From 80ed715f5a11411a3396e98fab7906c89884938b Mon Sep 17 00:00:00 2001
From: Matt Huszagh <huszaghmatt@gmail.com>
Date: Tue, 12 Jan 2021 16:07:04 -0800
Subject: [PATCH] shift sin/cos before demodulation product to avoid i64

---
 dsp/src/lib.rs |  6 ------
 src/main.rs    | 16 ++++++----------
 2 files changed, 6 insertions(+), 16 deletions(-)

diff --git a/dsp/src/lib.rs b/dsp/src/lib.rs
index 2f02601..3f090b5 100644
--- a/dsp/src/lib.rs
+++ b/dsp/src/lib.rs
@@ -20,12 +20,6 @@ pub fn shift_round(x: i32, shift: usize) -> i32 {
     (x + (1 << (shift - 1))) >> shift
 }
 
-/// TODO consolidate with `shift_round`.
-#[inline(always)]
-pub fn shift_round64(x: i64, shift: usize) -> i64 {
-    (x + (1 << (shift - 1))) >> shift
-}
-
 /// Integer division, round up half.
 ///
 /// # Arguments
diff --git a/src/main.rs b/src/main.rs
index 3d484d8..22fe534 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -101,7 +101,7 @@ use dac::{Dac0Output, Dac1Output};
 use dsp::{
     divide_round, iir, iir_int,
     pll::PLL,
-    shift_round, shift_round64,
+    shift_round,
     trig::{atan2, cossin},
 };
 use pounder::DdsOutput;
@@ -1088,15 +1088,11 @@ const APP: () = {
 
                 let mut signal = (0_i32, 0_i32);
 
-                // TODO should we shift cos/sin first to avoid i64?
-                signal.0 = shift_round64(
-                    adc_samples[0][i] as i16 as i64 * cos as i64,
-                    16,
-                ) as i32;
-                signal.1 = shift_round64(
-                    adc_samples[0][i] as i16 as i64 * sin as i64,
-                    16,
-                ) as i32;
+                // shift cos/sin before multiplying to avoid i64 multiplication
+                signal.0 =
+                    adc_samples[0][i] as i16 as i32 * shift_round(cos, 16);
+                signal.0 =
+                    adc_samples[0][i] as i16 as i32 * shift_round(sin, 16);
 
                 signal.0 =
                     iir_lockin.update(&mut iir_state_lockin[0], signal.0);