refactor: switch to npins

niv is written in Haskell, while npins is written in Rust. I'm more
comfortable with Rust. npins also seems more actively maintained.

The files in npins/ were generated with:

    nix-shell -p npins
    npins init --bare
    npins add --name nixos channel nixos-24.05

5 files changed, 101 insertions, 10 deletions

diff --git a/nixos/README.md b/nixos/README.md
index e96348b..6c4bcb1 100644
--- a/nixos/README.md
+++ b/nixos/README.md
@@ -4,11 +4,11 @@ This is a fairly straightforward setup for making a NixOS system configuration s
 
 This means:
 - `nix-channel` is disabled
-- Nixpkgs is managed with [niv](https://github.com/nmattia/niv) [^1]
+- Nixpkgs is managed with [npins](https://github.com/andir/npins) [^1]
 - The same Nixpkgs is used for the system and all Nix commands
 - This includes the Nixpkgs version, config and overlays
 
-[^1]: Yes niv is a third-party tool, but it's essentially just a nice wrapper around `nix-prefetch-url` and co.
+[^1]: Yes npins is a third-party tool, but it's essentially just a nice wrapper around `nix-prefetch-url` and co.
 
 ## Usage
 
@@ -31,10 +31,10 @@ We're assuming that you just installed NixOS by going through the [official inst
      ```
      nixos-generate-config --dir .
      ```
-3. Pin Nixpkgs to the [latest stable version](https://nixos.org/manual/nixos/stable/release-notes) using [niv](https://github.com/nmattia/niv):
+3. Pin Nixpkgs to the [latest stable version](https://nixos.org/manual/nixos/stable/release-notes) using npins:
    ```
-   nix-shell -p niv --run \
-     'niv init --nixpkgs NixOS/nixpkgs --nixpkgs-branch nixos-23.11'
+   nix-shell -p npins --run \
+     'npins init --bare && npins add --name nixos channel nixos-23.11'
    ```
 4. Remove all stateful channels:
    ```
@@ -52,11 +52,11 @@ Here are some changes you can make:
 - Change the NixOS configuration in `./configuration.nix`
 - Update the pinned Nixpkgs:
   ```
-  niv update nixpkgs
+  npins update nixos
   ```
 - Upgrade to a newer release:
   ```
-  niv update nixpkgs --branch nixos-23.11
+  npins add --name nixos --channel nixos-23.11
   ```
 - Change the Nixpkgs config by editing `nixpkgs/config.nix`
 - Add Nixpkgs overlays to `nixpkgs/overlays.nix`
diff --git a/nixos/nixpkgs/path.nix b/nixos/nixpkgs/path.nix
index ea4a74c..7ae177e 100644
--- a/nixos/nixpkgs/path.nix
+++ b/nixos/nixpkgs/path.nix
@@ -1,2 +1,2 @@
 # The Nixpkgs path to use
-(import ../nix/sources.nix).nixpkgs.outPath
+(import ../npins).nixos.outPath
diff --git a/nixos/npins/default.nix b/nixos/npins/default.nix
new file mode 100644
index 0000000..5e7d086
--- /dev/null
+++ b/nixos/npins/default.nix
@@ -0,0 +1,80 @@
+# Generated by npins. Do not modify; will be overwritten regularly
+let
+  data = builtins.fromJSON (builtins.readFile ./sources.json);
+  version = data.version;
+
+  mkSource =
+    spec:
+    assert spec ? type;
+    let
+      path =
+        if spec.type == "Git" then
+          mkGitSource spec
+        else if spec.type == "GitRelease" then
+          mkGitSource spec
+        else if spec.type == "PyPi" then
+          mkPyPiSource spec
+        else if spec.type == "Channel" then
+          mkChannelSource spec
+        else
+          builtins.throw "Unknown source type ${spec.type}";
+    in
+    spec // { outPath = path; };
+
+  mkGitSource =
+    {
+      repository,
+      revision,
+      url ? null,
+      hash,
+      branch ? null,
+      ...
+    }:
+    assert repository ? type;
+    # At the moment, either it is a plain git repository (which has an url), or it is a GitHub/GitLab repository
+    # In the latter case, there we will always be an url to the tarball
+    if url != null then
+      (builtins.fetchTarball {
+        inherit url;
+        sha256 = hash; # FIXME: check nix version & use SRI hashes
+      })
+    else
+      assert repository.type == "Git";
+      let
+        urlToName =
+          url: rev:
+          let
+            matched = builtins.match "^.*/([^/]*)(\\.git)?$" repository.url;
+
+            short = builtins.substring 0 7 rev;
+
+            appendShort = if (builtins.match "[a-f0-9]*" rev) != null then "-${short}" else "";
+          in
+          "${if matched == null then "source" else builtins.head matched}${appendShort}";
+        name = urlToName repository.url revision;
+      in
+      builtins.fetchGit {
+        url = repository.url;
+        rev = revision;
+        inherit name;
+        # hash = hash;
+      };
+
+  mkPyPiSource =
+    { url, hash, ... }:
+    builtins.fetchurl {
+      inherit url;
+      sha256 = hash;
+    };
+
+  mkChannelSource =
+    { url, hash, ... }:
+    builtins.fetchTarball {
+      inherit url;
+      sha256 = hash;
+    };
+in
+if version == 3 then
+  builtins.mapAttrs (_: mkSource) data.pins
+else
+  throw "Unsupported format version ${toString version} in sources.json. Try running `npins upgrade`"
diff --git a/nixos/npins/sources.json b/nixos/npins/sources.json
new file mode 100644
index 0000000..2deef59
--- /dev/null
+++ b/nixos/npins/sources.json
@@ -0,0 +1,11 @@
+{
+  "pins": {
+    "nixos": {
+      "type": "Channel",
+      "name": "nixos-24.05",
+      "url": "https://releases.nixos.org/nixos/24.05/nixos-24.05.6668.e8c38b73aeb2/nixexprs.tar.xz",
+      "hash": "0lhh36z3fvd3b64dz7an08y3c3shb67aj17ny9z28bs21i3dc5yh"
+    }
+  },
+  "version": 3
+}
\ No newline at end of file
diff --git a/nixos/sanix.nix b/nixos/sanix.nix
index 4895127..02d9ed5 100644
--- a/nixos/sanix.nix
+++ b/nixos/sanix.nix
@@ -1,7 +1,7 @@
 { lib, pkgs, config, ... }: {
   environment.systemPackages = with pkgs; [
-    # We're using niv to manage the systems Nixpkgs version, install it globally for ease
-    niv
+    # We're using npins to manage the systems Nixpkgs version, install it globally for ease
+    npins
   ];
 
   # Use the Nixpkgs config and overlays from the local files for this NixOS build