129 lines
3.7 KiB
Scheme
129 lines
3.7 KiB
Scheme
(library
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(btree)
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(export btree-value-type
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btree-dev
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btree-root
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btree-open
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btree-lookup
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btree-each
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le64-type)
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(import (block-io)
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(chezscheme)
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(binary-format)
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(list-utils))
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;;; Unlike the kernel or c++ versions, I'm going to leave it to the hiogher
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;;; levels to handle multi level btrees.
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(binary-format node-header
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(csum le32)
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(flags le32)
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(blocknr le64)
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(nr-entries le32)
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(max-entries le32)
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(value-size le32)
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(padding le32))
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;;; (unpacker bv offset)
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(define-record-type value-type (fields size unpacker))
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(define (max-entries vt)
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(/ (- metadata-block-size node-header-size)
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(+ (size-type le64)
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(value-type-size vt))))
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(define (key-offset index)
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(+ node-header-size (* (size-type le64) index)))
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(define (value-base header)
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(+ node-header-size
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(* (node-header-max-entries header)
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(size-type le64))))
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(define (value-offset header vt index)
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(+ (value-base header)
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(* (value-type-size vt) index)))
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(define-record-type btree
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(fields value-type dev root))
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(define (btree-open vt dev root)
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(make-btree vt dev root))
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(define le64-type
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(make-value-type (size-type le64)
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(lambda (bv offset)
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(unpack-type bv offset le64))))
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(define (internal-node? header)
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(bitwise-bit-set? (node-header-flags header) 0))
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(define (leaf-node? header)
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(bitwise-bit-set? (node-header-flags header) 1))
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(define (key-at node index)
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(unpack-type node (key-offset index) le64))
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(define (value-at header node index vt)
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((value-type-unpacker vt) node (value-offset header vt index)))
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;;; Performs a binary search looking for the key and returns the index of the
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;;; lower bound.
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(define (lower-bound node header key)
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(let loop ((lo 0) (hi (node-header-nr-entries header)))
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(if (<= (- hi lo) 1)
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lo
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(let* ((mid (+ lo (/ (- hi lo) 2)))
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(k (key-at node mid)))
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(cond
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((= key k) mid)
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((< k key) (loop mid hi))
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(else (loop lo mid)))))))
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;;;;----------------------------------------------
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;;;; Lookup
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;;;;----------------------------------------------
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(define (btree-lookup tree key default)
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(let ((dev (btree-dev tree))
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(vt (btree-value-type tree)))
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(let loop ((root (btree-root tree)))
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(let* ((node (read-block dev root))
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(header (node-header-unpack node 0))
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(index (lower-bound node header key)))
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(if (internal-node? header)
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(loop (value-at header node index le64-type))
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(if (= key (key-at node index))
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(value-at header node index vt)
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default))))))
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;;;;----------------------------------------------
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;;;; Walking the btree
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;;;;----------------------------------------------
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;;; Calls (fn key value) on every entry of the btree.
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(define (btree-each tree fn)
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(let ((vt (btree-value-type tree)))
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(define (visit-leaf node header)
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(let loop ((index 0))
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(when (< index (node-header-nr-entries header))
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(fn (key-at node index) (value-at header node index vt))
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(loop (+ 1 index)))))
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(define (visit-internal node header)
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(let loop ((index 0))
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(when (< index (node-header-nr-entries header))
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(visit-node (value-at header node index le64-type))
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(loop (+ 1 index)))))
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(define (visit-node root)
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(let* ((node (read-block (btree-dev tree) root))
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(header (node-header-unpack node 0)))
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((if (internal-node? header) visit-internal visit-leaf) node header)))
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(visit-node (btree-root tree)))))
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