//! What the file system of the working copy can represent (spec §7.7),
//! and which paths of a state it cannot.
//!
//! Executable bits and symbolic links exist on Unix only; elsewhere a
//! snapshot keeps the recorded executable bit and reads a symlink written
//! out as a plain file back as the symlink. macOS and Windows file systems
//! ignore case by default and Windows forbids some names, so paths that
//! differ only in case or use such names are reported before they are
//! written.

use std::collections::BTreeMap;

/// Capabilities of the working copy's file system.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Platform {
    pub exec_bits: bool,
    pub symlinks: bool,
    pub case_insensitive: bool,
    /// Windows' name rules: reserved device names, `<>:"\|?*`, trailing dots and spaces.
    pub windows_names: bool,
}

impl Platform {
    /// The platform oxy runs on, with its default file system.
    pub fn current() -> Platform {
        Platform {
            exec_bits: cfg!(unix),
            symlinks: cfg!(unix),
            case_insensitive: cfg!(any(target_os = "macos", target_os = "windows")),
            windows_names: cfg!(target_os = "windows"),
        }
    }

    pub fn windows() -> Platform {
        Platform {
            exec_bits: false,
            symlinks: false,
            case_insensitive: true,
            windows_names: true,
        }
    }

    pub fn macos() -> Platform {
        Platform {
            exec_bits: true,
            symlinks: true,
            case_insensitive: true,
            windows_names: false,
        }
    }

    /// Problems this platform would have with these paths.
    pub fn problems<'a>(&self, paths: impl IntoIterator<Item = &'a str>) -> Vec<String> {
        let paths: Vec<&str> = paths.into_iter().collect();
        let mut out = Vec::new();
        if self.case_insensitive {
            out.extend(
                case_collisions(&paths).into_iter().map(|(a, b)| {
                    format!("{a} and {b} differ only in case; one will hide the other")
                }),
            );
        }
        if self.windows_names {
            out.extend(
                paths
                    .iter()
                    .filter_map(|p| windows_name_problem(p).map(|why| format!("{p}: {why}"))),
            );
        }
        out
    }
}

/// Pairs of paths equal but for case (including a directory's case).
pub fn case_collisions(paths: &[&str]) -> Vec<(String, String)> {
    let mut seen: BTreeMap<String, &str> = BTreeMap::new();
    let mut out = Vec::new();
    // Every prefix counts: `Docs/a` and `docs/b` put two directories in one place.
    let mut prefixes: Vec<&str> = Vec::new();
    for p in paths {
        let mut end = 0;
        for part in p.split('/') {
            end += part.len();
            prefixes.push(&p[..end]);
            end += 1;
        }
    }
    prefixes.sort();
    prefixes.dedup();
    for p in prefixes {
        let key = p.to_lowercase();
        match seen.get(&key) {
            Some(first) if *first != p => out.push((first.to_string(), p.to_string())),
            Some(_) => {}
            None => {
                seen.insert(key, p);
            }
        }
    }
    out
}

/// `path.canonicalize()`, but on Windows without the verbatim `\\?\`
/// prefix when the plain form names the same file (oxygen#143): a root
/// shown as `\\?\C:\work\demo` in every message otherwise.
pub fn canonical(path: &std::path::Path) -> std::io::Result<std::path::PathBuf> {
    path.canonicalize().map(plain)
}

/// [`canonical`]'s rule alone: `\\?\C:\…` as `C:\…` and `\\?\UNC\…` as
/// `\\…`, when the result is under Windows' old length limit and has no
/// component the plain form would read differently (`.`, `..`, a trailing
/// dot or space, a reserved name). Anything else is left as it is.
pub fn plain(p: std::path::PathBuf) -> std::path::PathBuf {
    let Some(s) = p.to_str() else { return p };
    let unchanged_by_plain = |rest: &str| {
        rest.len() < 260
            && !rest.split('\\').any(|c| {
                c == "."
                    || c == ".."
                    || c.ends_with('.')
                    || c.ends_with(' ')
                    || (!c.is_empty() && windows_name_problem(c).is_some())
            })
    };
    if let Some(rest) = s.strip_prefix(r"\\?\UNC\") {
        if unchanged_by_plain(rest) {
            return std::path::PathBuf::from(format!(r"\\{rest}"));
        }
    } else if let Some(rest) = s.strip_prefix(r"\\?\") {
        let b = rest.as_bytes();
        let drive = b.len() >= 3 && b[0].is_ascii_alphabetic() && b[1] == b':' && b[2] == b'\\';
        if drive && unchanged_by_plain(&rest[3..]) {
            return std::path::PathBuf::from(rest);
        }
    }
    p
}

/// Why Windows would refuse a path, if it would.
pub fn windows_name_problem(path: &str) -> Option<&'static str> {
    const RESERVED: &[&str] = &[
        "CON", "PRN", "AUX", "NUL", "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8",
        "COM9", "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9",
    ];
    for part in path.split('/') {
        if part
            .chars()
            .any(|c| matches!(c, '<' | '>' | ':' | '"' | '\\' | '|' | '?' | '*') || (c as u32) < 32)
        {
            return Some("contains a character Windows does not allow in names");
        }
        if part.ends_with('.') || part.ends_with(' ') {
            return Some("a name ending in a dot or space cannot be created on Windows");
        }
        let stem = part.split('.').next().unwrap_or("").to_ascii_uppercase();
        if RESERVED.contains(&stem.as_str()) {
            return Some("a reserved device name on Windows");
        }
    }
    None
}

#[cfg(test)]
mod tests {
    use super::*;

    /// oxygen#143: Windows' verbatim prefix is dropped where the plain path
    /// says the same, and kept where it would not.
    #[test]
    fn a_verbatim_windows_path_is_shown_plain_when_it_can_be() {
        use std::path::PathBuf;
        let p = |s: &str| plain(PathBuf::from(s)).to_string_lossy().into_owned();
        assert_eq!(p(r"\\?\C:\work\demo"), r"C:\work\demo");
        assert_eq!(p(r"\\?\UNC\server\share\repo"), r"\\server\share\repo");
        assert_eq!(
            p(r"\\?\C:\work\CON"),
            r"\\?\C:\work\CON",
            "a reserved name stays verbatim"
        );
        assert_eq!(p(r"\\?\C:\work\trailing."), r"\\?\C:\work\trailing.");
        assert_eq!(
            p(r"\\?\Volume{1234}\x"),
            r"\\?\Volume{1234}\x",
            "not a drive path"
        );
        let long = format!(r"\\?\C:\{}", "a".repeat(300));
        assert_eq!(p(&long), long, "past the old length limit");
        assert_eq!(
            p("/home/andy/repo"),
            "/home/andy/repo",
            "nothing to do elsewhere"
        );
    }

    #[test]
    fn collisions_and_names() {
        assert_eq!(
            case_collisions(&["README.md", "readme.md", "src/a.rs"]),
            vec![("README.md".into(), "readme.md".into())]
        );
        assert_eq!(
            case_collisions(&["Docs/a", "docs/b"]),
            vec![("Docs".into(), "docs".into())]
        );
        assert!(case_collisions(&["a/b", "a/c", "b"]).is_empty());
        assert!(windows_name_problem("src/con.rs").is_some());
        assert!(windows_name_problem("notes/a:b.txt").is_some());
        assert!(windows_name_problem("trailing./x").is_some());
        assert!(windows_name_problem("src/console.rs").is_none());
        let w = Platform::windows();
        assert_eq!(w.problems(["A.txt", "a.txt", "aux.c"]).len(), 2);
        assert!(Platform {
            exec_bits: true,
            symlinks: true,
            case_insensitive: false,
            windows_names: false
        }
        .problems(["A", "a"])
        .is_empty());
    }
}