// SPDX-License-Identifier: MPL-2.0

//! The *libui-ng-sys* build script.

use std::{env, path::PathBuf, process};

use error_stack::{Result, ResultExt as _};

/// The error type returned by [`main`].
#[derive(thiserror::Error, Debug)]
enum Error {
    /// Failed to vendor a dependency into the build directory.
    #[error("failed to vendor {name} into {}", .out_dir.display())]
    VendorDep {
        name: &'static str,
        out_dir: PathBuf,
    },
    /// Failed to fix file permissions for Ninja.
    #[cfg(unix)]
    #[error("failed to fix file permissions for Ninja")]
    FixNinjaPermissions,
    /// Failed to build *libui-ng*.
    #[error("failed to build libui-ng")]
    BuildLibui,
    /// Failed to link required system libraries.
    #[error("failed to link required system libraries")]
    LinkSystemLibs,
    /// Failed to generate bindings to *libui-ng*.
    #[error("failed to generate bindings")]
    GenBindings,
}

fn main() -> process::ExitCode {
    match main_impl() {
        Ok(_) => process::ExitCode::SUCCESS,
        Err(e) => {
            eprintln!("{e:?}");

            process::ExitCode::FAILURE
        }
    }
}

fn main_impl() -> Result<(), Error> {
    let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
    let libui_dir = out_dir.join("libui-ng");
    let meson_dir = out_dir.join("meson");
    let ninja_dir = out_dir.join("ninja");

    // Cargo will prevent the crate from being published if this build script modifies files outside
    // `$OUT_DIR`. To work around this when building *libui-ng*, we copy vendored build dependencies
    // to `$OUT_DIR` as necessary.
    let vendor_dep = |name, dir| {
        dep::sync(name, dir)
            .change_context_lazy(|| Error::VendorDep { name, out_dir: out_dir.clone() })
    };

    vendor_dep("libui-ng", libui_dir.as_path())?;

    let strategy = if env::var("DOCS_RS").is_ok() {
        // Don't bother linking  with *libui-ng* at all; we're only generating documentation.
        Strategy::GenBindingsOnly
    } else if cfg!(feature = "probe-system-libui") {
        // We will probe for the system *libui-ng* library. If it exists, we'll link dynamically to
        // it, and if not, we'll build from source.

        let maybe_lib = pkg_config::Config::new()
            // We don't want *pkg-config* to emit Cargo metadata because we'll handle that
            // ourselves.
            .cargo_metadata(false)
            // Environment metadata is OK though.
            .env_metadata(true)
            // We will link dynamically, not statically.
            .statik(false)
            .probe("libui-ng");

        match maybe_lib {
            Ok(lib) => Strategy::UseSystem { link_paths: lib.link_paths },
            Err(_) => Strategy::BuildFromSource,
        }
    } else {
        Strategy::BuildFromSource
    };

    match strategy {
        Strategy::UseSystem { ref link_paths } => {
            // Tell Cargo where to find the system *libui-ng*.
            for path in link_paths {
                println!("cargo:rustc-link-search=native={}", path.display());
            }
            // Link *libui-ng* dynamically.
            println!("cargo:rustc-link-lib=dylib=ui");
        }
        Strategy::BuildFromSource => {
            let backend = build::Backend::from_cfg();

            // Meson is necessary for building.
            vendor_dep("meson", meson_dir.as_path())?;
            if matches!(backend, build::Backend::Ninja) {
                vendor_dep("ninja", ninja_dir.as_path())?;

                // When downloading crates from *crates.io*, file execute permissions are *not*
                // respected. This is a problem for Ninja, which attempts to execute a file named
                // *inline.sh*. For this reason, we manually mark it as executable on Unix
                // platforms. (This is not an issue on Windows.)
                #[cfg(unix)]
                unix::mark_executable(ninja_dir.join("src/inline.sh"))
                    .change_context(Error::FixNinjaPermissions)?;
            }
            // Build *libui-ng* from source.
            let libui_build_dir = backend
                .build_libui(&libui_dir, &meson_dir, &ninja_dir)
                .change_context(Error::BuildLibui)
                .map_err(|report| {
                    #[cfg(target_os = "windows")]
                    {
                        // This is a common source of build errors.
                        report.attach_printable(
                            "Have you tried building libui-ng-sys from the Microsoft Developer \
                            Command Prompt or Developer Powershell?"
                        )
                    }
                    #[cfg(not(target_os = "windows"))]
                    report
                })?;

            // Tell Cargo where to find the copy of *libui-ng* that we just built.
            println!("cargo:rustc-link-search=native={}", libui_build_dir.display());
            // Link *libui-ng* statically.
            println!("cargo:rustc-link-lib=static=ui");

            // Static libraries do not encode information on the system libraries that must be
            // linked, so we must tell Cargo (and, by extension, the dynamic linker) what we need.
            link_system_libs().change_context(Error::LinkSystemLibs)?;
        }
        Strategy::GenBindingsOnly => {
            // Do nothing; we will generate the bindings for all strategies in a moment.
        }
    }

    // Generate bindings.
    bindings::generate(&libui_dir, &out_dir).change_context(Error::GenBindings)?;

    // Recompile *libui-ng-sys* whenever this build script is modified.
    println!("cargo:rerun-if-changed=build.rs");

    Ok(())
}

#[derive(Clone)]
enum Strategy {
    UseSystem {
        link_paths: Vec<PathBuf>,
    },
    BuildFromSource,
    GenBindingsOnly,
}

#[derive(thiserror::Error, Debug)]
enum LinkSystemLibsError {
    #[cfg(target_os = "macos")]
    #[error("failed to link clang_rt")]
    ClangRt,
    #[cfg(target_os = "linux")]
    #[error("failed to link GTK")]
    Gtk,
}

fn link_system_libs() -> Result<(), LinkSystemLibsError> {
    #[allow(unused)]
    macro_rules! link {
        ($kind:literal : $($name:literal)*) => {
            $(
                println!(concat!("cargo:rustc-link-lib=", $kind, "=", $name));
            )*
        };
        ($($name:literal)*) => {
            $(
                println!(concat!("cargo:rustc-link-lib=", $name));
            )*
        };
    }

    #[cfg(target_os = "macos")]
    macro_rules! frameworks {
        ($($name:literal)*) => { link!("framework": $($name)*) };
    }

    #[allow(unused)]
    macro_rules! dylibs {
        ($($name:literal)*) => { link!("dylib": $($name)*) };
    }

    #[cfg(target_os = "macos")]
    {
        use error_stack::IntoReport as _;

        // See *dep/libui-ng/darwin/meson.build*.
        frameworks! {
            "AppKit"
            "Foundation"
        }

        #[derive(thiserror::Error, Debug)]
        enum LinkClangRtError {
            #[error("failed to spawn clang")]
            SpawnClang,
        }

        let clang_rt_dir = process::Command::new("clang")
            .arg("-print-runtime-dir")
            .output()
            .into_report()
            .change_context(LinkClangRtError::SpawnClang)
            .map(|out| unix::make_path_from_ascii(out.stdout))
            .change_context(LinkSystemLibsError::ClangRt)?;

        println!("cargo:rustc-link-search=native={}", clang_rt_dir.display());
        link!("clang_rt.osx");
    }
    #[cfg(target_os = "linux")]
    {
        gtk::link().change_context(LinkSystemLibsError::Gtk)?;
    }
    #[cfg(target_os = "windows")]
    {
        // See *dep/libui-ng/windows/meson.build*.
        dylibs! {
            "comctl32"
            "comdlg32"
            "d2d1"
            "dwrite"
            "gdi32"
            "kernel32"
            "msimg32"
            "ole32"
            "oleacc"
            "oleaut32"
            "user32"
            "uuid"
            "uxtheme"
            "windowscodecs"
        };
    }

    Ok(())
}

#[cfg(unix)]
mod unix {
    //! Functionality exclusive to Linux and macOS.

    use std::{
        ffi::OsString,
        fmt,
        fs,
        os::unix::{ffi::OsStringExt as _, fs::PermissionsExt as _},
        path::{Path, PathBuf},
    };

    use error_stack::{IntoReport as _, Result, ResultExt as _};

    #[derive(Debug)]
    pub struct MarkExecutableError {
        path: PathBuf,
    }

    impl std::error::Error for MarkExecutableError {}
    impl fmt::Display for MarkExecutableError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "failed to mark {} as executable", self.path.display())
        }
    }

    pub fn mark_executable(path: impl AsRef<Path>) -> Result<(), MarkExecutableError> {
        let path = path.as_ref();

        fs::set_permissions(path, fs::Permissions::from_mode(0o755))
            .into_report()
            .change_context_lazy(|| MarkExecutableError { path: path.to_path_buf() })
    }

    #[allow(dead_code)]
    pub fn make_path_from_ascii(mut path: Vec<u8>) -> PathBuf {
        if let Some(newline_idx) = memchr::memchr(b'\n', path.as_slice()) {
            // Exclude the newline and everything after.
            path.truncate(newline_idx);
        }

        PathBuf::from(OsString::from_vec(path))
    }
}

#[cfg(target_os = "linux")]
mod gtk {
    use error_stack::{IntoReport as _, Result};
    pub use pkg_config::Error;

    static MIN_VERSION: &str = "3.10.0";
    static PACKAGE: &str = "gtk+-3.0";

    pub fn find() -> Result<pkg_config::Library, Error> {
        probe(false)
    }

    pub fn link() -> Result<(), pkg_config::Error> {
        probe(true).map(|_| ())
    }

    fn probe(should_link: bool) -> Result<pkg_config::Library, Error> {
        pkg_config::Config::new()
            .atleast_version(MIN_VERSION)
            .print_system_cflags(should_link)
            .print_system_libs(should_link)
            .probe(PACKAGE)
            .into_report()
    }
}

mod dep {
    use std::{fs, os, path::Path};

    use error_stack::{IntoReport as _, Result, ResultExt as _};

    #[derive(thiserror::Error, Debug)]
    pub enum SyncError {
        #[error("fs::create_dir() failed")]
        CreateDir,
        #[error("fs::read_dir() failed")]
        ReadDir,
        #[error("failed to read directory entry")]
        DirEntry,
        #[error("fs::DirEntry::file_type() failed")]
        DirEntryFileType,
        #[error("fs::copy() failed")]
        Copy,
        #[error("fs::read_link() failed")]
        ReadLink,
        #[error("failed to create symbolic link")]
        Symlink,
    }

    pub fn sync(name: &str, to: &Path) -> Result<(), SyncError> {
        copy_dir_contents(Path::new("dep").join(name), to)
    }

    fn copy_dir_contents(
        from: impl AsRef<Path>,
        to: impl AsRef<Path>,
    ) -> Result<(), SyncError> {
        let to = to.as_ref();
        if !to.exists() {
            fs::create_dir(to).into_report().change_context(SyncError::CreateDir)?;
        }

        let maybe_entries = fs::read_dir(from.as_ref())
            .into_report()
            .change_context(SyncError::ReadDir)?;
        for maybe_entry in maybe_entries {
            let entry = maybe_entry.into_report().change_context(SyncError::DirEntry)?;
            let kind = entry
                .file_type()
                .into_report()
                .change_context(SyncError::DirEntryFileType)?;

            let from = entry.path();
            let to = to.join(entry.file_name());
            if kind.is_dir() {
                copy_dir_contents(from, to)?;
            } else if kind.is_file() {
                fs::copy(from, to).into_report().change_context(SyncError::Copy)?;
            } else if kind.is_symlink() {
                let original = fs::read_link(from)
                    .into_report()
                    .change_context(SyncError::ReadLink)?;

                #[cfg(windows)]
                {
                    use os::windows::fs;

                    if original.is_dir() {
                        fs::symlink_dir(original, to)
                    } else  {
                        fs::symlink_file(original, to)
                    }
                    .into_report()
                    .change_context(SyncError::Symlink)?;
                }
                #[cfg(not(windows))]
                {
                    #[cfg(unix)]
                    {
                        os::unix::fs::symlink(original, to)
                            .into_report()
                            .change_context(SyncError::Symlink)?;
                    }
                    #[cfg(not(unix))]
                    {
                        fs::copy(from, to).into_report().change_context(SyncError::Copy)?;
                    }
                }
            } else {
                panic!("what is this thing?");
            }
        }

        Ok(())
    }
}

mod build {
    use std::{env, path::{Path, PathBuf}, process};

    use error_stack::{IntoReport as _, Result, ResultExt as _};

    /// The error type returned by [`Backend`] functions.
    #[derive(thiserror::Error, Debug)]
    pub enum Error {
        /// Failed to setup *libui-ng*.
        #[error("failed to setup libui-ng")]
        SetupLibui,
        /// Failed to build Ninja.
        #[error("failed to build Ninja")]
        BuildNinja,
        /// Failed to compile *libui-ng*.
        #[error("failed to compile libui-ng")]
        CompileLibui,
        /// Failed to hard-link *ui.lib* to *libui.a* for compatibility with MSVC.
        #[cfg(target_os = "windows")]
        #[error("failed to fix libui-ng archive filename")]
        FixLibuiName,
    }

    #[derive(thiserror::Error, Debug)]
    pub enum PythonError {
        /// Failed to run Python.
        #[error("failed to run Python")]
        RunPython,
        /// The process run by Python failed.
        #[error("{:?}", out)]
        Python { out: process::Output },
    }

    impl Backend {
        pub fn from_cfg() -> Self {
            if cfg!(feature = "build-with-msvc") {
                Self::Msvc
            } else if cfg!(feature = "build-with-xcode") {
                Self::Xcode
            } else {
                Self::Ninja
            }
        }
    }

    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub enum Backend {
        Msvc,
        Ninja,
        Xcode,
    }

    impl Backend {
        /// Builds *libui*.
        pub fn build_libui(
            self,
            libui_dir: &Path,
            meson_dir: &Path,
            ninja_dir: &Path,
        ) -> Result<PathBuf, Error> {
            if let Self::Ninja = self {
                // This must precede setting up *libui* as Meson requires Ninja even in the
                // configuration phase.
                Self::build_ninja(ninja_dir).change_context(Error::BuildNinja)?;
            }

            self.setup_libui(libui_dir, meson_dir, ninja_dir).change_context(Error::SetupLibui)?;
            self.compile_libui(libui_dir, meson_dir, ninja_dir)
                .change_context(Error::CompileLibui)?;

            let build_dir = libui_dir.join("build/meson-out");
            #[cfg(target_os = "windows")]
            self.fix_libui_name(build_dir.as_path())
                .change_context(Error::FixLibuiName)?;

            Ok(build_dir)
        }

        fn ninja_path(ninja_dir: &Path) -> PathBuf {
            let ext = env::consts::EXE_EXTENSION;
            ninja_dir.join("ninja").with_extension(ext)
        }

        fn run_python(
            f: impl Fn(&mut process::Command),
            ninja_dir: Option<&Path>,
        ) -> Result<(), PythonError> {
            let mut cmd = process::Command::new("python3");
            f(&mut cmd);

            if let Some(dir) = ninja_dir {
                cmd.env("NINJA", Self::ninja_path(dir));
            }

            let out = cmd.output().into_report().change_context(PythonError::RunPython)?;
            if out.status.success() {
                Ok(())
            } else {
                Err(error_stack::report!(PythonError::Python { out }))
            }
        }

        /// Builds Ninja.
        fn build_ninja(ninja_dir: &Path) -> Result<(), PythonError> {
            if Self::ninja_path(ninja_dir).exists() {
                // We'll give the benefit of the doubt that this is actually a complete, working
                // binary.
                return Ok(());
            }

            Self::run_python(
                |cmd| {
                    cmd
                        .arg("configure.py")
                        .arg("--bootstrap")
                        .current_dir(ninja_dir);
                },
                None,
            )
        }

        /// Prepares *libui* to be built.
        fn setup_libui(
            &self,
            libui_dir: &Path,
            meson_dir: &Path,
            ninja_dir: &Path,
        ) -> Result<(), PythonError> {
            Self::run_python(
                |cmd| {
                    cmd
                        .arg(meson_dir.join("meson.py"))
                        .arg("setup")
                        .arg("--default-library=static")
                        .arg("--buildtype=release")
                        .arg(format!("--optimization={}", Self::optimization_level()))
                        .arg(format!("--backend={}", self.as_str()))
                        .arg(libui_dir.join("build"))
                        .arg(libui_dir);
                },
                Some(ninja_dir),
            )
        }

        #[allow(dead_code)]
        fn is_debug() -> bool {
            !matches!(env::var("DEBUG").as_deref(), Ok("0" | "false"))
        }

        fn optimization_level() -> String {
            let level = env::var("OPT_LEVEL").expect("$OPT_LEVEL is unset");
            match level.as_str() {
                // Meson doesn't support "-Oz"; we'll try the next-closest option.
                "z" => String::from("s"),
                _ => level,
            }
        }

        fn as_str(&self) -> &'static str {
            match self {
                Self::Msvc => "vs",
                Self::Ninja => "ninja",
                Self::Xcode => "xcode",
            }
        }

        fn compile_libui(
            &self,
            libui_dir: &Path,
            meson_dir: &Path,
            ninja_dir: &Path,
        ) -> Result<(), PythonError> {
            Self::run_python(
                |cmd| {
                    cmd
                        .arg(meson_dir.join("meson.py"))
                        .arg("compile")
                        .arg(format!("-C={}", libui_dir.join("build").display()));
                },
                Some(ninja_dir),
            )
        }

        #[cfg(target_os = "windows")]
        fn fix_libui_name(&self, build_dir: &Path) -> Result<(), std::io::Error> {
            // Meson unconditionally names the library "libui.a", which prevents MSVC's `link.exe`
            // from finding it; we must manually rename it to "ui.lib".
            //
            // Note: we must do this for all Windows builds, not just where `self = Self::Msvc`;
            // whether `rustc` uses MSVC is independent of whether we build *libui-ng* with MSVC.

            use std::fs;

            let old_path = build_dir.join("libui.a");
            let new_path = build_dir.join("ui.lib");

            if old_path.exists() {
                if new_path.exists() {
                    let _ = fs::remove_file(new_path.as_path());
                }
                fs::hard_link(old_path, new_path)?;
            } else {
                // If it doesn't exist, it's probably because we built with MSVC, so the file is
                // already named "ui.lib". Let's make sure.
                assert_eq!(*self, Self::Msvc);
            }

            Ok(())
        }
    }
}

mod bindings {
    use std::{borrow::Cow, fmt, iter, path::{Path, PathBuf}};

    use error_stack::{IntoReport as _, Result, ResultExt as _};

    /// The error type returned by binding functions.
    #[derive(thiserror::Error, Debug)]
    pub enum Error {
        /// Failed to generate bindings.
        #[error("failed to generate bindings")]
        Generate,
        /// Failed to write bindings to a file.
        #[error("failed to write bindings to {}", .to.display())]
        WriteToFile {
            to: PathBuf,
        },
    }

    /// Generates bindings to *libui* and writes them to the given directory.
    pub fn generate(libui_dir: &Path, out_dir: &Path) -> Result<(), Error> {
        Header::main().generate(libui_dir, out_dir)?;
        Header::control_sigs().generate(libui_dir, out_dir)?;

        #[cfg(target_os = "macos")]
        Header::darwin().generate(libui_dir, out_dir)?;
        #[cfg(target_os = "linux")]
        Header::unix().generate(libui_dir, out_dir)?;
        #[cfg(target_os = "windows")]
        Header::windows().generate(libui_dir, out_dir)?;

        Ok(())
    }

    struct Header {
        lang: Language,
        include_stmts: Vec<IncludeStmt>,
        filename: String,
        blocklists_main: bool,
    }

    impl Header {
        fn main() -> Self {
            Self {
                lang: Language::C,
                include_stmts: vec![
                    IncludeStmt {
                        flavor: IncludeStmtFlavor::Include,
                        scope: IncludeStmtScope::Local,
                        arg: "ui.h".to_string(),
                    },
                ],
                filename: "bindings".to_string(),
                blocklists_main: false,
            }
        }

        fn control_sigs() -> Self {
            Self {
                lang: Language::C,
                include_stmts: vec![
                    IncludeStmt {
                        flavor: IncludeStmtFlavor::Include,
                        scope: IncludeStmtScope::Local,
                        arg: "common/controlsigs.h".to_string(),
                    },
                ],
                filename: "bindings-control-sigs".to_string(),
                blocklists_main: true,
            }
        }

        #[cfg(target_os = "macos")]
        fn darwin() -> Self {
            Self::ext(
                "darwin",
                Language::ObjC,
                iter::once({
                    IncludeStmt {
                        flavor: IncludeStmtFlavor::Import,
                        scope: IncludeStmtScope::System,
                        arg: "Cocoa/Cocoa.h".into(),
                    }
                })
            )
        }

        #[cfg(target_os = "linux")]
        fn unix() -> Self {
            Self::ext(
                "unix",
                Language::C,
                iter::once({
                    IncludeStmt {
                        flavor: IncludeStmtFlavor::Include,
                        scope: IncludeStmtScope::System,
                        arg: "gtk/gtk.h".into(),
                    }
                })
            )
        }

        #[cfg(target_os = "windows")]
        fn windows() -> Self {
            Self::ext(
                "windows",
                Language::C,
                iter::once({
                    IncludeStmt {
                        flavor: IncludeStmtFlavor::Include,
                        scope: IncludeStmtScope::System,
                        arg: "windows.h".into(),
                    }
                })
            )
        }

        fn ext(
            name: impl fmt::Display,
            lang: Language,
            deps: impl IntoIterator<Item = IncludeStmt>,
        ) -> Self {
            Self {
                lang,
                include_stmts: iter::once({
                    IncludeStmt {
                        flavor: IncludeStmtFlavor::Include,
                        scope: IncludeStmtScope::Local,
                        arg: "ui.h".to_string(),
                    }
                })
                .chain(deps)
                .chain(iter::once({
                    IncludeStmt {
                        flavor: IncludeStmtFlavor::Include,
                        scope: IncludeStmtScope::Local,
                        arg: format!("ui_{}.h", name),
                    }
                }))
                .collect(),
                filename: format!("bindings-{}", name),
                blocklists_main: true,
            }
        }

        fn generate(self, libui_dir: &Path, out_dir: &Path) -> Result<(), Error> {
            static LIBUI_REGEX: &str = "ui(?:[A-Z][a-z0-9]*)*";

            let mut builder = bindgen::builder()
                .header_contents("wrapper.h", &self.contents(libui_dir))
                .parse_callbacks(Box::new(bindgen::CargoCallbacks))
                .allowlist_recursively(false)
                .allowlist_function(LIBUI_REGEX)
                .allowlist_type(LIBUI_REGEX)
                .allowlist_var(LIBUI_REGEX);

            // Note: Virtually every wrapper except that for "ui.h" should blocklist "ui.h".
            if self.blocklists_main {
                builder = builder.blocklist_file(".*ui\\.h");
            }

            let bindings = builder
                .clang_args(ClangArgs::new().as_args())
                .clang_arg("-x")
                .clang_arg(match self.lang {
                    Language::C => "c",
                    #[cfg(target_os = "macos")]
                    Language::ObjC => "objective-c",
                })
                .layout_tests(false)
                .generate()
                .map_err(|_| Error::Generate)?;
            bindings.emit_warnings();

            let to = PathBuf::from(format!("{}.rs", self.filename));

            bindings
                .write_to_file(out_dir.join(to.as_path()))
                .into_report()
                .change_context_lazy(|| Error::WriteToFile { to })
        }

        fn contents(&self, libui_dir: &Path) -> String {
            self
                .include_stmts
                .iter()
                .map(|stmt| stmt.to_string(libui_dir))
                .collect::<Vec<String>>()
                .join("\n")
        }
    }

    enum Language {
        C,
        #[cfg(target_os = "macos")]
        ObjC,
    }

    struct IncludeStmt {
        flavor: IncludeStmtFlavor,
        scope: IncludeStmtScope,
        arg: String,
    }

    enum IncludeStmtFlavor {
        Include,
        #[cfg(target_os = "macos")]
        Import,
    }

    enum IncludeStmtScope {
        System,
        Local,
    }

    impl IncludeStmt {
        fn to_string(&self, libui_dir: &Path) -> String {
            format!(
                "#{} {}",
                match self.flavor {
                    IncludeStmtFlavor::Include => "include",
                    #[cfg(target_os = "macos")]
                    IncludeStmtFlavor::Import => "import",
                },
                match self.scope {
                    IncludeStmtScope::System => format!("<{}>", self.arg),
                    IncludeStmtScope::Local => format!(
                        "\"{}\"",
                        libui_dir.join(&self.arg).display(),
                    ),
                },
            )
        }
    }

    struct ClangArgs {
        defines: Vec<ClangDefine>,
        include_paths: Vec<PathBuf>,
        isysroot: Option<Cow<'static, Path>>,
    }

    struct ClangDefine {
        key: String,
        value: Option<String>,
    }

    impl ClangArgs {
        fn new() -> Self {
            #[cfg(target_os = "macos")]
            return Self::macos();
            #[cfg(target_os = "linux")]
            return Self::linux();
            #[cfg(target_os = "windows")]
            return Self::windows();
            #[allow(unreachable_code)]
            {
                unimplemented!("unsupported target OS");
            }
        }

        #[cfg(target_os = "macos")]
        fn macos() -> Self {
            use std::process;

            static DEFAULT_SDK_PATH: &str = concat!(
                "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer",
                "/SDKs/MacOSX.sdk",
            );

            let sdk_path = process::Command::new("xcrun")
                .args(["--sdk", "macosx", "--show-sdk-path"])
                .output()
                .ok()
                .map(|out| crate::unix::make_path_from_ascii(out.stdout))
                .map(Cow::Owned)
                .unwrap_or_else(|| Cow::Borrowed(Path::new(DEFAULT_SDK_PATH)));

            Self {
                defines: Vec::new(),
                include_paths: Vec::new(),
                // On macOS, *bindgen* looks in the wrong location for system headers, so we need to
                // manually specify it here.
                //
                // For more information, see
                // <https://zameermanji.com/blog/2021/7/13/using-bindgen-with-system-frameworks-on-macos/>.
                isysroot: Some(sdk_path),
            }
        }

        #[cfg(target_os = "linux")]
        fn linux() -> Self {
            let gtk = crate::gtk::find().unwrap();

            let defines = gtk
                .defines
                .into_iter()
                .map(|(key, value)| {
                    ClangDefine { key, value }
                })
                .collect();

            Self {
                defines,
                include_paths: gtk.include_paths,
                isysroot: None,
            }
        }

        #[cfg(target_os = "windows")]
        fn windows() -> Self {
            Self {
                defines: Vec::new(),
                include_paths: Vec::new(),
                isysroot: None,
            }
        }

        fn as_args(self) -> Vec<String> {
            let defines = self
                .defines
                .into_iter()
                .flat_map(|define| {
                    vec![
                        "-D".to_string(),
                        format!(
                            "{}{}",
                            define.key,
                            define.value.map(|it| format!("={}", it)).unwrap_or_default(),
                        ),
                    ]
                });

            let includes = self
                .include_paths
                .into_iter()
                .flat_map(|path| {
                    vec![
                        "-I".to_string(),
                        path.display().to_string(),
                    ]
                });

            let isysroot = self
                .isysroot
                .into_iter()
                .flat_map(|path| {
                    vec![
                        "-isysroot".to_string(),
                        path.display().to_string(),
                    ]
                });

            defines.chain(includes).chain(isysroot).collect()
        }
    }
}