ensure_capacity(instrs, capacity, *count)) return false;
+  uint8_t *lit_data = malloc(lit_len);
+  if (!lit_data) return false;
+  memcpy(lit_data, data + start, lit_len);
+  (*instrs)[*count].type = DELTA_INSTR_LITERAL;
+  (*instrs)[*count].literal.data = lit_data;
+  (*instrs)[*count].literal.length = lit_len;
+  (*count)++;
+  return true;
+}
+
+Delta *delta_compute(const void *new_file_data, uint64_t new_file_size,
+                      const DeltaSignature *sig, uint32_t block_size) {
+  if (!new_file_data || !sig || new_file_size == 0 || block_size == 0)
+    return NULL;
+
+  const uint8_t *new_data = (const uint8_t *)new_file_data;
+
+  uint32_t capacity = 64;
+  uint32_t count = 0;
+  DeltaInstruction *instrs = malloc(capacity * sizeof(DeltaInstruction));
+  if (!instrs) return NULL;
+
+  uint64_t literal_start = 0;
+  bool has_literal = false;
+
+  uint64_t i = 0;
+
+  uint32_t s1 = 1, s2 = 0;
+  bool rolling_valid = false;
+
+  while (i < new_file_size) {
+    uint32_t window_len = (uint32_t)((new_file_size - i < block_size)
+                                         ? (new_file_size - i)
+                                         : block_size);
+    bool full_window = (window_len == block_size);
+
+    uint32_t adler;
+    if (rolling_valid && full_window) {
+      uint8_t old_byte = new_data[i - 1];
+      uint8_t new_byte = new_data[i + block_size - 1];
+      s1 = (s1 + DELTA_ADLER32_MODULUS - old_byte + new_byte) %
+           DELTA_ADLER32_MODULUS;
+      s2 = (s2 + DELTA_ADLER32_MODULUS -
+            (uint32_t)((uint64_t)block_size * old_byte % DELTA_ADLER32_MODULUS) +
+            s1 - 1) %
+           DELTA_ADLER32_MODULUS;
+      adler = (s2 << 16) | s1;
+    } else {
+      s1 = 1;
+      s2 = 0;
+      for (uint32_t k = 0; k < window_len; k++) {
+        s1 = (s1 + new_data[i + k]) % DELTA_ADLER32_MODULUS;
+        s2 = (s2 + s1) % DELTA_ADLER32_MODULUS;
+      }
+      adler = (s2 << 16) | s1;
+      rolling_valid = full_window;
+    }
+
+    bool matched = false;
+    for (uint32_t j = 0; j < sig->block_count; j++) {
+      if (adler == sig->blocks[j].adler32 && full_window) {
+        uint32_t xxh = delta_xxhash32(new_data + i, window_len);
+        if (xxh == sig->blocks[j].xxhash) {
+          if (has_literal) {
+            if (!flush_literal(&instrs, &capacity, &count, new_data,
+                               literal_start, i)) {
+              free(instrs);
+              return NULL;
+            }
+            has_literal = false;
+          }
+
+          if (!ensure_capacity(&instrs, &capacity, count)) {
+            free(instrs);
+            return NULL;
+          }
+          instrs[count].type = DELTA_INSTR_BLOCK_MATCH;
+          instrs[count].match.block_index = j;
+          instrs[count].match.block_offset = 0;
+          instrs[count].match.length = window_len;
+          count++;
+
+          i += window_len;
+          rolling_valid = false;
+          matched = true;
+          break;
+        }
+      }
+    }
+
+    if (!matched) {
+      if (!has_literal) {
+        literal_start = i;
+        has_literal = true;
+      }
+      i++;
+    }
+  }
+
+  if (has_literal) {
+    if (!flush_literal(&instrs, &capacity, &count, new_data,
+                       literal_start, new_file_size)) {
+      free(instrs);
+      return NULL;
+    }
+  }
+
+  Delta *delta = malloc(sizeof(Delta));
+  if (!delta) {
+    for (uint32_t k = 0; k < count; k++) {
+      if (instrs[k].type == DELTA_INSTR_LITERAL)
+        free(instrs[k].literal.data);
+    }
+    free(instrs);
+    return NULL;
+  }
+
+  delta->new_file_size = new_file_size;
+  delta->instruction_count = count;
+  delta->instructions = instrs;
+  delta->delta_size = 0;
+
+  for (uint32_t k = 0; k < count; k++) {
+    delta->delta_size += 1;
+    if (instrs[k].type == DELTA_INSTR_BLOCK_MATCH) {
+      delta->delta_size += sizeof(uint32_t) * 3;
+    } else {
+      delta->delta_size += sizeof(uint32_t) + instrs[k].literal.length;
+    }
+  }
+
+  return delta;
+}
+
+Data *delta_serialize(const Delta *delta) {
+  if (!delta) return NULL;
+
+  uint64_t total = sizeof(uint64_t) + sizeof(uint32_t) + delta->delta_size;
+  uint8_t *buf = malloc((size_t)total);
+  if (!buf) return NULL;
+
+  size_t pos = 0;
+  memcpy(buf + pos, &delta->new_file_size, sizeof(uint64_t));
+  pos += sizeof(uint64_t);
+  memcpy(buf + pos, &delta->instruction_count, sizeof(uint32_t));
+  pos += sizeof(uint32_t);
+
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    uint8_t type = (uint8_t)delta->instructions[i].type;
+    memcpy(buf + pos, &type, sizeof(uint8_t));
+    pos += sizeof(uint8_t);
+
+    if (delta->instructions[i].type == DELTA_INSTR_BLOCK_MATCH) {
+      memcpy(buf + pos, &delta->instructions[i].match.block_index, sizeof(uint32_t));
+      pos += sizeof(uint32_t);
+      memcpy(buf + pos, &delta->instructions[i].match.block_offset, sizeof(uint32_t));
+      pos += sizeof(uint32_t);
+      memcpy(buf + pos, &delta->instructions[i].match.length, sizeof(uint32_t));
+      pos += sizeof(uint32_t);
+    } else {
+      memcpy(buf + pos, &delta->instructions[i].literal.length, sizeof(uint32_t));
+      pos += sizeof(uint32_t);
+      memcpy(buf + pos, delta->instructions[i].literal.data,
+             delta->instructions[i].literal.length);
+      pos += delta->instructions[i].literal.length;
+    }
+  }
+
+  return data_create(buf, (size_t)total);
+}
+
+Delta *delta_deserialize(const Data *data) {
+  if (!data || data->size < sizeof(uint64_t) + sizeof(uint32_t))
+    return NULL;
+
+  const uint8_t *buf = (const uint8_t *)data->data;
+  size_t pos = 0;
+
+  Delta *delta = malloc(sizeof(Delta));
+  if (!delta) return NULL;
+
+  memcpy(&delta->new_file_size, buf + pos, sizeof(uint64_t));
+  pos += sizeof(uint64_t);
+  memcpy(&delta->instruction_count, buf + pos, sizeof(uint32_t));
+  pos += sizeof(uint32_t);
+
+  delta->instructions = malloc(delta->instruction_count * sizeof(DeltaInstruction));
+  if (!delta->instructions) {
+    free(delta);
+    return NULL;
+  }
+
+  delta->delta_size = 0;
+
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (pos >= data->size) {
+      for (uint32_t k = 0; k < i; k++) {
+        if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
+          free(delta->instructions[k].literal.data);
+      }
+      free(delta->instructions);
+      free(delta);
+      return NULL;
+    }
+
+    uint8_t type;
+    memcpy(&type, buf + pos, sizeof(uint8_t));
+    pos += sizeof(uint8_t);
+
+    delta->delta_size += 1;
+
+    if (type == DELTA_OP_BLOCK_MATCH) {
+      if (pos + sizeof(uint32_t) * 3 > data->size) {
+        free(delta->instructions);
+        free(delta);
+        return NULL;
+      }
+      delta->instructions[i].type = DELTA_INSTR_BLOCK_MATCH;
+      memcpy(&delta->instructions[i].match.block_index, buf + pos, sizeof(uint32_t));
+      pos += sizeof(uint32_t);
+      memcpy(&delta->instructions[i].match.block_offset, buf + pos, sizeof(uint32_t));
+      pos += sizeof(uint32_t);
+      memcpy(&delta->instructions[i].match.length, buf + pos, sizeof(uint32_t));
+      pos += sizeof(uint32_t);
+      delta->delta_size += sizeof(uint32_t) * 3;
+    } else if (type == DELTA_OP_LITERAL) {
+      if (pos + sizeof(uint32_t) > data->size) {
+        free(delta->instructions);
+        free(delta);
+        return NULL;
+      }
+      memcpy(&delta->instructions[i].literal.length, buf + pos, sizeof(uint32_t));
+      pos += sizeof(uint32_t);
+
+      uint32_t lit_len = delta->instructions[i].literal.length;
+      if (pos + lit_len > data->size) {
+        free(delta->instructions);
+        free(delta);
+        return NULL;
+      }
+      delta->instructions[i].literal.data = malloc(lit_len);
+      if (!delta->instructions[i].literal.data) {
+        free(delta->instructions);
+        free(delta);
+        return NULL;
+      }
+      memcpy(delta->instructions[i].literal.data, buf + pos, lit_len);
+      pos += lit_len;
+      delta->delta_size += sizeof(uint32_t) + lit_len;
+    } else {
+      for (uint32_t k = 0; k < i; k++) {
+        if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
+          free(delta->instructions[k].literal.data);
+      }
+      free(delta->instructions);
+      free(delta);
+      return NULL;
+    }
+  }
+
+  return delta;
+}
+
+void *delta_apply(const void *old_data, uint64_t old_size, const Delta *delta,
+                  uint32_t block_size) {
+  if (!old_data || !delta) return NULL;
+
+  void *output = malloc((size_t)delta->new_file_size);
+  if (!output) return NULL;
+
+  uint8_t *out = (uint8_t *)output;
+  uint8_t *old = (uint8_t *)old_data;
+  uint64_t out_pos = 0;
+
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (delta->instructions[i].type == DELTA_INSTR_BLOCK_MATCH) {
+      uint64_t src_offset = (uint64_t)delta->instructions[i].match.block_index *
+                             block_size;
+      src_offset += delta->instructions[i].match.block_offset;
+      uint32_t len = delta->instructions[i].match.length;
+
+      if (src_offset + len > old_size) {
+        free(output);
+        return NULL;
+      }
+      memcpy(out + out_pos, old + src_offset, len);
+      out_pos += len;
+    } else {
+      uint32_t len = delta->instructions[i].literal.length;
+      memcpy(out + out_pos, delta->instructions[i].literal.data, len);
+      out_pos += len;
+    }
+  }
+
+  if (out_pos != delta->new_file_size) {
+    free(output);
+    return NULL;
+  }
+
+  return output;
+}
+
+void delta_destroy(Delta *delta) {
+  if (!delta) return;
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (delta->instructions[i].type == DELTA_INSTR_LITERAL)
+      free(delta->instructions[i].literal.data);
+  }
+  free(delta->instructions);
+  free(delta);
+}
+
+bool delta_should_attempt(uint64_t old_size, uint64_t new_size, uint64_t max_file_size) {
+  if (old_size < DELTA_MIN_FILE_SIZE || new_size < DELTA_MIN_FILE_SIZE)
+    return false;
+  if (old_size > max_file_size || new_size > max_file_size)
+    return false;
+  double large = (old_size > new_size) ? (double)old_size : (double)new_size;
+  double small = (old_size > new_size) ? (double)new_size : (double)old_size;
+  if (small == 0 || large / small > DELTA_MAX_SIZE_RATIO)
+    return false;
+  return true;
+}
+
+bool delta_is_worthwhile(const Delta *delta, uint64_t new_file_size) {
+  if (!delta || delta->instruction_count == 0) return false;
+
+  bool has_match = false;
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (delta->instructions[i].type == DELTA_INSTR_BLOCK_MATCH) {
+      has_match = true;
+      break;
+    }
+  }
+  if (!has_match) return false;
+
+  double ratio = (double)delta->delta_size / (double)new_file_size;
+  return ratio < DELTA_FALLBACK_RATIO;
+}
diff --git a/src/shared/delta.h b/src/shared/delta.h
new file mode 100644
index 0000000..209162d
--- /dev/null
+++ b/src/shared/delta.h
@@ -0,0 +1,81 @@
+#ifndef DELTA_H
+#define DELTA_H
+
+#include "data.h"
+#include <stdbool.h>
+#include <stdint.h>
+#include <stddef.h>
+
+#define DELTA_BLOCK_SIZE_DEFAULT  8192U
+#define DELTA_BLOCK_SIZE_MIN      1024U
+#define DELTA_BLOCK_SIZE_MAX      65536U
+#define DELTA_MIN_FILE_SIZE       16384ULL
+#define DELTA_MAX_FILE_SIZE       (256ULL * 1024 * 1024)
+#define DELTA_MAX_SIZE_RATIO      10.0
+#define DELTA_FALLBACK_RATIO      0.7
+#define DELTA_ADLER32_MODULUS     65521U
+
+#define DELTA_OP_BLOCK_MATCH  0x01
+#define DELTA_OP_LITERAL      0x02
+
+typedef struct {
+  uint32_t adler32;
+  uint32_t xxhash;
+} DeltaBlockSig;
+
+typedef struct {
+  uint64_t file_size;
+  uint32_t block_size;
+  uint32_t block_count;
+  DeltaBlockSig *blocks;
+} DeltaSignature;
+
+typedef enum {
+  DELTA_INSTR_BLOCK_MATCH = 0x01,
+  DELTA_INSTR_LITERAL     = 0x02
+} DeltaInstrType;
+
+typedef struct {
+  DeltaInstrType type;
+  union {
+    struct {
+      uint32_t block_index;
+      uint32_t block_offset;
+      uint32_t length;
+    } match;
+    struct {
+      uint8_t *data;
+      uint32_t length;
+    } literal;
+  };
+} DeltaInstruction;
+
+typedef struct {
+  uint64_t new_file_size;
+  uint32_t instruction_count;
+  DeltaInstruction *instructions;
+  uint64_t delta_size;
+} Delta;
+
+DeltaSignature *delta_signature_create(const void *old_file_data,
+                                        uint64_t old_file_size,
+                                        uint32_t block_size);
+Data *delta_signature_serialize(const DeltaSignature *sig);
+DeltaSignature *delta_signature_deserialize(const Data *data);
+void delta_signature_destroy(DeltaSignature *sig);
+
+Delta *delta_compute(const void *new_file_data, uint64_t new_file_size,
+                      const DeltaSignature *sig, uint32_t block_size);
+Data *delta_serialize(const Delta *delta);
+Delta *delta_deserialize(const Data *data);
+void *delta_apply(const void *old_data, uint64_t old_size, const Delta *delta,
+                  uint32_t block_size);
+void delta_destroy(Delta *delta);
+
+bool delta_should_attempt(uint64_t old_size, uint64_t new_size, uint64_t max_file_size);
+bool delta_is_worthwhile(const Delta *delta, uint64_t new_file_size);
+
+uint32_t delta_adler32(const void *data, uint32_t len);
+uint32_t delta_xxhash32(const void *data, uint32_t len);
+
+#endif
diff --git a/src/shared/file.c b/src/shared/file.c
index b73c12d..6eeabf5 100644
--- a/src/shared/file.c
+++ b/src/shared/file.c
@@ -10,6 +10,7 @@
 #include <unistd.h>
 
 #include "compression.h"
+#include "delta.h"
 #include "log.h"
 #include "config.h"
 #include "data.h"
@@ -136,13 +137,154 @@ bool file_save_to_disk(const char* root_directory, File* file) {
   return ok;
 }
 
+static void *old_data_from_path(const char *full_path, unsigned long long old_size) {
+  void *data = malloc((size_t)old_size);
+  if (!data) return NULL;
+  FILE *fp = fopen(full_path, "rb");
+  if (!fp) { free(data); return NULL; }
+  size_t nread = fread(data, 1, (size_t)old_size, fp);
+  fclose(fp);
+  if (nread != (size_t)old_size) { free(data); return NULL; }
+  return data;
+}
+
+static File *receive_delta_file(int fd, const Config *config, const char *check_path,
+                                 void *old_data, unsigned long long old_size) {
+  if (!old_data) return NULL;
+
+  DeltaSignature *sig = delta_signature_create(old_data, old_size,
+                                                config->delta_block_size);
+  if (!sig) { free(old_data); return NULL; }
+
+  Data *sig_data = delta_signature_serialize(sig);
+  if (!sig_data) { delta_signature_destroy(sig); free(old_data); return NULL; }
+
+  bool sig_sent = send_status(fd, STATUS_DELTA_SIGNATURE) &&
+                  send_data(fd, sig_data);
+  data_destroy(sig_data);
+
+  if (!sig_sent) { delta_signature_destroy(sig); free(old_data); return NULL; }
+
+  Status resp;
+  if (!receive_status(fd, &resp)) {
+    delta_signature_destroy(sig);
+    free(old_data);
+    return NULL;
+  }
+
+  if (resp == STATUS_DELTA_DATA) {
+    Data *delta_data = receive_data(fd);
+    if (!delta_data) {
+      delta_signature_destroy(sig);
+      free(old_data);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
+
+    Data *raw_delta = delta_data;
+    if (config->use_compression) {
+      raw_delta = data_decompress(delta_data);
+      data_destroy(delta_data);
+      if (!raw_delta) {
+        free(old_data);
+        delta_signature_destroy(sig);
+        send_status(fd, STATUS_ERROR);
+        return NULL;
+      }
+    }
+
+    Delta *delta = delta_deserialize(raw_delta);
+    data_destroy(raw_delta);
+    if (!delta) {
+      free(old_data);
+      delta_signature_destroy(sig);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
+
+    void *new_data = delta_apply(old_data, old_size, delta,
+                                  config->delta_block_size);
+    uint64_t new_size = delta->new_file_size;
+    delta_destroy(delta);
+
+    if (!new_data) {
+      free(old_data);
+      delta_signature_destroy(sig);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
+
+    File *file = file_create(check_path);
+    if (!file) {
+      free(new_data);
+      free(old_data);
+      delta_signature_destroy(sig);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
+
+    if (config->use_metadata) {
+      int meta_ok = 1;
+      file->metadata = metadata_receive(fd, &meta_ok);
+      if (!meta_ok) {
+        file_destroy(file);
+        free(new_data);
+        free(old_data);
+        delta_signature_destroy(sig);
+        send_status(fd, STATUS_ERROR);
+        return NULL;
+      }
+    }
+
+    data_destroy(file->data);
+    file->data = data_create(new_data, (size_t)new_size);
+
+    free(old_data);
+    delta_signature_destroy(sig);
+    return file;
+  }
+
+  if (resp == STATUS_NEXT) {
+    delta_signature_destroy(sig);
+    free(old_data);
+
+    File *file = file_create(check_path);
+    if (!file) { send_status(fd, STATUS_ERROR); return NULL; }
+
+    if (config->use_metadata) {
+      int meta_ok = 1;
+      file->metadata = metadata_receive(fd, &meta_ok);
+      if (!meta_ok) { file_destroy(file); send_status(fd, STATUS_ERROR); return NULL; }
+    }
+
+    Data *file_data = receive_data(fd);
+    if (file_data == NULL) {
+      file_destroy(file);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
+
+    if (config->use_compression) {
+      Data *uncompressed = data_decompress(file_data);
+      data_destroy(file_data);
+      if (uncompressed == NULL) { file_destroy(file); send_status(fd, STATUS_ERROR); return NULL; }
+      file_data = uncompressed;
+    }
+
+    data_destroy(file->data);
+    file->data = file_data;
+    return file;
+  }
+
+  delta_signature_destroy(sig);
+  free(old_data);
+  return NULL;
+}
+
 File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   *skipped = false;
   char* check_path = receive_str(fd);
-  if (check_path == NULL) {
-    send_status(fd, STATUS_ERROR);
-    return NULL;
-  }
+  if (check_path == NULL) { send_status(fd, STATUS_ERROR); return NULL; }
 
   unsigned long long check_size;
   long long check_mtime;
@@ -155,34 +297,48 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
 
   char* full_path = path_cat(config->receive_root_directory, check_path);
   struct stat st;
-  bool match = false;
-  if (full_path && stat(full_path, &st) == 0 && (unsigned long long)st.st_size == check_size &&
-      (long long)st.st_mtime == check_mtime) {
-    match = true;
-  }
-  free(full_path);
+  bool has_old_file = (full_path && stat(full_path, &st) == 0);
+  unsigned long long old_size = has_old_file ? (unsigned long long)st.st_size : 0;
+
+  bool match = has_old_file &&
+               (unsigned long long)st.st_size == check_size &&
+               (long long)st.st_mtime == check_mtime;
 
   if (match) {
-    if (!send_status(fd, STATUS_OK)) {
-      free(check_path);
-      return NULL;
-    }
+    if (!send_status(fd, STATUS_OK)) { free(full_path); free(check_path); return NULL; }
+    free(full_path);
     free(check_path);
     *skipped = true;
     return NULL;
   }
 
-  if (!send_status(fd, STATUS_NEXT)) {
-    free(check_path);
-    return NULL;
+  bool try_delta = config->use_delta && has_old_file &&
+                   delta_should_attempt(old_size, check_size, config->delta_max_file_size);
+
+  if (try_delta) {
+    void *old_data = old_data_from_path(full_path, old_size);
+    File *delta_file = receive_delta_file(fd, config, check_path,
+                                           old_data, old_size);
+    if (delta_file) {
+      free(full_path);
+      free(check_path);
+      return delta_file;
+    }
+    try_delta = false;
+  }
+
+  if (!try_delta) {
+    if (!send_status(fd, STATUS_NEXT)) {
+      free(full_path);
+      free(check_path);
+      return NULL;
+    }
   }
 
   File* file = file_create(check_path);
   free(check_path);
-  if (file == NULL) {
-    send_status(fd, STATUS_ERROR);
-    return NULL;
-  }
+  free(full_path);
+  if (file == NULL) { send_status(fd, STATUS_ERROR); return NULL; }
 
   if (config->use_metadata) {
     int meta_ok = 1;
@@ -242,11 +398,10 @@ bool to_disk(const char* path, const void* data, unsigned long long data_size) {
   return true;
 }
 
-bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, bool send_path) {
-  if (send_path && !send_str(file_descriptor, file->path))
-    return false;
-  if (use_metadata && !metadata_send(file_descriptor, file->metadata))
-    return false;
+bool file_send_sendfile(File *file, int file_descriptor, bool use_metadata, int compression_level, bool send_path) {
+  (void)compression_level;
+  if (send_path && !send_str(file_descriptor, file->path)) return false;
+  if (use_metadata && !metadata_send(file_descriptor, file->metadata)) return false;
 
   int fd = open(file->path, O_RDONLY);
   if (fd == -1) {
diff --git a/src/shared/file.h b/src/shared/file.h
index f2d692e..60e799c 100644
--- a/src/shared/file.h
+++ b/src/shared/file.h
@@ -26,7 +26,8 @@ bool file_load_data(File* file);
 File* file_receive(const Config* config, int file_descriptor);
 bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
                             int compression_level, bool send_path);
-bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, bool send_path);
+bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata,
+                        int compression_level, bool send_path);
 size_t file_content_to_buffer(File* file);
 FileMetadata* file_metadata_create(const struct stat* stats);
 void file_metadata_destroy(void* metadata);
diff --git a/src/shared/protocol.c b/src/shared/protocol.c
index d050869..4d91ce6 100644
--- a/src/shared/protocol.c
+++ b/src/shared/protocol.c
@@ -128,6 +128,10 @@ static const char* status_to_string(Status status) {
     return "CHUNK";
   case STATUS_CHECK:
     return "CHECK";
+  case STATUS_DELTA_SIGNATURE:
+    return "DELTA_SIGNATURE";
+  case STATUS_DELTA_DATA:
+    return "DELTA_DATA";
   default:
     return "UNKNOWN";
   }
diff --git a/src/shared/protocol.h b/src/shared/protocol.h
index 0289ab4..a7854f1 100644
--- a/src/shared/protocol.h
+++ b/src/shared/protocol.h
@@ -15,7 +15,9 @@ enum NET_STATUS {
   STATUS_NEXT,
   STATUS_CHUNK,
   STATUS_MANIFEST,
-  STATUS_CHECK
+  STATUS_CHECK,
+  STATUS_DELTA_SIGNATURE,
+  STATUS_DELTA_DATA
 };
 
 void io_set_fds(int read_fd, int write_fd);
diff --git a/tests/runner.c b/tests/runner.c
index b05f007..775bc9c 100644
--- a/tests/runner.c
+++ b/tests/runner.c
@@ -2,6 +2,7 @@
 #include "test_chunk.h"
 #include "test_compression.h"
 #include "test_config.h"
+#include "test_delta.h"
 #include "test_queue.h"
 #include "test_scanner.h"
 #include "test_shared_utils.h"
@@ -23,6 +24,7 @@ int main() {
   RUN_TEST(test_config);
   RUN_TEST(test_compression);
   RUN_TEST(test_scanner);
+  RUN_TEST(test_delta);
 
   printf("\n\033[1;36m=== TEST SUMMARY ===\033[0m\n");
   printf("Total Tests Run: %d\n", tests_run);
diff --git a/tests/test_delta.c b/tests/test_delta.c
new file mode 100644
index 0000000..4fbbe26
--- /dev/null
+++ b/tests/test_delta.c
@@ -0,0 +1,334 @@
+#include "test_utils.h"
+#include "delta.h"
+#include <string.h>
+#include <stdlib.h>
+
+static void test_adler32_basic() {
+  const char *data = "Hello";
+  uint32_t h = delta_adler32(data, 5);
+  EXPECT_TRUE(h != 0);
+  uint32_t h2 = delta_adler32(data, 5);
+  EXPECT_EQ_INT((int)h, (int)h2);
+}
+
+static void test_adler32_different_data() {
+  const char *a = "AAAA";
+  const char *b = "BBBB";
+  uint32_t ha = delta_adler32(a, 4);
+  uint32_t hb = delta_adler32(b, 4);
+  EXPECT_TRUE(ha != hb);
+}
+
+static void test_xxhash32_basic() {
+  const char *data = "Hello";
+  uint32_t h = delta_xxhash32(data, 5);
+  EXPECT_TRUE(h != 0);
+  uint32_t h2 = delta_xxhash32(data, 5);
+  EXPECT_EQ_INT((int)h, (int)h2);
+}
+
+static void test_xxhash32_different_data() {
+  const char *a = "AAAA";
+  const char *b = "BBBB";
+  uint32_t ha = delta_xxhash32(a, 4);
+  uint32_t hb = delta_xxhash32(b, 4);
+  EXPECT_TRUE(ha != hb);
+}
+
+static void test_signature_roundtrip() {
+  char old_data[4096];
+  for (int i = 0; i < 4096; i++) old_data[i] = (char)(i % 256);
+
+  DeltaSignature *sig = delta_signature_create(old_data, 4096, 1024);
+  EXPECT_NOT_NULL(sig);
+  EXPECT_EQ_INT((int)sig->block_count, 4);
+  EXPECT_EQ_INT((int)sig->block_size, 1024);
+
+  Data *serialized = delta_signature_serialize(sig);
+  EXPECT_NOT_NULL(serialized);
+
+  DeltaSignature *deserialized = delta_signature_deserialize(serialized);
+  EXPECT_NOT_NULL(deserialized);
+  EXPECT_EQ_INT((int)deserialized->block_count, (int)sig->block_count);
+  EXPECT_EQ_INT((int)deserialized->block_size, (int)sig->block_size);
+
+  for (uint32_t i = 0; i < sig->block_count; i++) {
+    EXPECT_EQ_INT((int)deserialized->blocks[i].adler32, (int)sig->blocks[i].adler32);
+    EXPECT_EQ_INT((int)deserialized->blocks[i].xxhash, (int)sig->blocks[i].xxhash);
+  }
+
+  delta_signature_destroy(sig);
+  data_destroy(serialized);
+  delta_signature_destroy(deserialized);
+}
+
+static void test_delta_identical_files() {
+  char data[2048];
+  for (int i = 0; i < 2048; i++) data[i] = (char)(i % 128);
+
+  DeltaSignature *sig = delta_signature_create(data, 2048, 512);
+  EXPECT_NOT_NULL(sig);
+
+  Delta *delta = delta_compute(data, 2048, sig, 512);
+  EXPECT_NOT_NULL(delta);
+
+  bool has_match = false;
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (delta->instructions[i].type == DELTA_INSTR_BLOCK_MATCH) {
+      has_match = true;
+      break;
+    }
+  }
+  EXPECT_TRUE(has_match);
+
+  bool all_match = true;
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (delta->instructions[i].type != DELTA_INSTR_BLOCK_MATCH) {
+      all_match = false;
+      break;
+    }
+  }
+  EXPECT_TRUE(all_match);
+
+  delta_signature_destroy(sig);
+  delta_destroy(delta);
+}
+
+static void test_delta_small_edit() {
+  char old_data[4096];
+  char new_data[4096];
+  for (int i = 0; i < 4096; i++) {
+    old_data[i] = (char)(i % 256);
+    new_data[i] = old_data[i];
+  }
+  new_data[100] = 'X';
+  new_data[101] = 'Y';
+  new_data[102] = 'Z';
+
+  DeltaSignature *sig = delta_signature_create(old_data, 4096, 1024);
+  EXPECT_NOT_NULL(sig);
+
+  Delta *delta = delta_compute(new_data, 4096, sig, 1024);
+  EXPECT_NOT_NULL(delta);
+
+  uint64_t total_literal = 0;
+  uint32_t match_count = 0;
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (delta->instructions[i].type == DELTA_INSTR_LITERAL)
+      total_literal += delta->instructions[i].literal.length;
+    else
+      match_count++;
+  }
+  EXPECT_TRUE(match_count > 0);
+  EXPECT_TRUE(total_literal < 4096);
+
+  void *reconstructed = delta_apply(old_data, 4096, delta, 1024);
+  EXPECT_NOT_NULL(reconstructed);
+  EXPECT_EQ_INT(memcmp(reconstructed, new_data, 4096), 0);
+
+  free(reconstructed);
+  delta_signature_destroy(sig);
+  delta_destroy(delta);
+}
+
+static void test_delta_completely_different() {
+  char old_data[4096];
+  char new_data[4096];
+  for (int i = 0; i < 4096; i++) {
+    old_data[i] = (char)(i * 7 + 3);
+    new_data[i] = (char)(i * 13 + 97);
+  }
+
+  DeltaSignature *sig = delta_signature_create(old_data, 4096, 1024);
+  EXPECT_NOT_NULL(sig);
+
+  Delta *delta = delta_compute(new_data, 4096, sig, 1024);
+  EXPECT_NOT_NULL(delta);
+
+  bool has_match = false;
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (delta->instructions[i].type == DELTA_INSTR_BLOCK_MATCH) {
+      has_match = true;
+      break;
+    }
+  }
+  EXPECT_TRUE(!has_match);
+  EXPECT_TRUE(!delta_is_worthwhile(delta, 4096));
+
+  delta_signature_destroy(sig);
+  delta_destroy(delta);
+}
+
+static void test_delta_serialize_roundtrip() {
+  char old_data[4096];
+  char new_data[4096];
+  for (int i = 0; i < 4096; i++) {
+    old_data[i] = (char)(i % 256);
+    new_data[i] = old_data[i];
+  }
+  new_data[500] = 'A';
+  new_data[501] = 'B';
+
+  DeltaSignature *sig = delta_signature_create(old_data, 4096, 1024);
+  Delta *delta = delta_compute(new_data, 4096, sig, 1024);
+  EXPECT_NOT_NULL(delta);
+
+  Data *serialized = delta_serialize(delta);
+  EXPECT_NOT_NULL(serialized);
+
+  Delta *deserialized = delta_deserialize(serialized);
+  EXPECT_NOT_NULL(deserialized);
+  EXPECT_EQ_INT((int)deserialized->new_file_size, (int)delta->new_file_size);
+  EXPECT_EQ_INT((int)deserialized->instruction_count, (int)delta->instruction_count);
+
+  void *reconstructed = delta_apply(old_data, 4096, deserialized, 1024);
+  EXPECT_NOT_NULL(reconstructed);
+  EXPECT_EQ_INT(memcmp(reconstructed, new_data, 4096), 0);
+
+  free(reconstructed);
+  data_destroy(serialized);
+  delta_destroy(deserialized);
+  delta_signature_destroy(sig);
+  delta_destroy(delta);
+}
+
+static void test_delta_file_growth() {
+  char old_data[2048];
+  char new_data[3072];
+  for (int i = 0; i < 2048; i++) old_data[i] = (char)(i % 256);
+  memcpy(new_data, old_data, 2048);
+  for (int i = 2048; i < 3072; i++) new_data[i] = (char)(i % 256);
+
+  DeltaSignature *sig = delta_signature_create(old_data, 2048, 512);
+  EXPECT_NOT_NULL(sig);
+
+  Delta *delta = delta_compute(new_data, 3072, sig, 512);
+  EXPECT_NOT_NULL(delta);
+
+  void *reconstructed = delta_apply(old_data, 2048, delta, 512);
+  EXPECT_NOT_NULL(reconstructed);
+  EXPECT_EQ_INT(delta->new_file_size, 3072);
+  EXPECT_EQ_INT(memcmp(reconstructed, new_data, 3072), 0);
+
+  free(reconstructed);
+  delta_signature_destroy(sig);
+  delta_destroy(delta);
+}
+
+static void test_delta_file_shrink() {
+  char old_data[3072];
+  char new_data[2048];
+  for (int i = 0; i < 3072; i++) old_data[i] = (char)(i % 256);
+  for (int i = 0; i < 2048; i++) new_data[i] = old_data[i];
+
+  DeltaSignature *sig = delta_signature_create(old_data, 3072, 512);
+  EXPECT_NOT_NULL(sig);
+
+  Delta *delta = delta_compute(new_data, 2048, sig, 512);
+  EXPECT_NOT_NULL(delta);
+
+  void *reconstructed = delta_apply(old_data, 3072, delta, 512);
+  EXPECT_NOT_NULL(reconstructed);
+  EXPECT_EQ_INT(delta->new_file_size, 2048);
+  EXPECT_EQ_INT(memcmp(reconstructed, new_data, 2048), 0);
+
+  free(reconstructed);
+  delta_signature_destroy(sig);
+  delta_destroy(delta);
+}
+
+static void test_should_attempt() {
+  EXPECT_TRUE(delta_should_attempt(100000, 100000, DELTA_MAX_FILE_SIZE));
+  EXPECT_TRUE(!delta_should_attempt(100, 100, DELTA_MAX_FILE_SIZE));
+  EXPECT_TRUE(!delta_should_attempt(100000, 10, DELTA_MAX_FILE_SIZE));
+  EXPECT_TRUE(!delta_should_attempt(300000000, 300000000, DELTA_MAX_FILE_SIZE));
+  EXPECT_TRUE(delta_should_attempt(50000, 60000, DELTA_MAX_FILE_SIZE));
+  EXPECT_TRUE(!delta_should_attempt(50000, 600000, DELTA_MAX_FILE_SIZE));
+  EXPECT_TRUE(delta_should_attempt(50000, 60000, 500000));
+  EXPECT_TRUE(!delta_should_attempt(100000, 100000, 50000));
+}
+
+static void test_is_worthwhile() {
+  Delta d;
+  d.instruction_count = 1;
+  DeltaInstruction instr;
+  instr.type = DELTA_INSTR_BLOCK_MATCH;
+  d.instructions = &instr;
+
+  d.delta_size = 100;
+  EXPECT_TRUE(delta_is_worthwhile(&d, 1000));
+
+  d.delta_size = 800;
+  EXPECT_TRUE(!delta_is_worthwhile(&d, 1000));
+
+  d.instruction_count = 1;
+  instr.type = DELTA_INSTR_LITERAL;
+  EXPECT_TRUE(!delta_is_worthwhile(&d, 1000));
+
+  EXPECT_TRUE(!delta_is_worthwhile(NULL, 1000));
+}
+
+static void test_large_file_delta() {
+  uint32_t block_size = 8192;
+  uint64_t old_size = 200000;
+  uint64_t new_size = 200000;
+
+  void *old_data = malloc((size_t)old_size);
+  void *new_data = malloc((size_t)new_size);
+  EXPECT_TRUE(old_data != NULL && new_data != NULL);
+
+  for (uint64_t i = 0; i < old_size; i++)
+    ((uint8_t *)old_data)[i] = (uint8_t)(i % 251);
+  memcpy(new_data, old_data, (size_t)old_size);
+
+  uint64_t offset = 100000;
+  uint32_t change_len = 4096;
+  for (uint32_t i = 0; i < change_len; i++)
+    ((uint8_t *)new_data)[offset + i] = (uint8_t)((i * 7 + 13) % 256);
+
+  DeltaSignature *sig = delta_signature_create(old_data, old_size, block_size);
+  EXPECT_TRUE(sig != NULL);
+  EXPECT_TRUE(sig->block_count == (uint32_t)((old_size + block_size - 1) / block_size));
+
+  Delta *delta = delta_compute(new_data, new_size, sig, block_size);
+  EXPECT_TRUE(delta != NULL);
+
+  uint64_t total_literal = 0;
+  uint32_t match_count = 0;
+  for (uint32_t i = 0; i < delta->instruction_count; i++) {
+    if (delta->instructions[i].type == DELTA_INSTR_LITERAL)
+      total_literal += delta->instructions[i].literal.length;
+    else
+      match_count++;
+  }
+  EXPECT_TRUE(match_count > 0);
+  EXPECT_TRUE(delta->delta_size < new_size / 2);
+
+  void *result = delta_apply(old_data, old_size, delta, block_size);
+  EXPECT_TRUE(result != NULL);
+  EXPECT_TRUE(delta->new_file_size == new_size);
+  EXPECT_TRUE(memcmp(result, new_data, (size_t)new_size) == 0);
+
+  free(result);
+  delta_destroy(delta);
+  delta_signature_destroy(sig);
+  free(old_data);
+  free(new_data);
+}
+
+void test_delta() {
+  test_adler32_basic();
+  test_adler32_different_data();
+  test_xxhash32_basic();
+  test_xxhash32_different_data();
+  test_signature_roundtrip();
+  test_delta_identical_files();
+  test_delta_small_edit();
+  test_delta_completely_different();
+  test_delta_serialize_roundtrip();
+  test_delta_file_growth();
+  test_delta_file_shrink();
+  test_should_attempt();
+  test_is_worthwhile();
+  test_large_file_delta();
+}
diff --git a/tests/test_delta.h b/tests/test_delta.h
new file mode 100644
index 0000000..2ea7aec
--- /dev/null
+++ b/tests/test_delta.h
@@ -0,0 +1,6 @@
+#ifndef TEST_DELTA_H
+#define TEST_DELTA_H
+
+void test_delta(void);
+
+#endif