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fix: get_next_power_exponent off by 1 #11169

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Merged
benesjan merged 3 commits into from
Jan 13, 2025
Merged

fix: get_next_power_exponent off by 1 #11169

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3aefaa3
fix: get_next_power_exponent off by 1
benesjan Jan 10, 2025
fb5b31b
optimization
benesjan Jan 10, 2025
d4b3da4
Revert "optimization"
benesjan Jan 10, 2025
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21 changes: 18 additions & 3 deletions noir-projects/noir-protocol-circuits/crates/types/src/merkle_tree/variable_merkle_tree.nr
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Original file line number Diff line number Diff line change
Expand Up @@ -20,7 +20,8 @@ pub struct VariableMerkleTree {
/// The smallest exponent `n` where `2^n >= input`
unconstrained fn get_next_power_exponent(input: u32, start: u8) -> u8 {
let mut next_power_exponent = 0;
if input <= 2 << start {
// We check if input is less than or equal to 2^start.
if input <= (1 << start) {
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@benesjan benesjan Jan 10, 2025

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This was a crux of the bug because 2 << start is not equal to 2^start but to 2^(start+1).

next_power_exponent = start;
} else {
next_power_exponent = get_next_power_exponent(input, start + 1);
Expand Down Expand Up @@ -49,7 +50,7 @@ fn get_prev_power_2(value: u32) -> u32 {
get_next_power_exponent(value, 0)
};

let next_power_2 = 2 << next_power_exponent;
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@benesjan benesjan Jan 10, 2025

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Here it didn't cause an issue because we were shifting 2 and not 1.

let next_power_2 = 1 << next_power_exponent;
let prev_power_2 = next_power_2 / 2;
assert(prev_power_2 < value);
assert(value <= next_power_2);
Expand Down Expand Up @@ -122,7 +123,8 @@ impl VariableMerkleTree {

pub mod tests {
use crate::{
hash::accumulate_sha256, merkle_tree::variable_merkle_tree::VariableMerkleTree,
hash::accumulate_sha256,
merkle_tree::variable_merkle_tree::{get_next_power_exponent, VariableMerkleTree},
tests::fixtures::merkle_tree::generate_full_sha_tree,
};

Expand All @@ -140,6 +142,19 @@ pub mod tests {
items
}

#[test]
unconstrained fn test_get_next_power_exponent() {
assert_eq(get_next_power_exponent(0, 0), 0);
assert_eq(get_next_power_exponent(1, 0), 0);
assert_eq(get_next_power_exponent(2, 0), 1);
assert_eq(get_next_power_exponent(3, 0), 2);
assert_eq(get_next_power_exponent(4, 0), 2);
assert_eq(get_next_power_exponent(5, 0), 3);
assert_eq(get_next_power_exponent(8, 0), 3);
assert_eq(get_next_power_exponent(9, 0), 4);
assert_eq(get_next_power_exponent(16, 0), 4);
}

#[test]
fn test_0_elems() {
let items = [0; 100];
Expand Down