6soz/src/system/nes/mapper/vrc4.zig

const types = @import("types.zig");
const Mirroring = types.Mirroring;

const Vrc4 = @This();

prg_banks: [2]u8 = [_]u8{ 0, 1 },
chr_banks_lo: [8]u4 = [_]u4{0} ** 8,
chr_banks_hi: [8]u5 = [_]u5{0} ** 8,

prg_mode: u1 = 0,
mirroring_mode: Mirroring = .vertical,

irq_reload: u8 = 0,
irq_counter: u8 = 0,
irq_enabled: bool = false,
irq_enable_on_ack: bool = false,
irq_mode: u1 = 0, // 0 = cycle, 1 = scanline
irq_prescaler: i16 = 341,
irq_assert: bool = false,

shift_a: u3,
shift_b: u3,

prg_bank_count: usize,
chr_bank_count: usize,

pub fn init(
    prg_size: usize,
    chr_size: usize,
    mapper_id: u8,
) Vrc4 {
    const shift_a: u3 = if (mapper_id == 21) 1 else 0;
    const shift_b: u3 = if (mapper_id == 21) 2 else 2;

    return .{
        .prg_bank_count = prg_size / 0x2000,
        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
        .shift_a = shift_a,
        .shift_b = shift_b,
    };
}

pub fn cpuMapRead(
    self: *const Vrc4,
    address: u16,
) ?usize {
    if (address < 0x8000) return null;

    const offset = @as(usize, address & 0x1fff);
    const last_bank = self.prg_bank_count - 1;
    const second_last_bank = self.prg_bank_count - 2;

    const r0 = @as(usize, self.prg_banks[0] & 0x1f) % self.prg_bank_count;
    const r1 = @as(usize, self.prg_banks[1] & 0x1f) % self.prg_bank_count;

    const bank_idx = (address - 0x8000) / 0x2000;
    var bank: usize = 0;

    if (self.prg_mode == 0) {
        switch (bank_idx) {
            0 => bank = r0,
            1 => bank = r1,
            2 => bank = second_last_bank,
            3 => bank = last_bank,
            else => unreachable,
        }
    } else {
        switch (bank_idx) {
            0 => bank = second_last_bank,
            1 => bank = r1,
            2 => bank = r0,
            3 => bank = last_bank,
            else => unreachable,
        }
    }

    return bank * 0x2000 + offset;
}

pub fn cpuWrite(
    self: *Vrc4,
    address: u16,
    value: u8,
) void {
    if (address < 0x8000) return;

    const reg_addr = address & 0xf000;
    const a = @as(u1, @truncate((address >> self.shift_a) & 1));
    const b = @as(u1, @truncate((address >> self.shift_b) & 1));
    const line = (@as(u2, b) << 1) | a;

    switch (reg_addr) {
        0x8000 => {
            self.prg_banks[0] = value & 0x1f;
        },
        0x9000 => {
            if (line == 0) {
                switch (@as(u2, @truncate(value & 3))) {
                    0 => self.mirroring_mode = .vertical,
                    1 => self.mirroring_mode = .horizontal,
                    2 => self.mirroring_mode = .single_screen_lower,
                    3 => self.mirroring_mode = .single_screen_upper,
                }
            } else if (line == 1 or line == 2) {
                self.prg_mode = @truncate((value >> 1) & 1);
            }
        },
        0xa000 => {
            self.prg_banks[1] = value & 0x1f;
        },
        0xb000, 0xc000, 0xd000, 0xe000 => {
            const slot_base = ((reg_addr - 0xb000) / 0x1000) * 2;
            const is_hi = (line & 2) != 0;
            const is_odd = (line & 1) != 0;
            const slot = slot_base + (if (is_odd) @as(usize, 1) else 0);

            if (!is_hi) {
                self.chr_banks_lo[slot] = @truncate(value & 0x0f);
            } else {
                self.chr_banks_hi[slot] = @truncate(value & 0x1f);
            }
        },
        0xf000 => {
            switch (line) {
                0 => self.irq_reload = (self.irq_reload & 0xf0) | (value & 0x0f),
                1 => self.irq_reload = (self.irq_reload & 0x0f) | ((value & 0x0f) << 4),
                2 => {
                    self.irq_mode = @truncate((value >> 2) & 1);
                    self.irq_enabled = (value & 0x02) != 0;
                    self.irq_enable_on_ack = (value & 0x01) != 0;
                    if (self.irq_enabled) {
                        self.irq_counter = self.irq_reload;
                        self.irq_prescaler = 341;
                    }
                    self.irq_assert = false;
                },
                3 => {
                    self.irq_enabled = self.irq_enable_on_ack;
                    self.irq_assert = false;
                },
            }
        },
        else => {},
    }
}

pub fn ppuMapRead(
    self: *const Vrc4,
    address: u16,
) ?usize {
    if (address >= 0x2000) return null;

    const slot = address / 0x0400;
    const offset = @as(usize, address & 0x03ff);

    const bank_val = (@as(usize, self.chr_banks_hi[slot]) << 4) | @as(usize, self.chr_banks_lo[slot]);
    const bank = bank_val % self.chr_bank_count;

    return bank * 0x0400 + offset;
}

pub fn ppuMapWrite(
    self: *const Vrc4,
    address: u16,
) ?usize {
    return self.ppuMapRead(address);
}

pub fn mirroring(
    self: *const Vrc4,
) Mirroring {
    return self.mirroring_mode;
}

pub fn irqAsserted(
    self: *const Vrc4,
) bool {
    return self.irq_assert;
}