From 3cd02a44cf870eebeff677fb7dced7acc11770e6 Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Fri, 21 Oct 2022 05:12:44 -0300
Subject: [PATCH] fix: clean up frequency timer implementations

---
 src/apu.zig | 40 ++++++++++++++++++----------------------
 1 file changed, 18 insertions(+), 22 deletions(-)

diff --git a/src/apu.zig b/src/apu.zig
index 3b0c025..5c5334e 100644
--- a/src/apu.zig
+++ b/src/apu.zig
@@ -966,24 +966,20 @@ const WaveDevice = struct {
 
     fn reloadTimer(self: *Self, value: u11) void {
         self.sched.removeScheduledEvent(.{ .ApuChannel = 2 });
-        const timer = (2048 - @as(u64, value)) * 4;
-        self.timer = @truncate(u11, timer);
 
-        self.sched.push(.{ .ApuChannel = 2 }, timer * tickInterval);
+        self.timer = (@as(u16, 2048) - value) * 2;
+        self.sched.push(.{ .ApuChannel = 2 }, @as(u64, self.timer) * tickInterval);
     }
 
     fn handleTimerOverflow(self: *Self, cnt_freq: io.Frequency, cnt_sel: io.WaveSelect, late: u64) void {
-        const timer = (2048 - @as(u64, cnt_freq.frequency.read())) * 2;
-
-        self.timer = @truncate(u12, timer);
-
         if (cnt_sel.dimension.read()) {
             self.offset = (self.offset + 1) % 0x40; // 0x20 bytes (both banks), which contain 2 samples each
         } else {
             self.offset = (self.offset + 1) % 0x20; // 0x10 bytes, which contain 2 samples each
         }
 
-        self.sched.push(.{ .ApuChannel = 2 }, timer * tickInterval -| late);
+        self.timer = (@as(u16, 2048) - cnt_freq.frequency.read()) * 2;
+        self.sched.push(.{ .ApuChannel = 2 }, @as(u64, self.timer) * tickInterval -| late);
     }
 
     fn sample(self: *const Self, cnt: io.WaveSelect) u4 {
@@ -1046,7 +1042,7 @@ const SquareWave = struct {
 
     pos: u3,
     sched: *Scheduler,
-    timer: u12,
+    timer: u16,
 
     pub fn init(sched: *Scheduler) Self {
         return .{
@@ -1089,22 +1085,19 @@ const SquareWave = struct {
     }
 
     fn handleTimerOverflow(self: *Self, comptime kind: ChannelKind, cnt: io.Frequency, late: u64) void {
-        const timer = (2048 - @as(u64, cnt.frequency.read())) * 4;
-
-        self.timer = @truncate(u12, timer);
         self.pos +%= 1;
 
-        self.sched.push(.{ .ApuChannel = if (kind == .Ch1) 0 else 1 }, timer * tickInterval -| late);
+        self.timer = (@as(u16, 2048) - cnt.frequency.read()) * 4;
+        self.sched.push(.{ .ApuChannel = if (kind == .Ch1) 0 else 1 }, @as(u64, self.timer) * tickInterval -| late);
     }
 
     fn reloadTimer(self: *Self, comptime kind: ChannelKind, value: u11) void {
         self.sched.removeScheduledEvent(.{ .ApuChannel = if (kind == .Ch1) 0 else 1 });
 
-        const tmp: u64 = (2048 - @as(u64, value)) * 4; // What Freq Timer should be assuming no weird behaviour
-        const timer = (tmp & ~@as(u64, 0x3)) | self.timer & 0x3; // Keep the last two bits from the old timer
-        self.timer = @truncate(u12, timer);
+        const tmp = (@as(u16, 2048) - value) * 4; // What Freq Timer should be assuming no weird behaviour
+        self.timer = (tmp & ~@as(u16, 0x3)) | self.timer & 0x3; // Keep the last two bits from the old timer;
 
-        self.sched.push(.{ .ApuChannel = if (kind == .Ch1) 0 else 1 }, timer * tickInterval);
+        self.sched.push(.{ .ApuChannel = if (kind == .Ch1) 0 else 1 }, @as(u64, self.timer) * tickInterval);
     }
 
     fn sample(self: *const Self, cnt: io.Duty) u1 {
@@ -1163,14 +1156,17 @@ const Lfsr = struct {
         self.sched.removeScheduledEvent(.{ .ApuChannel = 3 });
 
         const div = Self.divisor(poly.div_ratio.read());
-        const timer = @as(u64, div << poly.shift.read());
-
-        self.sched.push(.{ .ApuChannel = 3 }, timer * tickInterval);
+        const timer = div << poly.shift.read();
+        self.sched.push(.{ .ApuChannel = 3 }, @as(u64, timer) * tickInterval);
     }
 
     fn handleTimerOverflow(self: *Self, poly: io.PolyCounter, late: u64) void {
+        // Obscure: "Using a noise channel clock shift of 14 or 15
+        // results in the LFSR receiving no clocks."
+        if (poly.shift.read() >= 14) return;
+
         const div = Self.divisor(poly.div_ratio.read());
-        const timer = @as(u64, div << poly.shift.read());
+        const timer = div << poly.shift.read();
 
         const tmp = (self.shift & 1) ^ ((self.shift & 2) >> 1);
         self.shift = (self.shift >> 1) | (tmp << 14);
@@ -1178,7 +1174,7 @@ const Lfsr = struct {
         if (poly.width.read())
             self.shift = (self.shift & ~@as(u15, 0x40)) | tmp << 6;
 
-        self.sched.push(.{ .ApuChannel = 3 }, timer * tickInterval -| late);
+        self.sched.push(.{ .ApuChannel = 3 }, @as(u64, timer) * tickInterval -| late);
     }
 
     fn divisor(code: u3) u16 {