Tags: algorithms 


# Picking Up The Pieces

writeup by [haskal](https://awoo.systems) for [BLÅHAJ](https://blahaj.awoo.systems)

**403 points**
**93 solves**

>Can you help me? I had a flag that I bought at the store, but on the way home, I dropped parts of
>it on some roads! Now some roads have strings of text, and I can't tell which are part of my flag.
>I'm very smart and efficient (but somehow not smart or efficient enough to keep my flag), so I know
>that I took the fastest way from the store back to my house.
>I have a map with a list of all 200000 roads between intersections, and what strings are on them.
>The format of the map is <intersection 1> <intersection 2> <distance> <string on the road>. My
>house is at intersection 1 and the store is at intersection 200000.

provided file: [map.txt](https://git.lain.faith/BLAHAJ/writeups/raw/branch/writeups/2020/rgbctf/picking-pieces/map.txt)

## writeup

consider the intersections and roads to be a (weighted) graph and use dijkstra's algorithm to find
the shortest path. here is a solution in racket (because racket is good and you should use it)

(ctftime, being bad and awful doesn't highlight this correctly. see original writeup link)
#lang racket

(require graph)

;; this enables looking up the string labels when we're done
(define label-lookup (make-hash))

;; str -> (list num str str)
(define (parse-line line)
(match-define (list from to distance label) (string-split line " "))
(hash-set! label-lookup (list from to) label)
(hash-set! label-lookup (list to from) label)
(list (string->number distance) from to))

(displayln "reading file")
(define input
(map parse-line (file->lines "map.txt")))

(displayln "creating graph")
(define g (weighted-graph/undirected input))

(displayln "running coding and algorithms")
(define-values [dist pred] (dijkstra g "1"))
(displayln "done")

;; prints the path in reverse order then the labels in forward order
(define (walk x)
(displayln x)
(define the-pred (hash-ref pred x #f))
(unless (false? the-pred)
(walk the-pred)
(displayln (hash-ref label-lookup (list the-pred x)))))

(walk "200000")

Original writeup (https://git.lain.faith/BLAHAJ/writeups/src/branch/writeups/2020/rgbctf/picking-pieces/README.md).