Refactor marginal_memory() hex to bin lookup table to be a true static (

Qiskit#8223)

* Refactor marginal_memory() hex to bin lookup table to be a true static

In the recently merged Qiskit#8051 we create a lookup table in Rust to speed
up the hex->bin conversion used internally as part of the
marginal_memory() function. This was previously done using the
lazy_static crate which is used to lazily evaluate dynamic code to
create a static at runtime on the first access. The typical use case for
this is to create a static Vec or HashMap. However for the
marginal_counts() usage we didn't need to do this because we were
creating a fixed size array so the static can be evaulated at compile
time assuming the array is constructed with a const function. This
commit removes the lazy_static usage and switches to a true static to
further improve the performance of the lookup table by avoiding the
construction overhead.

* Reduce number of empty entries in LUT

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Cargo.toml

@@ -19,7 +19,6 @@ indexmap = "1.9.1"
 ahash = "0.7.6"
 num-complex = "0.4"
 num-bigint = "0.4"
-lazy_static = "1.4.0"
 
 [dependencies.pyo3]
 version = "0.16.5"

src/lib.rs

@@ -10,9 +10,6 @@
 // copyright notice, and modified files need to carry a notice indicating
 // that they have been altered from the originals.
 
-#[macro_use]
-extern crate lazy_static;
-
 use std::env;
 
 use pyo3::prelude::*;

src/results/converters.rs

@@ -10,35 +10,35 @@
 // copyright notice, and modified files need to carry a notice indicating
 // that they have been altered from the originals.
 
-lazy_static! {
-    static ref HEX_TO_BIN_LUT: [&'static str; 256] = {
-        let mut lookup = [""; 256];
-        lookup[b'0' as usize] = "0000";
-        lookup[b'1' as usize] = "0001";
-        lookup[b'2' as usize] = "0010";
-        lookup[b'3' as usize] = "0011";
-        lookup[b'4' as usize] = "0100";
-        lookup[b'5' as usize] = "0101";
-        lookup[b'6' as usize] = "0110";
-        lookup[b'7' as usize] = "0111";
-        lookup[b'8' as usize] = "1000";
-        lookup[b'9' as usize] = "1001";
-        lookup[b'A' as usize] = "1010";
-        lookup[b'B' as usize] = "1011";
-        lookup[b'C' as usize] = "1100";
-        lookup[b'D' as usize] = "1101";
-        lookup[b'E' as usize] = "1110";
-        lookup[b'F' as usize] = "1111";
-        lookup[b'a' as usize] = "1010";
-        lookup[b'b' as usize] = "1011";
-        lookup[b'c' as usize] = "1100";
-        lookup[b'd' as usize] = "1101";
-        lookup[b'e' as usize] = "1110";
-        lookup[b'f' as usize] = "1111";
-        lookup
-    };
+const fn generate_lookup_table() -> [&'static str; 103] {
+    let mut lookup = [""; 103];
+    lookup[b'0' as usize] = "0000";
+    lookup[b'1' as usize] = "0001";
+    lookup[b'2' as usize] = "0010";
+    lookup[b'3' as usize] = "0011";
+    lookup[b'4' as usize] = "0100";
+    lookup[b'5' as usize] = "0101";
+    lookup[b'6' as usize] = "0110";
+    lookup[b'7' as usize] = "0111";
+    lookup[b'8' as usize] = "1000";
+    lookup[b'9' as usize] = "1001";
+    lookup[b'A' as usize] = "1010";
+    lookup[b'B' as usize] = "1011";
+    lookup[b'C' as usize] = "1100";
+    lookup[b'D' as usize] = "1101";
+    lookup[b'E' as usize] = "1110";
+    lookup[b'F' as usize] = "1111";
+    lookup[b'a' as usize] = "1010";
+    lookup[b'b' as usize] = "1011";
+    lookup[b'c' as usize] = "1100";
+    lookup[b'd' as usize] = "1101";
+    lookup[b'e' as usize] = "1110";
+    lookup[b'f' as usize] = "1111";
+    lookup
 }
 
+static HEX_TO_BIN_LUT: [&str; 103] = generate_lookup_table();
+
 #[inline]
 pub fn hex_to_bin(hex: &str) -> String {
     hex[2..]