Median example

ceno_host/tests/test_elf.rs

@@ -99,7 +99,7 @@ fn test_sorting() -> Result<()> {
     let mut hints = CenoStdin::default();
     let mut rng = rand::thread_rng();
 
-    // Provide some random numbers to sort:
+    // Provide some random numbers to sort.
     hints.write(&(0..1000).map(|_| rng.gen::<u32>()).collect::<Vec<_>>())?;
 
     let all_messages = ceno_host::run(CENO_PLATFORM, ceno_examples::sorting, &hints);
@@ -109,6 +109,26 @@ fn test_sorting() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_median() -> Result<()> {
+    use rand::Rng;
+    let mut hints = CenoStdin::default();
+    let mut rng = rand::thread_rng();
+
+    // Provide some random numbers to find the median of.
+    let mut nums = (0..1000).map(|_| rng.gen::<u32>()).collect::<Vec<_>>();
+    hints.write(&nums)?;
+    nums.sort();
+    hints.write(&nums[nums.len() / 2])?;
+
+    let all_messages = ceno_host::run(CENO_PLATFORM, ceno_examples::median, &hints);
+    assert!(!all_messages.is_empty());
+    for (i, msg) in enumerate(&all_messages) {
+        println!("{i}: {msg}");
+    }
+    Ok(())
+}
+
 fn run(state: &mut VMState) -> Result<Vec<StepRecord>> {
     let steps = state.iter_until_halt().collect::<Result<Vec<_>>>()?;
     eprintln!("Emulator ran for {} steps.", steps.len());

examples-builder/build.rs

@@ -16,6 +16,7 @@ const EXAMPLES: &[&str] = &[
     "ceno_rt_panic",
     "hints",
     "sorting",
+    "median",
 ];
 const CARGO_MANIFEST_DIR: &str = env!("CARGO_MANIFEST_DIR");
 

guest/examples/examples/median.rs

@@ -0,0 +1,20 @@
+//! Find the median of a collection of numbers.
+//!
+//! Of course, we are asking our good friend, the host, for help, but we still need to verify the answer.
+#![no_main]
+#![no_std]
+
+extern crate ceno_rt;
+use ceno_rt::println;
+use core::fmt::Write;
+use rkyv::{Archived, vec::ArchivedVec};
+
+ceno_rt::entry!(main);
+fn main() {
+    let numbers: &ArchivedVec<u32> = ceno_rt::read();
+    let median_candidate: &Archived<u32> = ceno_rt::read();
+    let median_candidate = &&median_candidate.to_native();
+    let smaller = numbers.into_iter().filter(move |x| x < median_candidate).count();
+    assert_eq!(smaller, numbers.len() / 2);
+    println!("{}", median_candidate);
+}