The two functions were near duplicates: walk to the key, then look for the
nearest leaf on one side. Only the scan direction, the comparison deciding a
leaf qualifies, and whether to take the first or last leaf below the subtree
differed, exactly the distinction `BelowDirection` already draws for
`belowHelper`, so the pair collapse into one parameterised walk. Also drops
the stale `// TODO: what to return here?` above `lowerBound`'s `return end()`:
no predecessor is the correct answer for the smallest key, and the tests pin
it.
Existing coverage only exercised `boundHelper`'s inner-node branch, every test
map had at least three items, so the root was always an inner node and the
leaf branch at the top of the function was never reached with a real answer to
give. Adds coverage for a single-item map, the smallest map that reaches that
branch, and an empty map, where the scan must find nothing on every branch
before falling through to `end()`.
Fixes the single-item test's own comment, which claimed `root_` becomes a
leaf, when in fact `root_` stays the inner node it was constructed with for
any map built via `addItem`; only a single-item map synced from a peer
(`addRootNode`) ever replaces `root_` with a leaf directly. The same comment
also claimed the leaf branch settles every probe before `root_`'s inner-node
scan could run, which only holds for a probe the leaf qualifies against: for
the rest the leaf is popped and that scan is exactly what reaches `end()`.
The test name said `leaf root` for the same reason, and now names the leaf
below the root.
`belowHelper` built each stack entry's `SHAMapNodeID` from `branch`, the branch
used to reach the subtree root, rather than `childBranch`, the branch it had
just descended. The resulting IDs carried a correct depth but named a
different subtree, and nothing rejected them: such an ID has a legal depth and
a legal mask, so only comparing it against an actual leaf key exposes the
mismatch. The affected stacks feed read-only traversals whose consumers use
only the depth, so no ledger state, hash, or peer message was affected, but
any future consumer of `getNodeID()` would have silently received the wrong
position.
Rather than fix the one call, make the mistake unrepresentable.
`NodePathStack` replaces the bare `std::stack` and refuses to accept an ID at
all: every push takes the branch being descended and derives the ID itself, so
a node and its ID cannot disagree. `isPrefixOf` assertions on each push catch a
wrong branch at the point it happens rather than wherever the ID is later
read. Leaf entries now keep the depth they were reached at instead of a
normalized `kLeafDepth`, which is what lets those assertions hold:
`addGiveItem` splits a leaf from the depth it actually sits at.
The new traversal tests fail on the previous code: reverting the branch
derivation trips the leaf-key assertion on the first iteration. Also adds a
`deepFanOutKeysAtLeafDepth` helper and mirrors them against it, since the
existing `deepFanOutKeys`'s fan-out at the 6th nibble keeps its tree only
about 6 levels deep and never exercised the depth-63/64 code these tests are
meant to protect, plus a case that collapses the entire depth-63 chain of
single-child inner nodes into a leaf on the final delete, which the
every-other-key deletion pattern the other new tests use never triggers.
