2 files changed, 82 insertions, 14 deletions

diff --git a/2018/src/benchmark.rs b/2018/src/benchmark.rs
new file mode 100644
index 0000000..76de9e4
--- /dev/null
+++ b/2018/src/benchmark.rs
@@ -0,0 +1,78 @@
+use std::fmt;
+use std::time::{Instant, Duration};
+
+const MIN_DUR: f64 = 1.0;
+const MIN_TIMES: usize = 4;
+
+pub struct Bench {
+    pub executions: usize,
+    pub duration: f64,
+    pub duration_stddev: f64,
+}
+
+impl fmt::Display for Bench {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{} secs ± {} ({}%)",
+               self.duration, self.duration_stddev, self.duration_stddev / self.duration * 100.0)
+    }
+}
+
+fn duration_secs(d: Duration) -> f64 {
+    d.as_secs() as f64 + d.subsec_nanos() as f64 / 1.0e9
+}
+
+// Returns estimate of single-iteration time
+fn warmup<A, F: Fn() -> A>(func: F) -> f64 {
+    let mut ntimes = 1;
+    let mut time_taken = 0.0;
+
+    while time_taken < 0.5 {
+        let start = Instant::now();
+        for _ in 0..ntimes {
+            func();
+        }
+        let end = Instant::now();
+        time_taken = duration_secs(end - start);
+
+        ntimes *= 2;
+    }
+
+    time_taken / ntimes as f64
+}
+
+pub fn benchmark<A, F: Fn() -> A>(func: F) -> Bench {
+    let estimate = warmup(&func);
+    let ntimes_guess = if MIN_TIMES as f64 * estimate >= MIN_DUR {
+        MIN_TIMES
+    } else {
+        (MIN_DUR / estimate).ceil() as usize
+    };
+
+    let nparts = if ntimes_guess < 10 {
+        ntimes_guess
+    } else {
+        std::cmp::max(10, ntimes_guess / 10)
+    };
+    let nperpart = (ntimes_guess + nparts - 1) / nparts;
+
+    let ntimes = nparts * nperpart;
+
+    // eprintln!("BENCH: estimate={} ntimes_guess={} nparts={} nperpart={} ntimes={}", estimate, ntimes_guess, nparts, nperpart, ntimes);
+
+    let mut times = vec![0.0; nparts];
+
+    for i in 0..nparts {
+        let start = Instant::now();
+        for _ in 0..nperpart {
+            func();
+        }
+        let end = Instant::now();
+
+        times[i] = duration_secs(end - start);
+    }
+
+    let mean = times.iter().map(|&t| t).sum::<f64>() / ntimes as f64;
+    let stddev = times.iter().map(|t| (t - mean) * (t - mean)).sum::<f64>() / (ntimes - 1) as f64;
+
+    Bench { executions: 0, duration: mean, duration_stddev: stddev }
+}
diff --git a/2018/src/main.rs b/2018/src/main.rs
index 447005e..3e44615 100644
--- a/2018/src/main.rs
+++ b/2018/src/main.rs
@@ -2,7 +2,6 @@ use std::io;
 use std::io::{BufRead, BufReader, Error, ErrorKind, Read};
 use std::fs::File;
 use std::process::exit;
-use std::time::Instant;
 use argparse::{ArgumentParser, StoreTrue, Store};
 
 mod day1;
@@ -19,6 +18,7 @@ mod day11;
 mod day12;
 mod day13;
 mod day14;
+mod benchmark;
 
 static NUM_DAYS: i32 = 14;
 
@@ -50,23 +50,13 @@ fn file_for_day(day: i32) -> io::Result<BufReader<File>> {
 }
 
 fn benchmark_day(day: i32) -> io::Result<f64> {
-    let num_iters = 100;
-
     let mut input = Vec::new();
     file_for_day(day)?.read_to_end(&mut input)?;
 
-    let start = Instant::now();
-    for _i in 0..num_iters {
-        day_switch(day, BufReader::new(&input[..]))?;
-    }
-    let end = Instant::now();
-
-    let dur = (end - start) / num_iters;
-    let secs = dur.as_secs() as f64 + dur.subsec_micros() as f64 / 1000000.0;
-
-    println!("Day {}: {} secs", day, secs);
+    let res = benchmark::benchmark(|| day_switch(day, BufReader::new(&input[..])).unwrap());
+    println!("Day {}: {}", day, res);
 
-    Ok(secs)
+    Ok(res.duration)
 }
 
 fn benchmark_all_days() -> io::Result<()> {