From 0d39732300332e15dd0b1a87b351b0971cfada57 Mon Sep 17 00:00:00 2001
From: nicoo <nicoo@debian.org>
Date: Wed, 29 Jul 2020 20:51:05 +0200
Subject: [PATCH] factor::Decomposition: Inline a small number (4) of factors
MIME-Version: 1.0
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This avoids allocating on the heap when factoring most numbers,
without using much space on the stack.

This is ~3.5% faster than the previous commit, and ~8.3% faster than “master”.
---
 Cargo.lock                  |  7 +++++++
 src/uu/factor/Cargo.toml    |  1 +
 src/uu/factor/src/factor.rs | 10 ++++++++--
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 6b54012e6..74216eefa 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1243,6 +1243,11 @@ dependencies = [
  "generic-array 0.8.3 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "smallvec"
+version = "1.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "strsim"
 version = "0.8.0"
@@ -1639,6 +1644,7 @@ dependencies = [
  "quickcheck 0.9.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "rand 0.7.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "rand_chacha 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "smallvec 1.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "uucore 0.0.4 (git+https://github.com/uutils/uucore.git?branch=canary)",
  "uucore_procs 0.0.4 (git+https://github.com/uutils/uucore.git?branch=canary)",
 ]
@@ -2682,6 +2688,7 @@ dependencies = [
 "checksum sha1 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)" = "2579985fda508104f7587689507983eadd6a6e84dd35d6d115361f530916fa0d"
 "checksum sha2 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)" = "7d963c78ce367df26d7ea8b8cc655c651b42e8a1e584e869c1e17dae3ccb116a"
 "checksum sha3 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)" = "26405905b6a56a94c60109cfda62610507ac14a65be531f5767dec5c5a8dd6a0"
+"checksum smallvec 1.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "3757cb9d89161a2f24e1cf78efa0c1fcff485d18e3f55e0aa3480824ddaa0f3f"
 "checksum strsim 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "8ea5119cdb4c55b55d432abb513a0429384878c15dde60cc77b1c99de1a95a6a"
 "checksum syn 0.11.11 (registry+https://github.com/rust-lang/crates.io-index)" = "d3b891b9015c88c576343b9b3e41c2c11a51c219ef067b264bd9c8aa9b441dad"
 "checksum syn 1.0.48 (registry+https://github.com/rust-lang/crates.io-index)" = "cc371affeffc477f42a221a1e4297aedcea33d47d19b61455588bd9d8f6b19ac"
diff --git a/src/uu/factor/Cargo.toml b/src/uu/factor/Cargo.toml
index c4dbc96cc..a37ee5249 100644
--- a/src/uu/factor/Cargo.toml
+++ b/src/uu/factor/Cargo.toml
@@ -18,6 +18,7 @@ num-traits = "0.2" # used in src/numerics.rs, which is included by build.rs
 [dependencies]
 num-traits = "0.2"
 rand = { version="0.7", features=["small_rng"] }
+smallvec = { version="0.6.13, < 1.0" }
 uucore = { version="0.0.4", package="uucore", git="https://github.com/uutils/uucore.git", branch="canary" }
 uucore_procs = { version="0.0.4", package="uucore_procs", git="https://github.com/uutils/uucore.git", branch="canary" }
 
diff --git a/src/uu/factor/src/factor.rs b/src/uu/factor/src/factor.rs
index f6da886a1..b6a38ab79 100644
--- a/src/uu/factor/src/factor.rs
+++ b/src/uu/factor/src/factor.rs
@@ -7,6 +7,7 @@
 
 extern crate rand;
 
+use smallvec::SmallVec;
 use std::cell::RefCell;
 use std::fmt;
 
@@ -16,11 +17,16 @@ use crate::{miller_rabin, rho, table};
 type Exponent = u8;
 
 #[derive(Clone, Debug)]
-struct Decomposition(Vec<(u64, Exponent)>);
+struct Decomposition(SmallVec<[(u64, Exponent); NUM_FACTORS_INLINE]>);
+
+// The number of factors to inline directly into a `Decomposition` object.
+// As a consequence of the Erdős–Kac theorem, the average number of prime
+//  factors of integers < 10²⁵ ≃ 2⁸³ is 4, so we can use that value.
+const NUM_FACTORS_INLINE: usize = 4;
 
 impl Decomposition {
     fn one() -> Decomposition {
-        Decomposition(Vec::new())
+        Decomposition(SmallVec::new())
     }
 
     fn add(&mut self, factor: u64, exp: Exponent) {