add get_x_offset

src/iir.rs

@@ -27,7 +27,7 @@ fn macc(y0: f32, x: &[f32], a: &[f32]) -> f32 {
 }
 
 impl IIR {
-    pub fn set_pi(&mut self, kp: f32, ki: f32, g: f32) -> Result<(), &'static str> {
+    pub fn set_pi(&mut self, kp: f32, ki: f32, g: f32) -> Result<(), &str> {
         let ki = copysign(ki, kp);
         let g = copysign(g, kp);
         let (a1, b0, b1) =
@@ -52,6 +52,14 @@ impl IIR {
         Ok(())
     }
 
+    pub fn get_x_offset(&self) -> Result<f32, &str> {
+        let b: f32 = self.ba[..3].iter().sum();
+        if abs(b) < f32::EPSILON {
+            return Err("b is zero")
+        }
+        Ok(self.y_offset/b)
+    }
+
     pub fn set_x_offset(&mut self, xo: f32) {
         let b: f32 = self.ba[..3].iter().sum();
         self.y_offset = xo*b;

src/main.rs

@@ -563,6 +563,7 @@ fn main() -> ! {
 
         IIR_CH[1].set_pi(-0.1, -10.*t, 0.).expect("bad coefficients");
         IIR_CH[1].set_x_offset(0.1*SCALE);
+        IIR_CH[1].get_x_offset().expect("bad coefficients");
     }
 
     cortex_m::interrupt::free(|cs| {