test(nes): consolidate and simplify inline subsystem unit tests

src/system/nes/mapper/gxrom.zig

@@ -0,0 +1,91 @@
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
+
+// ponytail: Mapper 66 (GxROM / GNROM) - 32KB PRG bank in bits 4-5, 8KB CHR bank in bits 0-1
+pub const Gxrom = @This();
+
+prg_bank: u8 = 0,
+chr_bank: u8 = 0,
+mirroring_mode: Mirroring,
+
+pub fn init(initial_mirroring: Mirroring) Gxrom {
+    return .{ .mirroring_mode = initial_mirroring };
+}
+
+pub fn cpuRead(self: *const Gxrom, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_32k_banks = @max(1, prg_rom.len / 0x8000);
+        const bank = @as(usize, self.prg_bank) % total_32k_banks;
+        const offset = (bank * 0x8000) + (address - 0x8000);
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn cpuWrite(self: *Gxrom, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        self.prg_bank = (value >> 4) & 0x03;
+        self.chr_bank = value & 0x03;
+        return true;
+    }
+    return false;
+}
+
+pub fn ppuRead(self: *const Gxrom, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else if (chr_rom.len > 0) {
+            const total_8k_banks = @max(1, chr_rom.len / 0x2000);
+            const bank = @as(usize, self.chr_bank) % total_8k_banks;
+            const offset = (bank * 0x2000) + address;
+            if (offset < chr_rom.len) return chr_rom[offset];
+        }
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *Gxrom, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Gxrom) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqLine(_: *const Gxrom) bool {
+    return false;
+}
+
+pub fn notifyPpuAddress(_: *Gxrom, _: u16) void {}
+
+test "gxrom bank switching" {
+    var gx = Gxrom.init(.horizontal);
+    var ram: [0x2000]u8 = undefined;
+
+    // Write 0x21: PRG bank 2, CHR bank 1
+    _ = gx.cpuWrite(0x8000, 0x21, &ram);
+    try std.testing.expectEqual(@as(u8, 2), gx.prg_bank);
+    try std.testing.expectEqual(@as(u8, 1), gx.chr_bank);
+}