Egraphs
 EGraph
 My EGraph Blog Posts
 Union Finds
 Hash Cons
 Ematching
 Equality Saturation
 Proof Production
 E Graph Tactics
 Applications
 PEG Program Expression Graphs
 Egglog
 Lambda Encoding
 Contextual EGraphs
 Equational Logic
 CHR egraphs
 Misc
EGraph
What is an egraph?
What is a summer’s day? A rainbow?
Egraphs are a data structure that efficiently holds terms and equalities between terms.
It is useful for algebraic simplification, program transformations and proving equivalences in equational reasoning
Destructive term rewriting can be used in
My EGraph Blog Posts
 Egraphs in Julia (Part I)
 EGraph Pattern Matching (Part II)
 Progress on Automated Reasoning for Catlab with Metatheory.jl Egraphs
 Rewriting Monoidal Categories in the Browser with Egg
 Union Find Dicts: Dictionaries Keyed on Equivalence Classes
 A Simplified Egraph Implementation
 Partial Evaluation of a Pattern Matcher for Egraphs
 Encoding Egraphs to Souffle Datalog
 Egglog: a Prolog Syntax for Egg, Checkpoint I
 JuliaCon 2021 Talk on Metatheory.jl and Snippets From the Cutting Room Floor
 Egglog 2: Automatically Proving the Pullback of a Monic is Monic
 Egglog Examples: Pullbacks, SKI, Lists, and Arithmetic
 Naive Egraph Rewriting in Souffle Datalog
 A Questionable Idea: Hacking findParent into Souffle with User Defined Functors
 Embedding Egraph Rewriting and Egglog in Constraint Handling Rules
Union Finds
Union finds are also called disjoint set datastructures.
You can take a set an group it into equivalence classes. One place this is useful is finding the connected components of a graph.
There are different styles of union finds and both make one thing in different and interesting ways.
At a level of fairly high abstraction, a union find is a contracting map. You iterate the map to a fixed point. Each equivalence class is the basin of a fixed point.
See coq post https://www.philipzucker.com/simplecoqunionfind/
How essential is path compression? It is the thing that gets you that inverse ackermann complexity. It requires mutation so far as I know.
Another way of constructing disjoint sets is to note that preimages of any map form disjoint sets.
Is the inverse acckerman complxity a requirement for anything I might call a union find? I don’t think so.
Union finds can be fully normalized to flatness.
Reference union finds
A chain of pointers that lead up to a root. Often the root is a pointer to itself.
# no path compression
# `is` is python reference equality
class BadUF():
def __init__(self):
self.parent = self
def find(self):
x = self
while x.parent is not x:
x = x.parent
return x
def union(self,rhs):
p = self.find()
p2 = rhs.find()
p.parent = p2
return p2
x = BadUF()
y = BadUF()
z = BadUF()
x.union(y)
print(x.find() is y.find())
print(x.find() is z.find())
Union find arrays and ints
What is a pointer but an index? What is an index but a pointer? In some sense your ram is just a giant array that you hand out. And vice versa you can build the analog of pointer based algorithms by using an array as ram.
In some languages you don’t have access to raw pointers easily. The array style also makes it easier/more natural to copy union finds. This style also makes it more clear how to talk about the union find in a purely functional way.
class BadUF():
# no path compression. no depth stroage.
def __init__(self):
self.parent = []
def find(self,i):
p = self.parent[i]
while p != self.parent[p]: # walrus?
p = self.parent[p]
return p
def makeset(self):
x = len(self.parent) # right?
self.parent.append(x)
return x
def set_size(self,n):
while len(self.parent) < n:
self.makeset()
return
def union(self,i,j):
i = self.find(i)
j = self.find(j)
self.parent[i] = j
return j
uf = BadUF()
x = uf.makeset()
y = uf.makeset()
z = uf.makeset()
uf.union(x,y)
assert(uf.find(x) == uf.find(y))
assert(uf.find(x) != uf.find(z))
Variations
Union Find Dicts: Dictionaries Keyed on Equivalence Classes. You can make a dict from union find to lattices. It needs to know how to merge stuff in the range. Relatedly, you could also have union find keyed on union find.
Union find with group elements on edges. kmett talk. Yihong points out http://poj.org/problem?id=2492 as a competitive programming exercise that can use this. “generalized union find” mentioned in CHR book
Scoped Union find
Colored Union find
Data Structures and Algorithms for Disjoint Set Union Problem “deunions” and backtracking. hmm. 1989
simple union finds incudes description of Rem’s algorithm
parwary thesis  Parallel Graph Algorithms for Combinatorial Scientific Computing Compression methods:
 compress all to root
 splitting  compress to grandparent
 halving compress every other to grandparent
Relationship to minimum spanning tree  Kruskal uses union find to determine whether you need an edge or not
Applications
connected components of graph
reducibility of loops Testing Flow Graph Reducibility  tarjan
https://www.cs.princeton.edu/courses/archive/spring07/cos226/lectures/01unionfind.pdf “ ! Network connectivity. ! Percolation. ! Image processing. ! Least common ancestor. ! Equivalence of finite state automata. ! HinleyMilner polymorphic type inference. ! Kruskal’s minimum spanning tree algorithm. ! Games (Go, Hex) ! Compiling equivalence statements in Fortran “ quickfind vs quickunion
percolation. estimate percolation via monte carlo. quick test of connectivity between conducting edges. That’s neat.
https://en.wikipedia.org/wiki/Tarjan%27s_offline_lowest_common_ancestors_algorithm
value numbering
Hash Cons
Ematching
Searching over the egraph. It is both simple and complex. It is a brother of pattern matching. The branching factor of each eclass means that it is more nondeterministic than tree pattern matching. It is similar to database search. Relational Ematching.
Consider
 pattern matching at the root of a single tree
 pattern matching at ever node in a tree

pattern matching over a database of trees (forest)
 pattern matching over a DAG
 pattern matching over a graph (hard in principle (subgraph isomorphism), in practice maybe not that bad)
Simplify de Moura and Bjorner Their VM is quire similar to WAM.
Equality Saturation
Proof Production
Proof producing congruence closure
A union find is a data structure useful for finding connected components in a graph. The “proof” that two vertices are connected is the path between them. We need to maintain an incremental spanning tree of some kind.
We also need to record “reasons” why each edge got added. Reasons may be rewrite rules, or congruence closure.
A proof might be:
 a term rewriting sequence
 a grounded confluent terminating term rewriting system
 The ground equalities to assert to an egraph that are enough to push the proof through with no egraph rewrite rules
See also Z3’s proof production
E Graph Tactics
 Samuel Gruetter and Dustin Jamner  Coq.
congruence
tactic is an egraph. Too slow? code of coq tactic. Deciding Equality in the Constructor Theory  Lean? lean tatic
Axiom i64 : Type.
Axiom add : i64 > i64 > i64.
Axioms add_comm : forall x y, add x y = add y x.
Axioms add_assoc : forall x y z, add (add x y) z = add x (add y z).
Axioms a b c d : i64.
Axiom num : nat > i64.
Definition add_rule_set := (add_comm , add_assoc).
Fixpoint addl (n : nat) : i64 :=
match n with
 O => num 0
 S n => add (num (S n)) (addl n)
end.
Fixpoint addr (n : nat) : i64 :=
match n with
 O => num 0
 S n => add (addr n) (num (S n))
end.
Global Hint Rewrite add_assoc : base0. (* add_comm is no good. nonterminating as a rewrite*)
Goal addl 10 = addr 10. simpl.
(* congruence with (add_comm (num 0) (num 1)). autorewrite with base0. *)
(* pose add_comm. pose add_assoc. *)
pose add_rule_set as rs. destruct rs.
congruence. Qed.
Applications
 Denali https://courses.cs.washington.edu/courses/cse501/15sp/papers/joshi.pdf
 Herbie  improve accuracy of floating point expressions
 Szalinksi shrink 3d cad programs
 Vectorization for Digital Signal Processors via Equality Saturation
 Highperformance symbolicnumerics via multiple dispatch

PEG Program Expression Graphs
https://ztatlock.net/pubs/2009poplequalitysaturationoptimizationsegraphs.pdf https://rosstate.org/publications/eqsat/
Control flow graph (CFG) is just like, a thing. Its denotational semantics are a bit confused.
Egraphs like talking about functions. How to encode something cfg like into functions?
Dataflow graphs are basically just a DAG functional representation. So that intra block dataflow is not really a problem.
Being in SSA make one close to purely functional. YOu never assign to the sam name twice (although let binding shadowing could also model this purely functionally).
SSA Phi nodes are slightly bizarre. Gated Phi nodes make explicit the condition on which the merging happens. This makes them more like a purely functional ifthenelse.
One can consider using streams as a functional representation of loop behavior. There are operations like tail, head, other on the stream.
PEGs have some clues on how to treat summation symbols $\Sigma$ in an egraph. Surely you can write a sum function asa CFG. Partial Sums / indefinite integrals rather than closed sum, definite integrals.
Is there a relationship between pegs and timely dataflow?
You only Grep once by koppel  uses PEGlike representation for semantic code search.
RVSDG https://github.com/jameysharp/optir/
Loops in egraphs and Landin’s knot.
Equality Saturation: a New Approach to Optimization
Quiche quiche python egraph for manipulating python AST
Tree Automata
https://github.com/ondrik/libvata Tree Automata Techniques and Applications
EGraphs, VSAs, and Tree Automata: a Rosetta Stone slides merge only rules
https://en.wikipedia.org/wiki/Tree_automaton
Egglog
See language/egglog
First class sets
GATs
# I should try souffle using subsumption UF. Maybe it'll work now?
class BadUF():
# no path compression. no depth stroage.
def __init__(self):
self.parent = []
def find(self,i):
p = self.parent[i]
while p != self.parent[p]: # walrus?
p = self.parent[p]
return p
def makeset(self):
x = len(self.parent) # right?
self.parent.append(x)
return x
def set_size(self,n):
while len(self.parent) < n:
self.makeset()
return
def union(self,i,j):
i = self.find(i)
j = self.find(j)
self.parent[i] = j
return j
uf = BadUF()
uf.set_size(5)
'''
add = {(0,1,2), (1,0,3), (0,1,4)} # x + y, y + x
while True:
newadd = {(y,x,xy) for (x,y,xy) in add if (y,x,xy) not in add}
if newadd.issubset(add):
break
add.update(newadd)
# rebuild
while True:
# normalize
add = {(uf.find(x), uf.find(y), uf.find(xy)) for (x,y,xy) in add}
# congruence close
fresh = { (x,y,uf.union(xy,xy1)) for (x,y,xy) in add for (x1,y1,xy1) in add if x == x1 and y == y1 and xy != xy1}
if len(fresh) == 0:
break
print(add)
'''
add = {(0,1,2), (1,0,3), (0,1,4)} # x + y, y + x
while True:
newadd = {(y,x,xy) for (x,y,xy) in add if (y,x,xy) not in add}
if newadd.issubset(add):
break
add.update(newadd)
# rebuild
while True:
# normalize
add = {(uf.find(x), uf.find(y), uf.find(xy)) for (x,y,xy) in add}
addind = {(x,y) : xy for (x,y,xy) in add}
# congruence close
fresh = [uf.union(xy,addind[(x,y)]) for (x,y,xy) in add if addind[(x,y)] != xy]
if len(fresh) == 0:
break
print(add)
'''
add = {(0,1) : 2, (1,0) : 3 , (0,1) : 4} # x + y, y + x
while True:
newadd = {(y,x,xy) for (x,y),xy in add.items() if (y,x) not in add or add[(y,x)] != xy}
if newadd.issubset(add):
break
add.update(newadd)
# rebuild
while True:
# normalize
add = {(uf.find(x), uf.find(y), uf.find(xy)) for (x,y,xy) in add}
# congruence close
fresh = { (x,y,uf.union(xy,xy1)) for (x,y,xy) in add for (x1,y1,xy1) in add if x == x1 and y == y1 and xy != xy1}
if len(fresh) == 0:
break
'''
print(add)
Lambda Encoding
Might work:
 Combinators. SKI or categorical. Many projects are doing something like this. Relational Algebra or matrix algebra are the same thing.
 Succ lifting. pavel oleg. Do de bruijn but let Succ float out of Var. Let’s you do de bruijn index lifting as local rewrite rule. Succ has interpretation as a higher order function.
Arguable:
 Co debruijn. Hash Cons modulo alpha. Mcbride’s Do Be doo be doo. Doesn’t seem to work. Correlated rewrites required
 Graph compilation. Optimal graph reduction
Contextual EGraphs
No one know what this means. Everyone wants it.
Perhaps related to backtracking
Equational Logic
Maude Goguen theorem proving and algebra
rigid euniifcation https://pure.uvt.nl/ws/portalfiles/portal/311063/80170.pdf Equational Proofs in Tableaux en Logic Programming de Kogel, E.A. Another fairly extnsive work that seems a lot like egglog.
CHR egraphs
: use_module(library(chr)).
: initialization(main,main).
% hmm are these annotations ok?
: chr_constraint make(+int), find(+int,), root(+int,+int), union(+int,+int),
link(+int,+int), pto(+int,+int), counter(+int).
make(A), counter(N) <=> N = A, N1 is N + 1, counter(N1), root(A,0).
union(A,B) <=> find(A,X), find(B,Y), link(X,Y).
pto(A, B), find(A,X) <=> find(B,X), pto(A,X).
root(A,_) \ find(A,X) <=> X=A.
link(A,A) <=> true.
link(A,B), root(A,N), root(B,M) <=> N>=M  pto(B,A), K is max(N,M+1), root(A,K).
link(B,A), root(A,N), root(B,M) <=> N>=M  pto(B,A), K is max(N,M+1), root(A,K).
main(_) : counter(0), make(A), make(B), make(C), union(A,B), find(B,X), find(C,Y),
print(X),print(Y), union(A,B), chr_show_store(true).
norm @ pto(A, A1), pto(B,B1), pto(E,E1) \ eclass(A + B, E) <=> eclass(A1 + B1, E1).
cong @ eclass(T,E1) \ eclass(T, E2) <=> E1 < E2  pto(E2, E1).
: use_module(library(chr)).
: initialization(main,main).
: chr_option(optimize,full). % Thanks Tom Schrijvers for the tip
: chr_constraint eclass(?,).
: use_module(library(chr)).
: initialization(main,main).
: chr_option(optimize,full). % Thanks Tom Schrijvers for the tip
: chr_constraint eclass(?,), eclass2(?,), collect/1, kill/0, count/1.
cong @ eclass(T, E1) \ eclass(T, E2) <=> E1 = E2.
% rewrite rules.
% how can we run only once?
% How can we not make a counter inc if already exists? Maybe that doesn't matter much.
comm @ eclass(X + Y, E) ==> eclass2(Y + X, E).
assocl @ eclass(X + YZ, E), eclass(Y + Z, YZ) ==> eclass2(X + Y, XY), eclass2(XY + Z, E). % if I put counter(N) in here, can refire?
assocr @ eclass(X + Y, XY), eclass(XY + Z, E) ==> eclass2(X + YZ, E), eclass2(Y + Z, YZ).
% To collect up new eclasses
collect @ eclass2(T,E), collect(L) <=> L = [eclass3(T,E)  L1], collect(L1).
done @ collect(L) <=> L = [].
% helpers to cleanup eclass2
kill @ kill \ eclass2(_,_) <=> true.
killdone @ kill <=> true.
% helper to count eclasses
count @ count(N), eclass(_,_) <=> N1 is N + 1, count(N1).
% Take rhs list and inject them as CHR constraints
process([]).
process([eclass3(T, E) L]) : eclass(T,E), process(L).
% Do N rewriting runs
batch() : collect(L), process(L).
batch(0).
batch(N) : batch(), N1 is N 1, batch(N1).
init_add(N) : eclass(N,E), N1 is N  1, init_add_aux(N1,E).
init_add_aux(0,_).
init_add_aux(N,E) :
eclass(N, EN), eclass(EN + E, E2), N1 is N1, init_add_aux(N1, E2).
insert( T , E) :
ground(T),
var(E),
T =.. [F  Args],
length(Args, N), length(Es, N),
T2 =.. [F  Es],
eclass(T2, E),
maplist(insert, Args, Es).
main(_) :
N = 6,
init_add(N),
Num is 3**(N)  2**(N+1) + 1 + N, print(Num),
BNum is N,
time(batch(BNum)), kill, count(0), chr_show_store(true).
/*
Output:
608
% 397,754,165 inferences, 41.712 CPU in 41.732 seconds (100% CPU, 9535677 Lips)
count(608)
N=5 is under a second. Not good scaling.
*/
Misc
What would be a mvp egraph in C specialized for the comm/assoc problem look like. Use reference based union find with tag bits?
Is ematching a wam? I suppose so. Ematching over a fixed egraph is easily expressible in prolog.
We could implement egraph as assert
We can’t use unification vars?
Does tabling help?
f(1,2,3).
g(1,2,3).
? plus(A,B,AB), plus(AB,C,ABC)
SketchGuided Equality Saturation Scaling Equality Saturation to Complex Optimizations in Languages with Bindings de buijn indexes with extraction. Rise compiler
EGRAPH 2022 workshop youtube feed
Yihong trick to make ematching faster
wasm mutate  fuzzing wasm with egraphs
abstract interp for egraphs improving interval boounds
sketch guided equality satruation Give sketches (~ patterns?) which one the ergaph reaches, transition to a new set of rewriting rules / clear the egraphs.
Caviar: an egraph based TRS for automatic code optimization halide
Automatic Datapath Optimization using EGraphs Samuel Coward, George A. Constantinides, Theo Drane
https://en.wikipedia.org/wiki/Egraph
EqualityBased Translation Validator for LLVM
Automatic Datapath Optimization using EGraphs RTL optimization
Improving MBA Deobfuscation using Equality Saturation
Fast equality saturation in Haskell
Deep Reinforcement Learning for Equality Saturation caviar TRS system for proving optimizations applied to Halide.
WebAssembly Diversification for Malware Evasion
MCTSGEB: Monte Carlo Tree Search is a Good Egraph Builder
An Evaluation Algorithm for Datalog with Equality Martin E. Bidlingmaier