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core/btree: Clean up B-Tree offset comments

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Pekka Enberg 2025-04-10 08:27:55 +03:00
parent 86a4d3e33b
commit 761c03f7c5
1 changed files with 20 additions and 18 deletions
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38
core/storage/btree.rs
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@ -46,32 +46,37 @@ use super::{
/// 0 1 2 3 4 5 6 7 8 11
///
pub mod offset {
/// type of btree page -> u8
/// Type of the B-Tree page (u8).
pub const BTREE_PAGE_TYPE: usize = 0;
/// pointer to first freeblock -> u16
/// The second field of the b-tree page header is the offset of the first freeblock, or zero if there are no freeblocks on the page.
/// A freeblock is a structure used to identify unallocated space within a b-tree page.
/// Freeblocks are organized as a chain.
/// A pointer to the first freeblock (u16).
///
/// To be clear, freeblocks do not mean the regular unallocated free space to the left of the cell content area pointer, but instead
/// blocks of at least 4 bytes WITHIN the cell content area that are not in use due to e.g. deletions.
/// This field of the B-Tree page header is an offset to the first freeblock, or zero if
/// there are no freeblocks on the page. A freeblock is a structure used to identify
/// unallocated space within a B-Tree page, organized as a chain.
///
/// Please note that freeblocks do not mean the regular unallocated free space to the left
/// of the cell content area pointer, but instead blocks of at least 4
/// bytes WITHIN the cell content area that are not in use due to e.g.
/// deletions.
pub const BTREE_FIRST_FREEBLOCK: usize = 1;
/// number of cells in the page -> u16
/// The number of cells in the page (u16).
pub const BTREE_CELL_COUNT: usize = 3;
/// pointer to first byte of cell allocated content from top -> u16
/// SQLite strives to place cells as far toward the end of the b-tree page as it can,
/// in order to leave space for future growth of the cell pointer array.
/// = the cell content area pointer moves leftward as cells are added to the page
/// A pointer to first byte of cell allocated content from top (u16).
///
/// SQLite strives to place cells as far toward the end of the b-tree page as it can, in
/// order to leave space for future growth of the cell pointer array. This means that the
/// cell content area pointer moves leftward as cells are added to the page.
pub const BTREE_CELL_CONTENT_AREA: usize = 5;
/// number of fragmented bytes -> u8
/// The number of fragmented bytes (u8).
///
/// Fragments are isolated groups of 1, 2, or 3 unused bytes within the cell content area.
pub const BTREE_FRAGMENTED_BYTES_COUNT: usize = 7;
/// if internalnode, pointer right most pointer (saved separately from cells) -> u32
/// The right-most pointer (saved separately from cells) (u32)
pub const BTREE_RIGHTMOST_PTR: usize = 8;
}
@ -2904,10 +2909,7 @@ impl BTreeCursor {
// set new page type
root_contents.write_u8(offset::BTREE_PAGE_TYPE, new_root_page_type);
root_contents.write_u32(offset::BTREE_RIGHTMOST_PTR, child.get().id as u32);
root_contents.write_u16(
offset::BTREE_CELL_CONTENT_AREA,
self.usable_space() as u16,
);
root_contents.write_u16(offset::BTREE_CELL_CONTENT_AREA, self.usable_space() as u16);
root_contents.write_u16(offset::BTREE_CELL_COUNT, 0);
root_contents.write_u16(offset::BTREE_FIRST_FREEBLOCK, 0);