//! Toolchain resolver (§22.2).
//!
//! Reads project metadata files to determine the pinned Python version, then
//! maps it to the closest Nyx reference image. Records `pin_origin` (where the
//! version was found) and a `toolchain_drift` flag when the resolved image is
//! not an exact match for the requested version.

use std::path::Path;

/// Resolved toolchain information for a target directory.
#[derive(Debug, Clone)]
pub struct ToolchainResolution {
    /// Nyx reference toolchain identifier (e.g. `"python-3.11"`).
    pub toolchain_id: String,
    /// Where the version pin was read from.
    pub pin_origin: PinOrigin,
    /// Whether the resolved toolchain differs from the exact pinned version.
    pub toolchain_drift: bool,
    /// Resolved semver string (e.g. `"3.11.5"`).
    pub version_string: String,
}

/// Where the toolchain version pin was discovered.
#[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum PinOrigin {
    /// `.python-version` file (pyenv).
    PythonVersion,
    /// `pyproject.toml` `[tool.python]` or `[project] requires-python`.
    PyprojectToml,
    /// `Pipfile` `[requires] python_version`.
    Pipfile,
    /// `runtime.txt` (Heroku-style).
    RuntimeTxt,
    /// No pin found; used the system default.
    SystemDefault,
}

/// Resolve the Python toolchain for `project_root`.
///
/// Reads project pin files in priority order:
/// `.python-version` > `pyproject.toml` > `Pipfile` > `runtime.txt` > default.
pub fn resolve_python(project_root: &Path) -> ToolchainResolution {
    if let Some(r) = try_python_version_file(project_root) {
        return r;
    }
    if let Some(r) = try_pyproject_toml(project_root) {
        return r;
    }
    if let Some(r) = try_pipfile(project_root) {
        return r;
    }
    if let Some(r) = try_runtime_txt(project_root) {
        return r;
    }
    default_python()
}

fn try_python_version_file(root: &Path) -> Option<ToolchainResolution> {
    let path = root.join(".python-version");
    let content = std::fs::read_to_string(&path).ok()?;
    let version = content.trim().to_owned();
    if version.is_empty() {
        return None;
    }
    Some(map_version(&version, PinOrigin::PythonVersion))
}

fn try_pyproject_toml(root: &Path) -> Option<ToolchainResolution> {
    let content = std::fs::read_to_string(root.join("pyproject.toml")).ok()?;
    // Look for `requires-python = ">=3.11"` or `python = "3.11"`.
    for line in content.lines() {
        let line = line.trim();
        if line.starts_with("requires-python") || (line.starts_with("python") && line.contains('=') && !line.starts_with("python_requires")) {
            if let Some(ver) = extract_version_from_toml_value(line) {
                return Some(map_version(&ver, PinOrigin::PyprojectToml));
            }
        }
    }
    None
}

fn try_pipfile(root: &Path) -> Option<ToolchainResolution> {
    let content = std::fs::read_to_string(root.join("Pipfile")).ok()?;
    let mut in_requires = false;
    for line in content.lines() {
        let line = line.trim();
        if line == "[requires]" {
            in_requires = true;
            continue;
        }
        if line.starts_with('[') {
            in_requires = false;
        }
        if in_requires && line.starts_with("python_version") {
            if let Some(ver) = extract_version_from_toml_value(line) {
                return Some(map_version(&ver, PinOrigin::Pipfile));
            }
        }
    }
    None
}

fn try_runtime_txt(root: &Path) -> Option<ToolchainResolution> {
    let content = std::fs::read_to_string(root.join("runtime.txt")).ok()?;
    let line = content.lines().next()?.trim();
    // e.g. "python-3.11.5"
    let version = line.strip_prefix("python-").unwrap_or(line);
    if version.is_empty() {
        return None;
    }
    Some(map_version(version, PinOrigin::RuntimeTxt))
}

fn default_python() -> ToolchainResolution {
    ToolchainResolution {
        toolchain_id: "python-3".to_owned(),
        pin_origin: PinOrigin::SystemDefault,
        toolchain_drift: false,
        version_string: "3".to_owned(),
    }
}

/// Extract the bare version string from a TOML assignment like:
///   `requires-python = ">=3.11"`  → `"3.11"`
///   `python_version = "3.11"`     → `"3.11"`
fn extract_version_from_toml_value(line: &str) -> Option<String> {
    let after_eq = line.splitn(2, '=').nth(1)?;
    let raw = after_eq.trim().trim_matches('"').trim_matches('\'');
    // Strip leading comparators: >=, <=, ==, ~=, ^, >
    let ver = raw.trim_start_matches(|c: char| !c.is_ascii_digit());
    if ver.is_empty() {
        return None;
    }
    Some(ver.to_owned())
}

/// Map a raw version string to a Nyx reference toolchain ID.
///
/// Reference images: `python-3.8`, `python-3.9`, `python-3.10`,
/// `python-3.11`, `python-3.12`, `python-3.13`.
fn map_version(version: &str, origin: PinOrigin) -> ToolchainResolution {
    // Normalise: take major.minor from "3.11.5" → "3.11"
    let parts: Vec<&str> = version.splitn(3, '.').collect();
    let major = parts.first().copied().unwrap_or("3");
    let minor = parts.get(1).copied();

    let (toolchain_id, drift) = match (major, minor) {
        ("3", Some("8")) => ("python-3.8".to_owned(), false),
        ("3", Some("9")) => ("python-3.9".to_owned(), false),
        ("3", Some("10")) => ("python-3.10".to_owned(), false),
        ("3", Some("11")) => ("python-3.11".to_owned(), false),
        ("3", Some("12")) => ("python-3.12".to_owned(), false),
        ("3", Some("13")) => ("python-3.13".to_owned(), false),
        // Older 3.x → nearest supported is 3.8
        ("3", Some(m)) if m.parse::<u32>().map_or(false, |v| v < 8) => {
            ("python-3.8".to_owned(), true)
        }
        // Newer 3.x beyond catalog → use 3.13 as closest
        ("3", Some(_)) => ("python-3.13".to_owned(), true),
        ("3", None) => ("python-3".to_owned(), false),
        // Python 2 → unsupported, use system default as closest
        ("2", _) => ("python-3".to_owned(), true),
        _ => ("python-3".to_owned(), true),
    };

    ToolchainResolution {
        version_string: version.to_owned(),
        toolchain_id,
        pin_origin: origin,
        toolchain_drift: drift,
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::fs;
    use tempfile::TempDir;

    #[test]
    fn python_version_file_exact() {
        let dir = TempDir::new().unwrap();
        fs::write(dir.path().join(".python-version"), "3.11.5\n").unwrap();
        let r = resolve_python(dir.path());
        assert_eq!(r.toolchain_id, "python-3.11");
        assert!(!r.toolchain_drift);
        assert_eq!(r.pin_origin, PinOrigin::PythonVersion);
    }

    #[test]
    fn python_version_file_drift() {
        let dir = TempDir::new().unwrap();
        fs::write(dir.path().join(".python-version"), "3.7\n").unwrap();
        let r = resolve_python(dir.path());
        assert!(r.toolchain_drift);
    }

    #[test]
    fn pyproject_requires_python() {
        let dir = TempDir::new().unwrap();
        fs::write(dir.path().join("pyproject.toml"), "[project]\nrequires-python = \">=3.11\"\n").unwrap();
        let r = resolve_python(dir.path());
        assert_eq!(r.toolchain_id, "python-3.11");
        assert_eq!(r.pin_origin, PinOrigin::PyprojectToml);
    }

    #[test]
    fn pipfile_python_version() {
        let dir = TempDir::new().unwrap();
        fs::write(dir.path().join("Pipfile"), "[requires]\npython_version = \"3.10\"\n").unwrap();
        let r = resolve_python(dir.path());
        assert_eq!(r.toolchain_id, "python-3.10");
        assert_eq!(r.pin_origin, PinOrigin::Pipfile);
    }

    #[test]
    fn fallback_to_system_default() {
        let dir = TempDir::new().unwrap();
        let r = resolve_python(dir.path());
        assert_eq!(r.pin_origin, PinOrigin::SystemDefault);
    }
}