//! File checksum computation and formatting for `.changes` / `.buildinfo`
//! fields (MD5, SHA-1, SHA-256 + size), mirroring `Dpkg::Checksums`.

use std::collections::HashMap;
use std::io::Read;
use std::path::Path;

use md5::Md5;
use sha1::Sha1;
use sha2::{Digest, Sha256};

/// Checksums and size of a single file.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Entry {
    /// File size in bytes.
    pub size: u64,
    /// Lowercase hexadecimal MD5 digest.
    pub md5: String,
    /// Lowercase hexadecimal SHA-1 digest.
    pub sha1: String,
    /// Lowercase hexadecimal SHA-256 digest.
    pub sha256: String,
}

/// Compute all supported checksums of a file.
fn compute(path: &Path) -> Result<Entry, Box<dyn std::error::Error>> {
    let mut file = std::fs::File::open(path)
        .map_err(|e| format!("cannot open '{}' for checksumming: {}", path.display(), e))?;

    let mut md5_hasher = Md5::new();
    let mut sha1_hasher = Sha1::new();
    let mut sha256_hasher = Sha256::new();
    let mut size: u64 = 0;
    let mut buf = [0u8; 64 * 1024];

    loop {
        let n = file.read(&mut buf)?;
        if n == 0 {
            break;
        }
        md5_hasher.update(&buf[..n]);
        sha1_hasher.update(&buf[..n]);
        sha256_hasher.update(&buf[..n]);
        size += n as u64;
    }

    Ok(Entry {
        size,
        md5: hex::encode(md5_hasher.finalize()),
        sha1: hex::encode(sha1_hasher.finalize()),
        sha256: hex::encode(sha256_hasher.finalize()),
    })
}

/// A registry of checksummed files, keyed by the name they are distributed
/// under (which may differ from the on-disk path).
///
/// Insertion order is preserved, matching the order in which
/// `dpkg-genchanges` accumulates artifacts (dsc, tarballs, debs, buildinfo).
#[derive(Debug, Clone, Default)]
pub struct FileChecksums {
    entries: Vec<(String, Entry)>,
    index: HashMap<String, usize>,
}

impl FileChecksums {
    /// Create an empty registry.
    pub fn new() -> Self {
        Self::default()
    }

    /// Add a file, registering it under its own file name.
    pub fn add_file(&mut self, path: &Path) -> Result<(), Box<dyn std::error::Error>> {
        let key = path
            .file_name()
            .and_then(|n| n.to_str())
            .ok_or_else(|| format!("invalid file name: {}", path.display()))?
            .to_string();
        self.add_file_as(path, &key)
    }

    /// Add a file, registering it under an explicit distribution key.
    pub fn add_file_as(
        &mut self,
        path: &Path,
        key: &str,
    ) -> Result<(), Box<dyn std::error::Error>> {
        let entry = compute(path)?;
        self.insert_entry(key, entry);
        Ok(())
    }

    /// Insert a pre-computed entry (e.g. taken from a `.dsc` checksum field).
    /// Re-inserting an existing key updates it in place, keeping its position.
    pub fn insert_entry(&mut self, key: &str, entry: Entry) {
        if let Some(&pos) = self.index.get(key) {
            self.entries[pos].1 = entry;
            return;
        }
        self.index.insert(key.to_string(), self.entries.len());
        self.entries.push((key.to_string(), entry));
    }

    /// Remove a file from the registry. Returns true if it was present.
    pub fn remove(&mut self, key: &str) -> bool {
        match self.index.remove(key) {
            Some(pos) => {
                self.entries.remove(pos);
                // Reindex the shifted tail.
                for (i, (k, _)) in self.entries.iter().enumerate().skip(pos) {
                    self.index.insert(k.clone(), i);
                }
                true
            }
            None => false,
        }
    }

    /// Look up the entry for a given key.
    pub fn get(&self, key: &str) -> Option<&Entry> {
        self.index.get(key).map(|&pos| &self.entries[pos].1)
    }

    /// Iterate over `(key, entry)` pairs in insertion order.
    pub fn iter(&self) -> impl Iterator<Item = (&String, &Entry)> {
        self.entries.iter().map(|(k, e)| (k, e))
    }

    /// Number of registered files.
    pub fn len(&self) -> usize {
        self.entries.len()
    }

    /// True if no file is registered.
    pub fn is_empty(&self) -> bool {
        self.entries.is_empty()
    }

    /// Format a `Checksums-*` style field value: one `\n`-separated line per
    /// file of the form `" <hash> <size> <key>"`.
    fn format_field<F>(&self, hash_of: F) -> String
    where
        F: Fn(&Entry) -> &str,
    {
        let mut out = String::new();
        for (key, e) in self.iter() {
            out.push('\n');
            out.push_str(hash_of(e));
            out.push(' ');
            out.push_str(&e.size.to_string());
            out.push(' ');
            out.push_str(key);
        }
        out
    }

    /// Value for the `Checksums-Md5` field (empty string if no file).
    pub fn field_md5(&self) -> String {
        self.format_field(|e| &e.md5)
    }

    /// Value for the `Checksums-Sha1` field (empty string if no file).
    pub fn field_sha1(&self) -> String {
        self.format_field(|e| &e.sha1)
    }

    /// Value for the `Checksums-Sha256` field (empty string if no file).
    pub fn field_sha256(&self) -> String {
        self.format_field(|e| &e.sha256)
    }
}

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

    #[test]
    fn known_digests() {
        let dir = tempfile::tempdir().unwrap();
        let p = dir.path().join("sample.txt");
        std::fs::write(&p, b"hello world\n").unwrap();

        let mut cs = FileChecksums::new();
        cs.add_file(&p).unwrap();

        let e = cs.get("sample.txt").unwrap();
        // Verified with coreutils: echo "hello world" | md5sum / sha1sum / sha256sum
        assert_eq!(e.md5, "6f5902ac237024bdd0c176cb93063dc4");
        assert_eq!(e.sha1, "22596363b3de40b06f981fb85d82312e8c0ed511");
        assert_eq!(
            e.sha256,
            "a948904f2f0f479b8f8197694b30184b0d2ed1c1cd2a1ec0fb85d299a192a447"
        );
        assert_eq!(e.size, 12);
    }

    #[test]
    fn insertion_order_preserved() {
        let dir = tempfile::tempdir().unwrap();
        let a = dir.path().join("a.txt");
        let b = dir.path().join("b.txt");
        std::fs::write(&a, b"aaa").unwrap();
        std::fs::write(&b, b"bb").unwrap();

        let mut cs = FileChecksums::new();
        // Insert b first: insertion order (not alphabetical) must be kept,
        // matching dpkg's artifact accumulation order.
        cs.add_file(&b).unwrap();
        cs.add_file(&a).unwrap();

        let keys: Vec<&str> = cs.iter().map(|(k, _)| k.as_str()).collect();
        assert_eq!(keys, vec!["b.txt", "a.txt"]);

        assert_eq!(
            cs.field_md5(),
            "\n21ad0bd836b90d08f4cf640b4c298e7c 2 b.txt\n47bce5c74f589f4867dbd57e9ca9f808 3 a.txt"
        );
    }

    #[test]
    fn reinsert_updates_in_place() {
        let dir = tempfile::tempdir().unwrap();
        let a = dir.path().join("a.txt");
        std::fs::write(&a, b"aaa").unwrap();

        let mut cs = FileChecksums::new();
        cs.add_file(&a).unwrap();
        std::fs::write(&a, b"bbbb").unwrap();
        cs.add_file(&a).unwrap(); // updated in place, same position

        assert_eq!(cs.len(), 1);
        assert_eq!(cs.get("a.txt").unwrap().size, 4);
    }

    #[test]
    fn remove_keeps_order() {
        let dir = tempfile::tempdir().unwrap();
        let a = dir.path().join("a.txt");
        let b = dir.path().join("b.txt");
        let c = dir.path().join("c.txt");
        std::fs::write(&a, b"1").unwrap();
        std::fs::write(&b, b"2").unwrap();
        std::fs::write(&c, b"3").unwrap();

        let mut cs = FileChecksums::new();
        cs.add_file(&a).unwrap();
        cs.add_file(&b).unwrap();
        cs.add_file(&c).unwrap();
        assert!(cs.remove("b.txt"));
        assert!(!cs.remove("b.txt"));

        let keys: Vec<&str> = cs.iter().map(|(k, _)| k.as_str()).collect();
        assert_eq!(keys, vec!["a.txt", "c.txt"]);
    }
}