From ac9d0068861e97bc892bb8fe5608b3f4a2012d5f Mon Sep 17 00:00:00 2001
From: Justin Tracey <j2tracey@gmail.com>
Date: Sun, 6 Feb 2022 02:22:54 -0500
Subject: [PATCH] join: guess the number of fields in each line

This lets us use fewer reallocations when parsing each line.
The current guess is set to the maximum fields in a line so far. This is
a free performance win in the common case where each line has the same
number of fields, but comes with some memory overhead in the case where
there is a line with lots of fields at the beginning of the file, and
fewer later, but each of these lines are typically not kept for very
long anyway.
---
 src/uu/join/src/join.rs | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/uu/join/src/join.rs b/src/uu/join/src/join.rs
index dcd699438..861d1684a 100644
--- a/src/uu/join/src/join.rs
+++ b/src/uu/join/src/join.rs
@@ -283,8 +283,8 @@ struct Line {
 }
 
 impl Line {
-    fn new(string: Vec<u8>, separator: Sep) -> Self {
-        let mut field_ranges = Vec::new();
+    fn new(string: Vec<u8>, separator: Sep, len_guess: usize) -> Self {
+        let mut field_ranges = Vec::with_capacity(len_guess);
         let mut last_end = 0;
         if separator == Sep::Whitespaces {
             // GNU join uses Bourne shell field splitters by default
@@ -325,6 +325,7 @@ struct State<'a> {
     file_num: FileNum,
     print_unpaired: bool,
     lines: Split<Box<dyn BufRead + 'a>>,
+    max_len: usize,
     seq: Vec<Line>,
     line_num: usize,
     has_failed: bool,
@@ -355,6 +356,7 @@ impl<'a> State<'a> {
             file_num,
             print_unpaired,
             lines: f.split(line_ending as u8),
+            max_len: 1,
             seq: Vec::new(),
             line_num: 0,
             has_failed: false,
@@ -517,7 +519,11 @@ impl<'a> State<'a> {
         match self.lines.next() {
             Some(value) => {
                 self.line_num += 1;
-                Ok(Some(Line::new(value?, sep)))
+                let line = Line::new(value?, sep, self.max_len);
+                if line.field_ranges.len() > self.max_len {
+                    self.max_len = line.field_ranges.len();
+                }
+                Ok(Some(line))
             }
             None => Ok(None),
         }