btstack_util: implement btstack_virtual_memcpy that copies a chunk of data into a virtual buffer backed by a physical buffer.

src/btstack_util.c

@@ -559,3 +559,38 @@ void btstack_strcat(char * dst, uint16_t dst_size, const char * src){
     (void) memcpy( &dst[dst_len], src, bytes_to_copy);
     dst[dst_len + bytes_to_copy] = 0;
 }
+
+uint16_t btstack_virtual_memcpy(
+    const uint8_t * field_data, uint16_t field_len, uint16_t field_offset, // position of field in complete data block
+    uint8_t * buffer, uint16_t buffer_size, uint16_t buffer_offset){
+
+    uint16_t after_buffer = buffer_offset + buffer_size ;
+    // bail before buffer
+    if ((field_offset + field_len) < buffer_offset){
+        return 0;
+    }
+    // bail after buffer
+    if (field_offset >= after_buffer){
+        return 0;
+    }
+    // calc overlap
+    uint16_t bytes_to_copy = field_len;
+    
+    uint16_t skip_at_start = 0;
+    if (field_offset < buffer_offset){
+        skip_at_start = buffer_offset - field_offset;
+        bytes_to_copy -= skip_at_start;
+    }
+
+    uint16_t skip_at_end = 0;
+    if ((field_offset + field_len) > after_buffer){
+        skip_at_end = (field_offset + field_len) - after_buffer;
+        bytes_to_copy -= skip_at_end;
+    }
+    
+    btstack_assert((skip_at_end + skip_at_start) <= field_len);
+    btstack_assert(bytes_to_copy <= field_len);
+
+    memcpy(&buffer[(field_offset + skip_at_start) - buffer_offset], &field_data[skip_at_start], bytes_to_copy);
+    return bytes_to_copy;
+}

src/btstack_util.h

@@ -343,6 +343,36 @@ void btstack_strcat(char * dst, uint16_t dst_size, const char * src);
  */
 uint8_t btstack_clz(uint32_t value);
 
+/**
+ * @brief Copy chunk of data into a virtual buffer backed by a physical buffer.
+ * Used to provide chunk of data of larger buffer that is constructed on the fly, e.g. serializing data struct
+ * 
+ * For example, copy field2 to buffer, with buffer_offset = 11
+ * 
+ *           field1  field2  field3     field4        field5  filed6
+ * struct:  -------|-------|----------|-------------|-------|--------------
+ * buffer:             ------------------       
+ * result:             ----|         
+ * 
+ * When also copying field3 and field4 to buffer, with buffer_offset = 11
+ * 
+ *           field1  field2  field3     field4        field5  filed6
+ * struct:  -------|-------|----------|-------------|-------|--------------
+ * buffer:             ------------------       
+ * result:             ----|----------|--         
+ *
+ * @param field_data
+ * @param field_len
+ * @param field_offset  position of field in complete data block
+ * @param buffer_data
+ * @param buffer_len
+ * @param buffer_offset position of buffer in complete data block
+ * @return bytes_copied number of bytes actually stored in buffer
+ */
+uint16_t btstack_virtual_memcpy(
+    const uint8_t * field_data, uint16_t field_len, uint16_t field_offset, 
+    uint8_t * buffer, uint16_t buffer_size, uint16_t buffer_offset);
+
 
 /* API_END */