Issue #3457133 by mfb, mistergroove: Programatically using file hash to...

Issue #3457133 by mfb, mistergroove: Programatically using file hash to compare an external (temp) file (documentation)

README.md

@@ -84,6 +84,32 @@ $fids = \Drupal::entityQuery('file')
 ```
 
 
+## Duplicate file detection APIs
+
+In addition to being enabled via field widget settings, or on a site-wide basis,
+the `FileHashDedupe` validator can also be programmatically added to an upload
+form, or executed on a file object:
+
+```php
+$validators = ['FileHashDedupe' => []];
+$violations = \Drupal::service('file.validator')->validate($file, $validators);
+```
+
+The `strict` and `original` boolean options can optionally be added to the
+`FileHashDedupe` validator settings array. If `strict` is enabled, temporary
+files will also be searched for duplicates, in addition to permanent files. If
+`original` is enabled (and the setting to store an additional "original" hash
+for each uploaded file is enabled), original file hashes will also be searched
+for duplicates, in addition to current file hashes.
+
+The `FileHash::duplicateLookup()` method can also be called directly, given one
+of the enabled hash algorithms and a file object, to return a file ID:
+
+```php
+$fid = \Drupal::service('filehash')->duplicateLookup('sha256', $file);
+```
+
+
 ## Visual output
 
 You can use the included Identicon field formatter to visualize each file hash