// NOTE: This module is implemented and tested but not yet integrated into
// the production read path (FileReader uses mmap for line access).
// It is kept as a building block for a future pread-based reader that
// would avoid the SIGBUS risk of mmap.
// ─── read_cache.rs ─────────────────────────────────────────────────────────
// 16-slot LRU read cache with 4KB page-aligned keys.
// Reduces syscalls by caching recently-read 4KB blocks.
// Cross-block reads are handled via a spill buffer (not cached).
// ──────────────────────────────────────────────────────────────────────────

use std::fs::File;
use std::io;
use std::os::unix::fs::FileExt;

const LRU_SLOTS: usize = 16;
pub const BLOCK_ALIGN: usize = 4096;

struct CacheSlot {
    buf: Vec<u8>,
    block_offset: u64,
    len: usize,
    last_access: u64,
}

pub struct LruReadCache {
    slots: [CacheSlot; LRU_SLOTS],
    spill_buf: Vec<u8>,
    spill_len: usize,
    tick: u64,
}

pub type ReadCache = LruReadCache;

impl Default for LruReadCache {
    fn default() -> Self {
        Self {
            slots: std::array::from_fn(|_| CacheSlot {
                buf: vec![0u8; BLOCK_ALIGN],
                block_offset: 0,
                len: 0,
                last_access: 0,
            }),
            spill_buf: Vec::new(),
            spill_len: 0,
            tick: 0,
        }
    }
}

impl LruReadCache {
    pub fn new() -> Self {
        Self::default()
    }

    /// Read `len` bytes starting at `offset`. Returns a slice into the cache
    /// on a hit, or fills a cache slot on a miss. Cross-block reads go through
    /// the spill buffer and are not cached.
    pub fn get(&mut self, file: &File, offset: u64, len: usize) -> io::Result<&[u8]> {
        if len == 0 {
            return Ok(&[]);
        }

        let aligned_key = offset & !(BLOCK_ALIGN as u64 - 1);
        let request_end = offset.checked_add(len as u64).ok_or_else(|| {
            io::Error::new(io::ErrorKind::InvalidInput, "read range overflows u64")
        })?;
        let block_end = aligned_key.saturating_add(BLOCK_ALIGN as u64);

        if request_end > block_end {
            self.spill_buf.resize(len, 0);
            let bytes_read = file.read_at(&mut self.spill_buf[..len], offset)?;
            if bytes_read == 0 {
                return Err(io::Error::new(io::ErrorKind::UnexpectedEof, "read 0 bytes"));
            }
            if bytes_read < len {
                return Err(io::Error::new(io::ErrorKind::UnexpectedEof, "short read"));
            }
            self.spill_len = len;
            return Ok(&self.spill_buf[..len]);
        }

        let hit_idx = self.slots.iter().position(|slot| {
            let slot_end = slot.block_offset.saturating_add(slot.len as u64);
            slot.len > 0 && slot.block_offset == aligned_key && request_end <= slot_end
        });

        if let Some(idx) = hit_idx {
            self.slots[idx].last_access = self.tick;
            self.tick += 1;
            let start = (offset - self.slots[idx].block_offset) as usize;
            return Ok(&self.slots[idx].buf[start..start + len]);
        }

        let mut evict_idx = 0;
        let mut min_access = self.slots[0].last_access;
        for (i, slot) in self.slots.iter().enumerate() {
            if slot.last_access < min_access {
                min_access = slot.last_access;
                evict_idx = i;
            }
        }

        let slot = &mut self.slots[evict_idx];
        let bytes_read = file.read_at(&mut slot.buf, aligned_key)?;

        // Non-empty reads that return 0 are EOF. Zero-length reads are handled above
        // as a successful no-op.
        if bytes_read == 0 {
            return Err(io::Error::new(io::ErrorKind::UnexpectedEof, "read 0 bytes"));
        }

        slot.block_offset = aligned_key;
        slot.len = bytes_read;
        slot.last_access = self.tick;
        self.tick += 1;

        let bytes_end = aligned_key.saturating_add(bytes_read as u64);
        if request_end > bytes_end {
            return Err(io::Error::new(io::ErrorKind::UnexpectedEof, "short read"));
        }

        let start = (offset - slot.block_offset) as usize;
        Ok(&slot.buf[start..start + len])
    }

    /// Invalidate all cache slots and the spill buffer.
    pub fn clear(&mut self) {
        for slot in &mut self.slots {
            slot.block_offset = 0;
            slot.len = 0;
            slot.last_access = 0;
        }
        self.spill_len = 0;
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::io::Write;
    use tempfile::NamedTempFile;

    fn make_file(data: &[u8]) -> NamedTempFile {
        let mut f = NamedTempFile::new().unwrap();
        f.write_all(data).unwrap();
        f.flush().unwrap();
        f
    }

    #[test]
    fn cache_hit_returns_same_data() {
        // Read the same range twice — second read should be a cache hit.
        let f = make_file(b"Hello, World! This is a test of the cache.");
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let first = cache.get(&file, 0, 13).unwrap().to_vec();
        let second = cache.get(&file, 0, 13).unwrap().to_vec();
        assert_eq!(first, second);
        assert_eq!(&first, b"Hello, World!");
    }

    #[test]
    fn cache_miss_reads_correct_data() {
        // Two non-overlapping ranges — both must be misses but return correct data.
        let data = b"0123456789ABCDEFGHIJ";
        let f = make_file(data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let a = cache.get(&file, 0, 10).unwrap().to_vec();
        assert_eq!(&a, b"0123456789");

        let b = cache.get(&file, 10, 10).unwrap().to_vec();
        assert_eq!(&b, b"ABCDEFGHIJ");
    }

    #[test]
    fn cross_block_read_uses_spill_buffer() {
        // Read spanning a 4KB block boundary uses spill buffer, not cache.
        let data = vec![0xABu8; 8192];
        let f = make_file(&data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        // First, cache block 0.
        let a = cache.get(&file, 0, 100).unwrap().to_vec();
        assert_eq!(a, vec![0xABu8; 100]);

        // Now read spanning the boundary: offset=4000, len=200 spans [0,4096) and [4096,8192).
        let b = cache.get(&file, 4000, 200).unwrap().to_vec();
        assert_eq!(b, vec![0xABu8; 200]);
    }

    #[test]
    fn empty_file_read_fails() {
        let f = make_file(b"");
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        // Reading 1 byte from empty file should fail.
        let result = cache.get(&file, 0, 1);
        assert!(result.is_err());
    }

    #[test]
    fn clear_invalidates_cache() {
        let data = b"original data here";
        let f = make_file(data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        // Populate cache.
        let first = cache.get(&file, 0, 10).unwrap().to_vec();
        assert_eq!(&first, b"original d");

        // Invalidate.
        cache.clear();

        // After clear, reading same range should still work (re-reads from file).
        let after = cache.get(&file, 0, 10).unwrap().to_vec();
        assert_eq!(&after, b"original d");
    }

    // ─── New LRU-specific tests ───────────────────────────────────────────

    #[test]
    fn lru_multi_block_hit() {
        // Read 3 different aligned blocks, verify re-reading each hits cache.
        let data = vec![0u8; BLOCK_ALIGN * 4];
        let f = make_file(&data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        // Write distinct patterns to each block.
        drop(file);
        {
            use std::io::Seek;
            let mut f2 = std::fs::OpenOptions::new()
                .write(true)
                .open(f.path())
                .unwrap();
            f2.seek(std::io::SeekFrom::Start(0)).unwrap();
            f2.write_all(&[1u8; BLOCK_ALIGN]).unwrap();
            f2.seek(std::io::SeekFrom::Start(BLOCK_ALIGN as u64))
                .unwrap();
            f2.write_all(&[2u8; BLOCK_ALIGN]).unwrap();
            f2.seek(std::io::SeekFrom::Start((BLOCK_ALIGN * 2) as u64))
                .unwrap();
            f2.write_all(&[3u8; BLOCK_ALIGN]).unwrap();
        }
        let file = File::open(f.path()).unwrap();

        let block0 = cache.get(&file, 0, 16).unwrap().to_vec();
        let block1 = cache.get(&file, BLOCK_ALIGN as u64, 16).unwrap().to_vec();
        let block2 = cache
            .get(&file, (BLOCK_ALIGN * 2) as u64, 16)
            .unwrap()
            .to_vec();

        assert_eq!(block0, vec![1u8; 16]);
        assert_eq!(block1, vec![2u8; 16]);
        assert_eq!(block2, vec![3u8; 16]);

        // Re-read — should hit cache and return same data.
        let block0_again = cache.get(&file, 0, 16).unwrap().to_vec();
        let block1_again = cache.get(&file, BLOCK_ALIGN as u64, 16).unwrap().to_vec();
        let block2_again = cache
            .get(&file, (BLOCK_ALIGN * 2) as u64, 16)
            .unwrap()
            .to_vec();

        assert_eq!(block0_again, block0);
        assert_eq!(block1_again, block1);
        assert_eq!(block2_again, block2);
    }

    #[test]
    fn lru_eviction_order() {
        // Fill all 16 slots, re-access slot 0, add 17th block,
        // verify slot at offset 4096 (slot 1) evicted, not slot 0.
        let data = vec![0u8; BLOCK_ALIGN * 20];
        let f = make_file(&data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        // Fill all 16 slots with blocks 0..16.
        for i in 0..16u64 {
            cache.get(&file, i * BLOCK_ALIGN as u64, 1).unwrap();
        }

        // Re-access block 0 so it's not the LRU.
        cache.get(&file, 0, 1).unwrap();

        // Add 17th block — should evict block 1 (offset 4096), which is the oldest
        // since block 0 was re-accessed.
        cache.get(&file, 16 * BLOCK_ALIGN as u64, 1).unwrap();

        // Reading block 1 (offset 4096) should be a miss (evicted).
        // We verify by checking the cache slots: block 1 should not be cached.
        let has_block1 = cache
            .slots
            .iter()
            .any(|s| s.block_offset == BLOCK_ALIGN as u64 && s.len > 0);
        assert!(!has_block1, "block 1 should have been evicted");

        // Block 0 should still be cached.
        let has_block0 = cache.slots.iter().any(|s| s.block_offset == 0 && s.len > 0);
        assert!(has_block0, "block 0 should still be cached");
    }

    #[test]
    fn lru_clear_all_slots() {
        // Fill 3+ slots, call clear(), verify subsequent reads all miss.
        let data = vec![0x42u8; BLOCK_ALIGN * 4];
        let f = make_file(&data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        // Fill 3 slots.
        cache.get(&file, 0, 1).unwrap();
        cache.get(&file, BLOCK_ALIGN as u64, 1).unwrap();
        cache.get(&file, (BLOCK_ALIGN * 2) as u64, 1).unwrap();

        cache.clear();

        // All slots should be fully reset.
        for slot in &cache.slots {
            assert_eq!(slot.block_offset, 0);
            assert_eq!(slot.len, 0);
            assert_eq!(slot.last_access, 0);
        }
        assert_eq!(cache.spill_len, 0);

        // Re-read should still work (reads from file).
        let val = cache.get(&file, 0, 1).unwrap();
        assert_eq!(val[0], 0x42);
    }

    #[test]
    fn lru_aligned_keys() {
        // offset=100 and offset=200 both align to block 0 — should hit same slot.
        let data = vec![0xEEu8; BLOCK_ALIGN];
        let f = make_file(&data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let a = cache.get(&file, 100, 10).unwrap().to_vec();
        assert_eq!(a, vec![0xEEu8; 10]);

        let b = cache.get(&file, 200, 10).unwrap().to_vec();
        assert_eq!(b, vec![0xEEu8; 10]);

        // Both should be served from the same cache slot (block 0).
        let slot_count = cache
            .slots
            .iter()
            .filter(|s| s.block_offset == 0 && s.len > 0)
            .count();
        assert_eq!(slot_count, 1, "only one slot should hold block 0");
    }

    #[test]
    fn lru_cross_block_uses_spill_buffer() {
        // File [0xAA×4096, 0xBB×4096], get(file, 4090, 20) → [0xAA×6, 0xBB×14].
        let mut data = vec![0xAAu8; BLOCK_ALIGN];
        data.extend_from_slice(&vec![0xBBu8; BLOCK_ALIGN]);
        let f = make_file(&data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let result = cache.get(&file, 4090, 20).unwrap().to_vec();
        assert_eq!(&result[..6], &[0xAAu8; 6]);
        assert_eq!(&result[6..], &[0xBBu8; 14]);

        // Verify no cache slot holds block 0 or block 1.
        for slot in &cache.slots {
            assert!(
                slot.len == 0,
                "cross-block data should not be cached in slots"
            );
        }
    }

    #[test]
    fn lru_partial_last_block() {
        // 5000 byte file, get(file, 4096, 904) reads last 904 bytes,
        // then get(file, 4096, 100) hits cache.
        let data = vec![0x77u8; 5000];
        let f = make_file(&data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let first = cache.get(&file, 4096, 904).unwrap().to_vec();
        assert_eq!(first.len(), 904);
        assert_eq!(first, vec![0x77u8; 904]);

        // Second read of 100 bytes from same block should hit cache.
        let second = cache.get(&file, 4096, 100).unwrap().to_vec();
        assert_eq!(second.len(), 100);
        assert_eq!(second, vec![0x77u8; 100]);
    }

    #[test]
    fn lru_short_file_overread() {
        // 1 byte file, get(file, 0, 100) → Err (not panic).
        let f = make_file(b"X");
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let result = cache.get(&file, 0, 100);
        assert!(result.is_err());
    }

    #[test]
    fn lru_empty_file_returns_error() {
        // Empty file, get(file, 0, 1) → Err.
        let f = make_file(b"");
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let result = cache.get(&file, 0, 1);
        assert!(result.is_err());
    }

    #[test]
    fn long_line_spans_multiple_blocks() {
        // Create file with line >4KB: b'A'×4090 + "\n" + b'B'×4090 + "\n"
        let mut data = vec![b'A'; 4090];
        data.push(b'\n');
        data.extend_from_slice(&vec![b'B'; 4090]);
        data.push(b'\n');
        let f = make_file(&data);
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        // Read the first line: offset 0, len 4091 (fits in block 0 since 4091 <= 4096).
        let line1 = cache.get(&file, 0, 4091).unwrap().to_vec();
        assert_eq!(&line1[..4090], &[b'A'; 4090]);
        assert_eq!(line1[4090], b'\n');

        // Read the second line: offset 4091, len 4091 (starts in block 0, ends in block 1 — cross-block).
        let line2 = cache.get(&file, 4091, 4091).unwrap().to_vec();
        assert_eq!(&line2[..4090], &[b'B'; 4090]);
        assert_eq!(line2[4090], b'\n');
    }

    #[test]
    fn zero_len_read_is_noop_on_fresh_cache() {
        let f = make_file(b"");
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let result = cache.get(&file, 0, 0).unwrap();
        assert!(result.is_empty());
        assert_eq!(cache.tick, 0);
        assert!(cache.slots.iter().all(|s| s.len == 0));
    }

    #[test]
    fn zero_len_read_is_noop_on_populated_cache() {
        let f = make_file(b"abc");
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        cache.get(&file, 0, 1).unwrap();
        let tick_before = cache.tick;

        let result = cache.get(&file, 0, 0).unwrap();
        assert!(result.is_empty());
        assert_eq!(cache.tick, tick_before);
    }

    #[test]
    fn zero_len_read_at_max_offset_is_ok() {
        let f = make_file(b"");
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let result = cache.get(&file, u64::MAX, 0).unwrap();
        assert!(result.is_empty());
        assert_eq!(cache.tick, 0);
        assert!(cache.slots.iter().all(|s| s.len == 0));
    }

    #[test]
    fn nonzero_read_range_overflow_returns_invalid_input() {
        let f = make_file(b"abc");
        let file = File::open(f.path()).unwrap();
        let mut cache = ReadCache::new();

        let err = cache.get(&file, u64::MAX, 1).unwrap_err();
        assert_eq!(err.kind(), io::ErrorKind::InvalidInput);
    }
}