From: Brett Parker <iDunno@sommitrealweird.co.uk>
Date: Tue, 8 Dec 2020 20:55:31 +0000 (+0000)
Subject: Optimisation v1 for asteroid checks
X-Git-Url: https://git.sommitrealweird.co.uk/advent-of-code-2019.git/commitdiff_plain/667694b94a6758e3ce751c565309ae980e76a931?ds=sidebyside;hp=762c474265f967e401c9dfd52efdf81b2eeed9f0

Optimisation v1 for asteroid checks
---

diff --git a/day10/check_asteroids.sh b/day10/check_asteroids.sh
index e0bf959..8faf834 100755
--- a/day10/check_asteroids.sh
+++ b/day10/check_asteroids.sh
@@ -4,6 +4,7 @@ set -u
 set -e
 
 basedir=$(dirname $(readlink -f ${BASH_SOURCE}))
+
 filename=${1:-$basedir/3_4_8.txt}
 
 exec 3<$filename
@@ -25,9 +26,15 @@ while read -u 3 line; do
     ln=$((ln+1))
 done
 
+debug_line=true # default to not actually outputting any debug info
+
+if [ ${DEBUG:-0} -gt 0 ]; then
+    debug_line=debug_line
+fi
+
 debug_line() {
     line="$@"
-    #echo "$line" >&2
+    echo "$line" >&2
 }
 
 get_vector() {
@@ -47,7 +54,7 @@ check_collision() {
     local vector1=$1
     local vector2=$2
 
-    debug_line Comparing: $vector1 $vector2
+    $debug_line Comparing: $vector1 $vector2
 
     vector1_x=${vector1%,*}
     vector1_y=${vector1#*,}
@@ -65,38 +72,37 @@ check_collision() {
     vector2_y_val=${vector2_y#-}
 
     minimum_number=$vector2_x_val
-    if ( [ $vector2_x_val -gt $vector2_y_val ] && [ $vector2_y_val -gt 0 ] ) || 
-        [ $minimum_number -eq 0 ]; then
+    if [ $vector2_x_val -gt $vector2_y_val -a $vector2_y_val -gt 0 ] || [ $minimum_number -eq 0 ]; then
         minimum_number=$vector2_y_val
     fi
 
-    debug_line "Min number: $minimum_number"
+    $debug_line "Min number: $minimum_number"
 
     minstep_x=$vector2_x
     minstep_y=$vector2_y
     for (( a=$minimum_number; a>0; a-- )); do
-        if [ $((vector2_x % $a)) -eq 0 ] && [ $((vector2_y % $a)) -eq 0 ]; then
+        if [ $((vector2_x % $a)) -eq 0 -a $((vector2_y % $a)) -eq 0 ]; then
             minstep_x=$((vector2_x / $a))
             minstep_y=$((vector2_y / $a))
             break
         fi
     done
-    debug_line "Have minsteps: $minstep_x,$minstep_y"
+    $debug_line "Have minsteps: $minstep_x,$minstep_y"
 
     # in all other cases, we're at least heading in the right direction
     cur_x=$vector2_x
     cur_y=$vector2_y
 
-    debug_line "Looking for: $vector1_x,$vector1_y"
-    debug_line "Start: $cur_x,$cur_y"
+    $debug_line "Looking for: $vector1_x,$vector1_y"
+    $debug_line "Start: $cur_x,$cur_y"
 
     # from the starting point, add the min steps until we're past the other asteroid
     while keep_going $cur_x $cur_y $minstep_x $minstep_y $vector1_x $vector1_y; do
         cur_x=$((cur_x+$minstep_x))
         cur_y=$((cur_y+$minstep_y))
-        debug_line "Step: $cur_x,$cur_y"
-        if [ $cur_x -eq $vector1_x ] && [ $cur_y -eq $vector1_y ]; then
-            debug_line "Collision!"
+        $debug_line "Step: $cur_x,$cur_y"
+        if [ $cur_x -eq $vector1_x -a $cur_y -eq $vector1_y ]; then
+            $debug_line "Collision!"
             return 0
         fi
     done
@@ -104,6 +110,86 @@ check_collision() {
     return 1
 }
 
+fast_checks() {
+    asteroid=$1
+    start_x=${asteroid%,*}
+    start_y=${asteroid#*,}
+
+    hitblocker=0
+
+    max_x=$width
+    max_y=$ln
+
+    # first, head left of the asteroid
+    for (( x=$((start_x-1)); x>=0; x-- )); do
+        do_blocker_check $asteroid $x $start_y
+    done
+    hitblocker=0 # this gets updated in do_blocker_check, reset it
+
+    # now head right of the asteroid
+    for (( x=$(($start_x+1)); x<= $width; x++ )); do
+        do_blocker_check $asteroid $x $start_y
+    done
+    hitblocker=0 # this gets updated in do_blocker_check, reset it
+
+    # head upwards
+    for (( y=$((start_y-1)); y>=0; y-- )); do
+        do_blocker_check $asteroid $start_x $y
+    done
+    hitblocker=0 # this gets updated in do_blocker_check, reset it
+
+    # head down
+    for (( y=$((start_y+1)); y<=$ln; y++  )); do
+        do_blocker_check $asteroid $start_x $y
+    done
+}
+
+do_blocker_check() {
+    asteroid=$1
+    x=$2
+    y=$3
+
+    if [ ${asteroids[$x,$y]+a} ]; then
+        if [ $hitblocker -eq 0 ]; then
+            cansee[$asteroid,$x,$y]=1
+            cansee[$x,$y,$asteroid]=1
+            hitblocker=1
+        else
+            cantsee[$asteroid,$x,$y]=1
+            cantsee[$x,$y,$asteroid]=1
+        fi
+    fi
+}
+
+possibly_in_way() {
+    source_x=$1
+    source_y=$2
+    sink_x=$3
+    sink_y=$4
+    blocker_x=$5
+    blocker_y=$6
+
+    # first check the horizontal and vertical planes
+    if [[ ( $source_x -eq $sink_x && $sink_x -eq $blocker_x ) && ( $sink_y -gt $blocker_y && $blocker_y -gt $source_y ) || ( $sink_y -lt $blocker_y && $blocker_y -lt $source_y ) ]] ||
+       [[ ( $source_y -eq $sink_y && $sink_y -eq $blocker_y ) && ( $sink_x -gt $blocker_x && $blocker_x -gt $source_x ) || ( $sink_x -lt $blocker_x && $blocker_x -lt $source_x ) ]]; then
+       return 0
+    fi
+
+    # if our source asteroid is further to the right thank our sink, then to stand half a chance
+    # of being in the way, the blocker has to be x > $sink_x and x < source_x
+    # apply the same rule for y
+    # reverse the direction and do the same checks.
+    # So if source_x < sink_x then blocker_x has to be < sink_x > source_x
+    if [ $source_x -gt $sink_x -a $blocker_x -gt $sink_x -a $blocker_x -lt $source_x ] ||
+       [ $source_y -gt $sink_y -a $blocker_y -gt $sink_y -a $blocker_y -lt $source_y ] ||
+       [ $source_x -lt $sink_x -a $blocker_x -lt $sink_x -a $blocker_x -gt $source_x ] ||
+       [ $source_y -lt $sink_y -a $blocker_y -lt $sink_y -a $blocker_y -gt $source_y ]; then
+       return 0
+    fi
+
+    return 1
+}
+
 keep_going() {
     local cur_x=$1
     local cur_y=$2
@@ -112,19 +198,11 @@ keep_going() {
     local ast_x=$5
     local ast_y=$6
 
-    if [ $minstep_x -lt 0 ] && [ $cur_x -lt $ast_x ]; then
-        # past the asteroid, we missed
-        debug_line "stop cond 1"
-        return 1
-    elif [ $minstep_y -lt 0 ] && [ $cur_y -lt $ast_y ]; then
-        # past the asteroid, we missed
-        debug_line "stop cond 2"
-        return 1
-    elif [ $minstep_x -gt 0 ] && [ $cur_x -gt $ast_x ]; then
-        debug_line "stop cond 3"
-        return 1
-    elif [ $minstep_y -gt 0 ] && [ $cur_y -gt $ast_y ]; then
-        debug_line "stop cond 4"
+    if [ $minstep_x -lt 0 -a $cur_x -lt $ast_x ] ||
+       [ $minstep_y -lt 0 -a $cur_y -lt $ast_y ] ||
+       [ $minstep_x -gt 0 -a $cur_x -gt $ast_x ] ||
+       [ $minstep_y -gt 0 -a $cur_y -gt $ast_y ]; then
+        $debug_line "keepgoing finished"
         return 1
     fi
 
@@ -139,9 +217,13 @@ start_count=${#asteroids[@]}
 for asteroid in ${asteroid_loop[@]}; do
     ast1_x=${asteroid%,*}
     ast1_y=${asteroid#*,}
-
     cur_count=${#asteroid_loop[@]}
     percent=$((100-(100*$cur_count / $start_count)))
+
+    $debug_line "Doing fast checks"
+    fast_checks $asteroid
+    $debug_line "Running main loops"
+
     printf '\r%02d%% complete' $percent
     for asteroid2 in ${asteroid_loop[@]}; do
         if [ $asteroid2 == $asteroid ]; then
@@ -157,18 +239,25 @@ for asteroid in ${asteroid_loop[@]}; do
             continue
         fi
 
-        if ! [ ${cansee[$asteroid,$asteroid2]+a} ]; then # we already calculated this backwards
+        if ! [ ${cansee[$asteroid,$asteroid2]+a} ]; then # we already calculated this
             for asteroid3 in "${!asteroids[@]}"; do
                 if [ $asteroid == $asteroid3 ] || [ $asteroid2 == $asteroid3 ]; then
                     continue
                 fi
                 ast3_x=${asteroid3%,*}
                 ast3_y=${asteroid3#*,}
-                debug_line "Checking for collision between $asteroid and $asteroid2 by $asteroid3"
+
+                # if this asteroid can't be in the way, continue the loop
+                if ! possibly_in_way $ast1_x $ast1_y $ast2_x $ast2_y $ast3_x $ast3_y; then
+                    $debug_line "Apparently $ast1_x $ast1_y to $ast2_x $ast2_y cannot be blocked by $ast3_x $ast3_y"
+                    continue
+                fi
+
+                $debug_line "Checking for collision between $asteroid and $asteroid2 by $asteroid3"
                 vector2=$(get_vector $ast1_x $ast1_y $ast3_x $ast3_y)
                 if ( check_collision $vector1 $vector2 ); then
                     # path is blocked go on to next in outer loop
-                    debug_line "We hit $asteroid3 when looking for $asteroid2 from $asteroid"
+                    $debug_line "We hit $asteroid3 when looking for $asteroid2 from $asteroid"
                     cantsee[$asteroid2,$asteroid]=1
                     cantsee[$asteroid,$asteroid2]=1
                     continue 2
@@ -176,7 +265,7 @@ for asteroid in ${asteroid_loop[@]}; do
             done
         fi
         # nothing blocked the view of asteroid2 from asteroid1
-        debug_line "Apparently $asteroid can set $asteroid2"
+        $debug_line "Apparently $asteroid can see $asteroid2"
         asteroids[$asteroid]=$((${asteroids[$asteroid]}+1))
         asteroids[$asteroid2]=$((${asteroids[$asteroid2]}+1))
         cansee[$asteroid2,$asteroid]=1 # don't bother doing the expensive calculations on the return path
@@ -194,7 +283,7 @@ for asteroid in ${!asteroids[@]}; do
         best_count=$count
         best_ast=$asteroid
     fi
-    debug_line "$asteroid: ${asteroids[$asteroid]}"
+    $debug_line "$asteroid: ${asteroids[$asteroid]}"
 done
 
 echo "Best asteroid: $best_ast which can see $best_count asteroids"