## EECS 321 Programming Languages: Homework 7

### Two-Space Collection

Implement a two-space copying collector, as described in lecture 12. Use four state variables in your collector: the allocation pointer, the active semi-space, and two pointers that you use only during collection, one pointing to the left and one to the right side of the queue for copying the data. (It is possible to do this with only three state variables, and it might be possible with only two, but I recommend four.)

You may assume that your heap size is a multiple of 2 and contains at least 12 cells.

### Quality

You collector must be able to run these three programs without running out of space:

```#lang plai/gc2/mutator
(allocator-setup "gc.rkt" 400)
(define (count-down n)
(cond
[(zero? n) (count-down 20)]
[else (count-down (- n 1))]))
(count-down 0)```
```
#lang plai/gc2/mutator
(allocator-setup "gc.rkt" 400)
(define (mk-list n)
(cond
[(zero? n) '()]
[else (cons n (mk-list (- n 1)))]))
(define (forever)
(mk-list 10)
(forever))
(forever)```
```
#lang plai/gc2/mutator
(allocator-setup "gc.rkt" 400)
(define (proc-lst n)
(cond
[(zero? n) (lambda () 0)]
[else (let ([n1 (proc-lst (- n 1))])
(lambda () (+ (n1) n)))]))
(define (forever)
((proc-lst 10))
(forever))
(forever)```

### PLAI Allocation: a No No

Your garbage collector may not do any PLAI-level allocation (except stack space). This means that the functions cons, vector, map, and list are off limits. Similarly, the constructors generated by define-type are off limits (e.g., fun, id, num, add, and sub from earlier assignments). Closures with non-empty environments also require allocation, so you can only define functions at the top-level of the file (no lambda expressions in nested scopes).

The only exception: you may call get-root-set, which allocates at the PLAI level.

You may, of course, use functions that allocate in your tests to build up example heaps.

If you are unsure what primitives allocate memory, ask.

### plai/gc2 vs plai

You must use #lang plai/gc2/collector and #lang plai/gc2/mutator. The documentation explains what the primitives are and what functions you have to implement, but be careful that you read the versions with a gc2 in the name, not plai/mutator or plai/collector.

To find the right docs, open up the docs and click the up link at the top of the page until you get to a page named “Racket Documentation”. From there, click “Programming Languages: Application and Interpretation” and then either “GC Collector Language, 2” or “GC Mutator Language, 2”.

### Hint

If you insert calls to read in the middle of your collector, DrRacket will pause until you type something and hit return in the interactions window (but will still update the heap GUI). This allows you to see what is happening at various stages in the process of collection (or allocation) and thus can help you understand what is going on.

### Even Better Hint

Only use the previous hint to help you formulate test cases. The edit/debug/fix cycle leads to much pain that the edit/debug/write-test-cases/fix cycle avoids. (And, of course, only enter into this cycle with some baseline set of test cases that you've written up front.)

### Template

Below is a list of the functions required to implement a garbage collector, along with bogus implementations. See the docs for a better description of each of these functions (but beware that you find the gc2 versions of these functions.

#lang plai/gc2/collector

(define (init-allocator) (error 'this-collector-always-fails))
(define (gc:deref fl-loc) (error 'gc:deref))
(define (gc:alloc-flat fv) (error 'gc:alloc-flat))
(define (gc:cons hd tl) (error 'gc:cons))
(define (gc:first pr-loc) (error 'gc:first))
(define (gc:rest pr-loc) (error 'gc:rest))
(define (gc:flat? loc) (error 'gc:flat?))
(define (gc:cons? loc) (error 'gc:cons?))
(define (gc:set-first! pr-ptr new) (error 'gc:set-first!))
(define (gc:set-rest! pr-ptr new) (error 'gc:set-rest!))
(define (gc:closure code-ptr free-vars) (error 'gc:closure))
(define (gc:closure-code-ptr loc) (error 'gc:closure-code-ptr))
(define (gc:closure-env-ref loc i) (error 'gc:closure-env-ref))
(define (gc:closure? loc) (error 'gc:closure?))

### Handin

Hand in your garbage collector. It should begin with the line:

`#lang plai/gc2/collector`

 Last update: Sunday, February 19th, 2017stamourv@eecs.northwestern.edu