lib.path.difference: init

lib/path/default.nix

@@ -5,6 +5,11 @@ let
   inherit (builtins)
     isString
     isPath
+    isAttrs
+    dirOf
+    baseNameOf
+    typeOf
+    seq
     split
     match
     ;
@@ -18,6 +23,8 @@ let
     all
     concatMap
     foldl'
+    take
+    drop
     ;
 
   inherit (lib.strings)
@@ -33,10 +40,16 @@ let
     isValid
     ;
 
+  inherit (lib.attrsets)
+    mapAttrsToList
+    listToAttrs
+    nameValuePair
+    ;
+
   # Return the reason why a subpath is invalid, or `null` if it's valid
   subpathInvalidReason = value:
     if ! isString value then
-      "The given value is of type ${builtins.typeOf value}, but a string was expected"
+      "The given value is of type ${typeOf value}, but a string was expected"
     else if value == "" then
       "The given string is empty"
     else if substring 0 1 value == "/" then
@@ -100,6 +113,16 @@ let
     # An empty string is not a valid relative path, so we need to return a `.` when we have no components
     (if components == [] then "." else concatStringsSep "/" components);
 
+  # Takes a Nix path value and deconstructs it into the filesystem root
+  # (generally `/`) and a subpath
+  deconstructPath =
+    let
+      recurse = components: path:
+        # If the parent of a path is the path itself, then it's a filesystem root
+        if path == dirOf path then { root = path; inherit components; }
+        else recurse ([ (baseNameOf path) ] ++ components) (dirOf path);
+    in recurse [];
+
 in /* No rec! Add dependencies on this file at the top. */ {
 
   /* Append a subpath string to a path.
@@ -149,6 +172,120 @@ in /* No rec! Add dependencies on this file at the top. */ {
           ${subpathInvalidReason subpath}'';
     path + ("/" + subpath);
 
+  /* Determine the difference between multiple paths, including the longest
+    common prefix and the individual suffixes between them
+
+    The input is an attribute set of paths, where the keys are only for naming
+    and can be picked arbitrarily. The returned attribute set contains two
+    attributes:
+
+    - `commonPrefix`: A path value containing the common prefix between all the
+      given paths.
+
+    - `suffix`: An attribute set of normalised subpaths (see
+      `lib.path.subpath.normalise`). The keys are the same
+      as were given as the input, they can be used to easily match up the
+      suffixes to the inputs.
+
+    Throws an error if all paths don't share the same filesystem root.
+
+    Laws:
+
+    - The input paths can be recovered by appending each suffix to the common ancestor
+
+          forall paths, result = difference paths.
+            mapAttrs (_: append result.commonPrefix) result.suffix == paths
+
+    - The _longest_ common prefix is returned
+
+          forall paths, result = difference paths.
+            ! exists longerPrefix. hasProperPrefix result.commonPrefix longerPrefix && all (hasPrefix longerPrefix) (attrValues paths)
+
+    - Suffixes are normalised
+
+          forall paths, result = difference paths.
+            mapAttrs (_: subpath.normalise) result.suffix == result.suffix
+
+    Type:
+      difference :: AttrsOf Path -> { commonPrefix :: Path, suffix :: AttrsOf String }
+
+    Example:
+      difference { foo = ./foo; bar = ./bar; }
+      => { commonPrefix = ./.; suffix = { foo = "./foo"; bar = "./bar"; }; }
+
+      difference { foo = ./foo; bar = ./.; }
+      => { commonPrefix = ./.; suffix = { foo = "./foo"; bar = "./."; }; }
+
+      difference { foo = ./foo; bar = ./foo; }
+      => { commonPrefix = ./foo; suffix = { foo = "./."; bar = "./."; }; }
+
+      difference { foo = ./foo; bar = ./foo/bar; }
+      => { commonPrefix = ./foo; suffix = { foo = "./."; bar = "./bar"; }; }
+  */
+  difference =
+    # The attribute set of paths to calculate the difference between
+    paths:
+    let
+      # Deconstruct every path into its root and subpath
+      deconstructedPaths = mapAttrsToList (name: value:
+        # Check each item to be an actual path
+        assert assertMsg (isPath value) "lib.path.difference: Attribute ${name} is of type ${typeOf value}, but a path was expected";
+        deconstructPath value // { inherit name value; }
+      ) paths;
+
+      firstPath =
+        assert assertMsg
+          (paths != {})
+          "lib.path.difference: An empty attribute set was given, but a non-empty attribute set was expected";
+        head deconstructedPaths;
+
+      # The common root to all paths, errors if there are different roots
+      commonRoot =
+        # Fast happy path in case all roots are the same
+        if all (path: path.root == firstPath.root) deconstructedPaths then firstPath.root
+        # Slower sad path when that's not the case and we need to throw an error
+        else foldl'
+          (skip: path:
+            if path.root == firstPath.root then skip
+            else throw ''
+              lib.path.difference: Filesystem roots must be the same for all paths, but paths with different roots were given:
+                ${firstPath.name} = ${toString firstPath.value} (root ${toString firstPath.root})
+                ${toString path.name} = ${toString path.value} (root ${toString path.root})''
+          )
+          null
+          deconstructedPaths;
+
+      commonAncestorLength =
+        let
+          recurse = index:
+            let firstComponent = elemAt firstPath.components index; in
+            if all (path:
+                # If all paths have another level of components
+                length path.components > index
+                # And they all match
+                && elemAt path.components index == firstComponent
+              ) deconstructedPaths
+            then recurse (index + 1)
+            else index;
+        in
+          # Ensure that we have a common root before trying to find a common ancestor
+          # If we didn't do this one could evaluate `relativePaths` without an error even when there's no common root
+          seq commonRoot (recurse 0);
+
+      commonPrefix = commonRoot
+        + ("/" + joinRelPath (take commonAncestorLength firstPath.components));
+
+      suffix = listToAttrs (map (path:
+        nameValuePair path.name (joinRelPath (drop commonAncestorLength path.components))
+      ) deconstructedPaths);
+    in
+    assert assertMsg
+      (isAttrs paths)
+      "lib.path.difference: The given argument is of type ${typeOf paths}, but an attribute set was expected";
+    {
+      inherit commonPrefix suffix;
+    };
+
   /* Whether a value is a valid subpath string.
 
   - The value is a string

lib/path/tests/unit.nix

@@ -3,7 +3,7 @@
 { libpath }:
 let
   lib = import libpath;
-  inherit (lib.path) append subpath;
+  inherit (lib.path) append difference subpath;
 
   cases = lib.runTests {
     # Test examples from the lib.path.append documentation
@@ -40,6 +40,38 @@ let
       expected = false;
     };
 
+    # Test examples from the documentation
+    testDifferenceExample1 = {
+      expr = difference { foo = ./foo; bar = ./bar; };
+      expected = { commonPrefix = ./.; suffix = { foo = "./foo"; bar = "./bar"; }; };
+    };
+    testDifferenceExample2 = {
+      expr = difference { foo = ./foo; bar = ./.; };
+      expected = { commonPrefix = ./.; suffix = { foo = "./foo"; bar = "./."; }; };
+    };
+    testDifferenceExample3 = {
+      expr = difference { foo = ./foo; bar = ./foo; };
+      expected = { commonPrefix = ./foo; suffix = { foo = "./."; bar = "./."; }; };
+    };
+    testDifferenceExample4 = {
+      expr = difference { foo = ./foo; bar = ./foo/bar; };
+      expected = { commonPrefix = ./foo; suffix = { foo = "./."; bar = "./bar"; }; };
+    };
+    # Test that the invalid cases cause throws
+    testDifferenceNonAttrset = {
+      expr = (builtins.tryEval (difference 10).commonPrefix).success;
+      expected = false;
+    };
+    testDifferenceEmpty = {
+      expr = (builtins.tryEval (difference { }).commonPrefix).success;
+      expected = false;
+    };
+    testDifferenceNonPath = {
+      expr = (builtins.tryEval (difference { a = 10; }).commonPrefix).success;
+      expected = false;
+    };
+    # We can't test for multiple roots with the current Nix version though
+
     # Test examples from the lib.path.subpath.isValid documentation
     testSubpathIsValidExample1 = {
       expr = subpath.isValid null;