lib/trivial: move zipIntBits to its own file

The amount of implementation detail really should not be the first thing in a
prominent file called `trivial.nix`.

2 files changed, 49 insertions, 40 deletions

diff --git a/lib/trivial.nix b/lib/trivial.nix
index a36056253d79..b75e81eb7352 100644
--- a/lib/trivial.nix
+++ b/lib/trivial.nix
@@ -1,41 +1,5 @@
 { lib }:
-let
-  zipIntBits = f: x: y:
-    let
-      # (intToBits 6) -> [ 0 1 1 ]
-      intToBits = x:
-        if x == 0 || x == -1 then
-          []
-        else
-          let
-            headbit  = if (x / 2) * 2 != x then 1 else 0;          # x & 1
-            tailbits = if x < 0 then ((x + 1) / 2) - 1 else x / 2; # x >> 1
-          in
-            [headbit] ++ (intToBits tailbits);
-
-      # (bitsToInt [ 0 1 1 ] 0) -> 6
-      # (bitsToInt [ 0 1 0 ] 1) -> -6
-      bitsToInt = l: signum:
-        if l == [] then
-          (if signum == 0 then 0 else -1)
-        else
-          (builtins.head l) + (2 * (bitsToInt (builtins.tail l) signum));
-
-      xsignum = if x < 0 then 1 else 0;
-      ysignum = if y < 0 then 1 else 0;
-      zipListsWith' = fst: snd:
-        if fst==[] && snd==[] then
-          []
-        else if fst==[] then
-          [(f xsignum             (builtins.head snd))] ++ (zipListsWith' []                  (builtins.tail snd))
-        else if snd==[] then
-          [(f (builtins.head fst) ysignum            )] ++ (zipListsWith' (builtins.tail fst) []                 )
-        else
-          [(f (builtins.head fst) (builtins.head snd))] ++ (zipListsWith' (builtins.tail fst) (builtins.tail snd));
-    in
-      assert (builtins.isInt x) && (builtins.isInt y);
-      bitsToInt (zipListsWith' (intToBits x) (intToBits y)) (f xsignum ysignum);
-in
+
 rec {
 
   ## Simple (higher order) functions
@@ -71,13 +35,19 @@ rec {
   and = x: y: x && y;
 
   /* bitwise “and” */
-  bitAnd = builtins.bitAnd or zipIntBits (a: b: if a==1 && b==1 then 1 else 0);
+  bitAnd = builtins.bitAnd
+    or import ./zip-int-bits.nix
+      (a: b: if a==1 && b==1 then 1 else 0);
 
   /* bitwise “or” */
-  bitOr = builtins.bitOr or zipIntBits (a: b: if a==1 || b==1 then 1 else 0);
+  bitOr = builtins.bitOr
+    or import ./zip-int-bits.nix
+      (a: b: if a==1 || b==1 then 1 else 0);
 
   /* bitwise “xor” */
-  bitXor = builtins.bitXor or zipIntBits (a: b: if a!=b then 1 else 0);
+  bitXor = builtins.bitXor
+    or import ./zip-int-bits.nix
+      (a: b: if a!=b then 1 else 0);
 
   /* bitwise “not” */
   bitNot = builtins.sub (-1);
diff --git a/lib/zip-int-bits.nix b/lib/zip-int-bits.nix
new file mode 100644
index 000000000000..edbcdfe1e682
--- /dev/null
+++ b/lib/zip-int-bits.nix
@@ -0,0 +1,39 @@
+/* Helper function to implement a fallback for the bit operators
+   `bitAnd`, `bitOr` and `bitXOr` on older nix version.
+   See ./trivial.nix
+*/
+f: x: y:
+  let
+    # (intToBits 6) -> [ 0 1 1 ]
+    intToBits = x:
+      if x == 0 || x == -1 then
+        []
+      else
+        let
+          headbit  = if (x / 2) * 2 != x then 1 else 0;          # x & 1
+          tailbits = if x < 0 then ((x + 1) / 2) - 1 else x / 2; # x >> 1
+        in
+          [headbit] ++ (intToBits tailbits);
+
+    # (bitsToInt [ 0 1 1 ] 0) -> 6
+    # (bitsToInt [ 0 1 0 ] 1) -> -6
+    bitsToInt = l: signum:
+      if l == [] then
+        (if signum == 0 then 0 else -1)
+      else
+        (builtins.head l) + (2 * (bitsToInt (builtins.tail l) signum));
+
+    xsignum = if x < 0 then 1 else 0;
+    ysignum = if y < 0 then 1 else 0;
+    zipListsWith' = fst: snd:
+      if fst==[] && snd==[] then
+        []
+      else if fst==[] then
+        [(f xsignum             (builtins.head snd))] ++ (zipListsWith' []                  (builtins.tail snd))
+      else if snd==[] then
+        [(f (builtins.head fst) ysignum            )] ++ (zipListsWith' (builtins.tail fst) []                 )
+      else
+        [(f (builtins.head fst) (builtins.head snd))] ++ (zipListsWith' (builtins.tail fst) (builtins.tail snd));
+  in
+    assert (builtins.isInt x) && (builtins.isInt y);
+    bitsToInt (zipListsWith' (intToBits x) (intToBits y)) (f xsignum ysignum)