feat(sterni/nix): add trivial float library

Only implements the different conversion types from and to ints for now.
Unfortunately very reliant on builtins.{floor,ceil} which can't be
implemented purely except very inefficiently (to my knowledge), so it
only really works for C++ Nix >= 2.4. Tests are thus skipped for
C++ Nix 2.3.

Change-Id: Idcb1a11df11e214cdba3f2a0715472b370daa7dc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9008
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

users/sterni/nix/float/tests/default.nix

@@ -0,0 +1,49 @@
+{ depot, lib, ... }:
+
+let
+
+  inherit (depot.nix.runTestsuite)
+    runTestsuite
+    it
+    assertEq
+    ;
+
+  inherit (depot.users.sterni.nix)
+    float
+    ;
+
+  testsBuiltins = it "tests builtin operations" [
+    (assertEq "ceil pos" (float.ceil 1.5) 2)
+    (assertEq "ceil neg" (float.ceil (-1.5)) (-1))
+    (assertEq "floor pos" (float.floor 1.5) 1)
+    (assertEq "floor neg" (float.floor (-1.5)) (-2))
+  ];
+
+  testsConversionFrom = it "tests integer to float conversion" [
+    (assertEq "float.intToFloat is identity for floats" (float.intToFloat 1.3) 1.3)
+    (assertEq "float.intToFloat converts ints"
+      (builtins.all
+        (val: builtins.isFloat val)
+        (builtins.map float.intToFloat (builtins.genList (i: i - 500) 1000)))
+      true)
+  ];
+
+  exampleFloats = [ 0.5 0.45 0.3 0.1 200 203.457847 204.65547 (-1.5) (-2) (-1.3) (-0.45) ];
+  testsConversionTo = it "tests float to integer conversion" [
+    (assertEq "round"
+      (builtins.map float.round exampleFloats)
+      [ 1 0 0 0 200 203 205 (-2) (-2) (-1) 0 ])
+    (assertEq "truncate towards zero"
+      (builtins.map float.truncate exampleFloats)
+      [ 0 0 0 0 200 203 204 (-1) (-2) (-1) 0 ])
+  ];
+in
+
+runTestsuite "nix.num" ([
+  testsConversionFrom
+]
+  # Skip for e.g. C++ Nix < 2.4
+++ lib.optionals (builtins ? ceil && builtins ? floor) [
+  testsConversionTo
+  testsBuiltins
+])