# Fixing the hashing in "Hashing modulo α-equivalence"

Per Vognsen sent me a link to Maziarz et al’s Hashing Modulo Alpha-Equivalence because its Lemma 6.6 claims to solve a thorny problem we have both encountered several times.

Essentially, the lemma says that computing the natural recursive combination of hash values over $$2^b$$ bits for two distinct trees (ADT instances) $$a$$ and $$b$$ yields a collision probability at most $$\frac{|a| + |b|}{2^b}$$ if we use a random hash function (sure), and Section 6.2 claims without proof that the result can be safely extended to the unspecified “seeded” hash function they use.

That’s a minor result, and the paper’s most interesting contribution (to me) is an algorithmically efficient alternative to the locally nameless representation: rather than representing bindings with simple binders and complex references, as in de Bruijn indices (lambda is literally just a lambda literal, but references must count how many lambdas to go up in order to find the correct bindings), Maziarz and his coauthors use simple references (holes, all identical), and complex binders (each lambda tracks the set of paths from the lambda binding to the relevant holes).

The rest all flows naturally from this powerful idea.

Part of the naturally flowing rest are collision probability analyses for a few hashing-based data structures. Of course it’s not what PLDI is about, but that aspect of the paper makes it look like the authors are unaware of analysis and design tools for hashing based algorithms introduced in the 1970s (a quick Ctrl-F for “universal,” “Wegman,” or “Carter” yields nothing). That probably explains the reckless generalisation from truly random hash functions to practically realisable ones.

There are two core responsibilities for the hashing logic:

1. incrementally hash trees bottom up (leaf to root)
2. maintain the hash for a map of variable name to (hash of) trees (that may grow bottom-up as well)

As Per saliently put it, there are two options for formal analysis of collision probabilities here: we can either assume a cryptographic hash function like SHA-3 or BLAKE3, in which case any collision is world-breaking news, so all that matters is serialising data unambiguously when feeding bytes to the hash function, or we can work in the universal hashing framework.

Collision probability analysis for the former is trivial, so let’s assume we want the latter, pinpoint where the paper is overly optimistic, and figure out how to fix it.

## Incremental bottom-up hashing, without novelty

Let’s tackle the first responsibility: incrementally hashing trees bottom up.

The paper essentially says the following in Appendix A. Assume we have one truly random variable-arity hash function (“hash combiner”) $$f$$, and a tag for each constructor (e.g., $$s_{\texttt{Plus}}$$ for (Plus a b)); we can simply feed the constructor’s arity, its tag, and the subtrees’ hash values to $$f$$, e.g., $$f(2, s_{\texttt{Plus}}, hv_a, hv_b)$$… and goes on to show a surprisingly weak collision bound (the collision rate for two distinct trees grows with the sum of the size of both trees).1

A non-intuitive fact in hash-based algorithms is that results for truly random hash functions often fail to generalise for the weaker “salted” hash functions we can implement in practice. For example, linear probing hash tables need 5-universal hash functions2 in order to match the performance we expect from a naïve analysis with truly random hash functions. A 5-universal family of hash functions isn’t the kind of thing we use or come up with by accident (such families are parameterised by at least 5 words for word-sized outputs, and that’s a lot of salt).

The paper’s assumption that the collision bound it gets for a truly random function $$h$$ holds for practical salted/seeded hash functions is thus unwarranted (see, for examples, these counter examples for linear probing, or the seed-independent collisions that motivated the development of SipHash); strong cryptographic hash functions could work (find a collision, break Bitcoin), but we otherwise need a more careful analysis.

It so happens that we can easily improve on the collision bound with a classic incremental hashing approach: polynomial string hashing.

Polynomial string hash functions are computed over a fixed finite field $$\mathbb{F}$$ (e.g., arithmetic modulo a prime number $$p$$), and parameterised by a single point $$x \in \mathbb{F}$$.

Assuming a string of “characters” $$v_i \in \mathbb{F}$$ (e.g., we could hash strings of atomic bytes in arithmetic modulo a prime $$p \geq 256$$ by mapping each byte to the corresponding binary-encoded integer), the hash value is simply

$v_0 + v_1 x + v_2 x^2 \ldots + v_{n - 1} x^{n - 1},$

evaluated in the field $$\mathbb{F}$$, e.g., $$\mathbb{Z}/p\mathbb{Z}$$.

For more structured atomic (leaf) values, we can serialise to bits and make sure the field is large enough, or split longer bit serialised values into multiple characters. And of course, we can linearise trees to strings by encoding them in binary S-expressions, with dedicated characters for open ( and close ) parentheses.3

The only remaining problem is to commute hashing and string concatenation: given two subtrees a, b, we want to compute the hash value of (Plus a b), i.e., hash "(Plus " + a + " " + b + ")" in constant time, given something of constant size, like hash values for a and b.

Polynomials offer a lot of algebraic structure, so it shouldn’t be a surprise that there exists a solution.

In addition to computing h(a), i.e., $$\sum_{i=1}^{|a|} a_i x^i,$$ we will remember $$x^{|a|}$$, i.e., the product of x repeated for each “character” we fed to the hash function while hashing the subtree a. We can obviously compute that power in time linear in the size of a, although in practice we might prefer to first compute that size, and later exponentiate in logarithmic time with repeated squaring.

Equipped with this additional power of $$x\in\mathbb{F}$$, we can now compute the hash for the concatenation of two strings $$h(a \mathtt{++} b)$$ in constant time, given the hash and power of x for the constituent strings $$a$$ and $$b$$.

Expanding $$h(a \mathtt{++} b)$$ and letting $$m = |a|,$$ $$n = |b|$$ yields:

$a_0 + a_1 x + \ldots + a_{m - 1} x^{m - 1} + b_0 x^n + b_1 x^{n + 1} + \ldots + b_{n - 1} x^{m + n - 1},$

which we can rearrange as

$a_0 + a_1 x + \ldots + a_{m - 1} x^{m - 1} + x^m (b_0 + b_1 x + \ldots b_{n-1} x^{n-1},$

i.e.,

$h(a \mathtt{++} b) = h(a) + x^{|a|} h(b),$

and we already have all right-hand side three terms $$h(a),$$ $$x^{|a|},$$ and $$h(b).$$

Similarly, $$x^{|a \mathtt{++} b|} = x^{|a| + |b|} = x^a \cdot x^b,$$ computable in constant time as well.

This gives us an explicit representation for the hash summary of each substring, so it’s easy to handle, e.g., commutative and associative operators by sorting the pairs of $$(h(\cdot), x^{|\cdot|})$$ that correspond to each argument before hashing their concatenation.

TL;DR: a small extension of classic polynomial string hashing commutes efficiently with string concatenation.

And the collision rate? We compute the same polynomial string hash, so two distinct strings of length at most $$n$$ collide with probability at most $$n/|\mathbb{F}|$$ (with the expectation over the generation of the random point $$x \in \mathbb{F}$$;4 never worse than Lemma 6.6 of Maziarz et al, and up to twice as good.

Practical implementations of polynomial string hashing tend to evaluate the polynomial with Horner’s method rather than maintaining $$x^i$$. The result computes a different hash function, since it reverses the order of the terms in the polynomial, but that’s irrelevant for collision analysis. The concatenation trick is similarly little affected: we now want $$h(a \mathtt{++} b) = x^{|b|} h(a) + h(b)$$.

## Hashing unordered maps and sets

The term representation introduced in “Hashing Module Alpha-Equivalence” contains a map from variable name to a tree representation of the holes where the variable goes (like a DAWG representation for a set of words where each word is a path, except the paths only share as they get closer to the root of the tree… so maybe more like snoc lists with sharing).

We already know how to hash trees incrementally; the new challenge is in maintaining the hash value for a map.

Typically, one hashes unordered sets or maps by storing them in balanced trees sorted primarily on the key’s hash value, and secondarily on the key.5 We can also easily tweak arbitrary balanced trees to maintain the tree’s hash value as we add or remove entries: augment each node with the hash and power of x for the serialised representation of subtree rooted at the node.6

The paper instead takes the treacherously attractive approach of hashing individual key-value pairs, and combining them with an abelian group operator (commutative and associative, and where each element has an inverse)… in their case, bitwise xor over fixed-size words.

Of course, for truly random hash functions, this works well enough, and the proof is simple. Unfortunately, just because a practical hash function is well distributed for individial value does not mean pairs or triplets of values won’t show any “clumping” or pattern. That’s what $$k-$$universality is all about.

For key-value pairs, we can do something simple: associate one hash function from a (almost-xor)-universal family to each value, and use it to mix the associated value before xoring everything together.

It’s not always practical to associate one hash function with each key, but it does work for the data structure introduced in “Hashing modulo Alpha-Equivalence:” the keys are variable names, and these were regenerated arbitrarily to ensure uniqueness in a prior linear traversal of the expression tree. The “variable names” could thus include (or be) randomly generated parameters for a (almost-xor)-universal family.

Multiply-shift is universal, so that would work; other approaches modulo a Mersenne prime should also be safe to xor.

For compilers where hashing speed is more important than compact hash values, almost-universal families could make sense.

The simplest almost-xor-universal family of hash functions on contemporary hardware is probably PH, a 1-universal family that maps a pair of words $$(x_1, x_2)$$ to a pair of output words, and is parameterised on a pair of words $$(a_1, a_2)$$:

$\texttt{PH}_a(x) = (x_1 \oplus a_1) \odot (x_2 \oplus a_2),$

where $$\oplus$$ is the bitwise xor, and $$\odot$$ an unreduced carryless multiplication (e.g., x86 CLMUL).

Each instance of PH accepts a pair of $$w-$$bit words and returns a $$2w-$$bit result; that’s not really a useful hash function.

However, not only does PH guarantee a somewhat disappointing collision rate at most $$w^{-1}$$ for distinct inputs (expectation taken over the $$2w-$$bit parameter $$(a_1, a_2)$$), but, crucially, the results from any number of independently parameterised PH can be combined with xor and maintain that collision rate!

For compilers that may not want to rely on cryptographic extensions, the NH family also works, with $$\oplus$$ mapping to addition modulo $$2^w$$, and $$\odot$$ to full multiplication of two $$w-$$bit multiplicands into a single $$2w-$$bit product. The products have the similar property of colliding with probability $$w^{-1}$$ even once combined with addition modulo $$w^2$$.

Regardless of the hash function, it’s cute. Useful? Maybe not, when we could use purely functional balanced trees, and time complexity is already in linearithmic land.

## Unknown unknowns and walking across the campus

None of this takes away from the paper, which I found both interesting and useful (I intend to soon apply its insights), and it’s all fixable with a minimal amount of elbow grease… but the paper does make claims it can’t back, and that’s unfortunate when reaching out to people working on hash-based data structures would have easily prevented the issues.

I find cross-disciplinary collaboration most effective for problems we’re not even aware of, unknown unknowns for some, unknown knowns for the others. Corollary: we should especially ask experts for pointers and quick gut checks when we think it’s all trivial because we don’t see anything to worry about.

Thank you Per for linking to Maziarz et al’s nice paper and for quick feedback as I iterated on this post.

1. Perhaps not that surprising given the straightforward union bound.

2. Twisted tabular hashing also works despite not being quite 5-universal, and is already at the edge of practicality.

3. It’s often easier to update a hash value when appending a string, so reverse Polish notation could be a bit more efficient.

4. Two distincts inputs a and b define polynomials $$p_a$$ and $$p_b$$ of respective degree $$|a|$$ and $$|b|$$. They only collide for a seed $$x\in\mathbb{F}$$ when $$p_a(x) = p_b(x),$$ i.e., $$p_a(x) - p_b(x) = 0$$. This difference is a non-zero polynomial of degree at most $$\max(|a|, |b|),$$ so at most that many of the $$|\mathbb{F}|$$ potential values for $$x$$ will lead to a collision.

5. A more efficient option in practice, if maybe idiosyncratic, is to use Robin Hood hashing with linear probing to maintain the key-value pairs sorted by hash(key)` (and breaking improbable ties by comparing the keys themselves), but that doesn’t lend itself well to incremental hash maintenance.

6. Cryptographically-minded readers might find Incremental Multiset Hashes and their Application to Integrity Checking interesting.

Dec 29th, 2022