Merge fix/m23-single-frame-tail-overlap: [M23] small file single_frame_tail overlap fix

crates/bench/src/suites/jump.rs

@@ -1,5 +1,6 @@
 use std::collections::HashMap;
 
+use super::FRAME_LINES;
 use crate::metrics::MetricsCollector;
 use crate::mmap_reader::{
     MmapReaderPhaseAware, MmapReaderPlain, MmapReaderPopulate, MmapReaderRandom,
@@ -210,13 +211,17 @@ fn bench_reverse_scan<B: FileReaderBackend>(
     let path = &config.test_file;
     let reader = B::open(path).expect("Failed to open file");
     let total = reader.total_lines();
-    let iterations: usize = if config.quick_mode { 5 } else { 10 };
 
-    let mut latencies = Vec::with_capacity(iterations * 35);
+    if total <= FRAME_LINES {
+        reader.close();
+        return vec![];
+    }
+
+    let iterations: usize = if config.quick_mode { 5 } else { 10 };
+    let mut latencies = Vec::with_capacity(iterations * FRAME_LINES);
 
     for _ in 0..iterations {
-        // Read lines backwards from end: last line, last-1, ..., last-34
+        let start = total - FRAME_LINES;
-        let start = total.saturating_sub(35);
         for i in (start..total).rev() {
             let t = std::time::Instant::now();
             let _ = reader.get_line(i);
@@ -229,7 +234,7 @@ fn bench_reverse_scan<B: FileReaderBackend>(
     reader.close();
 
     let mut extra = HashMap::new();
-    extra.insert("lines_per_scan".into(), 35.0);
+    extra.insert("lines_per_scan".into(), FRAME_LINES as f64);
     extra.insert("iterations".into(), iterations as f64);
 
     vec![BenchmarkResult {

crates/bench/src/suites/mod.rs

@@ -5,3 +5,8 @@ pub mod memory;
 pub mod render;
 pub mod rotation;
 pub mod startup;
+
+/// Number of lines read per single-frame render benchmark.
+/// Also used as the scan window size for reverse-scan benchmarks.
+/// Shared across suites to ensure consistent workload sizing.
+pub(crate) const FRAME_LINES: usize = 35;

crates/bench/src/suites/render.rs

@@ -1,5 +1,6 @@
 use std::collections::HashMap;
 
+use super::FRAME_LINES;
 use crate::metrics::MetricsCollector;
 use crate::mmap_reader::{
     MmapReaderPhaseAware, MmapReaderPlain, MmapReaderPopulate, MmapReaderRandom,
@@ -88,15 +89,9 @@ fn bench_single_frame<B: FileReaderBackend>(
     let total = reader.total_lines();
     let mut results = Vec::new();
 
-    let positions = [
+    for (pos_name, start_line) in select_frame_positions(total) {
-        ("head", 0),
+        let mut latencies = Vec::with_capacity(FRAME_LINES);
-        ("middle", total / 2),
+        for i in 0..FRAME_LINES {
-        ("tail", total.saturating_sub(35)),
-    ];
-
-    for (pos_name, start_line) in positions {
-        let mut latencies = Vec::with_capacity(35);
-        for i in 0..35 {
             let line_idx = start_line + i;
             if line_idx >= total {
                 break;
@@ -126,6 +121,33 @@ fn bench_single_frame<B: FileReaderBackend>(
     results
 }
 
+/// Select non-overlapping benchmark positions for single-frame tests.
+///
+/// Returns `(label, start_line)` pairs. Positions are chosen so that no
+/// two frames overlap, skipping any position that cannot fit without
+/// colliding with a previously selected range.
+pub(crate) fn select_frame_positions(total: usize) -> Vec<(&'static str, usize)> {
+    let frame = FRAME_LINES;
+    if total == 0 {
+        return vec![];
+    }
+
+    let mut positions: Vec<(&'static str, usize)> = vec![("head", 0)];
+
+    // Need at least 3 full frames to place head, middle, and tail without overlap.
+    if total >= 3 * frame {
+        let head_end = frame;
+        let tail_start = total - frame;
+        let gap = tail_start - head_end;
+        // Center the middle frame within the gap, ensuring it fits entirely
+        let middle_start = head_end + gap.saturating_sub(frame) / 2;
+        positions.push(("middle", middle_start));
+        positions.push(("tail", tail_start));
+    }
+
+    positions
+}
+
 fn bench_continuous_scroll<B: FileReaderBackend>(
     backend: &str,
     variant: &str,
@@ -175,3 +197,119 @@ fn bench_continuous_scroll<B: FileReaderBackend>(
         extra,
     }]
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn starts(pos: &[(&str, usize)]) -> Vec<usize> {
+        pos.iter().map(|&(_, s)| s).collect()
+    }
+
+    fn labels<'a>(pos: &[(&'a str, usize)]) -> Vec<&'a str> {
+        pos.iter().map(|&(l, _)| l).collect()
+    }
+
+    fn ranges_overlap(a_start: usize, b_start: usize) -> bool {
+        let a_end = a_start + FRAME_LINES;
+        let b_end = b_start + FRAME_LINES;
+        a_start < b_end && b_start < a_end
+    }
+
+    #[test]
+    fn zero_lines_no_positions() {
+        assert!(select_frame_positions(0).is_empty());
+    }
+
+    #[test]
+    fn one_line_head_only() {
+        let pos = select_frame_positions(1);
+        assert_eq!(labels(&pos), vec!["head"]);
+        assert_eq!(starts(&pos), vec![0]);
+    }
+
+    #[test]
+    fn at_frame_lines_head_only() {
+        let pos = select_frame_positions(FRAME_LINES);
+        assert_eq!(labels(&pos), vec!["head"]);
+        assert_eq!(starts(&pos), vec![0]);
+    }
+
+    #[test]
+    fn just_above_threshold_head_middle_tail() {
+        let total = 3 * FRAME_LINES;
+        let pos = select_frame_positions(total);
+        assert_eq!(labels(&pos), vec!["head", "middle", "tail"]);
+        assert_eq!(starts(&pos)[0], 0);
+        assert_eq!(*starts(&pos).last().unwrap(), total - FRAME_LINES);
+        // Verify no overlap
+        for i in 0..pos.len() {
+            for j in (i + 1)..pos.len() {
+                assert!(
+                    !ranges_overlap(pos[i].1, pos[j].1),
+                    "overlap: {:?} @ {} vs {:?} @ {} (total={})",
+                    pos[i].0, pos[i].1, pos[j].0, pos[j].1, total
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn no_overlap_at_total_70() {
+        // total=70 < 3*35=105, so only head is returned
+        let pos = select_frame_positions(70);
+        assert_eq!(labels(&pos), vec!["head"]);
+    }
+
+    #[test]
+    fn no_overlap_at_total_104() {
+        let pos = select_frame_positions(104);
+        for i in 0..pos.len() {
+            for j in (i + 1)..pos.len() {
+                assert!(
+                    !ranges_overlap(pos[i].1, pos[j].1),
+                    "overlap at total=104: {:?} @ {} vs {:?} @ {}",
+                    pos[i].0, pos[i].1, pos[j].0, pos[j].1
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn no_overlap_at_total_105() {
+        let pos = select_frame_positions(105);
+        for i in 0..pos.len() {
+            for j in (i + 1)..pos.len() {
+                assert!(
+                    !ranges_overlap(pos[i].1, pos[j].1),
+                    "overlap at total=105: {:?} @ {} vs {:?} @ {}",
+                    pos[i].0, pos[i].1, pos[j].0, pos[j].1
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn middle_is_centered_between_head_and_tail() {
+        let total = 1000;
+        let pos = select_frame_positions(total);
+        assert_eq!(pos.len(), 3);
+
+        let (_, head_start) = pos[0];
+        let (_, middle_start) = pos[1];
+        let (_, tail_start) = pos[2];
+
+        assert_eq!(head_start, 0);
+        assert_eq!(tail_start, total - FRAME_LINES);
+
+        let head_end = head_start + FRAME_LINES;
+        let gap = tail_start - head_end;
+        assert_eq!(middle_start, head_end + gap.saturating_sub(FRAME_LINES) / 2);
+    }
+
+    #[test]
+    fn large_file_all_three_positions() {
+        let pos = select_frame_positions(1_000_000);
+        assert_eq!(labels(&pos), vec!["head", "middle", "tail"]);
+    }
+}