Specifically this required days of the week to be sorted by multiple price criteria.
[This is intended as a topic for discussion as opposed to a specific request for help]
I used the problem to develop a Lambda function that uses a binary tree to select the rows at the leaf nodes and perform a straightforward sort. The results then get stacked pairwise until the blue table of results is returned
BYROWλ
"Applies a user-defined function by row and collects array results into a stacked array"
=LET(resultϑ, BYROW(array, ThunkFnλ), BinaryTreeλ(ROWS(resultϑ), "", Derefϑarrλ))
The main bisection recursion is hidden from the user
BinaryTreeλ
"Generates binary numbers stacking blocks pairwise to an upper limit"
=LET(
maxNode, BASE(nodeCount - 1, 2),
maxLevel, LEN(maxNode),
level, LEN(parentNode),
maxChild, LEFT(maxNode, 1 + level),
IF(
level < maxLevel - 1,
LET(
childNode0, BinaryTreeλ(nodeCount, parentNode & 0, FNλ),
childNode1, BinaryTreeλ(nodeCount, parentNode & 1, FNλ),
IF((parentNode & 0) = maxChild, childNode0, VSTACK(childNode0, childNode1))
),
IF(
(parentNode & 0) = maxChild,
FNλ(parentNode & 0),
VSTACK(FNλ(parentNode & 0), FNλ(parentNode & 1))
)
)
)
but it is intended to non-problem specific and hence reusable.
A key feature is that the user provides a Lambda function that performs the required calculation as if BYROW would return the result, but the function is converted to one the returns a thunk and hence avoids the array of arrays problem.
ThunkFnλ
"Accepts a user-defined Lambda function that returns an array result and generates a related function that returns the corresponding thunk"
=LAMBDA(arr, LAMBDA(userFnλ(arr)))
Finally
Derefϑarrλ
"Dereferences a term from a thunk array using a zero-based binary pointer and expands the resulting scalar thunk"
=INDEX(resultϑ, DECIMAL(ptr, 2) + 1, 1)()
converts the binary node pointer to a decimal index.
This is all crazily heavy but, hopefully, would be simple to reuse for other problems since the main processing is problem independent.
