Add three benchmarks to the crate

Test decoding of single-pass and progressive JPEGs as well as
metadata extraction.

Add "cargo bench" to the CI, and cache previous results between runs
so that performance regressions can be easily spotted.

The photograph of the Eiffel Tower was taken by me; no rights reserved.

Author: lovasoa

.travis.yml

@@ -13,3 +13,6 @@ env:
 script:
   - cargo build --verbose --no-default-features --features "$FEATURES"
   - cargo test --verbose --no-default-features --features "$FEATURES"
+  - cargo bench --verbose --no-default-features --features "$FEATURES"
+
+cache: cargo

Cargo.toml

@@ -17,6 +17,11 @@ rayon = { version = "1.0", optional = true }
 [dev-dependencies]
 png = "0.14"
 walkdir = "2.0"
+criterion = "0.2"
+
+[[bench]]
+name = "decoding_benchmark"
+harness = false
 
 [features]
 default = ["rayon"]

appveyor.yml

@@ -17,3 +17,4 @@ build: false
 test_script:
   - cargo build --verbose
   - cargo test --verbose
+  - cargo bench --verbose

benches/decoding_benchmark.rs

@@ -0,0 +1,35 @@
+extern crate criterion;
+extern crate jpeg_decoder;
+
+
+use criterion::{black_box, Criterion, criterion_group, criterion_main};
+
+use jpeg_decoder as jpeg;
+use jpeg_decoder::ImageInfo;
+
+fn read_image(image: &[u8]) -> Vec<u8> {
+    jpeg::Decoder::new(black_box(image)).decode().unwrap()
+}
+
+fn read_metadata(image: &[u8]) -> ImageInfo {
+    let mut decoder = jpeg::Decoder::new(black_box(image));
+    decoder.read_info().unwrap();
+    decoder.info().unwrap()
+}
+
+pub fn criterion_benchmark(c: &mut Criterion) {
+    c.bench_function("decode a 512x512 JPEG", |b| b.iter(|| {
+        read_image(include_bytes!("tower.jpg"))
+    }));
+
+    c.bench_function("decode a 512x512 progressive JPEG", |b| b.iter(|| {
+        read_image(include_bytes!("tower_progressive.jpg"))
+    }));
+
+    c.bench_function("extract metadata from an image", |b| b.iter(|| {
+        read_metadata(include_bytes!("tower.jpg"))
+    }));
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);