Merge pull request #110 from sysprog21/fixedpoint-atan2

jserv · web-flow · commit 8a03ba42791b · 2025-10-18T01:31:13.000+08:00
Fix fixed-point conversion precision to avoid overflow
diff --git a/src/trig.c b/src/trig.c
@@ -148,7 +148,15 @@ static twin_angle_t twin_atan2_first_quadrant(twin_fixed_t y, twin_fixed_t x)
         }
     }
 
-    return (twin_angle_t) (double) angle / (32768.0) * TWIN_ANGLE_360;
+    /* Fixed-point conversion: angle / 32768 * TWIN_ANGLE_360
+     * Simplified: angle * 2^12 / 2^15 = angle / 8
+     * Use integer division (not right shift) to match original
+     * truncate-towards-zero behavior for negative values. Right shift would
+     * round towards negative infinity, causing precision errors for
+     * non-multiples of 8. Avoids overflow since no intermediate multiplication
+     * required.
+     */
+    return (twin_angle_t) (angle / 8);
 }
 
 twin_angle_t twin_atan2(twin_fixed_t y, twin_fixed_t x)

Original file line number	Diff line number	Diff line change
`@@ -148,7 +148,15 @@ static twin_angle_t twin_atan2_first_quadrant(twin_fixed_t y, twin_fixed_t x)`
`148`	`148`	`}`
`149`	`149`	`}`
`150`	`150`
`151`		`- return (twin_angle_t) (double) angle / (32768.0) * TWIN_ANGLE_360;`
	`151`	`+ /* Fixed-point conversion: angle / 32768 * TWIN_ANGLE_360`
	`152`	`+ * Simplified: angle * 2^12 / 2^15 = angle / 8`
	`153`	`+ * Use integer division (not right shift) to match original`
	`154`	`+ * truncate-towards-zero behavior for negative values. Right shift would`
	`155`	`+ * round towards negative infinity, causing precision errors for`
	`156`	`+ * non-multiples of 8. Avoids overflow since no intermediate multiplication`
	`157`	`+ * required.`
	`158`	`+ */`
	`159`	`+ return (twin_angle_t) (angle / 8);`
`152`	`160`	`}`
`153`	`161`
`154`	`162`	`twin_angle_t twin_atan2(twin_fixed_t y, twin_fixed_t x)`