Challenge #10: 👩‍💻 The elfish assembler

The elf programmers are creating a small magical assembler to control the machines in Santa Claus's workshop.

To assist them, we will implement a simple interpreter that supports the following magical instructions:

• MOV x y: Copies the value x (which can be a number or the content of a register) into register y
• INC x: Increments the content of register x by 1
• DEC x: Decrements the content of register x by 1
• JMP x y: If the value in register x is 0, then jumps to the instruction at index y and continues executing the program from there.

Expected behavior:

• If an attempt is made to access, increment, or decrement a register that has not been initialized, the default value 0 will be used.
• The jump with JMP is absolute and goes to the exact index indicated by y.
• Upon completion, the program should return the content of register A. If A did not have a defined value, it returns undefined.

const instructions = [
  'MOV -1 C', // copies -1 to register 'C',
  'INC C', // increments the value of register 'C'
  'JMP C 1', // jumps to the instruction at index 1 if 'C' is 0
  'MOV C A', // copies register 'C' to register 'A',
  'INC A' // increments the value of register 'A'
]

compile(instructions) // -> 2

/**
 Step-by-step execution:
 0: MOV -1 C -> The register C receives the value -1
 1: INC C    -> The register C becomes 0
 2: JMP C 1  -> C is 0, jumps to the instruction at index 1
 1: INC C    -> The register C becomes 1
 2: JMP C 1  -> C is 1, the instruction is ignored
 3: MOV C A  -> Copies register C to A. Now A is 1
 4: INC A    -> The register A becomes 2
 */

Note: Registers that have not been previously initialized are initialized to 0.