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

/// How many characters of the git commit that should be added to the version name
/// in dev builds.
const GIT_HASH_DEV_SUFFIX_LEN: usize = 6;

const ANDROID_VERSION_FILE_PATH: &str = "../dist-assets/android-product-version.txt";
const DESKTOP_VERSION_FILE_PATH: &str = "../dist-assets/desktop-product-version.txt";

#[derive(Debug, Copy, Clone)]
enum Target {
    Android,
    Desktop,
}

impl Target {
    pub fn current_target() -> Self {
        println!("cargo:rerun-if-env-changed=CARGO_CFG_TARGET_OS");
        match env::var("CARGO_CFG_TARGET_OS")
            .expect("CARGO_CFG_TARGET_OS should be set")
            .as_str()
        {
            "android" => Self::Android,
            "linux" | "windows" | "macos" => Self::Desktop,
            target_os => panic!("Unsupported target OS: {target_os}"),
        }
    }
}

fn main() {
    let product_version = get_product_version(Target::current_target());
    let android_product_version = get_product_version(Target::Android);

    let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
    fs::write(out_dir.join("product-version.txt"), product_version).unwrap();
    fs::write(
        out_dir.join("android-product-version.txt"),
        android_product_version,
    )
    .unwrap();
}

/// Returns the Mullvad product version from the corresponding metadata files,
/// depending on target platform.
fn get_product_version(target: Target) -> String {
    let version_file_path = match target {
        Target::Android => ANDROID_VERSION_FILE_PATH,
        Target::Desktop => DESKTOP_VERSION_FILE_PATH,
    };
    println!("cargo:rerun-if-changed={version_file_path}");
    let version = fs::read_to_string(version_file_path)
        .unwrap_or_else(|_| panic!("Failed to read {version_file_path}"))
        .trim()
        .to_owned();

    if let Some(dev_suffix) = get_dev_suffix(target, &version) {
        format!("{version}{dev_suffix}")
    } else {
        version
    }
}

/// Returns the development suffix for the current build. A build has a development
/// suffix if the build is not done on a git tag named `product_version`.
/// This also returns `None` if the `git` command can't run, or the code does
/// not live in a git repository.
fn get_dev_suffix(target: Target, product_version: &str) -> Option<String> {
    // Compute the expected tag name for the release named `product_version`
    let release_tag = match target {
        Target::Android => format!("android/{product_version}"),
        Target::Desktop => product_version.to_owned(),
    };

    // Get the git commit hashes for the latest release and current HEAD
    // Return `None` if unable to find the hash for HEAD.
    let head_commit_hash = git_rev_parse_commit_hash("HEAD")?;
    let product_version_commit_hash = git_rev_parse_commit_hash(&release_tag);

    // If we are currently building the release tag, there is no dev suffix
    if Some(&head_commit_hash) == product_version_commit_hash.as_ref() {
        return None;
    }
    Some(format!(
        "-dev-{}",
        &head_commit_hash[..GIT_HASH_DEV_SUFFIX_LEN]
    ))
}

/// Returns the commit hash for the commit that `git_ref` is pointing to.
///
/// Returns `None` if executing the `git rev-parse` command fails for some reason.
fn git_rev_parse_commit_hash(git_ref: &str) -> Option<String> {
    let git_dir = Path::new("..").join(".git");
    // If we build our output on information about HEAD we need to re-run if HEAD moves
    if git_ref == "HEAD" {
        let head_path = git_dir.join("HEAD");
        if head_path.exists() {
            println!("cargo:rerun-if-changed={}", head_path.display());
        }
    }
    // If we build our output on information about a git reference, we need to re-run
    // if it moves. Instead of trying to be smart, just re-run if any git reference moves.
    let git_refs_dir = git_dir.join("refs");
    if git_refs_dir.exists() {
        println!("cargo:rerun-if-changed={}", git_refs_dir.display());
    }

    let output = Command::new("git")
        .arg("rev-parse")
        .arg(format!("{git_ref}^{{commit}}"))
        .output()
        .ok()?;
    if !output.status.success() {
        return None;
    }
    Some(String::from_utf8(output.stdout).unwrap().trim().to_owned())
}