lib: add fromHexString

Co-authored-by: lucasew <lucas59356@gmail.com>

lib/default.nix

@@ -73,7 +73,7 @@ let
       info showWarnings nixpkgsVersion version isInOldestRelease
       mod compare splitByAndCompare seq deepSeq lessThan add sub
       functionArgs setFunctionArgs isFunction toFunction mirrorFunctionArgs
-      toHexString toBaseDigits inPureEvalMode isBool isInt pathExists
+      fromHexString toHexString toBaseDigits inPureEvalMode isBool isInt pathExists
       genericClosure readFile;
     inherit (self.fixedPoints) fix fix' converge extends composeExtensions
       composeManyExtensions makeExtensible makeExtensibleWithCustomName;

lib/tests/misc.nix

@@ -102,6 +102,7 @@ let
     testAllTrue
     toBaseDigits
     toHexString
+    fromHexString
     toInt
     toIntBase10
     toShellVars
@@ -286,6 +287,21 @@ runTests {
     expected = "FA";
   };
 
+  testFromHexStringFirstExample = {
+    expr = fromHexString "FF";
+    expected = 255;
+  };
+
+  testFromHexStringSecondExample = {
+    expr = fromHexString (builtins.hashString "sha256" "test");
+    expected = 9223372036854775807;
+  };
+
+  testFromHexStringWithPrefix = {
+    expr = fromHexString "0Xf";
+    expected = 15;
+  };
+
   testToBaseDigits = {
     expr = toBaseDigits 2 6;
     expected = [ 1 1 0 ];

lib/trivial.nix

@@ -1074,6 +1074,32 @@ in {
     then v
     else k: v;
 
+  /**
+    Convert a hexadecimal string to it's integer representation.
+
+    # Type
+
+    ```
+    fromHexString :: String -> [ String ]
+    ```
+
+    # Examples
+
+    ```nix
+    fromHexString "FF"
+    => 255
+
+    fromHexString (builtins.hashString "sha256" "test")
+    => 9223372036854775807
+    ```
+  */
+  fromHexString = value:
+  let
+    noPrefix = lib.strings.removePrefix "0x" (lib.strings.toLower value);
+  in let
+    parsed = builtins.fromTOML "v=0x${noPrefix}";
+  in parsed.v;
+
   /**
     Convert the given positive integer to a string of its hexadecimal
     representation. For example: