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Merge rust-bitcoin/rust-bitcoin#909: Make TaprootBuilder able to generate Huffman Tree 

ec17ec356d Move with_huffman_tree logic to TaprootBuilder (Jeremy Rubin)

Pull request description:

  .

ACKs for top commit:
  apoelstra:
    ACK ec17ec356d
  dr-orlovsky:
    utACK ec17ec356d

Tree-SHA512: 67a013124267f64bfae0b2007418ad59a42ae64d8b95e23c1d86cc7d96b0dd3b48deb255ce7bb839ef9a4d4f2e3a42d691d2d2430eb7791e01f992635773cc96
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Andrew Poelstra 2022-03-28 15:07:59 +00:00
parent 10949b7177 ec17ec356d
commit 7f53c2cdc1
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1 changed files with 53 additions and 40 deletions
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93
src/util/taproot.rs
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@ -192,24 +192,9 @@ pub struct TaprootSpendInfo {
}
impl TaprootSpendInfo {
/// Create a new [`TaprootSpendInfo`] from a list of script(with default script version) and
/// weights of satisfaction for that script. The weights represent the probability of
/// each branch being taken. If probabilities/weights for each condition are known,
/// constructing the tree as a Huffman tree is the optimal way to minimize average
/// case satisfaction cost. This function takes input an iterator of tuple(u64, &Script)
/// where usize represents the satisfaction weights of the branch.
/// For example, [(3, S1), (2, S2), (5, S3)] would construct a TapTree that has optimal
/// satisfaction weight when probability for S1 is 30%, S2 is 20% and S3 is 50%.
/// Create a new [`TaprootSpendInfo`] from a list of script(with default script version).
///
/// # Errors:
///
/// - When the optimal huffman tree has a depth more than 128
/// - If the provided list of script weights is empty
///
/// # Edge Cases:
/// - If the script weight calculations overflow, a sub-optimal tree may be generated. This
/// should not happen unless you are dealing with billions of branches with weights close to
/// 2^32.
/// See [`TaprootBuilder::with_huffman_tree`] for more detailed documentation
pub fn with_huffman_tree<C, I>(
secp: &Secp256k1<C>,
internal_key: UntweakedPublicKey,
@ -219,29 +204,7 @@ impl TaprootSpendInfo {
I: IntoIterator<Item=(u32, Script)>,
C: secp256k1::Verification,
{
let mut node_weights = BinaryHeap::<(Reverse<u64>, NodeInfo)>::new();
for (p, leaf) in script_weights {
node_weights.push((Reverse(p as u64), NodeInfo::new_leaf_with_ver(leaf, LeafVersion::TapScript)));
}
if node_weights.is_empty() {
return Err(TaprootBuilderError::IncompleteTree);
}
while node_weights.len() > 1 {
// Combine the last two elements and insert a new node
let (p1, s1) = node_weights.pop().expect("len must be at least two");
let (p2, s2) = node_weights.pop().expect("len must be at least two");
// Insert the sum of first two in the tree as a new node
// N.B.: p1 + p2 can not practically saturate as you would need to have 2**32 max u32s
// from the input to overflow. However, saturating is a reasonable behavior here as
// huffman tree construction would treat all such elements as "very likely".
let p = Reverse(p1.0.saturating_add(p2.0));
node_weights.push((p, NodeInfo::combine(s1, s2)?));
}
// Every iteration of the loop reduces the node_weights.len() by exactly 1
// Therefore, the loop will eventually terminate with exactly 1 element
debug_assert!(node_weights.len() == 1);
let node = node_weights.pop().expect("huffman tree algorithm is broken").1;
return Ok(Self::from_node_info(secp, internal_key, node));
TaprootBuilder::with_huffman_tree(script_weights)?.finalize(secp, internal_key)
}
/// Create a new key spend with internal key and proided merkle root.
@ -399,6 +362,56 @@ impl TaprootBuilder {
pub fn new() -> Self {
TaprootBuilder { branch: vec![] }
}
/// Create a new [`TaprootBuilder`] from a list of script(with default script version) and
/// weights of satisfaction for that script. The weights represent the probability of
/// each branch being taken. If probabilities/weights for each condition are known,
/// constructing the tree as a Huffman tree is the optimal way to minimize average
/// case satisfaction cost. This function takes input an iterator of tuple(u64, &Script)
/// where usize represents the satisfaction weights of the branch.
/// For example, [(3, S1), (2, S2), (5, S3)] would construct a TapTree that has optimal
/// satisfaction weight when probability for S1 is 30%, S2 is 20% and S3 is 50%.
///
/// # Errors:
///
/// - When the optimal huffman tree has a depth more than 128
/// - If the provided list of script weights is empty
///
/// # Edge Cases:
/// - If the script weight calculations overflow, a sub-optimal tree may be generated. This
/// should not happen unless you are dealing with billions of branches with weights close to
/// 2^32.
pub fn with_huffman_tree<I>(
script_weights: I,
) -> Result<Self, TaprootBuilderError>
where
I: IntoIterator<Item=(u32, Script)>,
{
let mut node_weights = BinaryHeap::<(Reverse<u64>, NodeInfo)>::new();
for (p, leaf) in script_weights {
node_weights.push((Reverse(p as u64), NodeInfo::new_leaf_with_ver(leaf, LeafVersion::TapScript)));
}
if node_weights.is_empty() {
return Err(TaprootBuilderError::IncompleteTree);
}
while node_weights.len() > 1 {
// Combine the last two elements and insert a new node
let (p1, s1) = node_weights.pop().expect("len must be at least two");
let (p2, s2) = node_weights.pop().expect("len must be at least two");
// Insert the sum of first two in the tree as a new node
// N.B.: p1 + p2 can not practically saturate as you would need to have 2**32 max u32s
// from the input to overflow. However, saturating is a reasonable behavior here as
// huffman tree construction would treat all such elements as "very likely".
let p = Reverse(p1.0.saturating_add(p2.0));
node_weights.push((p, NodeInfo::combine(s1, s2)?));
}
// Every iteration of the loop reduces the node_weights.len() by exactly 1
// Therefore, the loop will eventually terminate with exactly 1 element
debug_assert!(node_weights.len() == 1);
let node = node_weights.pop().expect("huffman tree algorithm is broken").1;
Ok(TaprootBuilder{branch: vec![Some(node)]})
}
/// Just like [`TaprootBuilder::add_leaf`] but allows to specify script version
pub fn add_leaf_with_ver(
self,