build: add a new target to create files for a new day

Makefile

@@ -1,7 +1,7 @@
 GO = go
 BIN = bin/aoc
 
-.PHONY: build test clean
+.PHONY: build test clean new
 
 build:
 	@mkdir -p $(dir $(BIN))
@@ -12,3 +12,49 @@ test:
 
 clean:
 	@rm -f $(BIN)
+
+new:
+	@if [ -z "$(filter-out $@,$(MAKECMDGOALS))" ]; then \
+		echo "Usage: make new 2020D9"; \
+		exit 1; \
+	fi
+	@DAY_ARG=$$(echo $(filter-out $@,$(MAKECMDGOALS)) | awk '{print $$1}'); \
+	YEAR=$$(echo $$DAY_ARG | sed 's/D.*//'); \
+	DAY_NUM=$$(echo $$DAY_ARG | sed 's/.*D//'); \
+	DAY_NAME=$$(case $$DAY_NUM in \
+		1) echo "One" ;; \
+		2) echo "Two" ;; \
+		3) echo "Three" ;; \
+		4) echo "Four" ;; \
+		5) echo "Five" ;; \
+		6) echo "Six" ;; \
+		7) echo "Seven" ;; \
+		8) echo "Eight" ;; \
+		9) echo "Nine" ;; \
+		10) echo "Ten" ;; \
+		11) echo "Eleven" ;; \
+		12) echo "Twelve" ;; \
+		13) echo "Thirteen" ;; \
+		14) echo "Fourteen" ;; \
+		15) echo "Fifteen" ;; \
+		16) echo "Sixteen" ;; \
+		17) echo "Seventeen" ;; \
+		18) echo "Eighteen" ;; \
+		19) echo "Nineteen" ;; \
+		20) echo "Twenty" ;; \
+		21) echo "TwentyOne" ;; \
+		22) echo "TwentyTwo" ;; \
+		23) echo "TwentyThree" ;; \
+		24) echo "TwentyFour" ;; \
+		25) echo "TwentyFive" ;; \
+		*) echo "Unknown" ;; \
+	esac); \
+	mkdir -p internal/$$YEAR/Day$$DAY_NAME; \
+	mkdir -p internal/data/$$YEAR/Day$$DAY_NAME; \
+	touch internal/$$YEAR/Day$$DAY_NAME/code.go; \
+	touch internal/$$YEAR/Day$$DAY_NAME/code_test.go; \
+	touch internal/data/$$YEAR/Day$$DAY_NAME/input.txt; \
+	echo "Created files for $$DAY_ARG"
+
+%:
+	@: