use std::collections::{BTreeSet, HashMap};
use std::fs::File;
use std::io;
use std::io::{Read, Seek, SeekFrom};
use std::ops::Deref;
use std::path::Path;
use std::str::FromStr;
use std::sync::RwLock;

use cfg_if::cfg_if;
use log::{debug, error};
use mime::Mime;
use mime_guess::from_ext;
use once_cell::sync::Lazy;
use walkdir::{DirEntry, WalkDir};

use crate::findings::{Findings, ScanError};
use crate::mime_db::MimeDb;
use crate::parameters::ScanOpts;
use crate::{String, MIMEDB};

static MIMEXT: Lazy<RwLock<HashMap<String, Option<Vec<String>>>>> = Lazy::new(|| RwLock::new(HashMap::new()));

cfg_if! {
	if #[cfg(windows)] {
		/// Determines whether or not a file is hidden by checking its win32 file attributes.
		pub fn is_hidden(entry: &DirEntry) -> bool {
		use std::os::windows::prelude::*;
		std::fs::metadata(entry.path()) // try to get metadata for file
			.map_or(
				false,                             // if getting metadata/attributes fails, assume it's not hidden
				|f| f.file_attributes() & 0x2 > 0, // flag for hidden - https://docs.microsoft.com/windows/win32/fileio/file-attribute-constants
			)
		}
	} else {
		/// Determines whether or not a file is hidden by checking for a leading full stop.
		pub fn is_hidden(entry: &DirEntry) -> bool {
		entry
			.file_name()
			.to_str()
			.map_or(false, |f| f.starts_with('.') && f != ".")
		}
	}
}

/// Returns `true` if a file matches the given criteria. This means checking whether the file's extension appears in
/// `exts` (if specified), potentially skipping over hidden files, and so on.
pub fn wanted_file(
	entry: &DirEntry,
	exts: Option<&BTreeSet<&str>>,
	exclude: Option<&BTreeSet<&str>>,
	scan_opts: &ScanOpts,
) -> bool {
	if entry.depth() == 0 {
		// the root directory should always be scanned.
		return true;
	}

	if !scan_opts.hidden && is_hidden(entry) {
		// skip hidden files and directories. this check is performed first because it's very lightweight.
		return false;
	}

	if entry.file_type().is_dir() {
		// always allow directories - there's no point doing file extension matching on something that isn't a file.
		return true;
	}

	if let Some(ext) = entry.path().extension() {
		// file has extension - discard invalid UTF-8 and normalise it to lowercase.
		let ext = ext.to_string_lossy().to_lowercase();
		let ext = ext.as_str();

		if scan_opts.ignore_unknown_exts && from_ext(ext).is_empty() {
			// unknown extension, skip.
			return false;
		}

		if let Some(exts) = exts {
			// only scan if the file has one of the specified extensions.
			exts.contains(&ext)
		} else {
			// no extensions specified - the file should be scanned unless its extension is on the exclude list.
			exclude.map_or(true, |exclude| !exclude.contains(&ext))
		}
	} else {
		// no file extension
		scan_opts.extensionless
	}
}

/// Inspects the given entry, returning a [`Findings`] on success and a [`ScanError`] on failure.
///
/// In the event of an IO error, the returned [`ScanError`] will be of type [`ScanError::File`]. Otherwise, a
/// [`ScanError::Mime`] will be returned, meaning that the file was scanned successfully, but a mimetype could not be
/// determined.
pub fn scan_file(entry: &DirEntry, canonical_paths: bool) -> Result<Findings, ScanError> {
	let path = entry.path();
	// try to determine mimetype for this entry
	let result = match mime_type(MIMEDB.deref(), path) {
		// an error occurred while trying to read the file
		Err(_) => return Err(ScanError::File(path)),
		// the file was read successfully, but we were unable to determine its mimetype
		Ok(None) => return Err(ScanError::Mime(path)),
		// a mimetype was found!
		Ok(Some(result)) => result,
	};

	// set of known extensions for the given mimetype
	let known_exts = mime_extension_lookup(result.essence_str().into());
	// file extension for this particular file
	let entry_ext = path.extension();

	let valid = match known_exts {
		// there is a known set of extensions for this mimetype, and the file has an extension
		Some(e) if entry_ext.is_some() => e.contains(&entry_ext.unwrap().to_string_lossy().to_lowercase().into()),
		// either this file has no extension, or there is no known set of extensions for this mimetype :(
		Some(_) | None => false,
	};

	let path = if canonical_paths {
		match std::fs::canonicalize(path) {
			Ok(path) => path,
			Err(_) => return Err(ScanError::File(entry.path())),
		}
	} else {
		path.to_path_buf() // :c
	};

	Ok(Findings {
		file: path,
		valid,
		mime: result,
	})
}

/// Takes a slice of [`DirEntry`]s and calls [`scan_file`] on each one, returning the results in a vector.
pub fn scan_from_walkdir(entries: &[DirEntry], canonical_paths: bool) -> Vec<Result<Findings, ScanError>> {
	cfg_if! {
		if #[cfg(feature = "multi-threaded")] {
			use rayon::prelude::*;

			// split the entries into chunks of 32, and iterate over each chunk of entries in a separate thread
			entries
				.par_chunks(32)
				.flat_map(|chunk| {
					chunk
						.iter() // iter over the chunk, which is a slice of DirEntry structs
						.map(|entry| scan_file(entry, canonical_paths))
						.collect::<Vec<_>>()
				})
				.collect()
		} else {
			entries.iter().map(|entry: &DirEntry| scan_file(entry, canonical_paths)).collect()
		}
	}
}

/// Scans a given directory with [`WalkDir`], filters with [`wanted_file`], checks for errors, and returns a vector of
/// [DirEntry]s.
pub fn scan_directory(
	dirs: &Path,
	exts: Option<&BTreeSet<&str>>,
	exclude: Option<&BTreeSet<&str>>,
	scan_opts: &ScanOpts,
) -> Option<Vec<DirEntry>> {
	let stepper = WalkDir::new(dirs).follow_links(scan_opts.follow_symlinks).into_iter();
	let mut probably_fatal_error = false;
	let entries: Vec<DirEntry> = stepper
		.filter_entry(|e| wanted_file(e, exts, exclude, scan_opts)) // filter out unwanted files
		.filter_map(|e| {
			if let Err(err) = &e {
				debug!("uh oh spaghettio!! {:#?}", e);
				// log errors to stdout, and remove them from the iterator
				let path = err.path().map_or("General error".into(), Path::to_string_lossy);

				if err.depth() == 0 {
					// if something goes wrong while trying to read the root directory, we're probably not going to get much done
					probably_fatal_error = true;
				}

				// TODO: is there a way to just say `map_or(x, |y| y).thing()` instead of `map_or(x.thing(), |y| y.thing())`?
				// i don't care whether i'm returning a walkdir error or an io error, i just care about whether or not it
				// implements ToString (which they both do). map_or doesn't work on trait objects though :(
				error!(
					"{}: {}",
					path,
					err.io_error().map_or(err.to_string(), |e| e.to_string())
				);
				return None;
			}
			e.ok()
		})
		// remove directories from the final list
		.filter(|e| !e.file_type().is_dir())
		// if fif is invoked without `-f` on a symlinked directory, it will recurse into the symlink (as desired) and ignore
		// any symlinks inside the symlinked root directory. however, the root directory will still be added to `entries` as
		// if it were a file to be scanned, and `scan_file` will fail to scan it, adding "Failed to read ~/whatever" to the
		// output. to avoid this, we can remove all symlinks from `entries` if `-f` is not set. i know this is kind of
		// confusing, but it's honestly kind of hard to explain... maybe a screenshot is better:
		// https://i.imgur.com/DYG7jlB.png
		// adding the symlink filter removes the line that's being pointed to in the image. 0u0
		.filter(|e| scan_opts.follow_symlinks || !e.file_type().is_symlink())
		.collect();

	if probably_fatal_error {
		None
	} else {
		Some(entries)
	}
}

/// The number of bytes to read initially.
///
/// Rather than reading the entire file all at once into a [`BUF_SIZE`] buffer, it tends to be faster to read a small
/// chunk of the file and trying to identify that, proceeding with the larger buffer if that fails. Many file formats
/// can be identified with the first few dozen bytes, so the "happy path" will likely be taken in the majority of cases.
pub const INITIAL_BUF_SIZE: usize = 128;

/// The number of bytes to read if the file couldn't be identified from its first [`INITIAL_BUF_SIZE`] bytes.
pub const BUF_SIZE: usize = 8192;

/// Tries to identify the mimetype of a file from a given path.
pub fn mime_type<T: MimeDb>(db: &T, path: &Path) -> io::Result<Option<Mime>> {
	let mut buffer = [0; INITIAL_BUF_SIZE];
	let mut file = File::open(path)?;

	// read a small amount to start with
	file.read(&mut buffer)?;

	let r = db.get_type(&buffer).filter(|mime|
		// some mime types should be investigated further, reading up to BUF_SIZE even if they've been determined already
		// one such type is XML - there's many more specific types that can be determined by reading further (such as SVG)
		mime != &mime::TEXT_XML
			// another is ZIP - many file formats (DOCX, ODT, JAR...) are just ZIP files with particular data structures.
			// determining that a file is in one of the MS office formats in particular requires looking quite far into the
			// file.
			&& mime != &Mime::from_str("application/zip").unwrap()
			// doc/ppt/xls files are a subset of what's known as an "OLE2 compound document storage", at least according to
			// shared-mime-info. if a pre-OOXML era MS office file is scanned and identified as x-ole-storage, reading further
			// will allow it to be detected correctly as the appropriate filetype.
			&& mime != &Mime::from_str("application/x-ole-storage").unwrap());

	if r.is_some() {
		return Ok(r);
	}

	// attempt to read up to the BUF_SIZE bytes of the file.
	// we've already read the first 128 bytes into a buffer, but i can't see an obvious way to reuse them in a way that's
	// faster than simply moving the seek position back to the start of the file and re-reading the whole BUF_SIZE bytes.
	// for example, reading (BUF_SIZE - INITIAL_BUF_SIZE) bytes, then concatenating the original INITIAL_BUF_SIZE buffer
	// with this new one would presumably be faster - but it's not. i think it's more expensive to create the iterator,
	// collect the contents, etc. i'll have to look into this more. i don't at all doubt that there IS a way to do this
	// efficiently, and i can think of a way in principle, but i'm not sure how to express it in a way that is both
	// idiomatic/safe and fast.
	let mut buffer = [0; BUF_SIZE];
	file.seek(SeekFrom::Start(0))?;
	file.read(&mut buffer)?;
	Ok(db.get_type(&buffer))
}

// Returns a list of known extensions for this mime type, if any.
// This function uses the [Mime]'s "essence" rather than the [Mime] itself - mime_guess::get_mime_extensions ignores
// the type suffix, treating "image/svg+xml" as "image/svg", and thus fails to find any extensions. Passing the
// essence_str (which includes the suffix) fixes this.
pub fn mime_extension_lookup(essence: String) -> Option<Vec<String>> {
	if let Ok(cache) = MIMEXT.read() {
		if let Some(exts) = cache.get(&essence) {
			return exts.clone();
		}
	}

	let essence = essence;
	let mut exts = mime_guess::get_mime_extensions_str(essence.as_str());
	if exts.is_none() {
		// no matches :c
		// mime_guess' database isn't exactly perfect... there are a lot of times where the db will return "some/x-thing"
		// but mime_guess only understands "some/thing", or vice-versa.
		// so, if there appear to be no extensions, try replacing "some/x-thing" with "some/thing", or "some/thing" with
		// "some/x-thing".
		if essence.contains("/x-") {
			// replace e.g. "application/x-gzip" with "application/gzip"
			exts = mime_guess::get_mime_extensions_str(&essence.replace("/x-", "/"));
		} else {
			// replace e.g. "video/mp2t" with "video/x-mp2t"
			exts = mime_guess::get_mime_extensions_str(&essence.replace("/", "/x-"));
		}
	}

	let exts = match exts {
		Some(exts) => {
			let possible_exts: Vec<String> = exts.iter().map(|e| String::from(*e)).collect();

			Some(if essence == mime::IMAGE_JPEG.essence_str() {
				// possible_exts starts with "jpe", because it's alphabetically before "jpeg" and "jpg". however, jpg/jpeg are
				// far more common than jpe, so it makes sense to suggest one of those rather than jpe. to do this, we can
				// add "jpg" to the start of the possible_exts list, ensuring that it will be the extension suggested by fif.
				[vec![String::from("jpg")], possible_exts].concat()
			} else if essence == mime::TEXT_XML.essence_str() || essence == "application/xml" {
				// a somewhat similar case arises with XML files - the first suggested extension is "asa", when it should
				// (in my opinion) be "xml".
				// there's also another problem: SVG files can easily be misidentified as XML files, because they usually
				// *are* valid XML - the more whitespace and comments an SVG file begins with, the more bytes must be read
				// before it's possible to determine that it's an SVG rather than an XML file. to "fix" this, we can add "svg"
				// as a valid extension for XML files, ensuring that SVG files misidentified as XML will still be considered
				// to have valid extensions.
				// TODO: if a file is detected as application/xml, but it has an extension like "xht" which corresponds to
				// "application/xhtml+xml", let it through - in other words, if it's identified as application/xml, but its
				// extension is classed as application/*+xml, consider it OK
				[vec![String::from("xml"), String::from("svg")], possible_exts].concat()
			} else if essence == "application/msword" {
				// classic office files considered harmful
				vec![String::from("doc"), String::from("xls"), String::from("ppt")]
			} else if essence == "application/zip" {
				// neither xdg-mime nor infer seem to be able to detect office XML files properly...
				[
					vec![
						String::from("zip"),
						String::from("docx"),
						String::from("xlsx"),
						String::from("pptx"),
					],
					possible_exts,
				]
				.concat()
			} else if essence == "application/x-ms-dos-executable" {
				// both .dll and .exe files are given the same mime type... but you definitely don't want to rename one to the
				// other!
				[vec![String::from("dll"), String::from("exe")], possible_exts].concat()
			} else {
				possible_exts
			})
		}
		None => None,
	};

	if let Ok(mut cache) = MIMEXT.write() {
		cache.insert(essence, exts.clone());
		exts
	} else {
		unreachable!()
	}
}