Advent_of_Code_2020/Day9-Encoding_Error at main · anacko/Advent_of_Code_2020 · GitHub

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Codes <- read.table("day09.csv")
Codes <- as.vector(Codes[,1])

# Part 1: Who is the First false number?

ValidateNumber <- function(i = 26, nPreamble = 25) {
    CurrentCheck <- i
    PossibleSums <- NULL

    for (j in (CurrentCheck-nPreamble):(CurrentCheck-1)) {
        for (i in (CurrentCheck-nPreamble):(CurrentCheck-1)) {
            PossibleSums <- c(PossibleSums, Codes[i]+Codes[j])
        }
    }
    isValid <- Codes[CurrentCheck] %in% PossibleSums
    return(isValid)
}

i = 26
CheckPosition <- ValidateNumber(i)
while(CheckPosition) {
    CheckPosition <- ValidateNumber(i)
    i <- i+1 }

InvalidPosition <- i-1  # invalid position is before the +1
ValidateNumber(InvalidPosition)
Codes[InvalidPosition] # first false

for (i in 26:length(Codes)) {
    if(ValidateNumber(i)==FALSE) print(Codes[i])
}
# No other numbers are invalid.

# Part 2: Searching for the invalid sequence (sums the invalid number)
for (k in 1:20) {
    for (i in 1:(InvalidPosition-1)) {
        if(sum(Codes[i:(i+k)]) == Codes[InvalidPosition]) {
            InvalidSequence <- Codes[i:(i+k)]
            print(InvalidSequence)
} } }

min(InvalidSequence) + max(InvalidSequence) # Encryption Weakness