move: advanced funcs to functional programming

27-functional-programming/log-parser-exp/groupby.go

@@ -0,0 +1,17 @@
+package main
+
+func groupBy(results []result, keyer groupFunc) []result {
+	grouped := make(map[string]result, len(results))
+
+	for _, cur := range results {
+		key := keyer(cur)
+		grouped[key] = cur.add(grouped[key])
+	}
+
+	out := results[:0]
+	for _, r := range grouped {
+		out = append(out, r)
+	}
+
+	return out
+}