//! Reference store — 32-byte key → 32-byte value mapping (Phase 5b).
//!
//! Used to map a [`Fingerprint`](crate::cluster::build_cache::Fingerprint)
//! to the [`BlobId`](crate::cluster::blob::BlobId) of its cached
//! artifact. Deliberately a dumb primitive: no versioning, no CRDT
//! semantics — Phase 3 will layer a richer metadata model on top,
//! but every real cargo-cache lookup we need in Phase 5 is a single
//! key → single value.
//!
//! # Layout
//!
//! ```text
//! <root>/
//!   refs/<kk>/<key_hex>.ref     — 32 raw bytes (the value)
//!   .tmp/                        — atomic-rename staging
//! ```
//!
//! `<kk>` is the first two hex chars of the key, keeping directory
//! fan-out bounded (256 entries per level). Writes go through
//! tempfile + rename so a mid-write crash leaves either a complete
//! file or nothing.

use anyhow::{bail, Context, Result};
use std::path::{Path, PathBuf};
use tokio::io::AsyncWriteExt;

/// A raw 32-byte key. Callers wrap semantically — the store itself
/// treats keys as opaque bytes.
pub type RefKey = [u8; 32];

/// A raw 32-byte value.
pub type RefValue = [u8; 32];

/// Directory-backed reference store.
#[derive(Debug, Clone)]
pub struct RefStore {
    root: PathBuf,
}

impl RefStore {
    /// Open (create if missing) a ref store rooted at `root`. Creates
    /// `refs/` and `.tmp/` subdirs. Safe to call on an existing store.
    pub fn open(root: PathBuf) -> Result<Self> {
        std::fs::create_dir_all(root.join("refs"))
            .with_context(|| format!("creating refs dir under {}", root.display()))?;
        std::fs::create_dir_all(root.join(".tmp"))
            .with_context(|| format!("creating .tmp dir under {}", root.display()))?;
        Ok(Self { root })
    }

    pub fn root(&self) -> &Path {
        &self.root
    }

    /// Look up a key. `None` when no ref has been set. Errors only on
    /// filesystem failures other than "not found".
    pub async fn get(&self, key: &RefKey) -> Result<Option<RefValue>> {
        let path = self.ref_path(key);
        match tokio::fs::read(&path).await {
            Ok(bytes) => {
                if bytes.len() != 32 {
                    bail!(
                        "ref at {} has wrong length {} (expected 32)",
                        path.display(),
                        bytes.len()
                    );
                }
                let mut out = [0u8; 32];
                out.copy_from_slice(&bytes);
                Ok(Some(out))
            }
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(None),
            Err(e) => Err(anyhow::Error::from(e))
                .with_context(|| format!("reading ref at {}", path.display())),
        }
    }

    /// Set a key → value mapping. Overwrites any existing value —
    /// callers who need last-writer-wins protection should implement
    /// it in the layer above (Phase 3 CRDT). Atomic on the filesystem:
    /// a mid-write crash leaves the previous value intact.
    pub async fn put(&self, key: &RefKey, value: &RefValue) -> Result<()> {
        let final_path = self.ref_path(key);
        if let Some(parent) = final_path.parent() {
            tokio::fs::create_dir_all(parent).await.with_context(|| {
                format!("creating ref bucket {}", parent.display())
            })?;
        }
        self.atomic_write(&final_path, value).await
    }

    /// Delete a key. Returns `true` if a ref was removed, `false` if
    /// no ref existed for the key.
    pub async fn delete(&self, key: &RefKey) -> Result<bool> {
        let path = self.ref_path(key);
        match tokio::fs::remove_file(&path).await {
            Ok(()) => Ok(true),
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(false),
            Err(e) => Err(anyhow::Error::from(e)),
        }
    }

    /// Whether a value is set for the given key.
    pub async fn contains(&self, key: &RefKey) -> Result<bool> {
        match tokio::fs::metadata(self.ref_path(key)).await {
            Ok(_) => Ok(true),
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(false),
            Err(e) => Err(anyhow::Error::from(e)),
        }
    }

    fn ref_path(&self, key: &RefKey) -> PathBuf {
        let hex = hex32(key);
        self.root
            .join("refs")
            .join(&hex[..2])
            .join(format!("{hex}.ref"))
    }

    async fn atomic_write(&self, final_path: &Path, bytes: &[u8]) -> Result<()> {
        let tmp_dir = self.root.join(".tmp");
        let tmp_name = format!(
            "{}.{}",
            std::process::id(),
            RANDOM_SUFFIX.fetch_add(1, std::sync::atomic::Ordering::Relaxed)
        );
        let tmp_path = tmp_dir.join(tmp_name);
        {
            let mut f = tokio::fs::File::create(&tmp_path).await.with_context(|| {
                format!("creating tmp ref {}", tmp_path.display())
            })?;
            f.write_all(bytes).await?;
            f.sync_all().await?;
        }
        tokio::fs::rename(&tmp_path, final_path)
            .await
            .with_context(|| {
                format!(
                    "renaming {} → {}",
                    tmp_path.display(),
                    final_path.display()
                )
            })?;
        Ok(())
    }
}

static RANDOM_SUFFIX: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);

fn hex32(bytes: &[u8; 32]) -> String {
    let mut out = String::with_capacity(64);
    for b in bytes {
        out.push_str(&format!("{b:02x}"));
    }
    out
}

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

    fn open() -> (tempfile::TempDir, RefStore) {
        let tmp = tempfile::TempDir::new().unwrap();
        let store = RefStore::open(tmp.path().to_path_buf()).unwrap();
        (tmp, store)
    }

    #[tokio::test]
    async fn open_creates_layout() {
        let (tmp, store) = open();
        assert!(tmp.path().join("refs").is_dir());
        assert!(tmp.path().join(".tmp").is_dir());
        assert_eq!(store.root(), tmp.path());
    }

    #[tokio::test]
    async fn get_returns_none_for_missing() {
        let (_tmp, store) = open();
        let key = [0u8; 32];
        assert_eq!(store.get(&key).await.unwrap(), None);
        assert!(!store.contains(&key).await.unwrap());
    }

    #[tokio::test]
    async fn put_and_get_round_trip() {
        let (_tmp, store) = open();
        let key = [0x11u8; 32];
        let value = [0x22u8; 32];
        store.put(&key, &value).await.unwrap();
        assert_eq!(store.get(&key).await.unwrap(), Some(value));
        assert!(store.contains(&key).await.unwrap());
    }

    #[tokio::test]
    async fn put_overwrites_prior_value() {
        let (_tmp, store) = open();
        let key = [0xaau8; 32];
        let v1 = [0x01u8; 32];
        let v2 = [0x02u8; 32];
        store.put(&key, &v1).await.unwrap();
        store.put(&key, &v2).await.unwrap();
        assert_eq!(store.get(&key).await.unwrap(), Some(v2));
    }

    #[tokio::test]
    async fn delete_removes_ref_and_reports() {
        let (_tmp, store) = open();
        let key = [0x55u8; 32];
        let value = [0x66u8; 32];
        store.put(&key, &value).await.unwrap();
        assert!(store.delete(&key).await.unwrap());
        assert_eq!(store.get(&key).await.unwrap(), None);
        // Second delete: returns false, not an error.
        assert!(!store.delete(&key).await.unwrap());
    }

    #[tokio::test]
    async fn distinct_keys_produce_distinct_files() {
        // Verify the on-disk layout by inspection — different keys
        // land in different bucket dirs (proves the fan-out heuristic).
        let (tmp, store) = open();
        let k1 = [0xffu8; 32];
        let k2 = [0x00u8; 32];
        store.put(&k1, &[0u8; 32]).await.unwrap();
        store.put(&k2, &[0u8; 32]).await.unwrap();
        assert!(tmp.path().join("refs/ff").exists());
        assert!(tmp.path().join("refs/00").exists());
    }

    #[tokio::test]
    async fn rejects_wrong_length_on_disk() {
        // Simulate corruption / hand-tampering: a file at the ref
        // path exists but isn't 32 bytes. Read must surface an error,
        // not silently return garbage.
        let (tmp, store) = open();
        let key = [0x7fu8; 32];
        let path = store.ref_path(&key);
        std::fs::create_dir_all(path.parent().unwrap()).unwrap();
        std::fs::write(&path, b"only 4b").unwrap();
        let err = store.get(&key).await.unwrap_err().to_string();
        assert!(err.contains("wrong length"), "unexpected: {err}");
    }
}