From 7e9dffdaff15969d4d15cc7e2fa0cd9153b92a85 Mon Sep 17 00:00:00 2001
From: dignifiedquire <me@dignifiedquire.com>
Date: Fri, 31 Jan 2025 12:27:20 +0100
Subject: [PATCH] fix: implement ln fallback locally

---
 Cargo.lock                 |  7 -------
 Cargo.toml                 |  1 -
 src/algorithms/generate.rs | 32 +++++++++++++++++++++++++++++++-
 3 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 86a58bb..042197b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -315,12 +315,6 @@ version = "0.2.169"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b5aba8db14291edd000dfcc4d620c7ebfb122c613afb886ca8803fa4e128a20a"
 
-[[package]]
-name = "libm"
-version = "0.2.11"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8355be11b20d696c8f18f6cc018c4e372165b1fa8126cef092399c9951984ffa"
-
 [[package]]
 name = "linux-raw-sys"
 version = "0.4.15"
@@ -525,7 +519,6 @@ dependencies = [
  "crypto-primes",
  "digest",
  "hex-literal",
- "libm",
  "pkcs1",
  "pkcs8",
  "proptest",
diff --git a/Cargo.toml b/Cargo.toml
index 4fe1d14..9f73a31 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,7 +24,6 @@ spki = { version = "0.8.0-rc.1", default-features = false, features = ["alloc"]
 zeroize = { version = "1.5", features = ["alloc"] }
 crypto-bigint = { version = "0.6.0", default-features = false, features = ["zeroize", "alloc"] }
 crypto-primes = { version = "0.6.0", default-features = false }
-libm = "0.2"
 
 # optional dependencies
 sha1 = { version = "=0.11.0-pre.4", optional = true, default-features = false, features = ["oid"] }
diff --git a/src/algorithms/generate.rs b/src/algorithms/generate.rs
index a03a230..7b28786 100644
--- a/src/algorithms/generate.rs
+++ b/src/algorithms/generate.rs
@@ -126,7 +126,21 @@ fn logf(val: f64) -> f64 {
 /// Natural logarithm for `f64`.
 #[cfg(not(feature = "std"))]
 fn logf(val: f64) -> f64 {
-    libm::logf(val as f32) as f64
+    logf_approx(val as f32) as f64
+}
+
+/// Ln implementation based on
+/// <https://gist.github.com/LingDong-/7e4c4cae5cbbc44400a05fba65f06f23>
+#[cfg(any(not(feature = "std"), test))]
+fn logf_approx(x: f32) -> f32 {
+    let bx: u32 = x.to_bits();
+    let ex: u32 = bx >> 23;
+    let t: i32 = (ex as i32) - 127;
+    let s: u32 = if t < 0 { -t as u32 } else { t as u32 };
+    let bx = 1065353216 | (bx & 8388607);
+    let x = f32::from_bits(bx);
+
+    -1.49278 + (2.11263 + (-0.729104 + 0.10969 * x) * x) * x + core::f32::consts::LN_2 * (s as f32)
 }
 
 fn generate_prime_with_rng<R: CryptoRngCore>(rng: &mut R, bit_length: u32) -> BoxedUint {
@@ -141,6 +155,7 @@ fn generate_prime_with_rng<R: CryptoRngCore>(rng: &mut R, bit_length: u32) -> Bo
 #[cfg(test)]
 mod tests {
     use super::*;
+    use rand::Rng;
     use rand_chacha::{rand_core::SeedableRng, ChaCha8Rng};
 
     const EXP: u64 = 65537;
@@ -186,4 +201,19 @@ mod tests {
     key_generation!(key_generation_multi_8_576, 8, 576);
     // TODO: reenable, currently slow
     // key_generation!(key_generation_multi_16_1024, 16, 1024);
+
+    #[test]
+    fn test_log_approx() {
+        let mut rng = ChaCha8Rng::from_seed([42; 32]);
+
+        for i in 0..100 {
+            println!("round {i}");
+            let prime_limit: f64 = rng.gen();
+            let a = logf(prime_limit);
+            let b = logf_approx(prime_limit as f32);
+
+            let diff = a - b as f64;
+            assert!(diff < 0.001, "{} != {}", a, b);
+        }
+    }
 }