Make code more reusable

source/app.d

@@ -6,6 +6,7 @@ import std.stdio;
 import std.getopt;
 
 import day1;
+import dayutil;
 
 immutable string progName = "aoc-2020";
 
@@ -13,25 +14,38 @@ void function(string[])[] programs = [
 	&day1.run
 ];
 
+void printUsage(string name) {
+	printUsage(name, null);
+}
+
+void printUsage(string name, string message) {
+	stderr.writeln("USAGE: %s [day] [part]".format(name));
+	if (message != null) {
+		stderr.writeln(message);
+	}
+	exit(-1);
+}
+
 void main(string[] args) {
 	int day;
 	if (args.length < 2) {
-		stderr.writeln("USAGE: %s [day]".format(args[0]));
-		exit(-1);
+		printUsage(args[0]);
 	}
 	try {
 		day = to!int(args[1]);
 	} catch (ConvException e) {
-		stderr.writeln("[day] is not an integer");
-		exit(-1);
+		printUsage(args[0], "[day] is not an integer");
 	}
 
 	if (day <= 0 || day > programs.length) {
-		stderr.writeln("Day must be between 1 and %d".format(programs.length - 1));
-		exit(-1);
+		printUsage(args[0], "[day] must be between 1 and %d".format(programs.length - 1));
 	}
 
 	
-	programs[day - 1](args[2..$]);
+	try {
+		programs[day - 1](args[2..$]);
+	} catch(ArgumentException e) {
+		printUsage(args[0], e.msg);
+	} 
 
 }

source/day1.d

@@ -3,20 +3,25 @@ import std.array;
 import std.conv;
 import std.exception;
 import std.format;
+import std.functional;
 import std.range;
 import std.stdio;
 
+import dayutil;
+
 immutable string progName = "aoc-2020";
 
 void run(string[] args) {
-	enforce(args.length == 1, "Please provide a part to run %s 1 [part]".format(progName));
-	int part = to!int(args[0]);
-	enforce(part > 0 &&  part <= 2, "Parts %d to %d supported".format(1, 2));
 
+	/* For each line on stdin, copy it, map it to an integer and sort it.
+	   Sorting a range makes it a SortedRange and functions like contains(range, elem)
+       will make use of optimised implementations, in the case of contains(range, elem)
+	   it will use a binary search instead of a linear search */
 	auto numbers = stdin.byLineCopy.map!(a => to!int(a)).array.sort;
 
-	auto fun = part == 1 ? &part1 : &part2;
-	writeln(fun(numbers));
+	int solution = parts!int(args, partial!(part1, numbers), partial!(part2, numbers));
+	enforce(solution >= 0, "No solutions found");
+	writeln(solution);
 }
 
 int part1(SortedRange!(int[]) numbers) {

source/dayutil.d

@@ -0,0 +1,29 @@
+import std.conv;
+import std.exception;
+import std.format;
+
+class ArgumentException : Exception{
+	mixin basicExceptionCtors;
+}
+
+/**
+ * Helper function for implementing 
+ *
+ * Looks at the first argument string, and calls the delegate dgs[i + 1], while making sure nothing
+   goes out bounds.
+ *
+ * Params:
+ *     args = Reference to string with arguments, usually from main.
+ *     dgs = list of delegates. Which one will be called depends on args[0]
+ */
+R parts(R)(ref string[] args, R delegate()[] dgs ...) {
+	ulong len = dgs.length;
+	enforce!ArgumentException(args.length >= 1, "Please provide a part to run as a command line argument");
+	int part = to!int(args[0]);
+	enforce!ArgumentException(part > 0 &&  part <= len, "This day supports parts %d to %d".format(1, len));
+
+	// Remove the first argument
+	args = args[1..$];
+	
+	return dgs[part - 1]();
+}