use std::convert::Infallible;

use bstr::{BStr, BString, ByteSlice};

use crate::Scheme;

/// The error returned by [parse()](crate::parse()).
#[derive(Debug, thiserror::Error)]
#[allow(missing_docs)]
pub enum Error {
    #[error("{} \"{url}\" is not valid UTF-8", kind.as_str())]
    Utf8 {
        url: BString,
        kind: UrlKind,
        source: std::str::Utf8Error,
    },
    #[error("{} {url:?} can not be parsed as valid URL", kind.as_str())]
    Url {
        url: String,
        kind: UrlKind,
        source: url::ParseError,
    },

    #[error("The host portion of the following URL is too long ({} bytes, {len} bytes total): {truncated_url:?}", truncated_url.len())]
    TooLong { truncated_url: BString, len: usize },
    #[error("{} \"{url}\" does not specify a path to a repository", kind.as_str())]
    MissingRepositoryPath { url: BString, kind: UrlKind },
    #[error("URL {url:?} is relative which is not allowed in this context")]
    RelativeUrl { url: String },
}

impl From<Infallible> for Error {
    fn from(_: Infallible) -> Self {
        unreachable!("Cannot actually happen, but it seems there can't be a blanket impl for this")
    }
}

///
#[derive(Debug, Clone, Copy)]
pub enum UrlKind {
    ///
    Url,
    ///
    Scp,
    ///
    Local,
}

impl UrlKind {
    fn as_str(&self) -> &'static str {
        match self {
            UrlKind::Url => "URL",
            UrlKind::Scp => "SCP-like target",
            UrlKind::Local => "local path",
        }
    }
}

pub(crate) enum InputScheme {
    Url { protocol_end: usize },
    Scp { colon: usize },
    Local,
}

pub(crate) fn find_scheme(input: &BStr) -> InputScheme {
    // TODO: url's may only contain `:/`, we should additionally check if the characters used for
    //       protocol are all valid
    if let Some(protocol_end) = input.find("://") {
        return InputScheme::Url { protocol_end };
    }

    if let Some(colon) = input.find_byte(b':') {
        // allow user to select files containing a `:` by passing them as absolute or relative path
        // this is behavior explicitly mentioned by the scp and git manuals
        let explicitly_local = &input[..colon].contains(&b'/');
        let dos_driver_letter = cfg!(windows) && input[..colon].len() == 1;

        if !explicitly_local && !dos_driver_letter {
            return InputScheme::Scp { colon };
        }
    }

    InputScheme::Local
}

pub(crate) fn url(input: &BStr, protocol_end: usize) -> Result<crate::Url, Error> {
    const MAX_LEN: usize = 1024;
    let bytes_to_path = input[protocol_end + "://".len()..]
        .iter()
        .filter(|b| !b.is_ascii_whitespace())
        .skip_while(|b| **b == b'/' || **b == b'\\')
        .position(|b| *b == b'/')
        .unwrap_or(input.len() - protocol_end);
    if bytes_to_path > MAX_LEN || protocol_end > MAX_LEN {
        return Err(Error::TooLong {
            truncated_url: input[..(protocol_end + "://".len() + MAX_LEN).min(input.len())].into(),
            len: input.len(),
        });
    }
    let (input, url) = input_to_utf8_and_url(input, UrlKind::Url)?;
    let scheme = url.scheme().into();

    if matches!(scheme, Scheme::Git | Scheme::Ssh) && url.path().is_empty() {
        return Err(Error::MissingRepositoryPath {
            url: input.into(),
            kind: UrlKind::Url,
        });
    }

    if url.cannot_be_a_base() {
        return Err(Error::RelativeUrl { url: input.to_owned() });
    }

    Ok(crate::Url {
        serialize_alternative_form: false,
        scheme,
        user: url_user(&url),
        password: url.password().map(Into::into),
        host: url.host_str().map(Into::into),
        port: url.port(),
        path: url.path().into(),
    })
}

pub(crate) fn scp(input: &BStr, colon: usize) -> Result<crate::Url, Error> {
    let input = input_to_utf8(input, UrlKind::Scp)?;

    // TODO: this incorrectly splits at IPv6 addresses, check for `[]` before splitting
    let (host, path) = input.split_at(colon);
    debug_assert_eq!(path.get(..1), Some(":"), "{path} should start with :");
    let path = &path[1..];

    if path.is_empty() {
        return Err(Error::MissingRepositoryPath {
            url: input.to_owned().into(),
            kind: UrlKind::Scp,
        });
    }

    // The path returned by the parsed url often has the wrong number of leading `/` characters but
    // should never differ in any other way (ssh URLs should not contain a query or fragment part).
    // To avoid the various off-by-one errors caused by the `/` characters, we keep using the path
    // determined above and can therefore skip parsing it here as well.
    let url = url::Url::parse(&format!("ssh://{host}")).map_err(|source| Error::Url {
        url: input.to_owned(),
        kind: UrlKind::Scp,
        source,
    })?;

    Ok(crate::Url {
        serialize_alternative_form: true,
        scheme: url.scheme().into(),
        user: url_user(&url),
        password: url.password().map(Into::into),
        host: url.host_str().map(Into::into),
        port: url.port(),
        path: path.into(),
    })
}

fn url_user(url: &url::Url) -> Option<String> {
    if url.username().is_empty() && url.password().is_none() {
        None
    } else {
        Some(url.username().into())
    }
}

pub(crate) fn file_url(input: &BStr, protocol_colon: usize) -> Result<crate::Url, Error> {
    let input = input_to_utf8(input, UrlKind::Url)?;
    let input_after_protocol = &input[protocol_colon + "://".len()..];

    let Some(first_slash) = input_after_protocol
        .find('/')
        .or_else(|| cfg!(windows).then(|| input_after_protocol.find('\\')).flatten())
    else {
        return Err(Error::MissingRepositoryPath {
            url: input.to_owned().into(),
            kind: UrlKind::Url,
        });
    };

    // We cannot use the url crate to parse host and path because it special cases Windows
    // driver letters. With the url crate an input of `file://x:/path/to/git` is parsed as empty
    // host and with `x:/path/to/git` as path. This behavior is wrong for Git which only follows
    // that rule on Windows and parses `x:` as host on Unix platforms. Additionally, the url crate
    // does not account for Windows special UNC path support.

    // TODO: implement UNC path special case
    let windows_special_path = if cfg!(windows) {
        // Inputs created via url::Url::from_file_path contain an additional `/` between the
        // protocol and the absolute path. Make sure we ignore that first slash character to avoid
        // producing invalid paths.
        let input_after_protocol = if first_slash == 0 {
            &input_after_protocol[1..]
        } else {
            input_after_protocol
        };
        // parse `file://x:/path/to/git` as explained above
        if input_after_protocol.chars().nth(1) == Some(':') {
            Some(input_after_protocol)
        } else {
            None
        }
    } else {
        None
    };

    let host = if windows_special_path.is_some() || first_slash == 0 {
        // `file:///path/to/git` or a windows special case was triggered
        None
    } else {
        // `file://host/path/to/git`
        Some(&input_after_protocol[..first_slash])
    };

    // default behavior on Unix platforms and if no Windows special case was triggered
    let path = windows_special_path.unwrap_or(&input_after_protocol[first_slash..]);

    Ok(crate::Url {
        serialize_alternative_form: false,
        host: host.map(Into::into),
        ..local(path.into())?
    })
}

pub(crate) fn local(input: &BStr) -> Result<crate::Url, Error> {
    if input.is_empty() {
        return Err(Error::MissingRepositoryPath {
            url: input.to_owned(),
            kind: UrlKind::Local,
        });
    }

    Ok(crate::Url {
        serialize_alternative_form: true,
        scheme: Scheme::File,
        password: None,
        user: None,
        host: None,
        port: None,
        path: input.to_owned(),
    })
}

fn input_to_utf8(input: &BStr, kind: UrlKind) -> Result<&str, Error> {
    std::str::from_utf8(input).map_err(|source| Error::Utf8 {
        url: input.to_owned(),
        kind,
        source,
    })
}

fn input_to_utf8_and_url(input: &BStr, kind: UrlKind) -> Result<(&str, url::Url), Error> {
    let input = input_to_utf8(input, kind)?;
    url::Url::parse(input)
        .map(|url| (input, url))
        .map_err(|source| Error::Url {
            url: input.to_owned(),
            kind,
            source,
        })
}