X-Git-Url: https://git.njae.me.uk/?p=advent-of-code-19.git;a=blobdiff_plain;f=advent19%2Fsrc%2Fadvent19.hs;fp=advent19%2Fsrc%2Fadvent19.hs;h=0cf73eec571b006ae66d6fe12ddbaae48d8016cb;hp=7543513837c4096960dfe2fea3d6d22e5f5c31b6;hb=0ec58176ef0b59be76d4c2798d4d3fabdb121a24;hpb=5571128ccde9145b55276adc236667be871253f2

diff --git a/advent19/src/advent19.hs b/advent19/src/advent19.hs
index 7543513..0cf73ee 100644
--- 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