use std::path::{Path, PathBuf};

use crate::data;

/// Instantiation
impl data::File<crate::MMap> {
    /// Try opening a data file at the given `path`.
    ///
    /// The `object_hash` is a way to read (and write) the same file format with different hashes, as the hash kind
    /// isn't stored within the file format itself.
    ///
    /// This constructor leaves allocation limiting disabled, allowing allocations of any size dictated by pack data.
    /// Use [`Self::from_data()`] together with [`File::with_alloc_limit_bytes()`][crate::data::File::with_alloc_limit_bytes()]
    /// when working with untrusted input.
    pub fn at(path: impl AsRef<Path>, object_hash: gix_hash::Kind) -> Result<Self, data::header::decode::Error> {
        Self::at_inner(path.as_ref(), object_hash)
    }

    fn at_inner(path: &Path, object_hash: gix_hash::Kind) -> Result<Self, data::header::decode::Error> {
        let data = crate::mmap::read_only(path).map_err(|e| data::header::decode::Error::Io {
            source: e,
            path: path.to_owned(),
        })?;
        Self::from_data(data, path.to_owned(), object_hash)
    }
}

impl<T> data::File<T>
where
    T: crate::FileData,
{
    /// Instantiate a data file from `data` as assumed to be read or memory-mapped from `path`.
    ///
    /// This constructor leaves allocation limiting disabled, allowing allocations of any size dictated by pack data.
    /// Call [`File::with_alloc_limit_bytes()`][crate::data::File::with_alloc_limit_bytes()] before decoding entries from untrusted input.
    pub fn from_data(data: T, path: PathBuf, object_hash: gix_hash::Kind) -> Result<Self, data::header::decode::Error> {
        use crate::data::header::N32_SIZE;
        let hash_len = object_hash.len_in_bytes();
        let pack_len = data.len();
        let id = gix_features::hash::crc32(path.as_os_str().to_string_lossy().as_bytes());
        if pack_len < N32_SIZE * 3 + hash_len {
            return Err(data::header::decode::Error::Corrupt(format!(
                "Pack data of size {pack_len} is too small for even an empty pack with shortest hash"
            )));
        }
        let (kind, num_objects) =
            data::header::decode(&data[..12].try_into().expect("enough data after previous check"))?;
        Ok(Self {
            data,
            path,
            id,
            version: kind,
            num_objects,
            object_hash,
            alloc_limit_bytes: None,
        })
    }

    /// Configure the maximum size of a single allocation caused by user-controlled on-disk pack data.
    ///
    /// Use `None` to disable the limit, which is also the default.
    ///
    /// This is currently enforced when decoding pack entries and resolving delta chains.
    /// Callers that allocate from pack metadata directly should consult [`File::alloc_limit_bytes()`][crate::data::File::alloc_limit_bytes]
    /// and apply the same limit themselves.
    pub fn with_alloc_limit_bytes(mut self, alloc_limit_bytes: Option<usize>) -> Self {
        self.alloc_limit_bytes = alloc_limit_bytes;
        self
    }
}