Removed needless scanning of each column

diff --git a/advent19/src/advent19.hs b/advent19/src/advent19.hs
index 7543513837c4096960dfe2fea3d6d22e5f5c31b6..0cf73eec571b006ae66d6fe12ddbaae48d8016cb 100644 (file)
--- a/advent19/src/advent19.hs
+++ b/advent19/src/advent19.hs
@@ -76,8 +76,8 @@ traceUpper machine prev x = u'
 traceLower :: Machine -> (Integer, Integer) -> Integer -> (Integer, Integer)
 traceLower machine (_, prev) x = (x, l')
     where (bic, foundU) = beamInColumn machine x
-          startU = max prev foundU
-          l = head $ dropWhile (\y -> tractorBeamAt machine x y) [startU..]
+          startL = if prev == 0 then foundU else prev
+          l = head $ dropWhile (\y -> tractorBeamAt machine x y) [startL..]
           l' = if prev == 0 && bic == False
                then 0
                else l - 1