Conditional editing of rows in a data frame

Dear R users,

I have a dataframe (main.table) with ~30,000 rows and 6 columns, of
which here are a few rows:

      id chr window         gene     xp.norm    xp.top
129 1_32   1     32       TAS1R1  1.28882115     FALSE
130 1_32   1     32       ZBTB48  1.28882115     FALSE
131 1_32   1     32       KLHL21  1.28882115     FALSE
132 1_32   1     32        PHF13  1.28882115     FALSE
133 1_33   1     33        PHF13  1.02727430     FALSE
134 1_33   1     33        THAP3  1.02727430     FALSE
135 1_33   1     33      DNAJC11  1.02727430     FALSE
136 1_33   1     33       CAMTA1  1.02727430     FALSE
137 1_34   1     34       CAMTA1  1.40312732      TRUE
138 1_35   1     35       CAMTA1  1.52104538     FALSE
139 1_36   1     36       CAMTA1  1.04853732     FALSE
140 1_37   1     37       CAMTA1  0.64794094     FALSE
141 1_38   1     38       CAMTA1  1.23026086      TRUE
142 1_38   1     38        VAMP3  1.23026086      TRUE
143 1_38   1     38         PER3  1.23026086      TRUE
144 1_39   1     39         PER3  1.18154967      TRUE
145 1_39   1     39         UTS2  1.18154967      TRUE
146 1_39   1     39      TNFRSF9  1.18154967      TRUE
147 1_39   1     39        PARK7  1.18154967      TRUE
148 1_39   1     39       ERRFI1  1.18154967      TRUE
149 1_40   1     40      no_gene  1.79796879     FALSE
150 1_41   1     41      SLC45A1  0.20193560     FALSE

I want to create two new columns, xp.bg and xp.n.top, using the
following criteria:

If gene is the same in consecutive rows, xp.bg is the minimum value of
xp.norm in those rows; if gene is not the same, xp.bg is simply the
value of xp.norm for that row;

Likewise, if there's a run of contiguous xp.top = TRUE values,
xp.n.top is the minimum value in that range, and if xp.top is false or
NA, xp.n.top is NA, or 0 (I don't care).

So, in the above example,
xp.bg for rows 136:141 should be 0.64794094, and is equal to xp.norm
for all other rows,
xp.n.top for row 137 is 1.40312732, 1.18154967 for rows 141:148, and
0/NA for all other rows.

Is there a way to combine indexing and if statements or some such to
accomplish this? I want to it this without using split(main.table,
main.table$gene), because there's about 20,000 unique entries for
gene, and one of the entries, no_gene, is repeated throughout. I
thought briefly of subsetting the rows where xp.top is TRUE, but I
then don't know how to set the range for min, so that it only looks at
what would originally have been consecutive rows, and searching the
help has not proved particularly useful.

Thanks in advance,
Irene Gallego Romero