R 相对于每组条件行计算每 n 行的斜率

我有以下名为df(以下为dput)的数据帧:

# A tibble: 14 × 5
   group date                indicator value diff_hours
   <chr> <dttm>              <lgl>     <dbl>      <dbl>
 1 A     2022-11-01 01:00:00 FALSE         2          4
 2 A     2022-11-01 02:00:00 FALSE         1          3
 3 A     2022-11-01 03:00:00 FALSE         4          2
 4 A     2022-11-01 04:00:00 FALSE         1          1
 5 A     2022-11-01 05:00:00 TRUE          3          0
 6 A     2022-11-01 06:00:00 FALSE         1          1
 7 A     2022-11-01 07:00:00 FALSE         3          2
 8 B     2022-11-01 01:00:00 FALSE         1          4
 9 B     2022-11-01 02:00:00 FALSE         2          3
10 B     2022-11-01 03:00:00 FALSE         3          2
11 B     2022-11-01 04:00:00 FALSE         1          1
12 B     2022-11-01 05:00:00 TRUE          4          0
13 B     2022-11-01 06:00:00 FALSE         1          1
14 B     2022-11-01 07:00:00 FALSE         5          2

我想计算相对于条件行indicator == TRUE每n行的斜率(lm(value ~ diff_hours)).具有TRUE的行的斜率应为NA.下面是名为df_desired的所需输出，其中n=2(见下图dput):

# A tibble: 14 × 6
# Groups:   group [2]
   group date                indicator value diff_hours slope
   <chr> <dttm>              <lgl>     <dbl>      <dbl> <dbl>
 1 A     2022-11-01 01:00:00 FALSE         2          4     1
 2 A     2022-11-01 02:00:00 FALSE         1          3     1
 3 A     2022-11-01 03:00:00 FALSE         4          2     3
 4 A     2022-11-01 04:00:00 FALSE         1          1     3
 5 A     2022-11-01 05:00:00 TRUE          3          0    NA
 6 A     2022-11-01 06:00:00 FALSE         1          1     2
 7 A     2022-11-01 07:00:00 FALSE         3          2     2
 8 B     2022-11-01 01:00:00 FALSE         1          4    -1
 9 B     2022-11-01 02:00:00 FALSE         2          3    -1
10 B     2022-11-01 03:00:00 FALSE         3          2     2
11 B     2022-11-01 04:00:00 FALSE         1          1     2
12 B     2022-11-01 05:00:00 TRUE          4          0    NA
13 B     2022-11-01 06:00:00 FALSE         1          1     4
14 B     2022-11-01 07:00:00 FALSE         5          2     4

例如，第1行和第2行的斜率为lm(c(2,1)~c(4,3))=1.所以我想知道，是否有人知道如何计算每n行相对于每组条件行的斜率？

Df和df_desired的dput:

df <- structure(list(group = c("A", "A", "A", "A", "A", "A", "A", "B", 
"B", "B", "B", "B", "B", "B"), date = structure(c(1667260800, 
1667264400, 1667268000, 1667271600, 1667275200, 1667278800, 1667282400, 
1667260800, 1667264400, 1667268000, 1667271600, 1667275200, 1667278800, 
1667282400), class = c("POSIXct", "POSIXt"), tzone = ""), indicator = c(FALSE, 
FALSE, FALSE, FALSE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, 
FALSE, TRUE, FALSE, FALSE), value = c(2, 1, 4, 1, 3, 1, 3, 1, 
2, 3, 1, 4, 1, 5), diff_hours = c(4, 3, 2, 1, 0, 1, 2, 4, 3, 
2, 1, 0, 1, 2)), class = c("grouped_df", "tbl_df", "tbl", "data.frame"
), row.names = c(NA, -14L), groups = structure(list(group = c("A", 
"B"), .rows = structure(list(1:7, 8:14), ptype = integer(0), class = c("vctrs_list_of", 
"vctrs_vctr", "list"))), class = c("tbl_df", "tbl", "data.frame"
), row.names = c(NA, -2L), .drop = TRUE))

df_desired <- structure(list(group = c("A", "A", "A", "A", "A", "A", "A", "B", 
"B", "B", "B", "B", "B", "B"), date = structure(c(1667260800, 
1667264400, 1667268000, 1667271600, 1667275200, 1667278800, 1667282400, 
1667260800, 1667264400, 1667268000, 1667271600, 1667275200, 1667278800, 
1667282400), class = c("POSIXct", "POSIXt"), tzone = ""), indicator = c(FALSE, 
FALSE, FALSE, FALSE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, 
FALSE, TRUE, FALSE, FALSE), value = c(2, 1, 4, 1, 3, 1, 3, 1, 
2, 3, 1, 4, 1, 5), diff_hours = c(4, 3, 2, 1, 0, 1, 2, 4, 3, 
2, 1, 0, 1, 2), slope = c(1, 1, 3, 3, NA, 2, 2, -1, -1, 2, 2, 
NA, 4, 4)), row.names = c(NA, -14L), class = c("grouped_df", 
"tbl_df", "tbl", "data.frame"), groups = structure(list(group = c("A", 
"B"), .rows = structure(list(1:7, 8:14), ptype = integer(0), class = c("vctrs_list_of", 
"vctrs_vctr", "list"))), class = c("tbl_df", "tbl", "data.frame"
), row.names = c(NA, -2L), .drop = TRUE))