R help - Oct 2018 - Random selection of points in SpatialPointsDataFrame with distance constraint

I'm trying to select points randomly (without replacement) from a
SpatialPointsDataFrame (input object) with a minimum distance of 3,000
meters between them. I want to get random 50% of all points and create a
vector object with lines index as result. I tried to use sample() function
but I still did not realised how to set the geographical constraint. I'll
appreciate any help.
> input
class       : SpatialPointsDataFrame
features    : 205
extent      : 203294.5, 259880.6, 7600123, 7668676  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=23 +south +ellps=GRS80 +units=m +no_defs
variables   : 3
names       :     Sites, Long_X,   Lat_Y
min values  : CPF - 050, 203295, 7600120
max values  : JES - S94, 259881, 7668680
> head(input)
  Sites Long_X   Lat_Y1 CPF - 050 235441 76171502 CPF - 052 234106
76157403 CPF - 054 232683 76142804 CPF - 056 233863 76144205 CPF - 058
234012 76128906 CPF - 062 236929 7612850

rdm_input <- sample(x= nrow(input), size=(0.5*205), replace= FALSE)[1]
167 189 79 80 126 129 144 100 4 109 170 72 123 73 132 93 169 5 134 176
196 158 152 183 23 136 180[28] 130 12 142 179 11 13 66 22 2 96 29 54
137 120 171 184 36 113 3 81 115  30 85 61 162 98 102[55] 103 181 90
133 56 174 76 201 150 197 14 86 118 121 28 97 160 178 1 186 141 163
172 32 65 168 74[82] 114 182 128 131 67 70 165 187 185 69 17 194 16
154 119 192 156 106  25 101 198 105 108 151 125 190 10[109] 84 38 51
161 40 94 45 19 145 75 42 122 117 49 87


MSc. Nielson Pasqualotto
Doutorando pelo Programa de P?s-Gradua??o em Ecologia Aplicada
(ESALQ/CENA/USP)
Bi?logo e Mestre em Ci?ncias pela Universidade de S?o Paulo (USP).
Laborat?rio de Ecologia e Conserva??o (LAEC).
Av. Bandeirantes, 3900. Bairro Monte Alegre, Ribeir?o Preto, S?o Paulo,
Brasil.

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