1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161

// Copyright (C) 2019-2023 Aleo Systems Inc.
// This file is part of the snarkVM library.

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at:
// http://www.apache.org/licenses/LICENSE-2.0

// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use super::*;

#[derive(Clone, Debug, PartialEq, Eq, Hash)]
pub struct KaryMerklePath<PH: PathHash, const DEPTH: u8, const ARITY: u8> {
    /// The leaf index for the path.
    leaf_index: u64,
    /// The `siblings` contains a list of sibling hashes from the leaf to the root.
    siblings: Vec<Vec<PH::Hash>>,
}

impl<PH: PathHash, const DEPTH: u8, const ARITY: u8> KaryMerklePath<PH, DEPTH, ARITY> {
    /// Returns a new instance of a Merkle path.
    pub fn try_from((leaf_index, siblings): (u64, Vec<Vec<PH::Hash>>)) -> Result<Self> {
        // Ensure the Merkle tree depth is greater than 0.
        ensure!(DEPTH > 0, "Merkle tree depth must be greater than 0");
        // Ensure the Merkle tree depth is less than or equal to 64.
        ensure!(DEPTH <= 64u8, "Merkle tree depth must be less than or equal to 64");
        // Ensure the Merkle tree arity is greater than 1.
        ensure!(ARITY > 1, "Merkle tree arity must be greater than 1");
        // Ensure the Merkle tree does not overflow a u128.
        ensure!((ARITY as u128).checked_pow(DEPTH as u32).is_some(), "Merkle tree size overflowed");
        // Ensure the leaf index is within the tree depth.
        ensure!((leaf_index as u128) < (ARITY as u128).saturating_pow(DEPTH as u32), "Out of bounds Merkle leaf index");
        // Ensure the Merkle path is the correct length.
        ensure!(siblings.len() == DEPTH as usize, "Found an incorrect Merkle path length");
        for sibling in &siblings {
            // Note: The ARITY is guaranteed to be greater than 1 (by the above check).
            ensure!(sibling.len() == (ARITY - 1) as usize, "Found an incorrect Merkle path arity");
        }
        // Return the Merkle path.
        Ok(Self { leaf_index, siblings })
    }
}

impl<PH: PathHash, const DEPTH: u8, const ARITY: u8> KaryMerklePath<PH, DEPTH, ARITY> {
    /// Returns the leaf index for the path.
    pub fn leaf_index(&self) -> u64 {
        self.leaf_index
    }

    /// Returns the siblings for the path.
    pub fn siblings(&self) -> &[Vec<PH::Hash>] {
        &self.siblings
    }

    /// Returns `true` if the Merkle path is valid for the given root and leaf.
    pub fn verify<LH: LeafHash<Hash = PH::Hash>>(
        &self,
        leaf_hasher: &LH,
        path_hasher: &PH,
        root: &PH::Hash,
        leaf: &LH::Leaf,
    ) -> bool {
        // Ensure the leaf index is within the tree depth.
        if (self.leaf_index as u128) >= (ARITY as u128).saturating_pow(DEPTH as u32) {
            eprintln!("Found an out of bounds Merkle leaf index");
            return false;
        }
        // Ensure the path length matches the expected depth.
        if self.siblings.len() != DEPTH as usize {
            eprintln!("Found an incorrect Merkle path length");
            return false;
        }

        // Initialize a tracker for the current hash, by computing the leaf hash to start.
        let mut current_hash = match leaf_hasher.hash_leaf(leaf) {
            Ok(candidate_leaf_hash) => candidate_leaf_hash,
            Err(error) => {
                eprintln!("Failed to hash the Merkle leaf during verification: {error}");
                return false;
            }
        };

        // Compute the ordering of the current hash and sibling hashes on each level.
        // The indicator index determines which sibling the current hash is.
        let Ok(indicator_indexes) = (0..DEPTH)
            .map(|i| {
                usize::try_from(self.leaf_index as u128 / (ARITY as u128).saturating_pow(i as u32) % (ARITY as u128))
            })
            .collect::<Result<Vec<_>, _>>()
        else {
            eprintln!("Found an incorrect Merkle leaf index");
            return false;
        };

        // Check levels between leaf level and root.
        for (indicator_index, sibling_hashes) in indicator_indexes.into_iter().zip_eq(&self.siblings) {
            // Construct the ordering of sibling hashes for this level.
            let mut sibling_hashes = sibling_hashes.clone();

            // Insert the current hash into the list of sibling hashes.
            sibling_hashes.insert(indicator_index, current_hash);

            // Update the current hash for the next level.
            match path_hasher.hash_children(&sibling_hashes) {
                Ok(hash) => current_hash = hash,
                Err(error) => {
                    eprintln!("Failed to hash the Merkle path during verification: {error}");
                    return false;
                }
            }
        }

        // Ensure the final hash matches the given root.
        current_hash == *root
    }
}

impl<PH: PathHash, const DEPTH: u8, const ARITY: u8> FromBytes for KaryMerklePath<PH, DEPTH, ARITY> {
    /// Reads in a Merkle path from a buffer.
    #[inline]
    fn read_le<R: Read>(mut reader: R) -> IoResult<Self> {
        // Read the leaf index.
        let leaf_index = u64::read_le(&mut reader)?;
        // Read the Merkle path siblings.
        let siblings = (0..DEPTH)
            .map(|_| (0..ARITY).map(|_| FromBytes::read_le(&mut reader)).collect::<IoResult<Vec<_>>>())
            .collect::<IoResult<Vec<_>>>()?;
        // Return the Merkle path.
        Self::try_from((leaf_index, siblings)).map_err(error)
    }
}

impl<PH: PathHash, const DEPTH: u8, const ARITY: u8> ToBytes for KaryMerklePath<PH, DEPTH, ARITY> {
    /// Writes the Merkle path to a buffer.
    #[inline]
    fn write_le<W: Write>(&self, mut writer: W) -> IoResult<()> {
        // Write the leaf index.
        self.leaf_index.write_le(&mut writer)?;
        // Write the Merkle path siblings.
        self.siblings
            .iter()
            .try_for_each(|siblings| siblings.iter().try_for_each(|sibling| sibling.write_le(&mut writer)))
    }
}

impl<PH: PathHash, const DEPTH: u8, const ARITY: u8> Serialize for KaryMerklePath<PH, DEPTH, ARITY> {
    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
        ToBytesSerializer::serialize_with_size_encoding(self, serializer)
    }
}

impl<'de, PH: PathHash, const DEPTH: u8, const ARITY: u8> Deserialize<'de> for KaryMerklePath<PH, DEPTH, ARITY> {
    fn deserialize<D: Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
        FromBytesDeserializer::<Self>::deserialize_with_size_encoding(deserializer, "K-ary Merkle path")
    }
}