feat(nix): add mergePatch r/2040

Change-Id: Id6a9ecbfb04886e6d96750b1451c29dc3f68154e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2307
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>

1 files changed, 186 insertions, 0 deletions

diff --git a/nix/mergePatch/default.nix b/nix/mergePatch/default.nix
new file mode 100644
index 0000000000..0f80b93d4c
--- /dev/null
+++ b/nix/mergePatch/default.nix
@@ -0,0 +1,186 @@
+{ lib, depot, ... }:
+/*
+  JSON Merge-Patch for nix
+  Spec: https://tools.ietf.org/html/rfc7396
+
+  An algorithm for changing and removing fields in nested objects.
+
+  For example, given the following original document:
+
+  {
+    a = "b";
+    c = {
+      d = "e";
+      f = "g";
+    }
+  }
+
+  Changing the value of `a` and removing `f` can be achieved by merging the patch
+
+  {
+    a = "z";
+    c.f = null;
+  }
+
+  which results in
+
+  {
+    a = "z";
+    c = {
+      d = "e";
+    };
+  }
+
+  Pseudo-code:
+    define MergePatch(Target, Patch):
+      if Patch is an Object:
+        if Target is not an Object:
+          Target = {} # Ignore the contents and set it to an empty Object
+        for each Name/Value pair in Patch:
+          if Value is null:
+            if Name exists in Target:
+              remove the Name/Value pair from Target
+          else:
+            Target[Name] = MergePatch(Target[Name], Value)
+        return Target
+      else:
+        return Patch
+*/
+
+let
+  foldlAttrs = op: init: attrs:
+    lib.foldl' op init
+      (lib.mapAttrsToList lib.nameValuePair attrs);
+
+  mergePatch = target: patch:
+    if lib.isAttrs patch
+    then
+      let target' = if lib.isAttrs target then target else {};
+      in foldlAttrs
+          (acc: patchEl:
+            if patchEl.value == null
+            then removeAttrs acc [ patchEl.name ]
+            else acc // {
+              ${patchEl.name} =
+                mergePatch
+                  (acc.${patchEl.name} or "unnused")
+                  patchEl.value;
+            })
+          target'
+          patch
+    else patch;
+
+  inherit (depot.nix.runTestsuite)
+    runTestsuite
+    it
+    assertEq
+    ;
+
+  tests =
+    let
+      # example target from the RFC
+      testTarget = {
+        a = "b";
+        c = {
+          d = "e";
+          f = "g";
+        };
+      };
+      # example patch from the RFC
+      testPatch = {
+        a = "z";
+        c.f = null;
+      };
+      emptyPatch = it "the empty patch returns the original target" [
+        (assertEq "id"
+          (mergePatch testTarget {})
+          testTarget)
+      ];
+      nonAttrs = it "one side is a non-attrset value" [
+        (assertEq "target is a value means the value is replaced by the patch"
+          (mergePatch 42 testPatch)
+          (mergePatch {} testPatch))
+        (assertEq "patch is a value means it replaces target alltogether"
+          (mergePatch testTarget 42)
+          42)
+      ];
+      rfcExamples = it "the examples from the RFC" [
+        (assertEq "a subset is deleted and overwritten"
+          (mergePatch testTarget testPatch) {
+            a = "z";
+            c = {
+              d = "e";
+            };
+          })
+        (assertEq "a more complicated example from the example section"
+          (mergePatch {
+            title = "Goodbye!";
+              author = {
+                givenName = "John";
+                familyName = "Doe";
+              };
+            tags = [ "example" "sample" ];
+            content = "This will be unchanged";
+          } {
+            title = "Hello!";
+            phoneNumber = "+01-123-456-7890";
+            author.familyName = null;
+            tags = [ "example" ];
+          })
+          {
+            title = "Hello!";
+            phoneNumber = "+01-123-456-7890";
+              author = {
+                givenName = "John";
+              };
+            tags = [ "example" ];
+            content = "This will be unchanged";
+          })
+      ];
+
+      rfcTests =
+        let
+          r = index: target: patch: res:
+            (assertEq "test number ${toString index}"
+              (mergePatch target patch)
+              res);
+          in it "the test suite from the RFC" [
+              (r 1  {"a" = "b";}       {"a" = "c";}       {"a" = "c";})
+              (r 2  {"a" = "b";}       {"b" = "c";}       {"a" = "b"; "b" = "c";})
+              (r 3  {"a" = "b";}       {"a" = null;}      {})
+              (r 4  {"a" = "b"; "b" = "c";}
+                    {"a" = null;}
+                    {"b" = "c";})
+              (r 5  {"a" = ["b"];}     {"a" = "c";}       {"a" = "c";})
+              (r 6  {"a" = "c";}       {"a" = ["b"];}     {"a" = ["b"];})
+              (r 7  {"a" = {"b" = "c";}; }
+                    {"a" = {"b" = "d"; "c" = null;};}
+                    {"a" = {"b" = "d";};})
+              (r 8  {"a" = [{"b" = "c";}];}
+                    {"a" = [1];}
+                    {"a" = [1];})
+              (r 9  ["a" "b"]          ["c" "d"]       ["c" "d"])
+              (r 10 {"a" = "b";}       ["c"]           ["c"])
+              (r 11 {"a" = "foo";}     null            null)
+              (r 12 {"a" = "foo";}     "bar"           "bar")
+              (r 13 {"e" = null;}      {"a" = 1;}      {"e" = null; "a" = 1;})
+              (r 14 [1 2]
+                    {"a" = "b"; "c" = null;}
+                    {"a" = "b";})
+              (r 15 {}
+                {"a" = {"bb" = {"ccc" = null;};};}
+                {"a" = {"bb" = {};};})
+            ];
+
+    in runTestsuite "mergePatch" [
+      emptyPatch
+      nonAttrs
+      rfcExamples
+      rfcTests
+    ];
+
+in {
+  __functor = _: mergePatch;
+
+  inherit tests;
+}