range check example

README.md

@@ -16,8 +16,15 @@ A good starting point for how to use Plonky2 for simple applications is the incl
 
 * [`factorial`](plonky2/examples/factorial.rs): Proving knowledge of 100!
 * [`fibonacci`](plonky2/examples/fibonacci.rs): Proving knowledge of the hundredth Fibonacci number
+* [`range_check`](plonky2/examples/range_check.rs): Proving that a field element is in a given range
 * [`square_root`](plonky2/examples/square_root.rs): Proving knowledge of the square root of a given field element
 
+To run an example, use
+
+```sh
+cargo run --example <example_name>
+```
+
 
 ## Building
 

plonky2/examples/range_check.rs

@@ -0,0 +1,38 @@
+#![allow(clippy::upper_case_acronyms)]
+
+use anyhow::Result;
+use plonky2::field::types::Field;
+use plonky2::iop::witness::{PartialWitness, WitnessWrite};
+use plonky2::plonk::circuit_builder::CircuitBuilder;
+use plonky2::plonk::circuit_data::CircuitConfig;
+use plonky2::plonk::config::{GenericConfig, PoseidonGoldilocksConfig};
+
+/// An example of using Plonky2 to prove that a given value lies in a given range.
+fn main() -> Result<()> {
+    const D: usize = 2;
+    type C = PoseidonGoldilocksConfig;
+    type F = <C as GenericConfig<D>>::F;
+
+    let config = CircuitConfig::standard_recursion_config();
+    let mut builder = CircuitBuilder::<F, D>::new(config);
+
+    // The secret value.
+    let value = builder.add_virtual_target();
+    builder.register_public_input(value);
+
+    let log_max = 6;
+    builder.range_check(value, log_max);
+
+    let mut pw = PartialWitness::new();
+    pw.set_target(value, F::from_canonical_usize(42));
+
+    let data = builder.build::<C>();
+    let proof = data.prove(pw)?;
+
+    println!(
+        "Value {} is less than 2^{}",
+        proof.public_inputs[0], log_max,
+    );
+
+    data.verify(proof)
+}