# Data Structures¶

CirKit (and RevKit) provide the analysis and manipulation of several data structures. These data structures are explained in this section. As described above, instances of these data structures are stored in individual stores. Not all data structures are available in both CirKit and RevKit. The following table gives an overview over the existing data structures, their access option for the store, and their availability in CirKit and RevKit.

Data structure Access option CirKit RevKit
Truth table -t --tt
Expression -e --expr
And-inverter graph (AIG) -a --aig
Majority-inverter graph (MIG) -m --mig
XOR majority graph (XMG) -x --xmg
Binary decision diagram (BDD) -b --bdd
Reversible circuit -c --circuit
Reversible specification -s --spec
BDD of a characteristic reversible function (RCBDD) -r --rcbdd

## Truth tables¶

Truth tables are bitstrings of length $2^k$ and represent Boolean functions over $k$ variables. The most significant bit is the first bit in the bitstring. For example, to load a truth table that represents the AND function $a land b$, type tt 1000. We assume that the least significant variable is $a$, then $b$, then $c$, and so on. The truth tables for $a$, $b$, and $c$ are therefore 10, 1100, and 11110000. In order to meet size requirements, truth tables can be extended. If, e.g., 1011 is the current truth table in store, the command tt -e 3 extends the truth table to be defined over 3 variables, yielding 10111011.

On can convert truth tables into AIGs using convert --tt_to_aig. This will construct an AIG in a very naïve way by constructing each minterm explicitly and then ORing them all. Conversely, one can obtain truth tables from AIGs using simulation. For this purpose use the command simulate with the flags -a to simulate from AIGs, -t to simulate to truth tables, and -n to store the simulatuion results. The following example illustrates the usage for the c17 benchmark from the ISCAS benchmark suite. It also employs NPN canonization on the resulting truth tables using the command npn.

### Example¶

cirkit> read_aiger c17.aig
cirkit> simulate -atn
[i] G16 : 1011100011111000101110001111100010111000111110001011100011111000 (B8F8B8F8B8F8B8F8)
[i] G17 : 0011001111111111001100001111000000110011111111110011000011110000 (33FF30F033FF30F0)
[i] runtime: 0.00 secs
cirkit> store -t
[i] truth tables in store:
0: 1011100011111000101110001111100010111000111110001011100011111000
*  1: 0011001111111111001100001111000000110011111111110011000011110000
cirkit> current -t 0
cirkit> npn -t --approach 0
[i] run-time: 0.89 secs
[i] NPN class for 1011100011111000101110001111100010111000111110001011100011111000 is 0000000000000000000000000000111111110000111100001111111111111111
[i] - phase: 1001010 perm: 5 4 1 3 0 2
cirkit> current -t 1
cirkit> npn -t --approach 0
[i] run-time: 0.89 secs
[i] NPN class for 0011001111111111001100001111000000110011111111110011000011110000 is 0000000000000000000000001111111100001111000011110000111111111111
[i] - phase: 1001010 perm: 5 0 1 2 4 3


The current truth table in the store corresponds to the last output of the AIG. Notice that truth table simulation only scales for AIGs with a small number of inputs. One can obtain a truth table from an expression using convert --expr_to_tt or its alias expr > tt.

Some truth table related commands are:

Command Description
tt Load and modify truth tables
npn NPN canonization (exact and heuristic)
convert --tt_to_aig, Alias: tt > aig Convert truth table to AIG
convert --expr_to_tt, Alias: expr > tt Convert expression to truth table
simulate -atn Simulates AIGs as truth table and stores them
simulate -mtn Simulates MIGs as truth table and stores them

## Expressions¶

Expressions provide an easy way to enter Boolean functions into CirKit. The expressions are multi-level expressions that can contain constants (0, 1), Boolean variables (a, b, c, ...), inversion (!), binary AND (()), binary OR ({}), binary XOR ([]), and ternary MAJ (<>). The whole syntax is given as follows:

expr ::= 0 | 1 | var | ! expr | ( expr expr ) | { expr expr } | [ expr expr ] | < expr expr expr >
var ::= a | b | c | ...


Note that a always corresponds to the least significant bit, b to the second least significant bit, and so on. Expressions can be loaded into its store (access flag -e) using the command expr. Expressions can be used as starting point to create truth tables (expr > tt) or binary decision diagrams (expr > bdd) for simple functions and avoid to create a file. The following example illustrates its usage.

### Example¶

cirkit> expr (ab)
cirkit> expr > tt
cirkit> print -t
1000
cirkit> expr !{ac}
cirkit> expr > tt
cirkit> print -t
00000101
cirkit> expr {{(ab)(ac)}(bc)}
cirkit> expr > tt
cirkit> print -t
11101000
cirkit> expr
cirkit> expr > tt
cirkit> print -t
11101000


Note that when loading !{ac} the resulting truth table represents a 3-variable Boolean function which does not functially depend on the value for b. The last two examples are both Boolean expressions for MAJ, the majority-of-three function.

Some commands related to expressions are:

Command Description
expr Load expressions
convert --expr_to_tt, Alias: expr > tt Convert expression to truth table
convert --expr_to_bdd, Alias: expr > bdd Convert expression to binary decision diagram

## And-inverter Graphs (AIG)¶

There are several ways to load an AIG into CirKit. If the AIG is represented as AIGER file with extension *.aig if in binary format and *.aag if in ASCII format, one can use the command read_aiger to parse the file and create an AIG in the store. If already an AIG is in the store, it will be overriden, unless one calls read_aiger -n. If the AIG is represented in Verilog such that ABC’s command %read is able to parse it, one can use read_verilog -a to read the Verilog file, convert it into an AIG and put it into the store. Also BENCH files can be read into AIGs with the command read_bench. The command tt > aig allows to translate the current truth table into an AIG. Internally, ABC’s API will be used for that purpose and the AIG is optimized using dc2.

This summary lists commands to load AIGs into CirKit:

Command Description
read_aiger Read AIG from binary or ASCII AIGER file
read_verilog -a Read AIG from Verilog file (using ABC’s %read command)
read_bench Read AIG from BENCH file
convert -tt_to_aig, Alias: tt > aig Convert truth table to AIG

### Manipulating the AIG¶

ABC is a powerful tool for AIG optimization and manipulation and using the tight integration of CirKit with ABC using the command abc, it is very easy to use ABC to optimize AIGs in CirKit directly. Hence, few commands in CirKit exist to perform AIG optimization, but mainly utility commands.

This list some commands in CirKit to manipulate an AIG:

Command Description
cone Extracts AIG based on output cones
cuts -a Performs cut enumeration
propagate Propagates constants through inputs
rename Renames inputs and outputs
shuffle -a Shuffles I/O of an AIG
strash Strashes an AIG (removes dangling nodes)
unate Computes unateness properties and functional dependencies of the AIG

### Writing an AIG¶

AIGs can be written into AIGER files using write_aiger or into Verilog files using write_verilog -a.

This summary lists commands to write AIGs:

Command Description
write_aiger Write AIG to ASCII file (ASCII if suffix is .aag)
write_verilog -a Write AIG to Verilog file