From b9e05f3cdfa464af67310252c2b9469d673c2529 Mon Sep 17 00:00:00 2001
From: timeshifter <timeshifter@sumatrae>
Date: Thu, 9 Jul 2026 19:12:38 +0200
Subject: [PATCH] parallelism

---
 benchmark.txt |   1 +
 src/main.rs   | 116 ++++++++++++++++++++++++++++++++++++--------------
 2 files changed, 86 insertions(+), 31 deletions(-)

diff --git a/benchmark.txt b/benchmark.txt
index 9db7c6f..dd39ac7 100644
--- a/benchmark.txt
+++ b/benchmark.txt
@@ -6,3 +6,4 @@
  71 mmap, reduced UTF8 parsing (names)
  50 manual number parsing
  48 unchecked unwrap
+  8 12-thread parallel 
diff --git a/src/main.rs b/src/main.rs
index dccdf77..c6fff00 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -2,51 +2,52 @@ use fxhash::FxHashMap;
 use memmap2::MmapOptions;
 
 fn main() {
-    let mut data = FxHashMap::default();
-
     let file = std::fs::File::open("measurements.txt").unwrap();
     // SAFETY: we are not going to modify the file while the program is running. No lock required
     // for now.
+
+    let mut datas = vec![];
+
     let mmap = unsafe { MmapOptions::new().map(&file) }.unwrap();
     mmap.advise(memmap2::Advice::Sequential).unwrap();
 
-    let mut pointer = 0;
-    let mut length = 0;
+    let length = mmap.len();
 
-    loop {
-        length = 0;
+    let nthreads = std::thread::available_parallelism().unwrap();
+    let (tx, rx) = std::sync::mpsc::sync_channel(nthreads.into());
 
-        while let Some(ch) = &mmap.get(pointer + length) {
-            if **ch == b'\n' {
-                break;
-            } else {
-                length += 1;
-            }
+    std::thread::scope(|scope| {
+        let mut handles = vec![];
+
+        let chunk_size = length / nthreads;
+        let mut start = 0;
+
+        for _ in 0..nthreads.into() {
+            let end = (start + chunk_size).min(length);
+            let end = find_newline_idx(&mmap, end);
+
+            let map = &mmap[start..end];
+
+            let this_tx = tx.clone();
+            let handle = scope.spawn(move || this_tx.send(single_thread(map)));
+            handles.push(handle);
+
+            start = end;
         }
+        drop(tx);
 
-        if length == 0 || pointer + length == mmap.len() {
-            break;
+        for h in handles {
+            h.join().unwrap().unwrap();
         }
-        let line = &mmap[pointer..pointer + length];
+    });
 
-        let mut split = line.split(|&c| c == b';');
-        let station = unsafe { split.next().unwrap_unchecked() };
-        let number = unsafe { split.next().unwrap_unchecked() };
-        let parsed = parse_number(number);
-        // panic!();
-
-        data.entry(station)
-            .and_modify(|values: &mut (i64, i64, usize, i64)| {
-                values.0 = values.0.min(parsed);
-                values.1 += parsed;
-                values.2 += 1;
-                values.3 = values.3.max(parsed);
-            })
-            .or_insert((parsed, parsed, 1_usize, parsed));
-
-        pointer += length + 1;
+    for data in rx.iter() {
+        datas.push(data);
     }
 
+    let data: FxHashMap<&[u8], (i64, i64, usize, i64)> =
+        datas.into_iter().flat_map(|d| d.into_iter()).collect();
+
     let mut stations: Vec<_> = data.keys().collect();
     stations.sort_unstable();
 
@@ -71,6 +72,59 @@ fn main() {
     println!("}}");
 }
 
+fn find_newline_idx(mmap: &memmap2::Mmap, start: usize) -> usize {
+    let mut idx = start;
+    while let Some(c) = mmap.get(idx) {
+        if *c == b'\n' {
+            return idx;
+        }
+        idx += 1;
+    }
+    mmap.len()
+}
+
+fn single_thread(mmap: &[u8]) -> FxHashMap<&[u8], (i64, i64, usize, i64)> {
+    let mut data = FxHashMap::default();
+
+    let mut pointer = 0;
+    let mut length = 0;
+
+    loop {
+        length = 0;
+
+        while let Some(ch) = &mmap.get(pointer + length) {
+            if **ch == b'\n' {
+                break;
+            } else {
+                length += 1;
+            }
+        }
+
+        if pointer + length == mmap.len() {
+            break;
+        }
+        let line = &mmap[pointer..pointer + length];
+
+        let mut split = line.split(|&c| c == b';');
+        let station = unsafe { split.next().unwrap_unchecked() };
+        let number = unsafe { split.next().unwrap_unchecked() };
+        let parsed = parse_number(number);
+
+        data.entry(station)
+            .and_modify(|values: &mut (i64, i64, usize, i64)| {
+                values.0 = values.0.min(parsed);
+                values.1 += parsed;
+                values.2 += 1;
+                values.3 = values.3.max(parsed);
+            })
+            .or_insert((parsed, parsed, 1_usize, parsed));
+
+        pointer += length + 1;
+    }
+
+    data
+}
+
 fn parse_number(number: &[u8]) -> i64 {
     unsafe {
         let sign = if *number.first().unwrap_unchecked() == b'-' {