fix(b+tree): lower_bound past the last key returns end(), and end() is a valid insert hint - #154
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…s a valid insert hint lower_bound expressed 'one past the end of a leaf' as the first value of the FOLLOWING leaf. Past the LAST leaf there is no following leaf, so it built an iterator from a null node slot - while end() is the last leaf at its num_vals offset. On a non-empty tree, for a key greater than every key present: tree.lower_bound( k ) == tree.end() was false, and tree.insert( tree.lower_bound( k ), k ) access-violated. So the ordinary sorted-container idiom failed for every append. end() is accepted as the append hint: it already works structurally - it is the last leaf at its num_vals offset, which insert() appends to - only the debug hint assertions dereferenced the hint unconditionally. The returned iterator now goes through make_iter( insert_pos_t ), which steps back from the next-insert position and so stays correct across a split that moved the value into the new node.
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lower_boundexpressed "one past the end of a leaf" as the first value of the following leaf. Past the last leaf there is no following leaf, so it built an iterator from a null node slot — whileend()is the last leaf at itsnum_valsoffset.On a non-empty tree, for a key greater than every key present:
So the ordinary sorted-container maintenance idiom failed for every append.
end()is now accepted as the append hint. It already worked structurally — it is the last leaf at itsnum_valsoffset, whichinsert()appends to; only the debug hint assertions dereferenced the hint unconditionally. The returned iterator now goes throughmake_iter( insert_pos_t ), which steps back from the next-insert position and so stays correct across a split that moved the value into the new node.Witness:
TEST( bp_tree, hinted_insert_boundaries ).Stacked on #153.