update: docs

README.md

@@ -2,20 +2,21 @@
 ![GitHub Release](https://img.shields.io/github/v/release/pnck/foo_input_ncm)
 ![GitHub Release Date](https://img.shields.io/github/release-date/pnck/foo_input_ncm)
 
-## Play NCM files directly with our favourite
+## Play NCM files directly with our favorite
 
 - On Windows
- ![screenshot-win](/doc/screenshot-win.png)
+ ![screenshot-win](/docs/screenshot-win.png)
 
 - Or on MacOS
- ![screenshot-mac](/doc/screenshot-mac.png)
+ ![screenshot-mac](/docs/screenshot-mac.png)
 
 
 ### Features
 - [x] Seamlessly load and play `.ncm` format files in _foobar2000_.
 - [x] Fully integrated with the powerful built-in _`metadb`_ system, enabling efficient searching and strong management capabilities.
 - [x] Unwrap/Convert `.ncm` to normal `.mp3` or `.flac` format with blazing speed.
-- [ ] ReplayGain and retagging support.
+- [x] Edit `replay gain`/`metadata`/`album art` in place.
+ - *Will be unrecognizable by the _NCM player_ after editing tags. Reset it by `Remove tags` utility. (Properties - Tools - Remove tags)
 
 ### Compatibility
 
@@ -35,6 +36,6 @@
 - [foo_input_msu](https://github.com/qwertymodo/foo_input_msu) by [qwertymodo](https://github.com/qwertymodo)
 - [foo_input_nemuc](https://github.com/hozuki/foo_input_nemuc) by [hozuki](https://github.com/hozuki)
 - [foo_input_spotify](https://github.com/FauxFaux/foo_input_spotify) by [FauxFaux](https://github.com/FauxFaux)
-- The very detailed [develop tutorio](http://yirkha.fud.cz/tmp/496351ef.tutorial-draft.html) for starters
+- The very detailed [development tutorial](http://yirkha.fud.cz/tmp/496351ef.tutorial-draft.html) for starters
 - Official component samples included in [foobar2000 sdk](https://www.foobar2000.org/SDK)
-- And the [ncmdump](https://github.com/anonymous5l/ncmdump) which encourages me doing a lot of research on the file structure
+- And the [ncmdump](https://github.com/anonymous5l/ncmdump) which encourages me to do a lot of research on the file structure

docs/reminders.md

@@ -0,0 +1,24 @@
+## Retagging
+
+By design, music meta information such as title/artist/album etc is embedded and re-encrypted into the `ncm` header. This decision was reached considering the following factors:
+
+- The audio content isn't _"clean"_. Meaning if we take the embedded tag of the audio as overwrites, some music might be "polluted" by the tags at the original state.
+
+- Audio content is not verified when played by _NCM player_. So there is no way to tell whether it's modified, nor to completely restore the file state.
+
+- Replay gain is written to the audio content because it's recoverable and **simply tested** that the file hash will turn back after clearing the custom gain information.
+
+- The meta information stored in the `ncm` header is essentially a standard stringified `JSON` object, allowing easy extension while preserving all original fields. Although modifying it would change the identification key, causing the _NCM player_ to refuse to playback, it's totally recoverable by deleting the overwrites.
+
+Currently, the downloaded `.ncm` files can be recognized and played by the _NCM player_ if:
+
+- It's put in a monitored directory.
+- The album art or the replay gain is edited, while other meta information isn't.
+- Other meta information is once edited, then cleared by `Remove tags` utility.
+
+## Extractor
+
+- The extraction menu is unavailable in old fb2k versions (v1.6), due to API incompatity.
+- The extraction process is multi-threaded. Spawn order: `UI => extractor(x1) => worker(xN)`.
+- The output file would be overwritten if existed, and no warning would be given in this case. See the comments at
+ `ncm_file::save_raw_audio()`.