Returns the monoid's accumulated measure across the whole (sub)tree at
the root of x. This is the cached value that internal operations use
for O(log n) locate and split.
Arguments
- x
A flexseq (or any immutables subclass).
- monoid_name
Name of an attached monoid (e.g.
.size, a custom name fromadd_monoids(), or a built-in like.pq_min/.oms_max_key).
Value
The cached monoid value at the root of x. Shape depends on
the monoid — typically atomic (e.g. a numeric sum) but may be a list
for built-ins that carry auxiliary state.
Details
On the full tree x, this is the aggregate over every element. After a
split — e.g. s <- split_by_predicate(x, p, "sum") — call
get_measure(s$left, "sum") to read the aggregate of just the left
side in O(1).
For per-element measures, see get_measures().
Examples
sum_m <- measure_monoid(`+`, 0, function(el) el)
x <- add_monoids(as_flexseq(c(3, 1, 4, 1, 5, 9, 2, 6)),
list(sum = sum_m))
get_measure(x, "sum") # total across the whole tree
#> [1] 31
get_measure(x, ".size") # built-in: element count
#> [1] 8
q <- priority_queue("a", "b", "c", priorities = c(5, 1, 3))
get_measure(q, ".pq_min") # built-in, list-valued
#> $has
#> [1] TRUE
#>
#> $priority
#> [1] 1
#>