extract method to compute the totp code based on a pre-computed hmac

This will make it easier to add support for differen hmac algorithms,
because we can reuse the method.

src/main.zig

@@ -99,16 +99,21 @@ const Authenticator = struct {
 
         //debug("intervalNumber: {d}\n", .{intervalNumber});
 
-        var out: [std.crypto.auth.hmac.HmacSha1.mac_length]u8 = undefined;
-        std.crypto.auth.hmac.HmacSha1.create(out[0..], &intervalAsU8Array, secret);
-        //debug("hmac: {X}\n", .{out});
+        if (std.mem.eql(u8, self.url.algorithm, "SHA1")) {
+            var hmac: [std.crypto.auth.hmac.HmacSha1.mac_length]u8 = undefined;
+            std.crypto.auth.hmac.HmacSha1.create(hmac[0..], &intervalAsU8Array, secret);
+            return try self.generateTotp(allocator, &hmac);
+        }
+        unreachable;
+    }
 
+    fn generateTotp(self: Authenticator, allocator: Allocator, hmac: []const u8) ![]const u8 {
         // take the 4 least significant bits of the hash and use them as byte offset
-        const leastSignificantByte = out[std.crypto.auth.hmac.HmacSha1.mac_length - 1];
+        const leastSignificantByte = hmac[hmac.len - 1];
         const byteIndex = leastSignificantByte & 0b1111;
         //debug("index: {d}\n", .{byteIndex});
 
-        const x: [4]u8 = [4]u8{ out[byteIndex], out[byteIndex + 1], out[byteIndex + 2], out[byteIndex + 3] };
+        const x: [4]u8 = [4]u8{ hmac[byteIndex], hmac[byteIndex + 1], hmac[byteIndex + 2], hmac[byteIndex + 3] };
 
         const tokenBase = std.mem.readInt(i32, &x, .big) & 0x7fffffff;
         //debug("tokenBase: {d}\n", .{tokenBase});