## Dustin Ingram

Writing — Speaking — Github — Twitter# Z-Machines and One-Eyed Robots

May 02 2013## The Problems

Today, John Graham-Cumming posted two problems he had to solve for his interview at Oxford. Both were relatively simple, but intriguing, and still stand as relevant interview questions thirty years later. I decided to implement the solutions to both problems in Python for a little practice.

### Z-Machines

This problem reminded me of a time I spend briefly programming numerical control machines, mostly because I’ve never had to program anything else using a jumping instruction set (besides a little BASIC on my TI-83).

The goal is to add two values in register `0`

and register `1`

and put the
result in register `2`

.

Here was my initial attempt (in the instruction set language):

```
0 Z2
1 Z3
2 Z4
3 I3
4 I2
5 J0,3 -> 3
6 I4
7 I2
8 J1,4 -> 6
```

This creates two “counter” registers (instructions 1 and 2), and essentially
has two loops (lines 3-5 and 6-8), which simultaneously increment the sum at
register `2`

and the value at the respective “counter” register. When both
counter registers match the registers of the original terms (`0`

and `1`

), the
result can be found in register `2`

.

John then asks when this program would fail. In this case, if either of the two initial values are zero, the first loop will continue incrementing the counter, but the result will never be zero, and the program will never terminate.

After a little thought, I was able to implement a second solution that can support zeros as well:

```
0 Z2
1 Z3
2 Z4
3 Z5
4 Z6
5 I6
6 J0,3 -> 8
7 J5,6 -> 11
8 I3
9 I2
10 J0,3 -> 8
11 J1,4 -> 13
12 J5,6 -> 16
13 I4
14 I2
15 J1,4 -> 13
```

This program creates the same counter registers as the original (lines 1 and 2). It creates two extra registers, and makes their values 0 and 1 (lines 3-5). These will be used to force a jump.

Line 6 checks if the first term is zero. If it isn’t, it goes into the loop which increments the sum (lines 8-10). If it is, the next instruction (line 7) forces a jump to line 11, essentially skipping the first loop.

Line 11 check if the second term is zero. Again, if it isn’t, it goes into a loop (lines 13-15), if it is, it forces a jump to line 16, which is the end of the program.

Still, the program might fail if the initial terms are negative, or if they are floating-point values. Since we only have instructions to increment integers and check for equality, I don’t see a way to support these.

My thoughts: The tediousness of determining instruction indices for the jumps makes me glad I live in the future.

Here’s the whole representation of the problem in Python:

```
#!/usr/bin/python
def z(a, c):
r[a] = 0
return c + 1
def i(a, c):
r[a] += 1
return c + 1
def j(a, c):
if r[a[0]] != r[a[1]]:
return a[2]
return c + 1
# Initialize an empty register
r = [None for _ in range(10)]
# Set the initial values
r[0] = 3
r[1] = 4
# Instruction set
b = [(z,2), (z,3), (z,4), (z,5), (z,6), (i,6),
(j,(0,3,8)), (j,(5, 6, 11)),
(i, 3), (i, 2), (j,(0,3,8)),
(j,(1,4,13)), (j,(5, 6, 17)),
(i, 4), (i, 2), (j,(1,4,13))]
# Initialize the counter to the first instruction
counter = 0
# Start through the instruction set
while counter < len(b):
f, n = b[counter]
counter = f(n, counter)
# Print the register at the end of the simulation
print(r)
# Print the resulting computation
print("%d + %d = %d" % (r[0], r[1], r[2]))
```

The resulting output:

```
[3, 4, 7, 3, 4, 0, 1, None, None, None]
3 + 4 = 7
```

Here’s the whole script as a gist.

### One-Eyed Robot

This problem is essentially a sorting problem, with a twist: you can only examine an individual value once.

The trick here is that since there are only three “values” (red, green, and blue), we can sort the buckets by always moving one of them to the front of the list, one of them to the end, and letting the third end up in the middle.

By maintaining a pointer to the “end” of the red values on the left, and a pointer to the “beginning” of the blue values on the right, we always know where to put a ball given it’s color.

The main algorithm is as follows, starting at the first bucket:

- If the ball is red, swap it with the first ball we know isn’t red, and go to the next bucket. Since we already checked all the balls before this point, we know that we’re swapping with a green ball;
- If the ball is blue, swap it with the last ball we know isn’t blue. Since this ball comes from the portion of the list we haven’t checked yet, stay at the current bucket, and examine the new ball;
- If the ball is green leave it where it is, and move on to the next bucket.

The key is to stop the algorithm once the bucket that we have to check next is one that we’ve already put a (blue) ball into.

Here’s the whole representation of the problem in Python:

```
#!/usr/bin/python
import random
class ball:
def __init__(self, color):
self.color = color
self.examined = 0
def look(self):
self.examined += 1
return self.color
class buckets:
def __init__(self, size=15):
self.line = [ball(random.choice(['r', 'g', 'b'])) for _ in range(size)]
self.i = 0
self.r = 0
self.b = size-1
def swap(self, i, j):
self.line[i], self.line[j] = self.line[j], self.line[i]
def check(self):
c = self.line[self.i].look()
if c == 'r':
self.swap(self.i, self.r)
self.r += 1
self.i += 1
elif c == 'b':
self.swap(self.i, self.b)
self.b -= 1
else:
self.i += 1
def solve(self):
while self.i <= self.b:
self.check()
b = buckets()
print([x.color for x in b.line])
b.solve()
print([x.color for x in b.line])
print([x.examined for x in b.line])
```

The resulting output:

```
['r', 'r', 'b', 'g', 'b', 'r', 'r', 'g', 'g', 'g', 'b', 'r', 'g', 'g', 'b']
['r', 'r', 'r', 'r', 'r', 'g', 'g', 'g', 'g', 'g', 'g', 'b', 'b', 'b', 'b']
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
```