Add how to switch on compile time known strings

src/switching_on_strings.zig

@@ -0,0 +1,38 @@
+const std = @import("std");
+
+// Switching on strings isn't allowed in zig, because all switch statement
+// end up turning into a [jump table](https://en.wikipedia.org/wiki/Branch_table)
+// but you can't do that with a string.
+
+// To start with you create an enum with the value of each item that you want
+// to support. Here I am using 4 values with numbers.
+const Number = enum {
+    One,
+    Two,
+    Three,
+    Four,
+
+    // Then we add a parse function, which allos the Number to be parsed into
+    // a Number via a StaticStringMap. The map itself is created at comptime,
+    // so is effectively hardcoded into the function
+    fn parse(s: []const u8) ?Number {
+        const map = std.StaticStringMap(Number).initComptime(.{
+            .{ "one", .One },
+            .{ "two", .Two },
+            .{ "three", .Three },
+            .{ "four", .Four },
+        });
+        return map.get(s);
+    }
+};
+
+test "Switch StaticStringMap" {
+    // Now when we actually do the switch, we parse it into an enum, and then
+    // can do an exhaustive switch on that. Since we could be passed something
+    // that doesn't exist, we have to also support null.
+    std.testing.expectEqual(switch (Number.parse("two")) {
+        null => "",
+        .One, .Two => "fizz",
+        .Three, .Four => "buzz",
+    }, "fizz");
+}