/// The error returned by the [`parse()`][crate::parse()] function.
#[derive(Debug, thiserror::Error)]
#[expect(missing_docs)]
pub enum Error {
    #[error("Empty refspecs are invalid")]
    Empty,
    #[error("Negative refspecs cannot have destinations as they exclude sources")]
    NegativeWithDestination,
    #[error("Negative specs must not be empty")]
    NegativeEmpty,
    #[error("Negative specs must not be object hashes")]
    NegativeObjectHash,
    /// Retained for compatibility; partial negative ref names are accepted and this is no longer returned.
    #[error("Negative specs must be full ref names, starting with \"refs/\"")]
    NegativePartialName,
    /// Retained for compatibility; negative ref patterns containing one `*` are accepted and this is no longer returned.
    #[error("Negative glob patterns are not allowed")]
    NegativeGlobPattern,
    /// Retained for compatibility; invalid fetch destinations are reported as [`Error::ReferenceName`] instead.
    #[error("Fetch destinations must be ref-names, like 'HEAD:refs/heads/branch'")]
    InvalidFetchDestination,
    #[error("Cannot push into an empty destination")]
    PushToEmpty,
    #[error("refspec patterns may only contain a single '*' character, found {pattern:?}")]
    PatternUnsupported { pattern: bstr::BString },
    #[error("Both sides of a two-sided specification need a pattern, like 'a/*:b/*'")]
    PatternUnbalanced,
    #[error(transparent)]
    ReferenceName(#[from] gix_validate::reference::name::Error),
}

/// Define how the parsed refspec should be used.
#[derive(PartialOrd, Ord, PartialEq, Eq, Copy, Clone, Hash, Debug)]
pub enum Operation {
    /// The `src` side is local and the `dst` side is remote.
    Push,
    /// The `src` side is remote and the `dst` side is local.
    Fetch,
}

pub(crate) mod function {
    use crate::{
        RefSpecRef,
        parse::{Error, Operation},
        types::Mode,
    };
    use bstr::{BStr, ByteSlice};

    /// Parse `spec` for use in `operation` and return it if it is valid.
    pub fn parse(mut spec: &BStr, operation: Operation) -> Result<RefSpecRef<'_>, Error> {
        fn fetch_head_only(mode: Mode) -> RefSpecRef<'static> {
            RefSpecRef {
                mode,
                op: Operation::Fetch,
                src: Some("HEAD".into()),
                dst: None,
            }
        }

        let mode = match spec.first() {
            Some(&b'^') => {
                spec = &spec[1..];
                Mode::Negative
            }
            Some(&b'+') => {
                spec = &spec[1..];
                Mode::Force
            }
            Some(_) => Mode::Normal,
            None => {
                return match operation {
                    Operation::Push => Err(Error::Empty),
                    Operation::Fetch => Ok(fetch_head_only(Mode::Normal)),
                };
            }
        };

        // Split on the last colon like `strrchr()` in Git's `parse_refspec()` does, so that a
        // push source may itself contain one - `:/message` and `<rev>:<path>` are both valid
        // revisions. With a single colon this is the same position as the first one.
        let (mut src, dst) = match spec.rfind_byte(b':') {
            Some(pos) => {
                if mode == Mode::Negative {
                    return Err(Error::NegativeWithDestination);
                }

                let (src, dst) = spec.split_at(pos);
                let dst = &dst[1..];
                let src = (!src.is_empty()).then(|| src.as_bstr());
                let dst = (!dst.is_empty()).then(|| dst.as_bstr());
                match (src, dst) {
                    (None, None) => match operation {
                        Operation::Push => (None, None),
                        Operation::Fetch => (Some("HEAD".into()), None),
                    },
                    (None, Some(dst)) => match operation {
                        Operation::Push => (None, Some(dst)),
                        Operation::Fetch => (Some("HEAD".into()), Some(dst)),
                    },
                    (Some(src), None) => match operation {
                        Operation::Push => return Err(Error::PushToEmpty),
                        Operation::Fetch => (Some(src), None),
                    },
                    (Some(src), Some(dst)) => (Some(src), Some(dst)),
                }
            }
            None => {
                let src = (!spec.is_empty()).then_some(spec);
                if Operation::Fetch == operation && mode != Mode::Negative && src.is_none() {
                    return Ok(fetch_head_only(mode));
                } else {
                    (src, None)
                }
            }
        };

        if let Some(spec) = src.as_mut()
            && *spec == "@"
        {
            *spec = "HEAD".into();
        }
        let (src, src_had_pattern) = validated(src, operation == Operation::Push && dst.is_some())?;
        let (dst, dst_had_pattern) = validated(dst, false)?;
        if mode != Mode::Negative
            && src_had_pattern != dst_had_pattern
            && !(operation == Operation::Push && dst.is_none())
        {
            return Err(Error::PatternUnbalanced);
        }

        if mode == Mode::Negative {
            match src {
                Some(spec) => {
                    if looks_like_object_hash(spec) {
                        return Err(Error::NegativeObjectHash);
                    }
                }
                None => return Err(Error::NegativeEmpty),
            }
        }

        Ok(RefSpecRef {
            op: operation,
            mode,
            src,
            dst,
        })
    }

    fn looks_like_object_hash(spec: &BStr) -> bool {
        spec.len() >= gix_hash::Kind::shortest().len_in_hex() && spec.iter().all(u8::is_ascii_hexdigit)
    }

    fn validate_partial_name_with_single_glob(spec: &BStr) -> Result<(), Error> {
        let mut buf = smallvec::SmallVec::<[u8; 256]>::with_capacity(spec.len());
        buf.extend_from_slice(spec);
        let glob_pos = buf.find_byte(b'*').expect("glob present");
        buf[glob_pos] = b'a';
        gix_validate::reference::name_partial(buf.as_bstr())?;
        Ok(())
    }

    /// Validate `spec`, and return it along with whether it holds a glob.
    ///
    /// `any_name` skips the check entirely, for the one side Git leaves unchecked.
    fn validated(spec: Option<&BStr>, any_name: bool) -> Result<(Option<&BStr>, bool), Error> {
        match spec {
            Some(spec) => {
                let glob_count = spec.iter().filter(|b| **b == b'*').take(2).count();
                if glob_count > 1 {
                    return Err(Error::PatternUnsupported { pattern: spec.into() });
                }
                let has_globs = glob_count > 0;
                if has_globs {
                    validate_partial_name_with_single_glob(spec)?;
                } else if !any_name {
                    gix_validate::reference::name_partial(spec)?;
                }
                Ok((Some(spec), has_globs))
            }
            None => Ok((None, false)),
        }
    }
}