use difference::Changeset;
use itertools::sorted;
use std::cmp::Ordering;
use std::ffi::{OsStr, OsString};
use std::iter::Peekable;
use std::path::{Path, PathBuf};
use std::str::from_utf8;
use thiserror::Error;
use walkdir::{DirEntry, WalkDir};

/// Errors which may be returned from `directory_compare`.
#[derive(Error, Debug)]
pub enum DirCompareError {
    /// An error occurred when walking over either of the
    /// input directories.
    #[error("Failed to walk over directory tree: {0}")]
    WalkDir(walkdir::Error),
    /// An entry, requested as part of the expected entry set, was
    /// not present in one of the directories.
    #[error("Missing Entry {0:#?}")]
    MissingEntry(OsString),
    /// The type of an entry differed (e.g., "file" vs "directory") for
    /// a compared path.
    #[error("Mismatched Entry Types: [{:#?}]: {:#?} vs [#{:#?}]: {:#?}",
        .lhs.path(),
        .lhs.file_type(),
        .rhs.path(),
        .rhs.file_type()
    )]
    EntryTypeMismatch { lhs: DirEntry, rhs: DirEntry },
    /// The contents of a file differ between the compared paths.
    #[error("File contents differ: {0}")]
    ContentsDiffer(String),
    /// A generic I/O error occurred while accessing the filesystem.
    #[error(transparent)]
    Io(#[from] std::io::Error),
}

// A wrapper class, containing both the original DirEntry (directly
// returned from Walkdir) and a path relative to the root directory.
#[derive(Clone, Debug)]
struct Entry {
    // Original DirEntry
    dirent: DirEntry,
    // Path relative to provided root directory.
    path: PathBuf,
}

// Move `iter` forward until it returns an entry matching `golden`.
// Returns the DirEntry which contains `golden`.
fn advance_iter<P, I>(
    source: P,
    iter: &mut Peekable<I>,
    golden: &OsStr,
) -> Result<Entry, DirCompareError>
where
    P: AsRef<Path>,
    I: Iterator<Item = walkdir::Result<Entry>>,
{
    loop {
        let result = iter.peek().ok_or_else(|| {
            DirCompareError::MissingEntry(source.as_ref().join(golden.to_owned()).into_os_string())
        })?;

        let item = match result {
            Ok(value) => value,
            Err(_) => {
                // Why this is safe:
                // - We successfully peeked the iterator, so this item exists.
                // - It is an error type.
                //
                // This hackery is a kludge to allow the "non-error" case
                // to continue operating on a non-owned reference, but
                // permitting the error case to take ownership of the error.
                let err = iter.next().unwrap().unwrap_err();
                return Err(DirCompareError::WalkDir(err));
            }
        };

        match item.path.as_path().cmp(Path::new(golden)) {
            Ordering::Less => {
                let _ = iter.next();
            }
            Ordering::Equal => {
                return Ok(item.clone());
            }
            Ordering::Greater => {
                return Err(DirCompareError::MissingEntry(
                    source.as_ref().join(golden.to_owned()).into_os_string(),
                ));
            }
        }
    }
}

fn compare_by_path(a: &DirEntry, b: &DirEntry) -> Ordering {
    a.path().cmp(b.path())
}

fn sorted_walkdir<P>(p: &P) -> impl Iterator<Item = walkdir::Result<Entry>> + '_
where
    P: AsRef<Path>,
{
    WalkDir::new(p)
        // Avoid returning the directory itself.
        .min_depth(1)
        // Return consistent order.
        .sort_by(compare_by_path)
        .into_iter()
        // Strip the provided prefix of the path; we want visibility
        // into the paths of files *relative* to the input path.
        .map(move |item| {
            item.map(|entry| {
                let path = entry.path().strip_prefix(p).unwrap().to_path_buf();
                Entry {
                    dirent: entry,
                    path,
                }
            })
        })
}

/// Compares the selected contents of two directories.
/// Checks all requested golden_paths within both directories.
///
/// The acronynms "lhs" and "rhs" are used as shorthands
/// for "{Left, Right} Hand Side".
///
/// ```
/// use directory_compare::directory_compare;
/// use tempdir::TempDir;
/// let lhs = TempDir::new("first_directory").unwrap();
/// let rhs = TempDir::new("second_directory").unwrap();
///
/// std::fs::write(lhs.path().join("my-file.txt"), "contents").unwrap();
/// std::fs::write(rhs.path().join("my-file.txt"), "contents").unwrap();
///
/// directory_compare(
///     &mut vec!["my-file.txt"].into_iter(),
///     lhs.path(),
///     rhs.path()
/// ).unwrap();
/// ```
pub fn directory_compare<P, I, Q, R>(
    golden_paths: &mut I,
    lhs: Q,
    rhs: R,
) -> Result<(), DirCompareError>
where
    P: AsRef<Path> + Ord,
    Q: AsRef<Path>,
    R: AsRef<Path>,
    I: Iterator<Item = P>,
{
    // Stable ordering is necessary for the following comparison
    // algorithm.
    let golden_paths = sorted(golden_paths);
    let mut lhs_iter = sorted_walkdir(&lhs).peekable();
    let mut rhs_iter = sorted_walkdir(&rhs).peekable();

    for golden in golden_paths {
        let golden_os_str = golden.as_ref().as_os_str();

        // Ignore all non-requested paths, advancing the iterators
        // past unrelated content.
        let lhs_entry = advance_iter(&lhs, &mut lhs_iter, golden_os_str)?;
        let rhs_entry = advance_iter(&rhs, &mut rhs_iter, golden_os_str)?;

        if lhs_entry.dirent.file_type() != rhs_entry.dirent.file_type() {
            return Err(DirCompareError::EntryTypeMismatch {
                lhs: lhs_entry.dirent,
                rhs: rhs_entry.dirent,
            });
        }

        if lhs_entry.dirent.file_type().is_file() {
            let lhs_contents = std::fs::read(lhs_entry.dirent.path())?;
            let rhs_contents = std::fs::read(rhs_entry.dirent.path())?;

            match (from_utf8(&lhs_contents), from_utf8(&rhs_contents)) {
                // Use a prettier, string-based diff for files which
                // are valid UTF-8.
                (Ok(lhs_str), Ok(rhs_str)) => {
                    let changes = Changeset::new(&lhs_str, &rhs_str, "\n");
                    if changes.distance != 0 {
                        return Err(DirCompareError::ContentsDiffer(format!(
                            "{:#?} != {:#?}:\n{}",
                            lhs_entry.dirent.path(),
                            rhs_entry.dirent.path(),
                            changes
                        )));
                    }
                }
                // For files which are invalid UTF-8, revert to a full hex-based
                // diff.
                //
                // TODO: We could try to only show the portion of the contents
                // which differ, rather than a full dump.
                (_, _) => {
                    if lhs_contents != rhs_contents {
                        return Err(DirCompareError::ContentsDiffer(format!(
                            "{:#?} != {:#?}:\n{}\n{}",
                            lhs_entry.dirent.path(),
                            rhs_entry.dirent.path(),
                            pretty_hex::pretty_hex(&lhs_contents),
                            pretty_hex::pretty_hex(&rhs_contents),
                        )));
                    }
                }
            }
        }
    }
    Ok(())
}

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

    #[test]
    fn compare_empty() -> Result<()> {
        let lhs = "testdata/lhs";
        let rhs = "testdata/rhs";
        let mut iter: std::vec::IntoIter<String> = vec![].into_iter();
        directory_compare(&mut iter, &lhs, &rhs)?;
        Ok(())
    }

    #[test]
    fn compare_file_same_contents() -> Result<()> {
        let lhs = "testdata/lhs";
        let rhs = "testdata/rhs";
        directory_compare(&mut vec!["file1.txt"].into_iter(), &lhs, &rhs)?;
        Ok(())
    }

    #[test]
    fn compare_multiple_files_same_contents() -> Result<()> {
        let lhs = "testdata/lhs";
        let rhs = "testdata/rhs";
        directory_compare(
            &mut vec!["file1.txt", "file2.txt", "subdirectory/file3.txt"].into_iter(),
            &lhs,
            &rhs,
        )?;
        Ok(())
    }

    #[test]
    fn compare_file_different_contents() -> Result<()> {
        let lhs = "testdata/lhs";
        let rhs = "testdata/rhs";
        let result = directory_compare(&mut vec!["differing.txt"].into_iter(), &lhs, &rhs);
        assert!(result.is_err());
        assert!(matches!(
            result.unwrap_err(),
            DirCompareError::ContentsDiffer(_)
        ));
        Ok(())
    }

    #[test]
    fn compare_directory_missing() -> Result<()> {
        let lhs = "testdata/lhs";
        let rhs = "testdata/rhs";
        let result = directory_compare(&mut vec!["lhs_only"].into_iter(), &lhs, &rhs);
        assert!(result.is_err());
        assert!(matches!(
            result.unwrap_err(),
            DirCompareError::MissingEntry(_)
        ));
        Ok(())
    }

    #[test]
    fn compare_file_differing_in_type() -> Result<()> {
        let lhs = "testdata/lhs";
        let rhs = "testdata/rhs";
        let result = directory_compare(&mut vec!["different_type"].into_iter(), &lhs, &rhs);
        assert!(result.is_err());
        assert!(matches!(
            result.unwrap_err(),
            DirCompareError::EntryTypeMismatch { lhs: _, rhs: _ }
        ));
        Ok(())
    }

    #[test]
    fn compare_differing_binary() -> Result<()> {
        let lhs = "testdata/lhs";
        let rhs = "testdata/rhs";
        let result = directory_compare(&mut vec!["binary"].into_iter(), &lhs, &rhs);
        assert!(result.is_err());
        assert!(matches!(
            result.unwrap_err(),
            DirCompareError::ContentsDiffer(_),
        ));
        Ok(())
    }
}