From 7f7f99f3505ba9bc3d330fe32d3e697a2ad45446 Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Mon, 10 May 2021 11:47:00 +0200
Subject: [PATCH] Implement `builtins.floor` and `builtins.ceil` using the C
 library functions internally

Closes #4782

Note: even though the type is internally called `NixFloat`, it's
actually a `double`.
---
 src/libexpr/primops.cc              | 40 +++++++++++++++++++++++++++++
 tests/lang/eval-okay-floor-ceil.exp |  1 +
 tests/lang/eval-okay-floor-ceil.nix |  9 +++++++
 3 files changed, 50 insertions(+)
 create mode 100644 tests/lang/eval-okay-floor-ceil.exp
 create mode 100644 tests/lang/eval-okay-floor-ceil.nix

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index fc4afaab8..3db9c6478 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -21,6 +21,8 @@
 #include <regex>
 #include <dlfcn.h>
 
+#include <math.h>
+
 
 namespace nix {
 
@@ -714,6 +716,44 @@ static RegisterPrimOp primop_addErrorContext(RegisterPrimOp::Info {
     .fun = prim_addErrorContext,
 });
 
+static void prim_ceil(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos));
+    mkInt(v, ceil(value));
+}
+
+static RegisterPrimOp primop_ceil({
+    .name = "__ceil",
+    .args = {"double"},
+    .doc = R"(
+        Converts an IEEE-754 double-precision floating-point number (*double*) to
+        the next higher integer.
+
+        If the datatype is neither an integer nor a "float", an evaluation error will be
+        thrown.
+    )",
+    .fun = prim_ceil,
+});
+
+static void prim_floor(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos));
+    mkInt(v, floor(value));
+}
+
+static RegisterPrimOp primop_floor({
+    .name = "__floor",
+    .args = {"double"},
+    .doc = R"(
+        Converts an IEEE-754 double-precision floating-point number (*double*) to
+        the next lower integer.
+
+        If the datatype is neither an integer nor a "float", an evaluation error will be
+        thrown.
+    )",
+    .fun = prim_floor,
+});
+
 /* Try evaluating the argument. Success => {success=true; value=something;},
  * else => {success=false; value=false;} */
 static void prim_tryEval(EvalState & state, const Pos & pos, Value * * args, Value & v)
diff --git a/tests/lang/eval-okay-floor-ceil.exp b/tests/lang/eval-okay-floor-ceil.exp
new file mode 100644
index 000000000..81f80420b
--- /dev/null
+++ b/tests/lang/eval-okay-floor-ceil.exp
@@ -0,0 +1 @@
+"23;24;23;23"
diff --git a/tests/lang/eval-okay-floor-ceil.nix b/tests/lang/eval-okay-floor-ceil.nix
new file mode 100644
index 000000000..d76a0d86e
--- /dev/null
+++ b/tests/lang/eval-okay-floor-ceil.nix
@@ -0,0 +1,9 @@
+with import ./lib.nix;
+
+let
+  n1 = builtins.floor 23.5;
+  n2 = builtins.ceil 23.5;
+  n3 = builtins.floor 23;
+  n4 = builtins.ceil 23;
+in
+  builtins.concatStringsSep ";" (map toString [ n1 n2 n3 n4 ])