Rename shared packages by day instead of vaguely by purpose.

2018/internal/day05/polymer.go

@@ -0,0 +1,54 @@
+// Functions for operating on polymers from day 5.
+package day05
+
+import (
+	"bytes"
+	"unicode"
+)
+
+func ApplyReactions(data []byte) []byte {
+	result := data
+	changed := true
+
+	for changed {
+		result, changed = React(result)
+	}
+
+	return result
+}
+
+// React finds substrings of the form "xX" or "Xx" and removes them, returning
+// the resulting bytes.
+func React(data []byte) ([]byte, bool) {
+	result := bytes.Buffer{}
+	changed := false
+
+	for i := 0; i < len(data); i++ {
+		if i < len(data)-1 &&
+			data[i] != data[i+1] &&
+			string(bytes.ToLower([]byte{data[i]})) == string(bytes.ToLower([]byte{data[i+1]})) {
+
+			// we've found a reduction; skip over it without writing
+			//			return append(data[:i], data[i+2:]...), true
+			i++
+			changed = true
+			continue
+		}
+
+		result.WriteByte(data[i])
+	}
+
+	// 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)
+}