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BF Interpreter

Language Reference

The brainfuck interpreter knows the following instructions:

Cmd Effect
+ Increases element under pointer
- Decrases element under pointer
> Increases pointer
< Decreases pointer
[ Starts loop, flag under pointer
] Indicates end of loop
. Outputs ASCII code under pointer
, Reads char and stores ASCII under ptr

Hello World

$ cat ../examples/hello_world.bf
++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.
$ ./bin/bf ../examples/hello_world.bf
Hello World!

2 + 2 = 4

$ cat ../examples/add.bf
,>,<[->+<]>>++++++[ - >++++++++< ]>[ - << - >> ]<< .
$ ./bin/bf ../examples/add.bf
22
4

Program purpose: Reads two numbers as ASCII characters, adds them (interpreting them as digit characters), and outputs the sum as an ASCII character.

Breakdown:

,>,< - Read two input characters into cells 0 and 1, then move back to cell 0

[->+<] - Loop that transfers the value in cell 0 to cell 1:

Decrement cell 0
Move right and increment cell 1
Move left and repeat until cell 0 is zero
Result: cell 0 = 0, cell 1 contains the sum
>> - Move to cell 2

++++++[ - >++++++++< ] - Multiply 6 × 8 = 48 in cell 2 (this is the ASCII offset for character '0'):

Set cell 2 to 6
Loop: decrement cell 2, move right, add 8 to cell 3, move left
After loop: cell 2 = 0, cell 3 = 48
>[ - << - >> ] - Subtract 48 from cell 1 (the sum):

Move to cell 3 (which holds 48)
While cell 3 > 0: decrement cell 3, move left twice, decrement cell 1, move right twice
Result: cell 1 now contains the numeric difference
<< . - Move back to cell 1 and output the character

Example: If you input '2' and '2' (ASCII 50), it calculates (50 + 50) - 48 = 52, which is ASCII '4'

Build and test

$ make
gcc -std=c99 -Wall -Wextra -O2 -Iinclude src/bf.c -o bin/bf
$ make test
Running tests...
=== Brainfuck Test Suite ===

Test 1: Reading valid file (hello_world.bf)
✓ PASS
Test 2: Output single character (+++++++++.)
✓ PASS (output: ASCII 9)
Test 3: Pointer movement (>+.)
✓ PASS (output: ASCII 1)
Test 4: Multiple increments
✓ PASS (output: ASCII 95, 96)
Test 5: Simple loop (++[>+<-]>.)
✓ PASS (output: ASCII 2)
Test 6: Loop multiplication (+++[>++<-]>.)
✓ PASS (output: ASCII 6)
Test 7: Nested loops (++[>++[>++<-]<-]>>.)
✓ PASS (output: ASCII 8)

=== Bracket Mismatch Tests ===
Test 8: Missing closing bracket ([++)
✓ PASS (exit code: 1)
Test 9: Missing opening bracket (++])
✓ PASS (exit code: 1)
Test 10: Multiple missing closing brackets ([[++)
✓ PASS (exit code: 2)
Test 11: Multiple missing opening brackets (++]])
✓ PASS (exit code: 2)
Test 12: Mixed mismatches ([[])
✓ PASS (exit code: 1)
Test 13: Properly balanced brackets ([]++.)
✓ PASS (exit code: 0, output: ASCII 2)

=== Addition Program Tests ===
Test 14: Addition (2+2=4)
✓ PASS (output: '4')