2020: rgbctf: picking up the pieces

2020/rgbctf/picking-pieces/README.md

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+# Picking Up The Pieces
+
+writeup by [haskal](https://awoo.systems) for [BLÅHAJ](https://blahaj.awoo.systems)
+
+**Misc**
+**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>
+
+## 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)
+
+```racket
+#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")
+```

2020/rgbctf/picking-pieces/pieces.rkt

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+#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")