Day 5, part 2.

2018/day05-2.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"fmt"
+
+	"internal/polymer"
+	"internal/util"
+)
+
+func main() {
+	data := util.ReadInputBytes()
+	result := [26][]byte{}
+
+	for i := 0; i < 26; i++ {
+		result[i] = polymer.StripElement(data, rune('a'+i))
+		result[i] = polymer.ApplyReactions(result[i])
+	}
+
+	shortest := len(result[0])
+	for i := 1; i < 26; i++ {
+		if len(result[i]) < shortest {
+			shortest = len(result[i])
+		}
+	}
+
+	fmt.Println(shortest)
+}

2018/internal/polymer/polymer.go

@@ -3,9 +3,10 @@ package polymer
 
 import (
 	"bytes"
+	"unicode"
 )
 
-func ApplyReactions(data []byte) string {
+func ApplyReactions(data []byte) []byte {
 	result := data
 	changed := true
 
@@ -13,7 +14,7 @@ func ApplyReactions(data []byte) string {
 		result, changed = React(result)
 	}
 
-	return string(result)
+	return result
 }
 
 // React finds substrings of the form "xX" or "Xx" and removes them, returning
@@ -40,3 +41,14 @@ func React(data []byte) ([]byte, bool) {
 	// No changes possible.
 	return result.Bytes(), changed
 }
+
+// StripElement removes all instances of letter or its upper-case counterpart
+// from the input.
+func StripElement(data []byte, strip rune) []byte {
+	return bytes.Map(func(r rune) rune {
+		if r == strip || r == unicode.ToUpper(strip) {
+			return -1
+		}
+		return r
+	}, data)
+}