const std = @import("std");
const builtin = @import("builtin");

/// danos is developed against Zig 0.16.x. Pre-1.0 Zig makes breaking API changes
/// between minor releases, and the .zon's `minimum_zig_version` only enforces a
/// floor — so reject anything off the 0.16 line to keep the build reproducible.
fn ensureZigVersion() void {
    const v = builtin.zig_version;
    if (v.major != 0 or v.minor != 16) {
        std.debug.print(
            "danos requires Zig 0.16.x, but this is {d}.{d}.{d}. " ++
                "Zig makes breaking changes between minor releases pre-1.0.\n",
            .{ v.major, v.minor, v.patch },
        );
        std.process.exit(1);
    }
}

/// Return the first path in `candidates` that exists on the build host, else the
/// first candidate as a fallback so a missing-firmware error still names a
/// concrete (and, by convention, the primary) path. Used to locate OVMF firmware
/// across distro/OS layouts without configuration.
fn firstExisting(io: std.Io, candidates: []const []const u8) []const u8 {
    for (candidates) |path| {
        std.Io.Dir.accessAbsolute(io, path, .{}) catch continue;
        return path;
    }
    return candidates[0];
}

pub fn build(b: *std.Build) void {
    ensureZigVersion();

    const target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});

    // Shared handoff definitions (BootInfo, Framebuffer, ...). No target is set,
    // so the module inherits the target of whichever binary imports it — the
    // freestanding kernel or the UEFI bootloader.
    const mod = b.addModule("danos", .{
        .root_source_file = b.path("src/root.zig"),
    });

    // Architecture-specific kernel code (CPU ops, entry, later GDT/IDT/paging).
    // The generic kernel imports this as "arch" and never names x86_64, so a new
    // architecture is a matter of pointing this module at a different directory.
    const arch_mod = b.addModule("arch", .{
        .root_source_file = b.path("src/arch/x86_64/cpu.zig"),
        .imports = &.{
            .{ .name = "danos", .module = mod }, // paging uses the shared BootInfo/memory-map types
        },
    });
    // CPU-exception stubs — real assembly, since they need cross-symbol
    // jumps/calls that Zig inline asm can't express (see the file's header).
    arch_mod.addAssemblyFile(b.path("src/arch/x86_64/isr.s"));

    // Compile-time config the kernel reads as `@import("build_options")`. The
    // QEMU test harness sets -Dtest-case=<name> to run one self-test at boot.
    const test_case = b.option([]const u8, "test-case", "Kernel self-test case to run at boot (see src/tests.zig)");
    const build_options = b.addOptions();
    build_options.addOption(?[]const u8, "test_case", test_case);
    const build_options_mod = build_options.createModule();

    // --- Kernel: freestanding x86_64 ELF, jumped to by the bootloader ---
    // SSE2 is part of the x86_64 baseline and UEFI leaves it enabled at handoff,
    // so we keep it: disabling it forces soft-float and makes the compiler unable
    // to encode the vector ops that std's formatting/runtime still emit.
    const kernel_target = b.resolveTargetQuery(.{
        .cpu_arch = .x86_64,
        .os_tag = .freestanding,
        .abi = .none,
    });

    const exe = b.addExecutable(.{
        .name = "danos",
        .root_module = b.createModule(.{
            .root_source_file = b.path("src/main.zig"),
            .target = kernel_target,
            .optimize = optimize,
            .code_model = .small, // kernel is linked in the low 2 GiB (see image_base)
            .red_zone = false, // interrupts would corrupt the SysV red zone
            .single_threaded = true, // no scheduler yet; avoids pulling in TLS/atomics
            .sanitize_c = .off, // the UBSan runtime needs f128/SSE support we don't provide
            .stack_check = false, // stack-probe calls have no runtime to land in
            .stack_protector = false,
            .imports = &.{
                .{ .name = "danos", .module = mod },
                .{ .name = "arch", .module = arch_mod },
                .{ .name = "build_options", .module = build_options_mod },
            },
        }),
    });
    exe.setLinkerScript(b.path("src/arch/x86_64/linker.ld"));
    exe.entry = .{ .symbol_name = "_start" };
    // Physical address the bootloader loads the kernel to (identity-mapped under
    // UEFI). Overrides Zig's default image base so the linker script's layout is
    // honoured; adjust here if it collides with firmware-reserved memory.
    exe.image_base = 0x100000; // 1 MiB

    b.installArtifact(exe);

    const efiexe = b.addExecutable(.{
        .name = "BOOTX64",
        .root_module = b.createModule(.{
            .root_source_file = b.path("src/efi.zig"),
            .target = b.resolveTargetQuery(.{
                .cpu_arch = .x86_64,
                .os_tag = .uefi,
            }),
            .optimize = optimize,
            .imports = &.{
                .{ .name = "danos", .module = mod },
            },
        }),
    });

    b.installArtifact(efiexe);

    // --- run-x86-64: boot the x86-64 kernel in QEMU via UEFI/OVMF ---
    // Firmware lives in different places per OS/distro, so probe the known
    // layouts (Arch, Debian/Ubuntu, Fedora, macOS Homebrew) and use the first
    // that exists. Override with -Dovmf-code / -Dovmf-vars if yours is elsewhere.
    const ovmf_code = b.option(
        []const u8,
        "ovmf-code",
        "Path to the OVMF_CODE firmware image",
    ) orelse firstExisting(b.graph.io, &.{
        "/usr/share/edk2/x64/OVMF_CODE.4m.fd", // Arch
        "/usr/share/OVMF/OVMF_CODE_4M.fd", // Debian/Ubuntu
        "/usr/share/OVMF/OVMF_CODE.fd", // older Debian/Ubuntu
        "/usr/share/edk2-ovmf/x64/OVMF_CODE.fd", // Fedora
        "/opt/homebrew/share/qemu/edk2-x86_64-code.fd", // macOS Homebrew (Apple Silicon)
        "/usr/local/share/qemu/edk2-x86_64-code.fd", // macOS Homebrew (Intel)
    });
    const ovmf_vars = b.option(
        []const u8,
        "ovmf-vars",
        "Path to the OVMF_VARS firmware image (a writable copy is made)",
    ) orelse firstExisting(b.graph.io, &.{
        "/usr/share/edk2/x64/OVMF_VARS.4m.fd", // Arch
        "/usr/share/OVMF/OVMF_VARS_4M.fd", // Debian/Ubuntu
        "/usr/share/OVMF/OVMF_VARS.fd", // older Debian/Ubuntu
        "/usr/share/edk2-ovmf/x64/OVMF_VARS.fd", // Fedora
        "/opt/homebrew/share/qemu/edk2-i386-vars.fd", // macOS Homebrew (Apple Silicon)
        "/usr/local/share/qemu/edk2-i386-vars.fd", // macOS Homebrew (Intel)
    });

    // Assemble an EFI System Partition layout: esp/EFI/BOOT/BOOTX64.efi
    const efi_install = b.addInstallArtifact(efiexe, .{
        .dest_dir = .{ .override = .{ .custom = "esp/EFI/BOOT" } },
    });
    // The bootloader loads the kernel by name from the volume root, so drop the
    // kernel ELF at esp/danos.
    const kernel_install = b.addInstallArtifact(exe, .{
        .dest_dir = .{ .override = .{ .custom = "esp" } },
    });

    // The firmware needs to write NVRAM, so give it a writable copy of the vars.
    const vars_copy = b.addSystemCommand(&.{ "cp", "-f", ovmf_vars });
    const vars_out = vars_copy.addOutputFileArg("OVMF_VARS.4m.fd");

    const run_efi = b.addSystemCommand(&.{
        "qemu-system-x86_64",
        "-machine",
        "q35",
        "-m",
        "128M",
        "-drive",
        b.fmt("if=pflash,format=raw,readonly=on,file={s}", .{ovmf_code}),
    });
    run_efi.addArg("-drive");
    run_efi.addPrefixedFileArg("if=pflash,format=raw,file=", vars_out);
    // Present the ESP directory to the guest as a FAT drive.
    run_efi.addArgs(&.{
        "-drive",
        b.fmt("format=raw,file=fat:rw:{s}/esp", .{b.install_path}),
        "-net",
        "none",
        // Emulated display advertising 1280x720 as its native (EDID preferred)
        // resolution, so the kernel's native-resolution switch has something to
        // find. `-vga none` avoids a second, default adapter.
        "-vga",
        "none",
        "-device",
        "VGA,edid=on,xres=1280,yres=720",
    });
    run_efi.step.dependOn(&efi_install.step);
    run_efi.step.dependOn(&kernel_install.step);

    const run_efi_step = b.step("run-x86-64", "Boot the x86-64 kernel in QEMU (UEFI/OVMF)");
    run_efi_step.dependOn(&run_efi.step);

    // const run_cmd = b.addRunArtifact(exe);
    // const run_step = b.step("run", "Run the app");
    // run_step.dependOn(&run_cmd.step);
    // run_cmd.step.dependOn(b.getInstallStep());
    //
    // if (b.args) |args| {
    //     run_cmd.addArgs(args);
    // }

    // Tests run on the host. The kernel and bootloader target freestanding/UEFI
    // and can't be executed natively, so only the shared module is unit-tested
    // here (compiled for the host rather than inheriting a freestanding target).
    const mod_tests = b.addTest(.{
        .root_module = b.createModule(.{
            .root_source_file = b.path("src/root.zig"),
            .target = target,
            .optimize = optimize,
        }),
    });

    const run_mod_tests = b.addRunArtifact(mod_tests);

    const test_step = b.step("test", "Run tests");
    test_step.dependOn(&run_mod_tests.step);
}