The Hanoi Omega-Automata Format

This document describes the Hanoi Omega-Automata (HOA) format. The name is a reference to the ATVA'13 conference, which was organized in Hanoi, and where the foundations of this format were laid.

The HTML and PDF versions of this document are updated from the sources on github.

Current tool support is described on a separate page.

All the examples shown here can also be downloaded separately.

For a short introduction we have a tool paper published at CAV'15. The slides from the lightning talk are available, but the poster covers much more of the format.

Current status

This is version 1 of the format. The document may evolve slightly to clarify some parts and fix typos, but you should expect no major semantic change.

If you see any problem, please report it on the issue tracker.

Change log:

• 2019-03-27: Clarify that states may have no outgoing edges (#70)
• 2019-03-27: Clarify that acc-sig can be specified for both states and edges (#69)
• 2015-06-21: Improve definition of accepting sets in the semantics (#48)
• 2015-05-26: Clarify notion of canonical encoding, better support for parity automata with property: colored, and cleaner definition of parity acceptance in corner cases (#46)
• 2015-05-20: More compact canonical encoding for parity acceptance, and canonical encoding for min odd and max even. (#42 and #43)
• 2015-04-17: Some clarification in case States: is missing. (#39)
• 2015-04-17: Fix transition-based semantics to deal with duplicate transitions. (#38)
• 2015-02-24: Clarify that HEADERNAME may not start with -. (#37)
• 2015-02-06: Version 1 published.

Goals

Design a format, inspired from ltl2dstar's format, but which:

• is more compact when a lot of atomic propositions are used, or when the automaton is not complete.
• supports non-deterministic and alternating omega-automata.
• supports state-labeled omega-automata.
• supports different types of acceptance conditions, preferably in a way that is easy to extend.
• supports transition-based acceptance.
• consider new lines as any other white-space characters, i.e., as token separators. All the examples below should work even after newlines have been removed or moved around (this typically happens if you copy/paste an automaton into a mailer that reformats paragraphs). A use case for not using any newline is when compiling results from experiments into a file, with one automaton per line (and maybe other measurements if that is a CSV file), for easier filtering with line-oriented tools such as grep/cut/sed.
• supports streams of omega-automata (i.e., multiple automata can be concatenated together and processed in batch).

Supported Types of Omega-Automata

The HOA format supports many types of finite automata over infinite words: automata with labels on transitions or labels on states, and with a wide range of acceptance conditions based on states or transitions appearing (in)finitely often in an automaton run. Instead of providing semantics for each supported type of omega-automata, we provide only semantics for the general case of alternating automata with labels on transitions and transition-based accepting conditions. Labels on states and state-based acceptance are formally seen as syntactic sugar (their semantics fully corresponding to the expected one). Automata in HOA format can have multiple initial states.

Preliminary Notes

$\def\AP{\mathit{AP}}\def\B{\mathbb{B}}\def\Fin{\mathsf{Fin}}\def\Inf{\mathsf{Inf}}$Input alphabets of all considered automata types consist of letters that are subsets of atomic propositions ($\AP$). To make the automata description more concise, we label states or transitions of automata with Boolean formulas over $\AP$ representing choices between multiple letters. A Boolean formula represents the set of letters satisfying the formula: a letter satisfies a formula if the valuation assigning True to all atomic propositions in the letter and False to all other atomic propositions is a model of the formula. We use $\B(\AP)$ to denote the set of Boolean formulas over $\AP$.

The format considers acceptance conditions built on a finite set ${S_0,S_1,\ldots,S_k}$ of acceptance sets. Each acceptance set $S_i$ is a subset of automata states and transitions. Loosely speaking, an acceptance condition says which acceptance sets should be visited infinitely often and which only finitely often by a run to be accepting. More precisely, an acceptance condition is a positive Boolean formula over atoms of the form $\Fin(S_i)$, $\Fin(\lnot S_i)$, $\Inf(S_i)$, or $\Inf(\lnot S_i)$. The atom $\Fin(S_i)$ indicates that all states and transitions in $S_i$ should occur at most finitely often in the run, while $\Inf(S_i)$ denotes that some state or transition of $S_i$ should be visited infinitely often. A state in an acceptance set is formally seen as an abbreviation for inclusion of all transitions leaving the state. The negation symbol $\lnot$ represents the complement of the set with respect to all transitions. Many examples of classical acceptance conditions (Büchi, Rabin, Streett, parity) will be given later.

The format has a common approach to atomic propositions, states, and acceptance sets: the number of propositions/states/sets, say $n$, is first declared and all propositions/states/sets are then referenced as $0,1,\ldots,n-1$.

Common Tokens

• STRING: a C-like double-quoted string "(\\.|[^\\"])*"

• INT: 0|[1-9][0-9]* A non-negative integer less than $2^{31}$ written in base 10 (with no useless 0 at the beginning).

• comments: /* … */ Comments may be introduced between any token by enclosing them with /* and */ (with proper nesting, i.e. /*a/*b*/c*/ is one comment). C++-style comments are not considered because they require newlines. Tools can use comments to output additional information (e.g. debugging data) that should be discarded upon reading.

• whitespace: [ \t\n\r] Except in double-quoted strings and comments, whitespace is used only for tokenization and can be discarded afterwards.

• BOOLEAN: [tf] The true and false Boolean constants.

• IDENTIFIER: [a-zA-Z_][0-9a-zA-Z_-]* An identifier made of letters, digits, - and _. Digits and - may not be used as first character, and t or f are not valid identifiers.

• ANAME: @[0-9a-zA-Z_-]+ An alias name, i.e., "@" followed by some alphanumeric characters, - or _. These are used to identify atomic propositions or subformulas.

• HEADERNAME: [a-zA-Z_][0-9a-zA-Z_-]*: Header names are similar to identifiers, except that they are immediately followed by a colon (i.e. no comment or space allowed). If an IDENTIFIER or a BOOLEAN is immediately followed by a colon, it should be considered as a HEADERNAME.

General Layout

This format is designed so that we can write tools that can process automata in batch. A tool could for instance get a stream of automata descriptions concatenated together as input and process these to output another stream of automata.

Every automaton is described in two parts: a header, that supplies meta-data about the automaton (such as number of states and acceptance condition), and a body, encoding the automaton as a labeled graph. The two parts are separated by --BODY--. The token --END-- marks the end of the automaton.

automaton ::= header "--BODY--" body "--END--"

Additionally, the --ABORT-- token may be used after any token of this grammar (even in the header) to indicate that the produced automaton should be discarded, for instance in case an error condition is detected during output. In a streaming scenario, a new automaton can start immediately after such an --ABORT-- token. Note that foo--ABORT-- is a valid identifier and shall not abort streaming: to ensure proper tokenization always add a space or newline before --ABORT-- to abort the output.

Header

header ::= format-version header-item*
format-version ::= "HOA:" IDENTIFIER
header-item ::= "States:" INT
             | "Start:" state-conj
             | "AP:" INT STRING*
             | "Alias:" ANAME label-expr
             | "Acceptance:" INT acceptance-cond
             | "acc-name:" IDENTIFIER (BOOLEAN|INT|IDENTIFIER)*
             | "tool:" STRING STRING?
             | "name:" STRING
             | "properties:" IDENTIFIER*
             | HEADERNAME (BOOLEAN|INT|STRING|IDENTIFIER)*

The header is a list of header-items (a HEADERNAME followed by some data). Except for the "HOA:" item, which should always come first, the items may occur in any order. Some HEADERNAMEs have predefined semantics (and might be mandatory) as specified below. This format also makes provision of additional (unspecified) header names to be used.

Any given HEADERNAME should occur at most once, except for Start:, Alias:, and properties:. The case of the HEADERNAME's initial is used to specify whether tools may safely ignore a header item they do not support: header items whose name start with an upper-case letter are expected to influence the semantic of the automaton: tools should at least warn about any such HEADERNAME they do not understand. A HEADERNAME whose initial is lowercase may be safely ignored without affecting the semantics.

Headers items HOA:, and Acceptance: must always be present.

HOA:

HOA: should always be the first token of the file. It is followed by an identifier that represents the version of the format. This document specifies the first version of this format so this header should appear as

HOA: v1

States:

This optional header item specifies the number of states in the automaton.

The states are assumed to be numbered consecutively from 0. For instance:

States: 10

specifies an automaton with 10 states numbered from 0 to 9.

An empty automaton, with no states, can be specified with States: 0.

It is recommended to specify the number of states whenever possible, so that readers may preallocate data structures and perform better error checking. Cases where States: could be missing are typically those where an automaton is constructed on-the-fly during its output, and the actual number of states is unknown before the end of the output.

Start:

This optional header item specifies the initial states. Multiple initial states can be specified by using several Start: headers with a different state number.

Alternating automata can start in a conjunction of states specified using the & operator.

header-item ::= … | "Start:" state-conj
state-conj ::= INT | state-conj "&" INT

If the Start: header item is omitted, then the automaton has no initial state and denotes an empty language.

AP:

AP: gives the number of atomic propositions, followed by unique names for each of these atomic propositions (using double-quoted C-strings). Atomic propositions are implicitly numbered from left to right, starting at 0.

For instance

AP: 3 "a" "proc@state" "a[x] >= 2"

specifies three atomic propositions:

• atomic proposition 0 is "a"
• atomic proposition 1 is "proc@state"
• atomic proposition 2 is "a[x] >= 2"

The number of double-quoted strings must match exactly the number given, and should all be different. This number may be 0, in which case it is not followed by any string, and this is equivalent to not using AP:.

Alias:

Aliases are used to name atomic propositions or common subformulas that will be used later as labels in the automaton. This format can be used without any aliases, referring to atomic propositions by their numbers. Naming atomic propositions using aliases can make the automaton more readable to the human, and naming subformulas that are used repeatedly can help making the output more concise.

headeritem ::= … | "Alias:" ANAME label-expr
label-expr ::= BOOLEAN | INT | ANAME | "!" label-expr
             | "(" label-expr ")"
             | label-expr "&" label-expr
             | label-expr "|" label-expr

The label-expr will also be used to label transitions in automata. INT refers to an atomic proposition number (as specified on the AP: line), ANAME refers to a previously defined alias, and BOOLEAN are the Boolean values (t or f). The Alias: line may appear multiple times, but it is forbidden to redefine an alias. The ! operator has priority over & which in turn has priority over |. Parentheses may be used for grouping.

Here are some examples of aliases:

AP: 3 "a" "proc@state" "a[x] >= 2"
Alias: @a 0
Alias: @ps 1
Alias: @a2 2
Alias: @c @ps|@a2

The first three aliases are just mnemonic names for the atomic propositions, while the last one replace some arbitrary subformula. Defining @c before the definition of @ps and @a2 would be incorrect.

Acceptance:

header-item ::= … | "Acceptance:" INT acceptance-cond

acceptance-cond ::= IDENTIFIER "(" "!"? INT ")"
                 | "(" acceptance-cond ")"
                 | acceptance-cond "&" acceptance-cond
                 | acceptance-cond "|" acceptance-cond
                 | BOOLEAN

The mandatory Acceptance: header item is used to specify the number of acceptance sets used by the automaton and how these acceptance sets are combined in the acceptance condition. If $m$ sets are declared, these sets are numbered from $0$ to $m-1$. In this version of the format, the IDENTIFIER used in acceptance-cond can only be Fin or Inf.

The acceptance condition is specified as a positive Boolean combination of expressions of the form Fin(x), Fin(!x), Inf(x), and Inf(!x) where:

• x is an integer in $[0,m)$ representing an accepting set,
• !x represents the complement of that set,
• Fin(x) and Inf(x) specify whether that set should be visited finitely or infinitely often.

The & operator has priority over |, and parentheses may be used for grouping.

Additionally the t and f Boolean constants can be used with their obvious meanings (t is always accepting while f is never accepting).

As explained previously, our semantics for acceptance are transition-based, so if an automaton uses both accepting states and accepting transitions, the acceptance of the states should be transferred to their outgoing transitions so that complementation of acceptance sets can be performed with respect to all transitions.

For instance

Acceptance: 2 Fin(!0) & Inf(1)

declares two acceptance sets. A run of the automaton is accepting if it visits the complement of the first set finitely often, and if it visits the second set infinitely often. More examples will be given in a later section.

An acceptance condition declaring $m$ sets needs not actually use all of these sets. In this case the unused sets can be ignored if they appear in the body of the automaton.

acc-name:

header-item ::= … | "acc-name:" IDENTIFIER (IDENTIFIER | INT)*

This optional header item gives a symbolic name to the acceptance conditions specified by the mandatory Acceptance: header item.

The first IDENTIFIER is a name, and the remaining IDENTIFIERs and INTs are parameters. The following acceptance names are fully specified in a later section:

• Buchi
• generalized-Buchi
• co-Buchi
• generalized-co-Buchi
• Streett
• Rabin
• generalized-Rabin
• parity
• all
• none

Additional (unspecified) acceptance names may be used when needed.

tool: and name:

header-item ::= …
             | "tool:" STRING STRING?
             | "name:" STRING

These optional header items can be used to record information about the tool used to produce the automaton, or to give a name to this automaton. The two arguments of tool: corresponds respectively to the tool's name and its (optional) version number.

For instance:

tool: "ltl-translate" "1.2-alpha"
name: "BA for GFa & GFb"

properties:

header-item ::= … | "properties:" IDENTIFIER*

The optional properties: header name can be followed by a list of identifiers that gives additional information about the automaton. Multiple properties: lines can be used, it has the same effect as listing all properties on one line. This information should be redundant in the sense that ignoring them should not impact the behavior of the automaton. For instance stating that an automaton is deterministic with

properties: deterministic

may enable tools that read the automaton to choose a better data structure to store this automaton, but ignoring this header item will not suddenly make the automaton non-deterministic.

The following properties have specified meanings, but additional may be added, and tools may simply ignore those they do not know:

• state-labels hints that the automaton uses only state labels
• trans-labels hints that the automaton uses only transition labels
• implicit-labels hints that the automaton uses only implicit transitions labels
• explicit-labels hints that the automaton uses only explicit transitions labels
• state-acc hints that the automaton uses only state-based acceptance specifications
• trans-acc hints that the automaton uses only transition-based acceptance specifications
• univ-branch hints that the automaton uses universal branching for at least one transition or for the initial state
• no-univ-branch hints that the automaton does not uses universal branching
• deterministic hints that the automaton is deterministic, i.e., it has at most one initial state, and the outgoing transitions of each state have disjoint labels (note that this also applies in the presence of universal branching)
• complete hints that the automaton is complete, i.e., it has at least one state, and the transition function is total
• unambiguous hints that the automaton is unambiguous, i.e., for each word there is at most one accepting run of the automaton (this also applies in the presence of universal branching)
• stutter-invariant hints that the automaton describes a stutter-invariant property
• weak hints that in each strongly connected component (in alternating automata, SCC can be defined in standard way if we see each universal branching transition as a set of non-branching transitions), all transitions (or all states) belong to the same accepting sets
• very-weak hints that the automaton is weak and every SCC has exactly one state
• inherently-weak hints that the automaton does not mix accepting cycles and non-accepting cycles in the same SCC
• terminal hints that the automaton is weak, that no non-accepting cycle can be reached from any accepting cycle, and that each SCC containing an accepting cycle is complete, i.e., the transition function is definied for each state of the SCC and each letter
• tight hints that the automaton is tight, i.e. for every word $w=a_0a_1\ldots$ accepted by the automaton there exists an accepting run over $w$ such that whenever two suffixes $w_i=a_ia_{i+1}\ldots$ and $w_j=a_ja_{j+1}\ldots$ of $w$ are identical, the run reaches identical states (or sets of the states in the case of alternating automaton) by reading the corresponding prefixes $a_0\ldots a_{i-1}$ and $a_0\ldots a_{j-1}$
• colored hints that each transition (or each state, for state-based acceptance) of the automaton belongs to exactly one acceptance set; this is typically the case in parity automata

Note that even if some property implies another one (for instance explicit-labels implies trans-labels) it is recommended to specify both.

Canonical acceptance specifications for classical conditions

The Acceptance: line is what defines the acceptance semantics of the automaton and the acc-name: has only informative value. However, tools that do not want to implement a parser for the Acceptance: line may decide to only support input HOA file that carry an acc-name: they support. To avoid surprises, when the acc-name: line is used, the mandatory Acceptance: line should match exactly the canonical acceptance formulas given in the following examples.

Note that the order of the terms in the canonical acceptance formulas matters even though the Boolean operators are commutative and associative (e.g., acc-name: Rabin 1 corresponds to Fin(0)&Inf(1) but not Inf(1)&Fin(0)). This restriction makes it easier to detect potential mismatches between acc-name: and Acceptance:.

Some of the examples below use Acceptance: specifications for which there are no acc-name: defined.

Büchi or co-Büchi

Those acceptance conditions are specified with

acc-name: Buchi
Acceptance: 1 Inf(0)

or

acc-name: co-Buchi
Acceptance: 1 Fin(0)

Generalized Büchi or generalized co-Büchi

A generalized Büchi automaton with three acceptance sets can be defined with:

acc-name: generalized-Buchi 3
Acceptance: 3 Inf(0)&Inf(1)&Inf(2)

A deterministic automaton with such an acceptance condition could be complemented without changing its transition structure by simply complementing the acceptance, giving a generalized co-Büchi automaton:

acc-name: generalized-co-Buchi 3
Acceptance: 3 Fin(0)|Fin(1)|Fin(2)

Streett acceptance

Pairs of acceptance sets ${(L_1,U_1),\ldots,(L_k,U_k)}$. A run is accepting for a pair $(L_i,U_i)$ iff the run visiting $L_i$ infinitely often implies that the run also visits $U_i$ infinitely often. A run is accepting iff it is accepting for all pairs. Assuming $k=3$ and numbering these 6 sets from left ($L_1$) to right ($U_3$), this corresponds to:

acc-name: Streett 3
Acceptance: 6 (Fin(0)|Inf(1))&(Fin(2)|Inf(3))&(Fin(4)|Inf(5))

The parameter 3 in acc-name: Streett 3 refers to the number of Streett pairs.

Rabin acceptance

There are several equivalent presentations of Rabin acceptance, and working with tools that use different definitions is often a source of confusion. Our notations of the acceptance condition accommodate all styles, while giving clear and unambiguous semantics.

J. Klein, in ltl2dstar, uses pairs ${(L_1,U_1),\ldots,(L_k,U_k)}$ where there should be some pair $(L_i,U_i)$ such that states in $L_i$ are visited infinitely often, but states in $U_i$ are visited finitely often. This is simply the complement of the Streett acceptance above:

Acceptance: 6 (Inf(0)&Fin(1))|(Inf(2)&Fin(3))|(Inf(4)&Fin(5))

C. Löding, in his diploma thesis, uses pairs ${(E_1,F_1),\ldots,(E_k,F_k)}$ where $E_i$ should be visited finitely often, and $F_i$ should be visited infinitely often. This is just a reordering of the previous pairs:

acc-name: Rabin 3
Acceptance: 6 (Fin(0)&Inf(1))|(Fin(2)&Inf(3))|(Fin(4)&Inf(5))

The parameter 3 in acc-name: Rabin 3 refers to the number of Rabin pairs. The reason this definition was choosen for acc-name: Rabin is that is seems to be the most commonly used.

S. Krishnan, in his ISAAC'94 paper, uses pairs ${(L_1,U_1),\ldots,(L_k,U_k)}$ such that the set of recurring states of a an accepting run should intersect $L_i$ and be included in $U_i$, for some pair $(L_i,U_i)$. A similar definition is used by Manna and Pnueli in their "Hierarchy of Temporal Properties" paper. This corresponds to:

Acceptance: 6 (Inf(0)&Fin(!1))|(Inf(2)&Fin(!3))|(Inf(4)&Fin(!5))

Generalized Rabin acceptance

Rabin acceptance has been generalized in works by Křetínský & Esparza or Babiak et al.. They both translate LTL formulas into generalized Rabin automata in which the acceptance condition may look like ${(E_1,{F_{11},F_{12},F_{13}}), (E_2,{F_{21},F_{22}})}$, and where a run is accepting if there exists some i such that the run visits finitely often the set $E_i$ and infinitely often all the sets $F_{ij}$. Such an acceptance condition can be specified with:

acc-name: generalized-Rabin 2 3 2
Acceptance: 7 (Fin(0)&Inf(1)&Inf(2)&Inf(3))|(Fin(4)&Inf(5)&Inf(6))

The first parameter of generalized-Rabin gives the number of generalized pairs and the following parameters give the number of $F_{ij}$ sets in the corresponding pairs.

Parity automata

For parity acceptance, acc-name: parity has three parameters to support combinations of min/max and even/odd, and to specify the range of acceptance sets used. In an automaton with max odd parity acceptance, for instance, a run is accepting if the maximum set number visited infinitely often along the run is odd.

A typical parity automaton should have property: colored, ensuring that each transition (or state) belongs to exactly one acceptance set. In this context the maximum or minimum set number seen infinitely often along a run always exists.

The canonical encodings for parity acceptance have been chosen so they behave nicely even in automata where property: colored does not hold, i.e., where some transitions (or states) may belong to multiple acceptance sets or none. In particular if $F$ is the set of numbers of the acceptance sets visited infinitely often by a run of an automaton with $n$ acceptance sets, we assume that $\min(\emptyset)=n$ and $\max(\emptyset)=-1$ for the purpose of deciding the parity of $\min(F)$ or $\max(F)$.

Here are the first instances of the min odd condition for different numbers of sets:

acc-name: parity min odd 0
Acceptance: 0 f

acc-name: parity min odd 1
Acceptance: 1 Fin(0)

acc-name: parity min odd 2
Acceptance: 2 Fin(0) & Inf(1)

acc-name: parity min odd 3
Acceptance: 3 Fin(0) & (Inf(1) | Fin(2))

acc-name: parity min odd 4
Acceptance: 4 Fin(0) & (Inf(1) | (Fin(2) & Inf(3)))

acc-name: parity min odd 5
Acceptance: 5 Fin(0) & (Inf(1) | (Fin(2) & (Inf(3) | Fin(4))))

The min even cases are dual:

acc-name: parity min even 0
Acceptance: 0 t

acc-name: parity min even 1
Acceptance: 1 Inf(0)

acc-name: parity min even 2
Acceptance: 2 Inf(0) | Fin(1)

acc-name: parity min even 3
Acceptance: 3 Inf(0) | (Fin(1) & Inf(2))

acc-name: parity min even 4
Acceptance: 4 Inf(0) | (Fin(1) & (Inf(2) | Fin(3)))

acc-name: parity min even 5
Acceptance: 5 Inf(0) | (Fin(1) & (Inf(2) | (Fin(3) & Inf(4))))

The max odd condition is similar to min odd, but encoded in the reverse direction:

acc-name: parity max odd 0
Acceptance: 0 t

acc-name: parity max odd 1
Acceptance: 1 Fin(0)

acc-name: parity max odd 2
Acceptance: 2 Inf(1) | Fin(0)

acc-name: parity max odd 3
Acceptance: 3 Fin(2) & (Inf(1) | Fin(0))

acc-name: parity max odd 4
Acceptance: 4 Inf(3) | (Fin(2) & (Inf(1) | Fin(0)))

acc-name: parity max odd 5
Acceptance: 5 Fin(4) & (Inf(3) | (Fin(2) & (Inf(1) | Fin(0))))

And again max even conditions are dual to max odd:

acc-name: parity max even 0
Acceptance: 0 f

acc-name: parity max even 1
Acceptance: 1 Inf(0)

acc-name: parity max even 2
Acceptance: 2 Fin(1) & Inf(0)

acc-name: parity max even 3
Acceptance: 3 Inf(2) | (Fin(1) & Inf(0))

acc-name: parity max even 4
Acceptance: 4 Fin(3) & (Inf(2) | (Fin(1) & Inf(0)))

acc-name: parity max even 5
Acceptance: 5 Inf(4) | (Fin(3) & (Inf(2) | (Fin(1) & Inf(0))))

Note that in all those acceptance specifications, Inf is always followed by |, Fin is always followed by &, and both are alternating.

Trivial acceptance conditions: all and none

When generalized-Büchi or Streett are used with 0 acceptance sets, they degenerate to conditions that accept all recognized words:

acc-name: generalized-Buchi 0
Acceptance: 0 t

or

acc-name: Streett 0
Acceptance: 0 t

We also have similar cases for parity acceptance:

acc-name: parity max odd 0
Acceptance: 0 t

acc-name: parity min even 0
Acceptance: 0 t

Such an all-accepting condition typically occurs when translating safety formulas, or when building monitors. In these specialized cases, it might not really make sense to name the acceptance generalized-Buchi or Streett. For this reason, we also support the name all as a synonym:

acc-name: all
Acceptance: 0 t

Similarly, but less interestingly, generalized-co-Buchi and Rabin all degenerate to f when using 0 acceptance pairs, and this acceptance condition can also be called none. The following acceptance specifications are equivalent and describe automata that will reject all words:

acc-name: generalized-co-Buchi 0
Acceptance: 0 f

acc-name: Rabin 0
Acceptance: 0 f

acc-name: generalized-Rabin 0
Acceptance: 0 f

acc-name: parity max even 0
Acceptance: 0 f

acc-name: parity min odd 0
Acceptance: 0 f

acc-name: none
Acceptance: 0 f

Other acceptance conditions

The HOA format is not restricted to the above classical acceptance conditions. Below are some examples of acceptance conditions for which we define no name for use in acc-name:. Tools should however feel free to introduce new names, should they need them.

A promise automaton generated by the tableau construction of ltl2tgba could be output with:

Acceptance: 3 Inf(!0) & Inf(!1) & Inf(!2)

(Spot actually makes an extra pass at the end of the translation to complement the acceptance sets in order to obtain the more usual generalized Büchi Inf(0)&Inf(1)&Inf(2) acceptance).

The product of a Rabin 2 automaton with a Streett 2 automaton, performed in a very straightforward way, will have an acceptance condition such as the following, where sets 1-4 are used as Rabin pairs, while sets 4-7 are used as Streett pairs.

Acceptance: 8 ((Inf(0)&Fin(1))|(Inf(2)&Fin(3)))&((Fin(4)|Inf(5))&(Fin(6)|Inf(7)))

Although it is not the case in the classical acceptance conditions, acceptance sets can be used multiple times in the same acceptance formula, possibly even with both Fin and Inf primitives. For instance when translating the LTL formula (GF(a) -> GF(b)) & (GF(b) -> GF(c)) into an automaton with Streett-like acceptance, it would make sense to use:

Acceptance: 3 (Fin(0)|Inf(1))&(Fin(1)|Inf(2))

this however does not fit our strict definition of acc-name: Streett 2 (which would be Acceptance 4: (Fin(0)|Inf(1))&(Fin(2)|Inf(3))).

Similarly, translating the LTL formulas GF(a) xor GF(b) could easily be done using an automaton with the following acceptance:

Acceptance: 2 (Fin(0)&Inf(1))|(Inf(0)&Fin(1))

Tool authors implementing support for generic acceptance conditions are encouraged to verify that they correctly deal with such cases, in which the same acceptance sets are used both in Fin and Inf primitives.

Body of the Automaton

The header is separated from the rest of the structure with --BODY--.

States are specified with the following grammar:

body             ::= (state-name edge*)*
// the optional dstring can be used to name the state for
// cosmetic or debugging purposes, as in ltl2dstar's format
state-name       ::= "State:" label? INT STRING? acc-sig?
acc-sig          ::= "{" INT* "}"
edge             ::= label? state-conj acc-sig?
label            ::= "[" label-expr "]"

The INT occurring in the state-name rule is the number of this state. States should be numbered from $0$ to $n-1$, where $n$ is the value given by the States: header item if it is present. If the States: header item is missing, $n-1$ should be assumed to be the highest state number listed either in the automaton body (either when defining a state, or when used as a destination of a transition) or as some initial state.

States may be listed in any order, but should all be listed (i.e., if the header has States: 10 then the body should have ten State: INT statements, with all numbers from 0 to 9). In addition to a number, a state may optionally be given a name (the STRING token) for cosmetic or practical purposes.

The INT* used in acc-sig represent the acceptance sets the state or edge belongs to. Since we use transition-based acceptance, when acc-sig is used on a state to declare membership to some acceptance sets, it is syntactic sugar for the membership of all the outgoing transitions to this set. For instance, State: 0 {1 3} says that all transitions leaving state 0 are in acceptance sets 1 and 3 (this is in addition to any acc-sig that could be specified at edge level).

The state-conj encodes the destination of an edge as a conjunction of state numbers. Non-alternating automata always use a single state number as destination. These conjunctions makes it possible to encode the universal branching of alternating automata, while disjunction is simply encoded as multiple transitions.

If a state has a label, no outgoing edge of this state should have a label: this should be used to represent state-labeled automata. In our semantics, we have to view this as syntactic sugar for all outgoing transitions being labeled by this very same label.

If an edge has a label, all edges of this state should have a label.

If one state has no label, all its edges should have labels, unless there is exactly $2^a$ edges listed, where $a$ is the number of atomic propositions. In the latter case, each edge corresponds to a transition, with the same order as in ltl2dstar. If a transition $t$ is the $i$-th transition of a state (starting with 0), then the label of $t$ can be deduced by interpreting $i$ as a bitset. The label is a set of atomic propositions such that the atomic proposition $j$ is in the set if the $j$-th least significant bit of $i$ is set to 1.

Note that a state may have no outgoing edge if it represents some dead-end.

Examples

Transition-based Rabin acceptance and explicit labels

HOA: v1
States: 2
Start: 0
acc-name: Rabin 1
Acceptance: 2 (Fin(0) & Inf(1))
AP: 2 "a" "b"
--BODY--
State: 0 "a U b"   /* An example of named state */
  [0 & !1] 0 {0}
  [1] 1 {0}
State: 1
  [t] 1 {1}
--END--

State-based Rabin acceptance and implicit labels

Because of implicit labels, the automaton necessarily has to be deterministic and complete.

HOA: v1
States: 3
Start: 0
acc-name: Rabin 1
Acceptance: 2 (Fin(0) & Inf(1))
AP: 2 "a" "b"
--BODY--
State: 0 "a U b" { 0 }
  2  /* !a  & !b */
  0  /*  a  & !b */
  1  /* !a  &  b */
  1  /*  a  &  b */
State: 1 { 1 }
  1 1 1 1       /* four transitions on one line */
State: 2 "sink state" { 0 }
  2 2 2 2
--END--

TGBA with implicit labels

HOA: v1
name: "GFa & GFb"
States: 1
Start: 0
acc-name: generalized-Buchi 2
Acceptance: 2 (Inf(0) & Inf(1))
AP: 2 "a" "b"
--BODY--
State: 0
  0       /* !a  & !b */
  0 {0}   /*  a  & !b */
  0 {1}   /* !a  &  b */
  0 {0 1} /*  a  &  b */
--END--

TGBA with explicit labels

HOA: v1
name: "GFa & GFb"
States: 1
Start: 0
acc-name: generalized-Buchi 2
Acceptance: 2 (Inf(0) & Inf(1))
AP: 2 "a" "b"
--BODY--
State: 0
[!0 & !1] 0
[0 & !1]  0 {0}
[!0 & 1]  0 {1}
[0 & 1]   0 {0 1}
--END--

TGBA with explicit labels using aliases

The following demonstrates the use of aliases to make the output slightly more readable (using @a instead of 0), and to abbreviate commonly used subformulas (@bc instead of 1 & 2).

HOA: v1
name: "GFa & GF(b & c)"
States: 1
Start: 0
acc-name: generalized-Buchi 2
Acceptance: 2 (Inf(0) & Inf(1))
AP: 3 "a" "b" "c"
Alias: @a 0
Alias: @bc 1 & 2
--BODY--
State: 0
[!@a & !@bc] 0
[@a & !@bc]  0 {0}
[!@a & @bc]  0 {1}
[@a & @bc]   0 {0 1}
--END--

Non-deterministic State-based Büchi automaton (à la Wring)

Encoding GFa using state labels requires multiple initial states.

HOA: v1
name: "GFa"
States: 2
Start: 0
Start: 1
acc-name: Buchi
Acceptance: 1 Inf(0)
AP: 1 "a"
--BODY--
State: [0] 0 {0}
  0 1
State: [!0] 1
  0 1
--END--

In this case, the acceptance and labels are carried by the states, so the only information given by the edges lists are the destinations states 0 1.

Note that even if a tool has no support for state labels or multiple initial states, the above automaton could easily be transformed into a transition-based one upon reading. It suffices to add a new initial state connected to all the original initial states, and then to move all labels onto incoming transitions. Acceptance sets can be moved to incoming or (more naturally) to outgoing transitions. For instance the following transition-based Büchi automaton is equivalent to the previous example:

HOA: v1
States: 3
Start: 0
acc-name: Buchi
Acceptance: 1 Inf(0)
AP: 1 "a"
--BODY--
State: 0
 [0] 1
 [!0]  2
State: 1  /* former state 0 */
 [0] 1 {0}
 [!0] 2 {0}
State: 2  /* former state 1 */
 [0] 1
 [!0] 2
--END--

Mixing state-based and transition-based acceptance

Here is a Büchi automaton for GFa | G(b <-> Xa).

HOA: v1
name: "GFa | G(b <-> Xa)"
Start: 0
acc-name: Buchi
Acceptance: 1 Inf(0)
AP: 2 "a" "b"
properties: explicit-labels trans-labels
--BODY--
State: 0
 [t] 1
 [1] 2
 [!1] 3
State: 1 "GFa"
 [0] 1 {0}
 [!0] 1
State: 2 "a & G(b <-> Xa)" {0}
 [0&1] 2
 [0&!1] 3
State: 3 "!a & G(b <-> Xa)" {0}
 [!0&1] 2
 [!0&!1] 3
--END--

In this automaton, marking states 2 and 3 as belonging to set 0 is equivalent to marking all their outgoing transitions as such:

HOA: v1
name: "GFa | G(b <-> Xa)"
Start: 0
acc-name: Buchi
Acceptance: 1 Inf(0)
AP: 2 "a" "b"
properties: explicit-labels trans-labels trans-acc
--BODY--
State: 0
 [t] 1
 [1] 2
 [!1] 3
State: 1 "GFa"
 [0] 1 {0}
 [!0] 1
State: 2 "a & G(b <-> Xa)"
 [0&1] 2 {0}
 [0&!1] 3 {0}
State: 3 "!a & G(b <-> Xa)"
 [!0&1] 2 {0}
 [!0&!1] 3 {0}
--END--

Mixing state-based and transition-based acceptance can also be done in the same state. For instance a state like:

State: 1 {1}
 [0] 2
 [!0] 3 {0}

is equivalent to

State: 1
 [0] 2 {1}
 [!0] 3 {0 1}

Alternating automata

Here is an example of alternating transition-based co-Büchi automaton encoding (Fa & G(b&Xc)) | c, it shows an example of multiple initial states (including a conjunct), and an example of conjunct destination.

HOA: v1
name: "(Fa & G(b&Xc)) | c"
States: 4
Start: 0&2
Start: 3
acc-name: co-Buchi
Acceptance: 1 Fin(0)
AP: 3 "a" "b" "c"
--BODY--
State: 0 "Fa"
[t] 0 {0}
[0] 1
State: 1 "true"
[t] 1
State: 2 "G(b&Xc)"
[1] 2&3
State: 3 "c"
[2] 1
--END--

Formal Semantics of Omega-Automata

The following definition specifies alternating automata with transition-based acceptance. Because of universal branching, the initial states and destination states of transitions are non-empty sets of states (i.e., elements of $2^Q\setminus{\emptyset}$) interpreted as conjunctions. Automata without universal branching use just elements of Q as initial or destination states.

Each omega-automaton described in this format can be seen as an automaton $\langle\AP,Q,m,R,I,\mathit{Acc}\rangle$ with labels on transitions and transition-based acceptance, where:

• $\AP$ is a finite set of atomic propositions. We use $\B(\AP)$ to denote the set of Boolean formulas over $\AP$.
• $Q$ is a finite set of states.
• $m$ is the number of acceptance sets.
• $R\subseteq Q\times\B(\AP)\times 2^{{0,1,\ldots,m-1}}\times(2^Q\setminus{\emptyset})$ is a transition relation. A quadruplet $(s,\ell,M,D)\in R$ represents a transition from $s$ to the conjunction of states in $D$, labeled by a Boolean formula $\ell\in\B(\AP)$, and belonging to the acceptance sets listed in $M\subseteq {0,1,\ldots,m-1}$, where $m$ is the declared number of acceptance sets. Let $s(t)$, $\ell(t)$, $M(t)$, and $D(t)$ denote the corresponding components of a transition $t$.
• $I\subseteq(2^Q\setminus{\emptyset})$ is a set of initial conjunctions of states.
• $\mathit{Acc}$ is a Boolean formula over ${\Fin(i),\Fin(\lnot i),\Inf(i),\Inf(\lnot i)\mid i\in {0,1,\ldots,m-1}}$.

The indices of the acceptance sets appearing in the transitions induce a list of acceptance sets $F=(S_0, ..., S_{m-1})$ where $S_i \subseteq R$ contains those transitions $t$ with $i \in M(t)$, i.e., $S_i = { t\in R \mid i\in M(t) }$.

The automaton is interpreted over infinite words, where letters are subsets of $\AP$. A run over a word $w=a_0 a_1\ldots$ is an infinite labeled directed acyclic graph $(V,E,\lambda)$ such that:

• $V$ is partitioned into $V_0\cup V_1\cup V_2\ldots$ where the sets $V_i$ are disjoint,
• for each edge $e\in E$ there exists $i\ge 0$ such that $e\in V_i\times V_{i+1}$,
• $\lambda:V\to R$ is a labeling function such that ${s(\lambda(x))\mid x\in V_0}\in I$ and, for each $x\in V_i$, $\ell_i(\lambda(x))$ evaluates to True in the valuation assigning True to atomic propositions in $a_i$ and False to all other atomic propositions, and successors of $x$ correspond to the target conjunction of states of the transition $\lambda(x)$, i.e., $D(\lambda(x))={s(\lambda(y))\mid (x,y)\in E}$. We say that the transition $\lambda(x)$ is applied to $x$.

Runs of automata without universal branching are simply infinite linear sequences of nodes.

A run is accepting if each branch of the run (i.e., each infinite oriented path starting in $V_0$) satisfies the acceptance condition $\mathit{Acc}$, where a branch satisfies

• $\Fin(i)$ if all transitions in $S_i$ are applied only to finitely many nodes on the branch.
• $\Fin(\lnot i)$ if all transitions outside $S_i$ are applied only to finitely many nodes on the branch.
• $\Inf(i)$ if some transition in $S_i$ is applied to infinitely many nodes on the branch.
• $\Inf(\lnot i)$ if some transition outside $S_i$ is applied to infinitely many nodes on the branch.

The automaton recognizes the language of all words for which there exists an accepting run of the automaton.

As mentioned above, the format also supports labels on states. This is formally seen as an abbreviation for the situation where all transitions leaving the state have this label. The format even admits automata combining states with labels, unlabeled states with labels on all outgoing transitions, and unlabeled states with implicitly labeled transitions.

The format supports both state-based and transition-based acceptance sets. In our transition-based semantics, as illustrated by our previous examples, marking a state as belonging to some acceptance set with (for instance) State: 0 {1 3} is syntactic sugar for marking all the outgoing transitions of state 0 as belonging to acceptance sets 1 and 3. This is especially important if one combines states and transitions in an acceptance set and use negation of this acceptance sets in the acceptance condition.

Semantics for Pure State-Based Acceptance

In tools that manipulate only state-based acceptance, acceptance will only be used for states, and therefore the transition-based semantics are inconvenient. For these tools, one can consider the following semantics.

The omega-automata are represented by a tuple $\langle\AP,Q,R,I,F,\mathit{Acc}\rangle$, where:

• $\AP$ is a finite set of atomic propositions.
• $Q$ is a finite set of states.
• $R\subseteq Q\times\B(\AP)\times(2^Q\setminus{\emptyset})$ is a transition relation,
• $I\subseteq(2^Q\setminus{\emptyset})$ is a set of initial conjunctions of states,
• $F={S_0,S_1,\ldots,S_{m-1}}$ is a finite set of acceptance sets. Each acceptance set $S_i\subseteq Q$ is a subset of states.
• $\mathit{Acc}$ is an acceptance condition.

In contrast to the automaton with transition-based acceptance, the acceptance sets $S_0, ..., S_{m-1}$ appear directly in the definition, with each $S_i$ being a subset of states. The acceptance condition is still a formula defined over $\Fin(i)$, $\Fin(\lnot i)$, $\Inf(i)$, or $\Inf(\lnot i)$, but this time each $i$ refers to the set $S_i$ of states that must occur infinitely or finitely often on each branch of an accepting run, and the complement operation $\lnot$ should be done with respect to $Q$ instead of $R$.

An automaton with state-based acceptance can be trivially converted to transition-based acceptance by shifting the acceptance set membership from each state to its outgoing transitions, and the two semantics are compatible in the sense that the two automata would recognize the same language. If the automaton has no dead states (i.e., states without successor), the result of such transformation can easily be reversed.

The two semantics disagree slightly on the handling of dead states. The state-based semantics allow dead states to appear in acceptance sets, while there is no way to do that with transition-based acceptance. This difference is inconsequential: a dead state is never going to contribute anything useful the recognized language.

Authors

This document has been put together by the following people:

• Alexandre Duret-Lutz
• František Blahoudek
• Jan Křetínský
• Jan Strejček
• Joachim Klein