From d0ea9631d2c9e0b1be5198061280a3894bce9287 Mon Sep 17 00:00:00 2001
From: Roderick van Domburg <roderick@vandomburg.net>
Date: Fri, 9 Apr 2021 19:31:26 +0200
Subject: [PATCH] Optimize requantizer to work in `f32`, then round

---
 audio/src/convert.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/audio/src/convert.rs b/audio/src/convert.rs
index 74a4d8f4..e291c804 100644
--- a/audio/src/convert.rs
+++ b/audio/src/convert.rs
@@ -21,7 +21,16 @@ macro_rules! convert_samples_to {
         $samples
             .iter()
             .map(|sample| {
-                (*sample as f64 * (std::$type::MAX as f64 + 0.5) - 0.5) as $type >> $drop_bits
+                // Losslessly represent [-1.0, 1.0] to [$type::MIN, $type::MAX]
+                // while maintaining DC linearity. There is nothing to be gained
+                // by doing this in f64, as the significand of a f32 is 24 bits,
+                // just like the maximum bit depth we are converting to.
+                let int_value = *sample * (std::$type::MAX as f32 + 0.5) - 0.5;
+
+                // Casting floats to ints truncates by default, which results
+                // in larger quantization error than rounding arithmetically.
+                // Flooring is faster, but again with larger error.
+                int_value.round() as $type >> $drop_bits
             })
             .collect()
     };