Initial code commit

proof of concept

FunctionalCuid.Tests/FunctionalCuid.Tests.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="Program.fs" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Expecto" Version="5.1.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\FunctionalCuid\FunctionalCuid.fsproj" />
+  </ItemGroup>
+
+</Project>

FunctionalCuid.Tests/Program.fs

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+
+open Expecto
+
+// these tests are bad, and I should feel bad...
+let tests =
+  testList "Smoke Tests" [
+    test "Generate a CUID" {
+      Cuid.cuid () |> ignore
+      }
+    test "Generate a slug" {
+      Cuid.slug () |> ignore
+      }
+    ]
+  
+[<EntryPoint>]
+let main argv =
+  runTestsWithArgs defaultConfig argv tests

FunctionalCuid/FunctionalCuid.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="Library.fs" />
+  </ItemGroup>
+
+</Project>

FunctionalCuid/Library.fs

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+module Cuid
+
+open System
+
+(*
+  Functional CUID
+  Collision-resistant Unique IDentifiers, written in F#
+
+  adapted from https://github.com/ericelliott/cuid
+  MIT License
+*)
+
+[<AutoOpen>]
+module private Support =
+  let blockSize = 4
+  let baseSize = 36UL
+  let discreteValues = pown baseSize blockSize
+
+  let pad size num =
+    let s = sprintf "000000000%s" num
+    s.Substring(s.Length - size)
+
+  let padToSize = pad blockSize
+
+  let base36Chars = "0123456789abcdefghijklmnopqrstuvwxyz"
+
+  let toBase36 nbr =
+    let rec convert nbr current =
+      match nbr with
+      | 0UL -> sprintf "%s0" current
+      | _ when nbr < baseSize -> sprintf "%c%s" base36Chars.[int nbr] current
+      | _ -> convert (nbr / baseSize) (sprintf "%c%s" base36Chars.[int (nbr % baseSize)] current)
+    convert nbr ""
+
+  let padBase36 = toBase36 >> padToSize
+
+  let rnd = Random ()
+
+  let randomBlock () =
+    (uint64 >> padBase36) (rnd.NextDouble () * float discreteValues)
+  
+  let mutable c = 0UL
+
+  let safeCounter () =
+    c <- if c < discreteValues then c else 0UL
+    c <- c + 1UL
+    c - 1UL
+
+  let epoch = DateTime (1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)
+
+  let timestampNow () =
+    (uint64 >> toBase36) (DateTime.Now - epoch).TotalMilliseconds
+  
+  let hostname =
+    try Environment.MachineName
+    with _ -> string (Random().Next ())
+
+  let fingerprint () =
+    let padTo2 = uint64 >> toBase36 >> pad 2
+    [ Diagnostics.Process.GetCurrentProcess().Id |> padTo2
+      hostname |> Seq.fold (fun acc chr -> acc + int chr) (hostname.Length + 36) |> padTo2
+      ]
+    |> List.reduce (+)
+
+  let rightChars chars (str : string) =
+    match chars with
+    | _ when chars > str.Length -> str
+    | _ -> str.[str.Length - chars..]
+
+/// Generate a CUID
+///
+/// The CUID is made up of 5 parts:
+/// - The letter "c" (is for both cookies and CUIDs; lowercase letter makes it HTML element ID friendly)
+/// - A timestamp (in milliseconds send the Unix epoch)
+/// - A sequential counter, used to prevent same-machine collisions
+/// - A fingerprint, generated from the hostname and process ID
+/// - 8 characters of random gibberish
+///
+/// The timestamp, fingerprint, and randomness are all encoded in base 36, using 0-9 and a-z.
+let cuid () =
+  [ "c"
+    timestampNow ()
+    (safeCounter >> padBase36) ()
+    fingerprint ()
+    randomBlock ()
+    randomBlock ()
+    ]
+  |> List.reduce (+)  
+
+/// Generate a slug
+///
+/// The slug is not as collision-resistant as the CUID, and is also not monotonically increasing, which is desirable
+/// for indexed database IDs; full CUIDs should be used in this case. A slug is made up of 4 parts:
+/// - The two right-most characters of the timestamp
+/// - The non-padded counter value (may be 1 to 4 characters)
+/// - The first and last characters of the fingerprint
+/// - 2 characters of random gibberish
+let slug () =
+  let print = fingerprint ()
+  [ (timestampNow >> rightChars 2) ()
+    (safeCounter >> string >> rightChars 4) ()
+    print.[0..0]
+    rightChars 1 print
+    (randomBlock >> rightChars 2) ()
+    ]
+  |> List.reduce (+)