move dcadjuster into its own module

src/chips/ay3891.zig

@@ -4,6 +4,7 @@
 const bitutils = @import("common").bitutils;
 const mask = bitutils.mask;
 const maskm = bitutils.maskm;
+const dcadjust = @import("common").dcadjust;
 
 /// map chip pin names to bit positions
 ///
@@ -91,7 +92,6 @@ pub fn Type(comptime cfg: Config) type {
         // misc constants
         const NUM_CHANNELS = 3;
         const FIXEDPOINT_SCALE = 16; // error accumulation precision boost
-        const DCADJ_BUFLEN = 128;
 
         // registers
         pub const REG = struct {
@@ -177,11 +177,7 @@ pub fn Type(comptime cfg: Config) type {
             volume: f32 = 0.0,
             value: f32 = 0.0,
             ready: bool = false, // true if a new sample value is ready
-            dcadj: struct {
-                sum: f32 = 0.0,
-                pos: u32 = 0.0,
-                buf: [DCADJ_BUFLEN]f32 = [_]f32{0.0} ** DCADJ_BUFLEN,
-            } = .{},
+            dcadj: dcadjust.Type(.{ .buf_len = 128 }) = .{},
         };
 
         tick_count: u38 = 0, // tick counter for internal clock division
@@ -244,6 +240,7 @@ pub fn Type(comptime cfg: Config) type {
 
         pub fn reset(self: *Self) void {
             self.active = false;
+            self.sample.dcadjust.reset();
             self.addr = 0;
             self.tick_count = 0;
             for (&self.regs) |r| {
@@ -345,28 +342,14 @@ pub fn Type(comptime cfg: Config) type {
                         }
                     }
                 }
-                self.sample.value = dcadjust(self, sm) * self.sample.volume;
+                self.sample.value = self.sample.dcadj.put(sm) * self.sample.volume;
                 self.sample.ready = true;
             } else {
                 self.sample.ready = false;
             }
             return bus;
         }
 
-        // DC adjustment filter from StSound, this moves an "offcenter"
-        // signal back to the zero-line (e.g. the volume-level output
-        // from the chip simulation which is >0.0 gets converted to
-        //a +/- sample value)
-        fn dcadjust(self: *Self, s: f32) f32 {
-            const pos = self.sample.dcadj.pos;
-            self.sample.dcadj.sum -= self.sample.dcadj.buf[pos];
-            self.sample.dcadj.sum += s;
-            self.sample.dcadj.buf[pos] = s;
-            self.sample.dcadj.pos = (pos + 1) & (DCADJ_BUFLEN - 1);
-            const div: f32 = @floatFromInt(DCADJ_BUFLEN);
-            return s - (self.sample.dcadj.sum / div);
-        }
-
         // write from data bus to register
         fn write(self: *Self, bus: Bus) void {
             const data = getData(bus);

src/common/common.zig

@@ -2,3 +2,4 @@ pub const bitutils = @import("bitutils.zig");
 pub const memory = @import("memory.zig");
 pub const glue = @import("glue.zig");
 pub const clock = @import("clock.zig");
+pub const dcadjust = @import("dcadjust.zig");

src/common/dcadjust.zig

@@ -0,0 +1,35 @@
+//! an audio DC adjuster (centers an off-center signal)
+//! see: https://github.com/arnaud-carre/StSound/blob/91d2d3c604386c637b038bba747c8da6c976c245/StSoundLibrary/Ym2149Ex.cpp#L67-L93
+const std = @import("std");
+const assert = std.debug.assert;
+const isPowerOfTwo = std.math.isPowerOfTwo;
+
+pub const Config = struct {
+    buf_len: comptime_int,
+};
+
+pub fn Type(cfg: Config) type {
+    assert(isPowerOfTwo(cfg.buf_len));
+
+    return struct {
+        const Self = @This();
+
+        sum: f32 = 0,
+        pos: u32 = 0,
+        buf: [cfg.buf_len]f32 = [_]f32{0.0} ** cfg.buf_len,
+
+        pub fn reset(self: *Self) void {
+            self.* = .{};
+        }
+
+        pub fn put(self: *Self, s: f32) f32 {
+            const pos = self.pos;
+            self.sum -= self.buf[pos];
+            self.sum += s;
+            self.buf[pos] = s;
+            self.pos = (pos + 1) & (cfg.buf_len - 1);
+            const div: f32 = @floatFromInt(cfg.buf_len);
+            return s - (self.sum / div);
+        }
+    };
+}