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

use tracing::{debug, info};

use uv_cache::Cache;
use uv_fs::{LockedFile, Simplified};

use crate::cfg::PyVenvConfiguration;
use crate::{find_default_python, find_requested_python, Error, Interpreter};

/// A Python environment, consisting of a Python [`Interpreter`] and its associated paths.
#[derive(Debug, Clone)]
pub struct PythonEnvironment {
    root: PathBuf,
    interpreter: Interpreter,
}

impl PythonEnvironment {
    /// Create a [`PythonEnvironment`] for an existing virtual environment.
    pub fn from_virtualenv(cache: &Cache) -> Result<Self, Error> {
        let Some(venv) = detect_virtual_env()? else {
            return Err(Error::VenvNotFound);
        };
        let venv = fs_err::canonicalize(venv)?;
        let executable = detect_python_executable(&venv);
        let interpreter = Interpreter::query(&executable, cache)?;

        debug_assert!(
            interpreter.base_prefix() == interpreter.base_exec_prefix(),
            "Not a virtualenv (Python: {}, prefix: {})",
            executable.display(),
            interpreter.base_prefix().display()
        );

        Ok(Self {
            root: venv,
            interpreter,
        })
    }

    /// Create a [`PythonEnvironment`] for a Python interpreter specifier (e.g., a path or a binary name).
    pub fn from_requested_python(python: &str, cache: &Cache) -> Result<Self, Error> {
        let Some(interpreter) = find_requested_python(python, cache)? else {
            return Err(Error::RequestedPythonNotFound(python.to_string()));
        };
        Ok(Self {
            root: interpreter.prefix().to_path_buf(),
            interpreter,
        })
    }

    /// Create a [`PythonEnvironment`] for the default Python interpreter.
    pub fn from_default_python(cache: &Cache) -> Result<Self, Error> {
        let interpreter = find_default_python(cache)?;
        Ok(Self {
            root: interpreter.prefix().to_path_buf(),
            interpreter,
        })
    }

    /// Create a [`PythonEnvironment`] from an existing [`Interpreter`] and root directory.
    pub fn from_interpreter(interpreter: Interpreter) -> Self {
        Self {
            root: interpreter.prefix().to_path_buf(),
            interpreter,
        }
    }

    /// Returns the location of the Python interpreter.
    pub fn root(&self) -> &Path {
        &self.root
    }

    /// Return the [`Interpreter`] for this virtual environment.
    pub fn interpreter(&self) -> &Interpreter {
        &self.interpreter
    }

    /// Return the [`PyVenvConfiguration`] for this virtual environment, as extracted from the
    /// `pyvenv.cfg` file.
    pub fn cfg(&self) -> Result<PyVenvConfiguration, Error> {
        Ok(PyVenvConfiguration::parse(self.root.join("pyvenv.cfg"))?)
    }

    /// Returns the location of the Python executable.
    pub fn python_executable(&self) -> &Path {
        self.interpreter.sys_executable()
    }

    /// Returns an iterator over the `site-packages` directories inside a virtual environment.
    ///
    /// In most cases, `purelib` and `platlib` will be the same, and so the iterator will contain
    /// a single element; however, in some distributions, they may be different.
    pub fn site_packages(&self) -> impl Iterator<Item = &Path> {
        std::iter::once(self.interpreter.purelib()).chain(
            if self.interpreter.purelib() == self.interpreter.platlib() {
                None
            } else {
                Some(self.interpreter.platlib())
            },
        )
    }

    /// Returns the path to the `bin` directory inside a virtual environment.
    pub fn scripts(&self) -> &Path {
        self.interpreter.scripts()
    }

    /// Grab a file lock for the virtual environment to prevent concurrent writes across processes.
    pub fn lock(&self) -> Result<LockedFile, std::io::Error> {
        if self.interpreter.is_virtualenv() {
            // If the environment a virtualenv, use a virtualenv-specific lock file.
            LockedFile::acquire(self.root.join(".lock"), self.root.simplified_display())
        } else {
            // Otherwise, use a global lock file.
            LockedFile::acquire(
                env::temp_dir().join(format!("uv-{}.lock", cache_key::digest(&self.root))),
                self.root.simplified_display(),
            )
        }
    }
}

/// Locate the current virtual environment.
pub(crate) fn detect_virtual_env() -> Result<Option<PathBuf>, Error> {
    if let Some(dir) = env::var_os("VIRTUAL_ENV").filter(|value| !value.is_empty()) {
        info!(
            "Found a virtualenv through VIRTUAL_ENV at: {}",
            Path::new(&dir).display()
        );
        return Ok(Some(PathBuf::from(dir)));
    }
    if let Some(dir) = env::var_os("CONDA_PREFIX").filter(|value| !value.is_empty()) {
        info!(
            "Found a virtualenv through CONDA_PREFIX at: {}",
            Path::new(&dir).display()
        );
        return Ok(Some(PathBuf::from(dir)));
    }

    // Search for a `.venv` directory in the current or any parent directory.
    let current_dir = env::current_dir().expect("Failed to detect current directory");
    for dir in current_dir.ancestors() {
        let dot_venv = dir.join(".venv");
        if dot_venv.is_dir() {
            if !dot_venv.join("pyvenv.cfg").is_file() {
                return Err(Error::MissingPyVenvCfg(dot_venv));
            }
            debug!("Found a virtualenv named .venv at: {}", dot_venv.display());
            return Ok(Some(dot_venv));
        }
    }

    Ok(None)
}

/// Returns the path to the `python` executable inside a virtual environment.
pub(crate) fn detect_python_executable(venv: impl AsRef<Path>) -> PathBuf {
    let venv = venv.as_ref();
    if cfg!(windows) {
        // Search for `python.exe` in the `Scripts` directory.
        let executable = venv.join("Scripts").join("python.exe");
        if executable.exists() {
            return executable;
        }

        // Apparently, Python installed via msys2 on Windows _might_ produce a POSIX-like layout.
        // See: https://github.com/PyO3/maturin/issues/1108
        let executable = venv.join("bin").join("python.exe");
        if executable.exists() {
            return executable;
        }

        // Fallback for Conda environments.
        venv.join("python.exe")
    } else {
        // Search for `python` in the `bin` directory.
        venv.join("bin").join("python")
    }
}