Locate First Predicate Flip Without Reconstructing Context Trees

Read-only analogue of [split_around_by_predicate()]: finds the distinguished element where the scan predicate flips, but does not rebuild left/right trees.

Usage

locate_by_predicate(
  t,
  predicate,
  monoid_name,
  accumulator = NULL,
  include_metadata = FALSE
)

Arguments

t

A `flexseq`.

predicate

Function on accumulated measure values.

monoid_name

Name of monoid from `attr(t, "monoids")`.

accumulator

Optional starting measure (defaults to monoid identity).

include_metadata

Logical; include left/hit/right measures and index.

Value

If `include_metadata = FALSE`: `list(found, elem)`. If `TRUE`: `list(found, elem, metadata = list(left_measure, hit_measure, right_measure, index))`.

Examples

x <- as_flexseq(letters[1:6])
x
#> FingerTree <size=6, named=no>
#>   monoids: none
#> 
#> [[1]]
#> [1] "a"
#> 
#> [[2]]
#> [1] "b"
#> 
#> [[3]]
#> [1] "c"
#> 
#> [[4]]
#> [1] "d"
#> 
#> [[5]]
#> [1] "e"
#> 
#> [[6]]
#> [1] "f"
#> 

loc <- locate_by_predicate(x, function(v) v >= 4, ".size")
loc
#> $found
#> [1] TRUE
#> 
#> $elem
#> [1] "d"
#> 

# include metadata with a custom monoid
sum_m <- measure_monoid(`+`, 0, as.numeric)
x2 <- as_flexseq(1:6, monoids = list(sum = sum_m))
loc2 <- locate_by_predicate(x2, function(v) v >= 10, "sum", include_metadata = TRUE)
loc2
#> $found
#> [1] TRUE
#> 
#> $elem
#> [1] 4
#> 
#> $metadata
#> $metadata$left_measure
#> [1] 6
#> 
#> $metadata$hit_measure
#> [1] 10
#> 
#> $metadata$right_measure
#> [1] 11
#> 
#> $metadata$index
#> [1] 4
#> 
#>