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day16_functions.R
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run_beam <- function(beam_paths, beam_directions, beam_field) {
past_commands <- c()
visited <- beam_field == "x"
while (length(beam_paths) != 0) {
new_paths <- list()
new_beam_directions <- vector("character", 0)
for (i in seq_along(beam_paths)) {
# Search for next mirror (or end of matrix)
## RIGHT -----------------------------------------------------------------
if (beam_directions[i] == "R") {
not_empty <- which(!beam_field[beam_paths[[i]][1], ] %in% c(".", "-"))
next_pos <- not_empty[not_empty > beam_paths[[i]][2]][1]
if (is.na(next_pos)) {
# Visit until end of matrix
visited[
matrix(
c(
rep(beam_paths[[i]][1], length(beam_paths[[i]][2]:ncol(visited))),
beam_paths[[i]][2]:ncol(visited)
),
ncol = 2
)
] <- TRUE
next
}
next_mirror <- beam_field[beam_paths[[i]][1], next_pos]
visited[
matrix(
c(
rep(beam_paths[[i]][1], length(beam_paths[[i]][2]:next_pos)),
beam_paths[[i]][2]:next_pos
),
ncol = 2
)
] <- TRUE
if (next_mirror == "|") {
new_paths <-
c(
new_paths,
list(c(beam_paths[[i]][1], next_pos), c(beam_paths[[i]][1], next_pos))
)
new_beam_directions <- c(new_beam_directions, c("U", "D"))
} else if (next_mirror == "/") {
new_paths <-
c(
new_paths,
list(c(beam_paths[[i]][1], next_pos))
)
new_beam_directions <- c(new_beam_directions, "U")
} else if (next_mirror == "\\") {
new_paths <-
c(
new_paths,
list(c(beam_paths[[i]][1], next_pos))
)
new_beam_directions <- c(new_beam_directions, "D")
}
## LEFT ----------------------------------------------------------------
} else if (beam_directions[i] == "L") {
not_empty <- which(!beam_field[beam_paths[[i]][1], ] %in% c(".", "-"))
next_pos <-
not_empty[not_empty < beam_paths[[i]][2]][
sum(not_empty < beam_paths[[i]][2])
]
if (length(next_pos) == 0) {
visited[
matrix(
c(
rep(beam_paths[[i]][1], length(1:beam_paths[[i]][2])),
1:beam_paths[[i]][2]
),
ncol = 2
)
] <- TRUE
next
}
next_mirror <- beam_field[beam_paths[[i]][1], next_pos]
visited[
matrix(
c(
rep(beam_paths[[i]][1], length(beam_paths[[i]][2]:next_pos)),
beam_paths[[i]][2]:next_pos
),
ncol = 2
)
] <- TRUE
if (next_mirror == "|") {
new_paths <-
c(
new_paths,
list(c(beam_paths[[i]][1], next_pos), c(beam_paths[[i]][1], next_pos))
)
new_beam_directions <- c(new_beam_directions, c("U", "D"))
} else if (next_mirror == "/") {
new_paths <-
c(
new_paths,
list(c(beam_paths[[i]][1], next_pos))
)
new_beam_directions <- c(new_beam_directions, "D")
} else if (next_mirror == "\\") {
new_paths <-
c(
new_paths,
list(c(beam_paths[[i]][1], next_pos))
)
new_beam_directions <- c(new_beam_directions, "U")
}
## UP ------------------------------------------------------------------
} else if (beam_directions[i] == "U") {
not_empty <- which(!beam_field[ , beam_paths[[i]][2]] %in% c(".", "|"))
next_pos <-
not_empty[not_empty < beam_paths[[i]][1]][
sum(not_empty < beam_paths[[i]][1])
]
if (length(next_pos) == 0) {
visited[
matrix(
c(
1:beam_paths[[i]][1],
rep(beam_paths[[i]][2], length(1:beam_paths[[i]][1]))
),
ncol = 2
)
] <- TRUE
next
}
next_mirror <- beam_field[next_pos, beam_paths[[i]][2]]
visited[
matrix(
c(
beam_paths[[i]][1]:next_pos,
rep(beam_paths[[i]][2], length(beam_paths[[i]][1]:next_pos))
),
ncol = 2
)
] <- TRUE
if (next_mirror == "-") {
new_paths <-
c(
new_paths,
list(c(next_pos, beam_paths[[i]][2]), c(next_pos, beam_paths[[i]][2]))
)
new_beam_directions <- c(new_beam_directions, c("R", "L"))
} else if (next_mirror == "/") {
new_paths <-
c(
new_paths,
list(c(next_pos, beam_paths[[i]][2]))
)
new_beam_directions <- c(new_beam_directions, "R")
} else if (next_mirror == "\\") {
new_paths <-
c(
new_paths,
list(c(next_pos, beam_paths[[i]][2]))
)
new_beam_directions <- c(new_beam_directions, "L")
}
## DOWN ----------------------------------------------------------------
} else if (beam_directions[i] == "D") {
not_empty <- which(!beam_field[ , beam_paths[[i]][2]] %in% c(".", "|"))
next_pos <- not_empty[not_empty > beam_paths[[i]][1]][1]
if (is.na(next_pos)) {
visited[
matrix(
c(
beam_paths[[i]][1]:nrow(visited),
rep(beam_paths[[i]][2], length(beam_paths[[i]][1]:nrow(visited)))
),
ncol = 2
)
] <- TRUE
next
}
next_mirror <- beam_field[next_pos, beam_paths[[i]][2]]
visited[
matrix(
c(
beam_paths[[i]][1]:next_pos,
rep(beam_paths[[i]][2], length(beam_paths[[i]][1]:next_pos))
),
ncol = 2
)
] <- TRUE
if (next_mirror == "-") {
new_paths <-
c(
new_paths,
list(c(next_pos, beam_paths[[i]][2]), c(next_pos, beam_paths[[i]][2]))
)
new_beam_directions <- c(new_beam_directions, c("R", "L"))
} else if (next_mirror == "/") {
new_paths <-
c(
new_paths,
list(c(next_pos, beam_paths[[i]][2]))
)
new_beam_directions <- c(new_beam_directions, "L")
} else if (next_mirror == "\\") {
new_paths <-
c(
new_paths,
list(c(next_pos, beam_paths[[i]][2]))
)
new_beam_directions <- c(new_beam_directions, "R")
}
}
}
# keep unique paths (throw out loops)
path_ids <-
paste0(
sapply(new_paths, function(x) paste0(x, collapse = "")), new_beam_directions
)
# Throw out loops
beam_paths <- new_paths[!path_ids %in% past_commands]
beam_directions <- new_beam_directions[!path_ids %in% past_commands]
path_ids <- path_ids[!path_ids %in% past_commands]
past_commands <- c(past_commands, path_ids)
}
return(visited)
}