dplyr：通过迭代向量来根据某些条件填充列

我正在寻找一种tidyverse解决方案，通过迭代向量，根据某种条件填充列.

Column prev represents previous state of a system, new represents the new state. prev and new are always in sequence, but some of them may loop, like in the example below.
0 and 100 values are start and finish states, the states represented here as letters are the important ones.

对于prev和new的特定组合(即，同时满足prev_condition和new_condition)，我需要迭代一个更大的向量vec(它的元素比可能的状态组合多)，并将值按顺序放入第to_do列

我想补充一点，prev和new的组合不需要是唯一的，这就是为什么我添加了第change列来指示发生的每个状态更改.

这里是一个简单的例子和一个解决方案在基地R.我真的希望一个优雅的tidyverse解决方案.非常感谢.

# Minimal reproducible data
df <- data.frame(prev = c("0", rep(letters[1:3], 2), rep(letters[4:10], 3)),
                 new = c(rep(letters[1:3], 2), rep(letters[4:10], 3), "100"),
                 change = 1:28,
                 to_do = rep(NA, 28))

# Vector for iteration
vec <- 1:30

# Conditions
prev_condition <- c(letters[4:6])   # prev state must be any of: "d" "e" "f"
new_condition <- c(letters[5:7])    # new state must be any of: "e" "f" "g"

# base R solution
n_row <- length(df[df$prev %in% prev_condition & df$new %in% new_condition, "to_do"])
df[df$prev %in% prev_condition & df$new %in% new_condition, "to_do"] <- vec[1:n_row]

df
#>    prev new change to_do
#> 1     0   a      1  <NA>
#> 2     a   b      2  <NA>
#> 3     b   c      3  <NA>
#> 4     c   a      4  <NA>
#> 5     a   b      5  <NA>
#> 6     b   c      6  <NA>
#> 7     c   d      7  <NA>
#> 8     d   e      8     I
#> 9     e   f      9     Y
#> 10    f   g     10     N
#> 11    g   h     11  <NA>
#> 12    h   i     12  <NA>
#> 13    i   j     13  <NA>
#> 14    j   d     14  <NA>
#> 15    d   e     15     W
#> 16    e   f     16     Q
#> 17    f   g     17     Z
#> 18    g   h     18  <NA>
#> 19    h   i     19  <NA>
#> 20    i   j     20  <NA>
#> 21    j   d     21  <NA>
#> 22    d   e     22     V
#> 23    e   f     23     C
#> 24    f   g     24     C
#> 25    g   h     25  <NA>
#> 26    h   i     26  <NA>
#> 27    i   j     27  <NA>
#> 28    j 100     28  <NA>

^{由reprex package(v2.0.1)于2022-05-09创建}