From 0fd04c359c531761d72212707e851e8115174b39 Mon Sep 17 00:00:00 2001
From: "Christoph J. Scherr" <Christoph.Scherr@newtec.de>
Date: Wed, 10 Jan 2024 16:19:03 +0100
Subject: [PATCH] bench-criterion base program

---
 Cargo.lock                          | 29 ++---------------------------
 Cargo.toml                          |  4 +++-
 members/bench-criterion/Cargo.toml  |  8 ++++++++
 members/bench-criterion/src/main.rs | 26 ++++++++++++++++++++++++++
 4 files changed, 39 insertions(+), 28 deletions(-)
 create mode 100644 members/bench-criterion/Cargo.toml
 create mode 100644 members/bench-criterion/src/main.rs

diff --git a/Cargo.lock b/Cargo.lock
index 1196d1f..acb6567 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3,34 +3,9 @@
 version = 3
 
 [[package]]
-name = "c-bindings"
+name = "bench-criterion"
 version = "0.1.0"
-dependencies = [
- "cc",
- "cty",
-]
-
-[[package]]
-name = "cc"
-version = "1.0.83"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f1174fb0b6ec23863f8b971027804a42614e347eafb0a95bf0b12cdae21fc4d0"
-dependencies = [
- "libc",
-]
-
-[[package]]
-name = "cty"
-version = "0.2.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b365fabc795046672053e29c954733ec3b05e4be654ab130fe8f1f94d7051f35"
-
-[[package]]
-name = "libc"
-version = "0.2.147"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b4668fb0ea861c1df094127ac5f1da3409a82116a4ba74fca2e58ef927159bb3"
 
 [[package]]
 name = "rs-basic"
-version = "0.1.0"
+version = "0.2.0"
diff --git a/Cargo.toml b/Cargo.toml
index 0108720..b9b9cb7 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,14 +2,16 @@
 publish = false
 members = [
 	".",
+	"members/bench-criterion",
 ]
 default-members = [
 	".",
+	"members/bench-criterion",
 ]
 
 [package]
 name = "rs-basic"
-version = "0.1.0"
+version = "0.2.0"
 edition = "2021"
 authors = ["Christoph J. Scherr <softwar@cscherr.de>"]
 publish = false
diff --git a/members/bench-criterion/Cargo.toml b/members/bench-criterion/Cargo.toml
new file mode 100644
index 0000000..568036e
--- /dev/null
+++ b/members/bench-criterion/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "bench-criterion"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/members/bench-criterion/src/main.rs b/members/bench-criterion/src/main.rs
new file mode 100644
index 0000000..f1fe01c
--- /dev/null
+++ b/members/bench-criterion/src/main.rs
@@ -0,0 +1,26 @@
+fn main() {
+    let n: f32 = std::env::args().collect::<Vec<String>>()[1]
+        .parse::<f32>()
+        .expect("bad input number");
+    println!("rsqrt:\t\t{}", inverse_sqrt(n));
+    println!("f_rsqrt:\t{}", fast_inverse_sqrt(n));
+}
+
+fn inverse_sqrt(n: f32) -> f32 {
+    1f32 / n.sqrt()
+}
+
+union MixedIntFloat {
+    f: f32,
+    i: u32,
+}
+
+/// see https://en.wikipedia.org/wiki/Fast_inverse_square_root
+fn fast_inverse_sqrt(n: f32) -> f32 {
+    let mut conv: MixedIntFloat = MixedIntFloat { f: n };
+    unsafe {
+        conv.i  = 0x5f3759df - (conv.i >> 1);
+        conv.f *= 1.5f32 - (n* 0.5f32 * conv.f * conv.f);
+        conv.f
+    }
+}