Fixed GH-17275: Fixed the calculation logic of dividend scale (#17279)

Fixes #17275
Closes #17279

Author: SakiTakamachi

NEWS

@@ -8,6 +8,8 @@ PHP                                                                        NEWS
     (Saki Takamachi)
   . Fixed bug GH-17064 (Correctly round rounding mode with zero edge case).
     (Saki Takamachi)
+  . Fixed bug GH-17275 (Fixed the calculation logic of dividend scale).
+    (Saki Takamachi)
 
 - Core:
   . Fixed bug OSS-Fuzz #382922236 (Duplicate dynamic properties in hooked object

ext/bcmath/libbcmath/src/div.c

@@ -427,24 +427,30 @@ bool bc_divide(bc_num numerator, bc_num divisor, bc_num *quot, size_t scale)
 		return true;
 	}
 
+	/* Length of numerator data that can be read */
+	size_t numerator_readable_len = numeratorend - numeratorptr + 1;
+
 	/* set scale to numerator */
 	if (numerator_scale > scale) {
 		size_t scale_diff = numerator_scale - scale;
 		if (numerator_bottom_extension > scale_diff) {
 			numerator_bottom_extension -= scale_diff;
 		} else {
 			numerator_bottom_extension = 0;
-			numeratorend -= scale_diff > numerator_top_extension ? scale_diff - numerator_top_extension : 0;
+			if (EXPECTED(numerator_readable_len > scale_diff)) {
+				numerator_readable_len -= scale_diff;
+				numeratorend -= scale_diff;
+			} else {
+				numerator_readable_len = 0;
+				numeratorend = numeratorptr;
+			}
 		}
 		numerator_top_extension = MIN(numerator_top_extension, scale);
 	} else {
 		numerator_bottom_extension += scale - numerator_scale;
 	}
 	numerator_scale = scale;
 
-	/* Length of numerator data that can be read */
-	size_t numerator_readable_len = numeratorend - numeratorptr + 1;
-
 	if (divisor_len > numerator_readable_len + numerator_bottom_extension) {
 		*quot = bc_copy_num(BCG(_zero_));
 		return true;

ext/bcmath/tests/gh17275.phpt

@@ -0,0 +1,14 @@
+--TEST--
+GH-17275 Incorrect result of bcdiv function
+--EXTENSIONS--
+bcmath
+--FILE--
+<?php
+var_dump(
+    bcdiv('0.03772321', '9650.0', 8),
+    bcdiv('0.03772321', '9650.0', 9),
+);
+?>
+--EXPECT--
+string(10) "0.00000390"
+string(11) "0.000003909"