//! HDF5 B-tree v1 parsing (type 0 for groups).

#[cfg(not(feature = "std"))]
use alloc::vec::Vec;

use crate::error::FormatError;
use crate::storage::{Storage, read_exact_at};

/// A parsed B-tree v1 node.
#[derive(Debug, Clone)]
pub struct BTreeV1Node {
    /// Node type: 0=group, 1=raw data chunks.
    pub node_type: u8,
    /// Node level: 0=leaf, >0=internal.
    pub node_level: u8,
    /// Number of entries used.
    pub entries_used: u16,
    /// Left sibling address, or None if undefined.
    pub left_sibling: Option<u64>,
    /// Right sibling address, or None if undefined.
    pub right_sibling: Option<u64>,
    /// Keys (entries_used + 1 values).
    pub keys: Vec<u64>,
    /// Child addresses (entries_used values).
    pub children: Vec<u64>,
}

/// Checks that `[offset, offset + needed)` fits within `data`, guarding the
/// addition against `usize` overflow from a crafted near-`usize::MAX` offset.
fn ensure_len(data: &[u8], offset: usize, needed: usize) -> Result<(), FormatError> {
    if offset
        .checked_add(needed)
        .is_none_or(|end| end > data.len())
    {
        return Err(FormatError::UnexpectedEof {
            expected: offset.saturating_add(needed),
            available: data.len(),
        });
    }
    Ok(())
}

fn read_offset(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
    let s = size as usize;
    if pos.checked_add(s).is_none_or(|end| end > data.len()) {
        return Err(FormatError::UnexpectedEof {
            expected: pos.saturating_add(s),
            available: data.len(),
        });
    }
    let slice = &data[pos..pos + s];
    Ok(match size {
        2 => u16::from_le_bytes([slice[0], slice[1]]) as u64,
        4 => u32::from_le_bytes([slice[0], slice[1], slice[2], slice[3]]) as u64,
        8 => u64::from_le_bytes([
            slice[0], slice[1], slice[2], slice[3], slice[4], slice[5], slice[6], slice[7],
        ]),
        _ => return Err(FormatError::InvalidOffsetSize(size)),
    })
}

fn is_undefined(data: &[u8], pos: usize, size: u8) -> bool {
    let s = size as usize;
    if ensure_len(data, pos, s).is_err() {
        return false;
    }
    data[pos..pos + s].iter().all(|&b| b == 0xFF)
}

impl BTreeV1Node {
    /// Parse a B-tree v1 node at the given offset in the file data.
    ///
    /// For type 0 (group) nodes, keys are offset_size bytes each (heap name offsets).
    pub fn parse(
        file_data: &[u8],
        offset: usize,
        offset_size: u8,
        length_size: u8,
    ) -> Result<BTreeV1Node, FormatError> {
        Self::parse_in(file_data, offset as u64, offset_size, length_size)
    }

    /// [`Self::parse`] over any [`Storage`]: one read of the node's header,
    /// one of its keys and children.
    pub fn parse_in<S: Storage + ?Sized>(
        file: &S,
        offset: u64,
        offset_size: u8,
        _length_size: u8,
    ) -> Result<BTreeV1Node, FormatError> {
        // signature(4) + node_type(1) + node_level(1) + entries_used(2) = 8
        // + left_sibling(offset_size) + right_sibling(offset_size)
        let os = offset_size as usize;
        let header_size = 8 + os * 2;
        let header = read_exact_at(file, offset, header_size)?;
        let file_data: &[u8] = &header;
        // The header's read checked that `offset + header_size` fits.
        let body_start = offset + header_size as u64;
        let offset = 0usize;

        if &file_data[offset..offset + 4] != b"TREE" {
            return Err(FormatError::InvalidBTreeSignature);
        }

        let node_type = file_data[offset + 4];
        let node_level = file_data[offset + 5];
        let entries_used = u16::from_le_bytes([file_data[offset + 6], file_data[offset + 7]]);

        let mut pos = offset + 8;
        let left_sibling = if is_undefined(file_data, pos, offset_size) {
            None
        } else {
            Some(read_offset(file_data, pos, offset_size)?)
        };
        pos += os;
        let right_sibling = if is_undefined(file_data, pos, offset_size) {
            None
        } else {
            Some(read_offset(file_data, pos, offset_size)?)
        };

        // For type 0: keys are offset_size bytes, children are offset_size bytes
        // Layout: key[0], child[0], key[1], child[1], ..., key[N-1], child[N-1], key[N]
        let eu = entries_used as usize;
        let key_size = os; // For type 0, key = offset_size
        let needed = eu * (key_size + os) + key_size; // eu children + (eu+1) keys
        let body = read_exact_at(file, body_start, needed)?;
        let file_data: &[u8] = &body;

        let mut keys = Vec::with_capacity(eu + 1);
        let mut children = Vec::with_capacity(eu);

        if os == 0 {
            // What reading the first key reports (and keeps `chunks_exact`
            // below from being given a zero size).
            return Err(FormatError::InvalidOffsetSize(offset_size));
        }
        // `needed` bytes: key[0], child[0], ..., child[eu - 1], key[eu].
        let (pairs, last) = file_data.split_at(eu * (key_size + os));
        for pair in pairs.chunks_exact(key_size + os) {
            keys.push(read_offset(pair, 0, offset_size)?);
            children.push(read_offset(pair, key_size, offset_size)?);
        }
        keys.push(read_offset(last, 0, offset_size)?);

        Ok(BTreeV1Node {
            node_type,
            node_level,
            entries_used,
            left_sibling,
            right_sibling,
            keys,
            children,
        })
    }
}

/// Maximum recursion depth for B-tree traversal (malformed data protection).
const MAX_BTREE_DEPTH: usize = 64;

/// Collect all leaf-level child addresses (SNOD addresses) by traversing the B-tree.
pub fn collect_symbol_table_nodes(
    file_data: &[u8],
    btree_address: u64,
    offset_size: u8,
    length_size: u8,
) -> Result<Vec<u64>, FormatError> {
    collect_symbol_table_nodes_in(file_data, btree_address, offset_size, length_size)
}

/// [`collect_symbol_table_nodes`] over any [`Storage`]: two reads per node.
pub fn collect_symbol_table_nodes_in<S: Storage + ?Sized>(
    file: &S,
    btree_address: u64,
    offset_size: u8,
    length_size: u8,
) -> Result<Vec<u64>, FormatError> {
    collect_symbol_table_nodes_inner(file, btree_address, offset_size, length_size, 0)
}

fn collect_symbol_table_nodes_inner<S: Storage + ?Sized>(
    file: &S,
    btree_address: u64,
    offset_size: u8,
    length_size: u8,
    depth: usize,
) -> Result<Vec<u64>, FormatError> {
    if depth > MAX_BTREE_DEPTH {
        return Err(FormatError::NestingDepthExceeded);
    }

    let node = BTreeV1Node::parse_in(file, btree_address, offset_size, length_size)?;

    if node.node_type != 0 {
        return Err(FormatError::InvalidBTreeNodeType(node.node_type));
    }

    if node.node_level == 0 {
        // Leaf: children are SNOD addresses
        Ok(node.children)
    } else {
        // Internal: recurse into children. After the first child that
        // fails, the others are only read (as `storage::touch` does), not
        // descended into; that error is returned.
        let mut result = Vec::new();
        let mut failed = None;
        for &child_addr in &node.children {
            if failed.is_some() {
                // Parsing reads the node's header, then its body.
                let _ = BTreeV1Node::parse_in(file, child_addr, offset_size, length_size);
                continue;
            }
            match collect_symbol_table_nodes_inner(
                file,
                child_addr,
                offset_size,
                length_size,
                depth + 1,
            ) {
                Ok(child_snods) => result.extend(child_snods),
                Err(e) => failed = Some(e),
            }
        }
        match failed {
            Some(e) => Err(e),
            None => Ok(result),
        }
    }
}

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

    fn write_offset(buf: &mut Vec<u8>, val: u64, size: u8) {
        match size {
            4 => buf.extend_from_slice(&(val as u32).to_le_bytes()),
            8 => buf.extend_from_slice(&val.to_le_bytes()),
            _ => panic!("test"),
        }
    }

    fn build_btree_node(
        node_type: u8,
        level: u8,
        keys: &[u64],
        children: &[u64],
        left: Option<u64>,
        right: Option<u64>,
        offset_size: u8,
    ) -> Vec<u8> {
        assert_eq!(keys.len(), children.len() + 1);
        let entries_used = children.len() as u16;
        let mut buf = Vec::new();
        buf.extend_from_slice(b"TREE");
        buf.push(node_type);
        buf.push(level);
        buf.extend_from_slice(&entries_used.to_le_bytes());
        let undef: u64 = if offset_size == 4 {
            0xFFFFFFFF
        } else {
            0xFFFFFFFFFFFFFFFF
        };
        write_offset(&mut buf, left.unwrap_or(undef), offset_size);
        write_offset(&mut buf, right.unwrap_or(undef), offset_size);
        for i in 0..children.len() {
            write_offset(&mut buf, keys[i], offset_size);
            write_offset(&mut buf, children[i], offset_size);
        }
        write_offset(&mut buf, *keys.last().unwrap(), offset_size);
        buf
    }

    #[test]
    fn parse_leaf_node() {
        let data = build_btree_node(0, 0, &[0, 5, 10], &[0x100, 0x200], None, None, 8);
        let node = BTreeV1Node::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(node.node_type, 0);
        assert_eq!(node.node_level, 0);
        assert_eq!(node.entries_used, 2);
        assert_eq!(node.keys, vec![0, 5, 10]);
        assert_eq!(node.children, vec![0x100, 0x200]);
        assert_eq!(node.left_sibling, None);
        assert_eq!(node.right_sibling, None);
    }

    #[test]
    fn parse_near_usize_max_offset_rejected_without_overflow() {
        let data = build_btree_node(0, 0, &[0, 5, 10], &[0x100, 0x200], None, None, 8);
        let result = BTreeV1Node::parse(&data, usize::MAX - 4, 8, 8);
        assert!(
            matches!(result, Err(FormatError::UnexpectedEof { .. })),
            "expected a clean UnexpectedEof, got {result:?}"
        );
    }

    #[test]
    fn parse_with_siblings_none() {
        let data = build_btree_node(0, 0, &[0, 8], &[0x300], None, None, 8);
        let node = BTreeV1Node::parse(&data, 0, 8, 8).unwrap();
        assert_eq!(node.left_sibling, None);
        assert_eq!(node.right_sibling, None);
    }

    #[test]
    fn parse_internal_node_and_collect() {
        // Build a 2-level tree: one internal node pointing to two leaf nodes
        let os: u8 = 8;
        let leaf1_offset: usize = 0;
        let leaf2_offset: usize = 256;
        let internal_offset: usize = 512;

        let leaf1 = build_btree_node(0, 0, &[0, 5], &[0xA00], None, None, os);
        let leaf2 = build_btree_node(0, 0, &[5, 10], &[0xB00], None, None, os);
        let internal = build_btree_node(
            0,
            1,
            &[0, 5, 10],
            &[leaf1_offset as u64, leaf2_offset as u64],
            None,
            None,
            os,
        );

        let mut file = vec![0u8; 1024];
        file[leaf1_offset..leaf1_offset + leaf1.len()].copy_from_slice(&leaf1);
        file[leaf2_offset..leaf2_offset + leaf2.len()].copy_from_slice(&leaf2);
        file[internal_offset..internal_offset + internal.len()].copy_from_slice(&internal);

        let snods = collect_symbol_table_nodes(&file, internal_offset as u64, os, os).unwrap();
        assert_eq!(snods, vec![0xA00, 0xB00]);
    }

    #[test]
    fn invalid_signature() {
        let mut data = build_btree_node(0, 0, &[0, 1], &[0x100], None, None, 8);
        data[0] = b'X';
        let err = BTreeV1Node::parse(&data, 0, 8, 8).unwrap_err();
        assert_eq!(err, FormatError::InvalidBTreeSignature);
    }

    #[test]
    fn collect_wrong_node_type() {
        let data = build_btree_node(1, 0, &[0, 1], &[0x100], None, None, 8);
        let mut file = vec![0u8; 512];
        file[..data.len()].copy_from_slice(&data);
        let err = collect_symbol_table_nodes(&file, 0, 8, 8).unwrap_err();
        assert_eq!(err, FormatError::InvalidBTreeNodeType(1));
    }

    #[test]
    fn parse_4byte_offsets() {
        let data = build_btree_node(0, 0, &[0, 4], &[0x50], None, None, 4);
        let node = BTreeV1Node::parse(&data, 0, 4, 4).unwrap();
        assert_eq!(node.entries_used, 1);
        assert_eq!(node.children, vec![0x50]);
    }

    /// Nodes and trees, cut at every length, parse identically through a
    /// `read_at`-only storage.
    #[test]
    fn storage_parse_matches_slice_parse() {
        use crate::storage::CountingStorage;
        let nodes = [
            build_btree_node(0, 0, &[0, 5, 10], &[0x100, 0x200], None, None, 8),
            build_btree_node(0, 0, &[0, 5], &[0x100], Some(0x40), Some(0x80), 4),
            build_btree_node(1, 2, &[0, 5], &[0x100], None, Some(0x80), 8),
        ];
        for (n, node) in nodes.iter().enumerate() {
            let os = if n == 1 { 4 } else { 8 };
            for cut in 0..=node.len() {
                let f = &node[..cut];
                let storage = CountingStorage::new(f.to_vec());
                let want = BTreeV1Node::parse(f, 0, os, 8);
                let got = BTreeV1Node::parse_in(&storage, 0, os, 8);
                assert_eq!(format!("{got:?}"), format!("{want:?}"));
            }
        }
        let leaf1 = build_btree_node(0, 0, &[0, 5], &[0xA00], None, None, 8);
        let leaf2 = build_btree_node(0, 0, &[5, 10], &[0xB00], None, None, 8);
        let internal = build_btree_node(0, 1, &[0, 5, 10], &[0, 256], None, None, 8);
        let mut file = vec![0u8; 512 + internal.len()];
        file[..leaf1.len()].copy_from_slice(&leaf1);
        file[256..256 + leaf2.len()].copy_from_slice(&leaf2);
        file[512..].copy_from_slice(&internal);
        for cut in [file.len(), 300, 260, 100, 10] {
            let mut f = file.clone();
            if cut < 512 {
                // Truncate the leaves, keep the root.
                f[cut..512].fill(0);
            }
            let storage = CountingStorage::new(f.clone());
            assert_eq!(
                collect_symbol_table_nodes_in(&storage, 512, 8, 8),
                collect_symbol_table_nodes(&f, 512, 8, 8)
            );
        }
        let storage = CountingStorage::new(file);
        collect_symbol_table_nodes_in(&storage, 512, 8, 8).unwrap();
        assert_eq!(storage.reads(), 6);
    }
}