From bf95eee3f19a256f1cb939d2d61cfcf58fb35d45 Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Mon, 5 Dec 2022 11:08:04 -0400
Subject: [PATCH] fix(apu): resolve bug in NR10 obscure behaviour

---
 src/core/apu/ToneSweep.zig    | 7 +++----
 src/core/apu/device/Sweep.zig | 6 ++++--
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/core/apu/ToneSweep.zig b/src/core/apu/ToneSweep.zig
index 0c63938..4ade2fa 100644
--- a/src/core/apu/ToneSweep.zig
+++ b/src/core/apu/ToneSweep.zig
@@ -94,10 +94,9 @@ pub fn sound1CntL(self: *const Self) u8 {
 pub fn setSound1CntL(self: *Self, value: u8) void {
     const new = io.Sweep{ .raw = value };
 
-    if (self.sweep.direction.read() and !new.direction.read()) {
-        // Sweep Negate bit has been cleared
-        // If At least 1 Sweep Calculation has been made since
-        // the last trigger, the channel is immediately disabled
+    if (!new.direction.read()) {
+        // If at least one (1) sweep calculation has been made with
+        // the negate bit set (since last trigger), disable the channel
 
         if (self.sweep_dev.calc_performed) self.enabled = false;
     }
diff --git a/src/core/apu/device/Sweep.zig b/src/core/apu/device/Sweep.zig
index fb5473f..605697d 100644
--- a/src/core/apu/device/Sweep.zig
+++ b/src/core/apu/device/Sweep.zig
@@ -31,7 +31,6 @@ pub fn tick(self: *Self, ch1: *ToneSweep) void {
     if (self.timer == 0) {
         const period = ch1.sweep.period.read();
         self.timer = if (period == 0) 8 else period;
-        if (!self.calc_performed) self.calc_performed = true;
 
         if (self.enabled and period != 0) {
             const new_freq = self.calculate(ch1.sweep, &ch1.enabled);
@@ -52,7 +51,10 @@ pub fn calculate(self: *Self, sweep: io.Sweep, ch_enable: *bool) u12 {
     const shadow_shifted = shadow >> sweep.shift.read();
     const decrease = sweep.direction.read();
 
-    const freq = if (decrease) shadow - shadow_shifted else shadow + shadow_shifted;
+    const freq = if (decrease) blk: {
+        self.calc_performed = true;
+        break :blk shadow - shadow_shifted;
+    } else shadow + shadow_shifted;
     if (freq > 0x7FF) ch_enable.* = false;
 
     return freq;