//! Memory-mapped disk format for DiskANN indices.
//!
//! **Goal:** Enable billion-scale search with minimal memory footprint.
//!
//! # File Format
//!
//! ```text
//! ┌─────────────────────────────────────────────────────────┐
//! │ Header (64 bytes)                                       │
//! ├─────────────────────────────────────────────────────────┤
//! │ Magic: "DISKANN\0" (8 bytes)                            │
//! │ Version: u32                                            │
//! │ Num nodes: u32                                          │
//! │ Dimensionality: u32                                     │
//! │ Max degree: u32                                         │
//! │ Medoid: u32                                             │
//! │ Graph offset: u64                                       │
//! │ Vectors offset: u64                                     │
//! │ Reserved: [u8; 24]                                      │
//! ├─────────────────────────────────────────────────────────┤
//! │ Graph Section (CSR format)                              │
//! │   - Offsets: [u32; num_nodes + 1]                       │
//! │   - Edges: [u32; total_edges]                           │
//! ├─────────────────────────────────────────────────────────┤
//! │ Vectors Section                                         │
//! │   - Vectors: [[f32; dim]; num_nodes]                    │
//! │   OR                                                    │
//! │   - Quantized codes: [[u8; code_size]; num_nodes]       │
//! └─────────────────────────────────────────────────────────┘
//! ```
//!
//! # Memory-Mapped Access
//!
//! - **Graph**: Read-only, random access to adjacency lists
//! - **Vectors**: Zero-copy access for distance computation
//! - **Page cache**: OS manages caching (hot nodes stay in RAM)
//!
//! # Benefits
//!
//! | Aspect | In-Memory | Memory-Mapped | Improvement |
//! |--------|-----------|---------------|-------------|
//! | Memory (1M vectors, 384 dims) | ~1.5 GB | ~50 MB | **30x less** |
//! | Startup time | 5-10s | <100ms | **50-100x faster** |
//! | Scalability | RAM-limited | Disk-limited | **10-100x more data** |

use crate::index::diskann::graph::CSRGraph;
use crate::types::error::{DatabaseError, Result};
use memmap2::{Mmap, MmapOptions};
use std::fs::File;
use std::io::{BufWriter, Write};
use std::path::Path;

/// Magic number for file format validation.
const MAGIC: &[u8; 8] = b"DISKANN\0";

/// Current file format version.
const VERSION: u32 = 1;

/// Header size in bytes.
const HEADER_SIZE: usize = 64;

/// DiskANN file header.
#[repr(C)]
#[derive(Debug, Clone, Copy)]
struct Header {
    /// Magic number (file format identifier)
    magic: [u8; 8],

    /// File format version
    version: u32,

    /// Number of nodes
    num_nodes: u32,

    /// Vector dimensionality
    dim: u32,

    /// Maximum out-degree
    max_degree: u32,

    /// Medoid node ID (entry point)
    medoid: u32,

    /// Byte offset to graph section
    graph_offset: u64,

    /// Byte offset to vectors section
    vectors_offset: u64,

    /// Reserved for future use
    _reserved: [u8; 24],
}

impl Header {
    /// Create a new header.
    fn new(num_nodes: u32, dim: u32, max_degree: u32, medoid: u32) -> Self {
        Self {
            magic: *MAGIC,
            version: VERSION,
            num_nodes,
            dim,
            max_degree,
            medoid,
            graph_offset: HEADER_SIZE as u64,
            vectors_offset: 0, // Set after graph is written
            _reserved: [0; 24],
        }
    }

    /// Validate header.
    ///
    /// # Errors
    ///
    /// Returns error if magic number or version is invalid
    fn validate(&self) -> Result<()> {
        todo!("Validate magic number and version")
    }

    /// Serialize header to bytes.
    fn to_bytes(&self) -> [u8; HEADER_SIZE] {
        todo!("Serialize header")
    }

    /// Deserialize header from bytes.
    ///
    /// # Errors
    ///
    /// Returns error if bytes are invalid
    fn from_bytes(bytes: &[u8; HEADER_SIZE]) -> Result<Self> {
        todo!("Deserialize header")
    }
}

/// Disk format writer for DiskANN index.
pub struct DiskFormat;

impl DiskFormat {
    /// Save index to disk in memory-mapped format.
    ///
    /// # Arguments
    ///
    /// * `path` - Output file path
    /// * `graph` - Graph structure (will be converted to CSR)
    /// * `vectors` - All vectors
    /// * `medoid` - Entry point node ID
    /// * `max_degree` - Maximum out-degree
    ///
    /// # Errors
    ///
    /// Returns error if:
    /// - File I/O fails
    /// - Vectors have inconsistent dimensions
    ///
    /// # Example
    ///
    /// ```rust,ignore
    /// DiskFormat::save("index.diskann", &graph, &vectors, medoid, 64)?;
    /// ```
    pub fn save(
        path: &str,
        graph: &CSRGraph,
        vectors: &[Vec<f32>],
        medoid: u32,
        max_degree: u32,
    ) -> Result<()> {
        todo!("Write header, graph, and vectors to file")
    }

    /// Write header to file.
    fn write_header(writer: &mut BufWriter<File>, header: &Header) -> Result<()> {
        todo!("Write header bytes")
    }

    /// Write graph (CSR format) to file.
    ///
    /// Returns byte offset where graph section ends.
    fn write_graph(writer: &mut BufWriter<File>, graph: &CSRGraph) -> Result<u64> {
        todo!("Write offsets and edges")
    }

    /// Write vectors to file.
    fn write_vectors(writer: &mut BufWriter<File>, vectors: &[Vec<f32>]) -> Result<()> {
        todo!("Write vectors as binary f32 arrays")
    }
}

/// Memory-mapped DiskANN index for zero-copy search.
pub struct MmapIndex {
    /// Memory-mapped file
    _mmap: Mmap,

    /// Parsed header
    header: Header,

    /// Graph section (CSR format)
    graph: CSRGraph,

    /// Vectors section (raw pointer into mmap)
    vectors_ptr: *const f32,
}

impl MmapIndex {
    /// Load index from disk with memory mapping.
    ///
    /// # Arguments
    ///
    /// * `path` - Index file path
    ///
    /// # Returns
    ///
    /// Memory-mapped index ready for search
    ///
    /// # Errors
    ///
    /// Returns error if:
    /// - File not found
    /// - File is corrupted (invalid header)
    /// - Memory mapping fails
    ///
    /// # Safety
    ///
    /// Memory-mapped data is immutable. Do not modify the file while index is loaded.
    ///
    /// # Example
    ///
    /// ```rust,ignore
    /// let index = MmapIndex::load("index.diskann")?;
    /// let results = index.search(&query, 10, 75)?;
    /// ```
    pub fn load(path: &str) -> Result<Self> {
        todo!("Open file, parse header, map graph and vectors")
    }

    /// Parse header from mmap.
    fn parse_header(mmap: &Mmap) -> Result<Header> {
        todo!("Read and validate header")
    }

    /// Parse graph (CSR) from mmap.
    fn parse_graph(mmap: &Mmap, header: &Header) -> Result<CSRGraph> {
        todo!("Read offsets and edges from mmap")
    }

    /// Get vector by node ID.
    ///
    /// # Arguments
    ///
    /// * `node_id` - Node ID
    ///
    /// # Returns
    ///
    /// Slice of vector components (zero-copy)
    ///
    /// # Safety
    ///
    /// Assumes vectors_ptr is valid and node_id < num_nodes
    pub fn vector(&self, node_id: u32) -> &[f32] {
        unsafe {
            let offset = node_id as usize * self.header.dim as usize;
            std::slice::from_raw_parts(self.vectors_ptr.add(offset), self.header.dim as usize)
        }
    }

    /// Get medoid (entry point).
    pub fn medoid(&self) -> u32 {
        self.header.medoid
    }

    /// Get graph structure.
    pub fn graph(&self) -> &CSRGraph {
        &self.graph
    }

    /// Get dimensionality.
    pub fn dim(&self) -> usize {
        self.header.dim as usize
    }
}

// MmapIndex is Send but not Sync (single-threaded mmap access)
unsafe impl Send for MmapIndex {}

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

    #[test]
    fn test_header_serialization() {
        todo!("Test header round-trip serialization")
    }

    #[test]
    fn test_header_validation() {
        todo!("Test header validation (magic, version)")
    }

    #[test]
    fn test_save_small_index() {
        todo!("Test saving small index to disk")
    }

    #[test]
    fn test_load_index() {
        todo!("Test loading index from disk")
    }

    #[test]
    fn test_save_load_roundtrip() {
        todo!("Test that save->load preserves data")
    }

    #[test]
    fn test_vector_access() {
        todo!("Test zero-copy vector access")
    }
}
