mtmd: add chunk save/load function (#26645)

* mtmd: add chunk save/load function

* nits

* add tests

* rn _MAX --> _COUNT

tests/test-mtmd-c-api.c

@@ -1,4 +1,6 @@
 #include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 #include <assert.h>
 
 #include "mtmd.h"
@@ -62,6 +64,72 @@ int main(void) {
         }
     }
 
+    // test chunk save/load round-trip
+    for (size_t i = 0; i < n_chunks; i++) {
+        const mtmd_input_chunk * chunk = mtmd_input_chunks_get(chunks, i);
+        assert(chunk != NULL);
+        enum mtmd_input_chunk_type type = mtmd_input_chunk_get_type(chunk);
+
+        // query the required buffer size (out_buf == NULL)
+        size_t expected_len = 0;
+        int32_t rc = mtmd_input_chunk_save(chunk, NULL, 0, &expected_len);
+        printf("    Chunk %zu: save query rc = %d, expected_len = %zu\n", i, rc, expected_len);
+        assert(rc == 0);
+        assert(expected_len > 0);
+
+        // saving into a too-small buffer must fail, not crash
+        char tiny_buf[1];
+        rc = mtmd_input_chunk_save(chunk, tiny_buf, sizeof(tiny_buf), NULL);
+        printf("    Chunk %zu: save into too-small buffer rc = %d (expect non-zero)\n", i, rc);
+        assert(rc != 0);
+
+        // save into a properly-sized buffer
+        char * buf = (char *) malloc(expected_len);
+        assert(buf != NULL);
+        rc = mtmd_input_chunk_save(chunk, buf, expected_len, NULL);
+        assert(rc == 0);
+
+        // loading from a truncated buffer must fail gracefully, not crash
+        if (expected_len > 1) {
+            mtmd_input_chunk * bad = mtmd_input_chunk_load(buf, expected_len - 1);
+            printf("    Chunk %zu: load from truncated buffer = %p (expect NULL)\n", i, (void *) bad);
+            assert(bad == NULL);
+        }
+
+        // load it back
+        mtmd_input_chunk * loaded = mtmd_input_chunk_load(buf, expected_len);
+        assert(loaded != NULL);
+
+        // metadata must match the original chunk
+        assert(mtmd_input_chunk_get_type(loaded) == type);
+        assert(mtmd_input_chunk_get_n_tokens(loaded) == mtmd_input_chunk_get_n_tokens(chunk));
+        assert(mtmd_input_chunk_get_n_pos(loaded) == mtmd_input_chunk_get_n_pos(chunk));
+
+        if (type == MTMD_INPUT_CHUNK_TYPE_TEXT) {
+            size_t n_tok_orig, n_tok_loaded;
+            const llama_token * tok_orig   = mtmd_input_chunk_get_tokens_text(chunk, &n_tok_orig);
+            const llama_token * tok_loaded = mtmd_input_chunk_get_tokens_text(loaded, &n_tok_loaded);
+            printf("    Chunk %zu: loaded %zu text tokens (orig %zu), first token %d (orig %d)\n",
+                i, n_tok_loaded, n_tok_orig,
+                n_tok_loaded > 0 ? tok_loaded[0] : -1,
+                n_tok_orig   > 0 ? tok_orig[0]   : -1);
+            assert(n_tok_orig == n_tok_loaded);
+            for (size_t j = 0; j < n_tok_orig; j++) {
+                assert(tok_orig[j] == tok_loaded[j]);
+            }
+        } else if (type == MTMD_INPUT_CHUNK_TYPE_IMAGE || type == MTMD_INPUT_CHUNK_TYPE_AUDIO) {
+            const char * id_orig   = mtmd_input_chunk_get_id(chunk);
+            const char * id_loaded = mtmd_input_chunk_get_id(loaded);
+            printf("    Chunk %zu: loaded id '%s' (orig '%s')\n", i, id_loaded, id_orig);
+            assert(id_orig != NULL && id_loaded != NULL);
+            assert(strcmp(id_orig, id_loaded) == 0);
+        }
+
+        mtmd_input_chunk_free(loaded);
+        free(buf);
+    }
+    printf("Chunk save/load round-trip OK\n");
+
     // Free the chunks
     mtmd_input_chunks_free(chunks);