make length of stock id configurable.

config-dist.php

@@ -101,6 +101,19 @@ Setting('DEFAULT_PERMISSIONS', ['ADMIN']);
 // 1D (=> Code128) or 2D (=> DataMatrix)
 Setting('GROCYCODE_TYPE', '1D');
 
+// Length for stock id tags. Stock id values are unique
+// to their product, so the same value for two different
+// products is fine. The values are generated using
+// a strong hash, so for most instances 6 or 8 digits
+// is enough to make the likelyhood of collisions
+// acceptably tiny, a value like 16 means we could
+// generate unique entries for every tin of tomatoes
+// ever made.
+//
+// Why make it shorter? So you can print shorter
+// grocycode barcodes. But probably not shorter than 6.
+Setting('STOCK_ID_LENGTH', 16);
+
 // Label printer settings
 // This is the URI that grocy will POST to when asked to print a label
 Setting('LABEL_PRINTER_WEBHOOK', '');

services/StockService.php

@@ -165,7 +165,27 @@ class StockService extends BaseService
 				$transactionId = uniqid();
 			}
 
-			$stockId = uniqid();
+            // uniqid() basically returns a clock-like counter,
+            // so the characters in the string do not contribute
+            // equally to the uniqueness, so we can't just take
+            // a prefix or suffix of the value to get shorter codes.
+            // Instead, we hash it, and take a prefix. Because we're
+            // using a strong hash algorithm, the hash values will
+            // be very uniform, so a quite short prefix is enough
+            // to give a pretty good guarantee of uniqueness.
+            //
+            // Further, when we lookup stock entries, e.g. to
+            // consume them, we first lookup by product id,
+            // so the product id forms a distict namespace,
+            // and the collisions can only occur within entries
+            // for a product id.
+            //
+            $stockIdLength = 16;
+            if (GROCY_STOCK_ID_LENGTH)
+            {
+                $stockIdLength = GROCY_STOCK_ID_LENGTH;
+            }
+			$stockId = substr(hash("sha256", uniqid()), 0, $stockIdLength);
 
 			$logRow = $this->getDatabase()->stock_log()->createRow([
 				'product_id' => $productId,