//! Build script, two jobs:
//!
//! 1. Compile the user programs in user/ into ELF executables in OUT_DIR.
//!    They're compiled with plain `rustc` for the built-in
//!    `x86_64-unknown-none` target (prebuilt `core`, so no build-std
//!    needed), as static executables (`relocation-model=static`) linked at a
//!    high user address -- which needs the `large` code model (the default
//!    ones assume code in the low/high 2 GiB).
//!
//! 2. Build the second QEMU disk, `target/disk.img` (64 MiB):
//!      * a GPT partition table (made with `sfdisk`) with one partition
//!        starting at sector 2048 (1 MiB, the usual alignment),
//!      * in it a FAT32 file system (`mkfs.vfat`), filled with `mtools`
//!        (`mmd`/`mcopy` write into an image file directly -- no root, no
//!        loop-mounting needed): everything in disk/ plus the user programs
//!        in /bin.
//!    The kernel finds the disk on the PCI bus, reads the GPT, mounts the
//!    FAT32 partition at /mnt/disk0 and loads programs from /mnt/disk0/bin.
//!    QEMU opens it with `snapshot=on`, so the kernel's writes never change
//!    this file (every boot and every test starts from the same disk).
//!
//! Needs dosfstools (mkfs.vfat), mtools (mmd, mcopy) and fdisk (sfdisk):
//!     sudo apt install dosfstools mtools fdisk

use std::{env, fs, io::Write, path::{Path, PathBuf}, process::{Command, Stdio}};

const PROGRAMS: &[&str] = &["hello", "evil", "spin", "echo", "demo", "child"];

const DISK_SECTORS: u64 = 64 * 1024 * 1024 / 512;
const PART_START: u64 = 2048;
/// Leave 1 MiB at the end too (the backup GPT lives in the last 33 sectors).
const PART_SECTORS: u64 = DISK_SECTORS - 2 * PART_START;

fn main() {
    let manifest = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
    let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
    let user_dir = manifest.join("user");
    println!("cargo:rerun-if-changed=user");
    println!("cargo:rerun-if-changed=disk");
    println!("cargo:rerun-if-changed=build.rs");

    for program in PROGRAMS {
        build_program(&user_dir, &out_dir, program);
    }
    build_disk(&manifest, &out_dir);
}

fn build_program(user_dir: &Path, out_dir: &Path, program: &str) {
    let rustc = env::var("RUSTC").unwrap_or_else(|_| "rustc".into());
    let status = Command::new(&rustc)
        .args(["--edition", "2024", "--target", "x86_64-unknown-none"])
        .args(["-C", "panic=abort", "-C", "opt-level=2", "-C", "debuginfo=0", "-C", "strip=debuginfo"])
        .args(["-C", "relocation-model=static", "-C", "code-model=large"])
        .arg("-C")
        .arg(format!("link-arg=-T{}", user_dir.join("link.ld").display()))
        .arg("-o")
        .arg(out_dir.join(program))
        .arg(user_dir.join(format!("{program}.rs")))
        .status()
        .expect("failed to run rustc for a user program");
    assert!(status.success(), "building user program {program} failed");
}

/// Find a tool in PATH or the sbin directories (often not in a user's PATH).
fn tool(name: &str) -> PathBuf {
    let path = env::var("PATH").unwrap_or_default();
    for dir in path.split(':').chain(["/usr/sbin", "/sbin", "/usr/bin"]) {
        let candidate = Path::new(dir).join(name);
        if candidate.is_file() {
            return candidate;
        }
    }
    panic!("`{name}` not found: install it with `sudo apt install dosfstools mtools fdisk`");
}

fn run(cmd: &mut Command, stdin: Option<&str>) {
    cmd.stdin(if stdin.is_some() { Stdio::piped() } else { Stdio::null() });
    let mut child = cmd.spawn().unwrap_or_else(|e| panic!("can't run {cmd:?}: {e}"));
    if let Some(input) = stdin {
        child.stdin.take().unwrap().write_all(input.as_bytes()).unwrap();
    }
    let status = child.wait().unwrap();
    assert!(status.success(), "{cmd:?} failed");
}

fn build_disk(manifest: &Path, out_dir: &Path) {
    let target_dir = env::var("CARGO_TARGET_DIR").map(PathBuf::from).unwrap_or_else(|_| manifest.join("target"));
    fs::create_dir_all(&target_dir).unwrap();
    let disk = target_dir.join("disk.img");
    let part = out_dir.join("part.img");

    // 1. The FAT32 file system, as its own image first. -s 1: one sector per
    //    cluster (FAT32 needs >= 65525 clusters, so small disks need small
    //    clusters). -n: volume label.
    let _ = fs::remove_file(&part);
    run(
        Command::new(tool("mkfs.vfat"))
            .args(["-F", "32", "-s", "1", "-n", "TYLERDISK", "-C"])
            .arg(&part)
            .arg((PART_SECTORS / 2).to_string()), // size in KiB
        None,
    );
    let mtools = |name: &str, args: &[&str]| {
        let mut cmd = Command::new(tool(name));
        cmd.env("MTOOLS_SKIP_CHECK", "1").arg("-i").arg(&part).args(args);
        run(&mut cmd, None);
    };
    // Copy disk/ (text files, subdirectories) to the root of the volume...
    for entry in fs::read_dir(manifest.join("disk")).unwrap() {
        let path = entry.unwrap().path();
        mtools("mcopy", &["-s", path.to_str().unwrap(), "::/"]);
    }
    // ...and the user programs to /bin.
    mtools("mmd", &["::/bin"]);
    for program in PROGRAMS {
        mtools("mcopy", &[out_dir.join(program).to_str().unwrap(), &format!("::/bin/{program}")]);
    }

    // 2. The whole disk: zeros + GPT partition table (sfdisk works on image
    //    files too), then the FAT32 image copied into the partition.
    let _ = fs::remove_file(&disk);
    fs::File::create(&disk).unwrap().set_len(DISK_SECTORS * 512).unwrap();
    let layout = format!(
        "label: gpt\nstart={PART_START}, size={PART_SECTORS}, type=EBD0A0A2-B9E5-4433-87C0-68B6B72699C7, name=\"fat32\"\n"
    );
    run(Command::new(tool("sfdisk")).arg("--quiet").arg(&disk), Some(&layout));
    let fat = fs::read(&part).unwrap();
    assert_eq!(fat.len() as u64, PART_SECTORS * 512);
    let mut file = fs::OpenOptions::new().write(true).open(&disk).unwrap();
    use std::io::{Seek, SeekFrom};
    file.seek(SeekFrom::Start(PART_START * 512)).unwrap();
    file.write_all(&fat).unwrap();
}