ci/eval: add rebuildsByPlatform to the comparison result

ci/eval/compare/default.nix

@@ -3,14 +3,66 @@
   jq,
   runCommand,
   writeText,
-  supportedSystems,
   ...
 }:
 { beforeResultDir, afterResultDir }:
 let
+  /*
+    Derivation that computes which packages are affected (added, changed or removed) between two revisions of nixpkgs.
+    Note: "platforms" are "x86_64-linux", "aarch64-darwin", ...
+
+    ---
+    Inputs:
+    - beforeResultDir, afterResultDir: The evaluation result from before and after the change.
+      They can be obtained by running `nix-build -A ci.eval.full` on both revisions.
+
+    ---
+    Outputs:
+      - changed-paths.json: Various information about the changes:
+        {
+          attrdiff: {
+            added: ["package1"],
+            changed: ["package2", "package3"],
+            removed: ["package4"],
+          },
+          labels: [
+            "10.rebuild-darwin: 1-10",
+            "10.rebuild-linux: 1-10"
+          ],
+          rebuildsByKernel: {
+            darwin: ["package1", "package2"],
+            linux: ["package1", "package2", "package3"]
+          },
+          rebuildCountByKernel: {
+            darwin: 2,
+            linux: 3,
+          },
+          rebuildsByPlatform: {
+            aarch64-darwin: ["package1", "package2"],
+            aarch64-linux: ["package1", "package2"],
+            x86_64-linux: ["package1", "package2", "package3"],
+            x86_64-darwin: ["package1"],
+          },
+        }
+      - step-summary.md: A markdown render of the changes
+
+    ---
+    Implementation details:
+
+    Helper functions can be found in ./utils.nix.
+    Two main "types" are important:
+
+    - `packagePlatformPath`: A string of the form "<PACKAGE_PATH>.<PLATFORM>"
+      Example: "python312Packages.numpy.x86_64-linux"
+
+    - `packagePlatformAttr`: An attrs representation of a packagePlatformPath:
+      Example: { name = "python312Packages.numpy"; platform = "x86_64-linux"; }
+  */
   inherit (import ./utils.nix { inherit lib; })
     diff
     groupByKernel
+    convertToPackagePlatformAttrs
+    groupByPlatform
     extractPackageNames
     getLabels
     uniqueStrings
@@ -20,21 +72,30 @@ let
   beforeAttrs = getAttrs beforeResultDir;
   afterAttrs = getAttrs afterResultDir;
 
+  # Attrs
+  # - keys: "added", "changed" and "removed"
+  # - values: lists of `packagePlatformPath`s
   diffAttrs = diff beforeAttrs afterAttrs;
 
   changed-paths =
     let
       rebuilds = uniqueStrings (diffAttrs.added ++ diffAttrs.changed);
+      rebuildsPackagePlatformAttrs = convertToPackagePlatformAttrs rebuilds;
 
-      rebuildsByKernel = groupByKernel rebuilds;
+      rebuildsByPlatform = groupByPlatform rebuildsPackagePlatformAttrs;
+      rebuildsByKernel = groupByKernel rebuildsPackagePlatformAttrs;
       rebuildCountByKernel = lib.mapAttrs (
         kernel: kernelRebuilds: lib.length kernelRebuilds
       ) rebuildsByKernel;
     in
     writeText "changed-paths.json" (
       builtins.toJSON {
-        attrdiff = lib.mapAttrs (_: v: extractPackageNames v) diffAttrs;
-        inherit rebuildsByKernel rebuildCountByKernel;
+        attrdiff = lib.mapAttrs (_: extractPackageNames) diffAttrs;
+        inherit
+          rebuildsByPlatform
+          rebuildsByKernel
+          rebuildCountByKernel
+          ;
         labels = getLabels rebuildCountByKernel;
       }
     );