Add JSON input (#3)

Co-authored-by: Smaug123 <patrick+github@patrickstevens.co.uk>
Reviewed-on: #3

AnkiStatic.Lib/Domain/Note.fs

@@ -0,0 +1,43 @@
+namespace AnkiStatic
+
+open System
+open System.Security.Cryptography
+open System.Text
+
+type Note<'Model> =
+    {
+        Guid : uint64
+        ModelId : 'Model
+        LastModified : DateTimeOffset
+        UpdateSequenceNumber : int
+        /// Serialised space-separated as a string, with a space at the start and end.
+        Tags : string list
+        /// Serialised as a string separated by the 0x1f character
+        Fields : string list
+        /// In the Sqlite table, this is an int field.
+        /// Sqlite is dynamically typed and accepts strings in an int field.
+        /// But it will sort "correctly" in the sense that integers are compared as integers
+        /// for the purpose of sorting in this way.
+        SortField : Choice<string, int>
+        /// Unused
+        Flags : int
+        /// Unused
+        Data : string
+    }
+
+    member this.Checksum : uint =
+        let fromBase256 (firstCount : int) (bytes : byte[]) : uint =
+            let mutable answer = 0u
+
+            for b = 0 to firstCount - 1 do
+                answer <- answer * 256u
+                answer <- answer + uint bytes.[b]
+
+            answer
+
+        use sha1 = SHA1.Create ()
+        // TODO: in the wild, this actually strips HTML first
+        this.Fields.[0] |> Encoding.UTF8.GetBytes |> sha1.ComputeHash |> fromBase256 4
+
+[<Measure>]
+type note