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Add an update method #1

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merged 2 commits into from
Jun 6, 2024
Merged

Add an update method #1

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7658c46
Add an update method
Quantumplation Jun 6, 2024
159cc7d
Remove TODO, formatting
Quantumplation Jun 6, 2024
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24 changes: 24 additions & 0 deletions on-chain/lib/aiken/merkle-patricia-forestry.ak
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Original file line number Diff line number Diff line change
Expand Up @@ -136,6 +136,30 @@ pub fn delete(
MerklePatriciaForestry { root: excluding(key, proof) }
}

/// **fails** when | the [Proof](#Proof) is invalid
/// --- | ---
/// **fails** when | there's no element in the trie at the given key
/// --- | ---
///
/// Update an element in the trie with a a new value. This requires a [Proof](#Proof)
/// of the old element, to ensure its in the list, and a [Proof](#Proof) of the new
/// element, to re-add it.
///
/// Can be thought of as a delete, followed by an insert, but is able to do it with one fewer
/// membership checks
pub fn update(
self: MerklePatriciaForestry,
key: ByteArray,
proof: Proof,
old_value: ByteArray,
new_value: ByteArray,
) {
// TODO: could we do this with a single traversal? `something(key, old_value, new_value, proof) -> (old_root, new_root)`?
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I've done -- at some point -- something similar for including / excluding since only the last step in the traversal really is different. But in the end, there's an extra cost that comes from having more arguments to keep track of and you still need the same number of hashes and prefix manipulation so there's almost no visible benefits on the execution costs in doing so. And, it obfuscates the code a lot more, so I gave up on the idea eventually.

expect including(key, old_value, proof) == self.root
// If we were doing a delete followed by an insert, we'd end up checking the `excluding` again here
MerklePatriciaForestry { root: including(key, new_value, proof) }
}

// -----------------------------------------------------------------------------
// ----------------------------------------------------------------------- Proof
// -----------------------------------------------------------------------------
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23 changes: 23 additions & 0 deletions on-chain/lib/aiken/merkle-patricia-forestry.tests.ak
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Expand Up @@ -256,6 +256,22 @@ test example_delete() {
}
}

test example_update() {
// https://media3.giphy.com/media/Bj5ILhCPm8EQ8/giphy.gif
mpf.update(trie(), banana, proof_banana(), "🍌", "🍆") == updated_banana()
}

test example_idempotent_update() {
and {
mpf.update(trie(), banana, proof_banana(), "🍌", "🍌") == trie(),
mpf.update(updated_banana(), banana, proof_banana(), "🍆", "🍆") == updated_banana(),
}
}

test example_fake_update() fail {
mpf.update(without_banana(), banana, proof_banana(), "🍌", "🍆") == updated_banana()
}

// -------------------- Some notable cases

test example_insert_whatever() {
Expand Down Expand Up @@ -470,6 +486,13 @@ fn without_banana() {
)
}

// The root hash we get with `banana` mapped to 🍆 instead
fn updated_banana() {
mpf.from_root(
#"9057d02799a012a9d47fab6f9f5c43b4b2bf94584b339e3b4d3969fd95d55972"
)
}

// ---------- blueberry

const blueberry = "blueberry[uid: 0]"
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