Optimize 2018 day 5 part 2 by doing pre-reactions

sim642 · sim642 · commit 4cb4289ffb30 · 2018-12-05T20:42:22.000+02:00
diff --git a/src/main/scala/eu/sim642/adventofcode2018/Day5.scala b/src/main/scala/eu/sim642/adventofcode2018/Day5.scala
@@ -22,8 +22,9 @@ object Day5 {
   def reactPolymerLength(s: String): Int = reactPolymer(s).length
 
   def bestPolymerLength(s: String): Int = {
-    val lowerUnits = s.toLowerCase.toSet
-    lowerUnits.map(lowerUnit => s.filterNot(_.toLower == lowerUnit)).map(reactPolymerLength).min
+    val preReacted = reactPolymer(s) // pre-react units which get reacted in all cases anyway, 5× shorter string used every time
+    val lowerUnits = preReacted.toLowerCase.toSet
+    lowerUnits.map(lowerUnit => preReacted.filterNot(_.toLower == lowerUnit)).map(reactPolymerLength).min
   }
 
 

Original file line number	Diff line number	Diff line change
`@@ -22,8 +22,9 @@ object Day5 {`
`22`	`22`	`def reactPolymerLength(s: String): Int = reactPolymer(s).length`
`23`	`23`
`24`	`24`	`def bestPolymerLength(s: String): Int = {`
`25`		`- val lowerUnits = s.toLowerCase.toSet`
`26`		`- lowerUnits.map(lowerUnit => s.filterNot(_.toLower == lowerUnit)).map(reactPolymerLength).min`
	`25`	`+ val preReacted = reactPolymer(s) // pre-react units which get reacted in all cases anyway, 5× shorter string used every time`
	`26`	`+ val lowerUnits = preReacted.toLowerCase.toSet`
	`27`	`+ lowerUnits.map(lowerUnit => preReacted.filterNot(_.toLower == lowerUnit)).map(reactPolymerLength).min`
`27`	`28`	`}`
`28`	`29`
`29`	`30`