perf: O(n) update of position

CHANGELOG.md

@@ -82,6 +82,8 @@ Types of changes
 - A `--threads` flag, so you can pick how many formatting threads to spawn.
   Defaults to the number of logical CPUs in your system.
 
+- Position counters were improved to offer an extra 1.13x speedup.
+
 ## [0.6.0] - 2022-02-25
 
 ### Added

README.md

@@ -203,15 +203,22 @@ Please see: [CHANGELOG.md](./CHANGELOG.md).
     - BogoMips: 7599.80
     - Cache Size: 16384 KB
 
+    Using:
+
+    ```bash
+    # x86_64-unknown-linux-gnu
+    $ time alejandra --threads $threads /path/to/nixpkgs
+    ```
+
     Results:
 
-    | Logical Cores | Seconds |
-    | :-----------: | :-----: |
-    |       1       |  15.1   |
-    |       2       |   7.9   |
-    |       4       |   5.4   |
-    |       8       |   4.1   |
-    |      16       |   3.6   |
+    | $threads | Seconds |
+    | :------: | :-----: |
+    |    1     |  13.4   |
+    |    2     |   6.9   |
+    |    4     |   3.6   |
+    |    8     |   2.6   |
+    |    16    |   3.1   |
 
 [^semantic-changes]: The methodology to claim this is:
 

src/alejandra_engine/src/position.rs

@@ -12,15 +12,13 @@ impl Default for Position {
 
 impl Position {
     pub fn update(&mut self, text: &str) {
-        let chars: Vec<char> = text.chars().collect();
-        let newlines = chars.iter().filter(|&c| *c == '\n').count();
-        self.line += newlines;
-        if newlines > 0 {
-            self.column = 0
+        for char in text.chars() {
+            if char == '\n' {
+                self.line += 1;
+                self.column = 0;
+            } else {
+                self.column += 1;
+            }
         }
-        self.column += match chars.iter().rposition(|c| *c == '\n') {
-            Some(pos) => chars.len() - pos - 1,
-            None => chars.len(),
-        };
     }
 }