R version 4.5.0 RC (2025-04-04 r88118 ucrt) -- "How About a Twenty-Six"
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Platform: x86_64-w64-mingw32/x64

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> library("zoo")

Attaching package: 'zoo'

The following objects are masked from 'package:base':

    as.Date, as.Date.numeric

> 
> target <- c(100, 1, 200, 2, 300)
> current <- na.fill0(c(NA, 1, NA, 2, NA), c(100, 200, 300))
> identical(target, current)
[1] TRUE
> 
> target <- structure(c(100, 1, 200, 2), na.action = 5L)
> current <- na.fill0(c(NA, 1, NA, 2, NA), list(100, 200, NULL))
> identical(target, current)
[1] TRUE
> 
> target <- structure(c(1, 1, 200, 2), na.action = 5L)
> current <- na.fill0(c(NA, 1, NA, 2, NA), list("extend", 200, NULL))
> identical(target, current)
[1] TRUE
> 
> target <- c(1, 1, 200, 2, 2)
> current <- na.fill0(c(NA, 1, NA, 2, NA), list("extend", 200, "extend"))
> identical(target, current)
[1] TRUE
> 
> target <- structure(c(1, 2), na.action = c(1L, 3L, 5L))
> current <- na.fill0(c(NA, 1, NA, 2, NA), list())
> identical(target, current)
[1] TRUE
> 
> target <- NULL
> current <- na.fill0(NULL, list(1))
> identical(target, current)
[1] TRUE
> 
> target <- 1
> current <- na.fill0(1, list(1))
> identical(target, current)
[1] TRUE
> 
> target <- 1
> current <- na.fill0(1, 2)
> identical(target, current)
[1] TRUE
> 
> target <- structure(c(17650, 17650, 0, 17651, 17651), class = "Date")
> current <- na.fill0(as.Date("2018-04-28") + c(NA, 1, NA, 2, NA), list("extend", as.Date(0)))
> identical(target, current)
[1] TRUE
> 
> target <- structure(c(0, 17650, 0, 17651, 0), class = "Date")
> current <- na.fill0(as.Date("2018-04-28") + c(NA, 1, NA, 2, NA), as.Date(0))
> identical(target, current)
[1] TRUE
> 
> proc.time()
   user  system elapsed 
   0.18    0.01    0.18