//! Where the kernel tree and the patches live, and the file that can say so.
//!
//! Everything else asks this module for paths, so the config file and
//! `spectral init` touch nothing else. The precedence is the one the roadmap
//! promised: the environment wins over the file, and the file wins over the
//! built-in default.

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

use serde::{Deserialize, Serialize};

use crate::cli::InitArgs;
use crate::error::{Error, Result, exit_code};

/// Environment variable that points at the kernel checkout.
pub const ENV_KERNEL_TREE: &str = "SPECTRAL_KERNEL";

/// Environment variable that points at the patch directory.
pub const ENV_PATCH_DIR: &str = "SPECTRAL_PATCH_DIR";

/// Tree used when nothing else says otherwise, relative to `$HOME`.
const DEFAULT_KERNEL_TREE: &str = ".spectral/linux";

/// Where written patches land by default, relative to `$HOME`.
const DEFAULT_PATCH_DIR: &str = ".spectral/patches";

/// The file, relative to the config directory.
const CONFIG_FILE: &str = "spectral/config.toml";

/// What the config file holds.
///
/// Unknown keys are refused rather than ignored: a mistyped key that silently
/// does nothing is worse than one that stops the run and says so.
#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct ConfigFile {
    /// Where the kernel tree is.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub kernel_tree: Option<PathBuf>,

    /// Where written patches go.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub patch_dir: Option<PathBuf>,

    /// Addresses every patch is copied to, on top of what get_maintainer says.
    #[serde(default, skip_serializing_if = "Vec::is_empty")]
    pub always_cc: Vec<String>,

    /// Who a patch is sent as.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub send_email: Option<SendEmail>,
}

/// The `[send_email]` table.
#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct SendEmail {
    /// `git send-email --from`, when it should differ from git's own identity.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub from: Option<String>,
}

/// Where a resolved path came from.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Origin {
    /// One of the two environment variables.
    Environment(&'static str),
    /// The config file.
    File,
    /// The built-in default, relative to `$HOME`.
    Default,
}

impl Origin {
    /// How the report names it.
    #[must_use]
    pub fn describe(self) -> String {
        match self {
            Self::Environment(variable) => format!("from ${variable}"),
            Self::File => "from the config file".to_owned(),
            Self::Default => "the built-in default".to_owned(),
        }
    }
}

#[derive(Debug, Clone)]
pub struct Config {
    kernel_tree: PathBuf,
    patch_dir: PathBuf,
    tree_origin: Origin,
    patch_origin: Origin,
    file: ConfigFile,
    file_path: PathBuf,
}

impl Config {
    /// Resolve both paths: environment first, then the config file, then the
    /// default under `$HOME`.
    ///
    /// The tree is not checked for existence here. Commands that need it call
    /// [`Config::require_kernel_tree`], which leaves `kernel quest` working
    /// before anything is cloned.
    pub fn load() -> Result<Self> {
        let home = home_dir();
        let file_path = config_path(&home);
        let file = read_file(&file_path)?;

        let (kernel_tree, tree_origin) = resolve(
            ENV_KERNEL_TREE,
            file.kernel_tree.clone(),
            home.join(DEFAULT_KERNEL_TREE),
        )?;
        let (patch_dir, patch_origin) = resolve(
            ENV_PATCH_DIR,
            file.patch_dir.clone(),
            home.join(DEFAULT_PATCH_DIR),
        )?;

        Ok(Self {
            kernel_tree,
            patch_dir,
            tree_origin,
            patch_origin,
            file,
            file_path,
        })
    }

    #[must_use]
    pub fn kernel_tree(&self) -> &Path {
        &self.kernel_tree
    }

    #[must_use]
    pub fn patch_dir(&self) -> &Path {
        &self.patch_dir
    }

    /// Where the tree path came from, for `doctor` to print.
    ///
    /// A path that was resolved for the user and silently turns out to be the
    /// wrong one is a worse report than one that says which knob set it.
    #[must_use]
    pub fn kernel_tree_origin(&self) -> String {
        self.tree_origin.describe()
    }

    /// Where the patch directory came from, for `doctor` to print.
    #[must_use]
    pub fn patch_dir_origin(&self) -> String {
        self.patch_origin.describe()
    }

    /// The identity a patch is sent as, when the config file names one.
    #[must_use]
    pub fn send_email_from(&self) -> Option<String> {
        self.file
            .send_email
            .as_ref()
            .and_then(|send_email| send_email.from.clone())
    }

    /// Addresses every patch is copied to.
    #[must_use]
    pub fn always_cc(&self) -> &[String] {
        &self.file.always_cc
    }

    /// The config file this run read, whether or not it was there.
    #[must_use]
    pub fn file_path(&self) -> &Path {
        &self.file_path
    }

    /// Resolve the tree and check that it really is a kernel source tree.
    pub fn require_kernel_tree(&self) -> Result<&Path> {
        if self.kernel_tree.join("scripts/checkpatch.pl").is_file() {
            Ok(&self.kernel_tree)
        } else if self.kernel_tree.exists() {
            Err(Error::NotAKernelTree {
                path: self.kernel_tree.clone(),
            })
        } else {
            Err(Error::KernelTreeMissing {
                path: self.kernel_tree.clone(),
                env: ENV_KERNEL_TREE,
            })
        }
    }
}

/// What `spectral init` did.
#[derive(Debug, Clone)]
pub struct Init {
    /// The file that was written, or that already said this.
    pub config_file: PathBuf,
    /// The tree the config now names.
    pub kernel_tree: PathBuf,
    /// Whether anything was written.
    pub wrote: bool,
}

/// `spectral init`: point the config at a tree, verifying or cloning one.
///
/// Idempotent on purpose. A second run that would change nothing says so and
/// writes nothing; a run that would change an existing answer is refused until
/// `--force` says it meant it.
pub fn init(args: &InitArgs) -> Result<Init> {
    let home = home_dir();
    let config_file = config_path(&home);
    let current = read_file(&config_file)?;

    let tree = match (&args.tree, &args.clone) {
        // The destination is named first: the refusal below has to happen
        // before a clone runs, not after one has already landed.
        (tree, Some(_)) => tree
            .clone()
            .unwrap_or_else(|| home.join(DEFAULT_KERNEL_TREE)),
        (Some(tree), None) => tree.clone(),
        (None, None) => Config::load()?.kernel_tree().to_path_buf(),
    };

    let mut next = current.clone();
    next.kernel_tree = Some(tree.clone());
    let changed = next != current;
    if changed && config_file.is_file() && !args.force {
        return Err(Error::ConfigSaysSomethingElse { path: config_file });
    }

    if let Some(url) = &args.clone {
        clone_tree(url, &tree, args.depth)?;
    }
    verify_tree(&tree)?;

    if changed {
        write_file(&config_file, &next)?;
    }

    Ok(Init {
        config_file,
        kernel_tree: tree,
        wrote: changed,
    })
}

/// Clone `url` into `into`.
///
/// A tree already sitting there is left alone: `init` run twice should not
/// re-clone anything.
fn clone_tree(url: &str, into: &Path, depth: Option<u32>) -> Result<()> {
    if into.join("scripts/checkpatch.pl").is_file() {
        return Ok(());
    }
    if let Some(parent) = into.parent() {
        std::fs::create_dir_all(parent)?;
    }

    let mut args = vec!["clone".to_owned()];
    if let Some(depth) = depth {
        args.push("--depth".to_owned());
        args.push(depth.to_string());
    }
    args.push(url.to_owned());
    args.push(into.display().to_string());

    let output =
        Command::new("git")
            .args(&args)
            .output()
            .map_err(|source| Error::CommandSpawn {
                program: "git clone".to_owned(),
                source,
            })?;
    if !output.status.success() {
        return Err(Error::ExternalCommand {
            program: "git".to_owned(),
            args: args.join(" "),
            code: exit_code(&output.status),
            stderr: String::from_utf8_lossy(&output.stderr)
                .trim_end()
                .to_owned(),
        });
    }

    Ok(())
}

/// Check that a path really is a kernel source tree.
fn verify_tree(tree: &Path) -> Result<()> {
    if tree.join("scripts/checkpatch.pl").is_file() {
        Ok(())
    } else if tree.exists() {
        Err(Error::NotAKernelTree {
            path: tree.to_path_buf(),
        })
    } else {
        Err(Error::KernelTreeMissing {
            path: tree.to_path_buf(),
            env: ENV_KERNEL_TREE,
        })
    }
}

/// Read the config file, or hand back an empty one when it is not there.
fn read_file(path: &Path) -> Result<ConfigFile> {
    let text = match std::fs::read_to_string(path) {
        Ok(text) => text,
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {
            return Ok(ConfigFile::default());
        }
        Err(error) => return Err(Error::Io(error)),
    };

    toml::from_str(&text).map_err(|source| Error::ConfigUnreadable {
        path: path.to_path_buf(),
        source,
    })
}

/// Write the config file, making its directory first.
fn write_file(path: &Path, file: &ConfigFile) -> Result<()> {
    let text = toml::to_string(file).map_err(|source| Error::ConfigUnwritable { source })?;
    if let Some(parent) = path.parent() {
        std::fs::create_dir_all(parent)?;
    }
    std::fs::write(path, text)?;

    Ok(())
}

/// Where the config file lives: `$XDG_CONFIG_HOME/spectral/config.toml`, or
/// `~/.config/spectral/config.toml` when that variable is unset.
#[must_use]
pub fn config_path(home: &Path) -> PathBuf {
    env::var_os("XDG_CONFIG_HOME")
        .map_or_else(|| home.join(".config"), PathBuf::from)
        .join(CONFIG_FILE)
}

/// Pick one path out of the environment, the file, and the default.
///
/// A relative path in the file is refused rather than resolved against
/// whichever directory the command happened to run in.
fn resolve(
    variable: &'static str,
    from_file: Option<PathBuf>,
    fallback: PathBuf,
) -> Result<(PathBuf, Origin)> {
    if let Some(value) = env::var_os(variable) {
        return Ok((PathBuf::from(value), Origin::Environment(variable)));
    }
    if let Some(path) = from_file {
        if !path.is_absolute() {
            return Err(Error::ConfigPathRelative { variable, path });
        }

        return Ok((path, Origin::File));
    }

    Ok((fallback, Origin::Default))
}

fn home_dir() -> PathBuf {
    env::var_os("HOME").map_or_else(|| PathBuf::from("."), PathBuf::from)
}