+\ ---- Garbage Collection ---- {{{
+
+variable gc-enabled
+false gc-enabled !
+
+variable gc-stack-depth
+
+: enable-gc
+ depth gc-stack-depth !
+ true gc-enabled ! ;
+
+: disable-gc
+ false gc-enabled ! ;
+
+: gc-enabled?
+ gc-enabled @ ;
+
+: pairlike? ( obj -- obj bool )
+ pair-type istype? if true exit then
+ string-type istype? if true exit then
+ symbol-type istype? if true exit then
+ compound-proc-type istype? if true exit then
+
+ false
+;
+
+: pairlike-marked? ( obj -- obj bool )
+ over nextfrees + @ 0=
+;
+
+: mark-pairlike ( obj -- obj )
+ over nextfrees + 0 swap !
+;
+
+: gc-unmark ( -- )
+ scheme-memsize 0 do
+ 1 nextfrees i + !
+ loop
+;
+
+: gc-mark-obj ( obj -- )
+
+ pairlike? invert if 2drop exit then
+ pairlike-marked? if 2drop exit then
+
+ mark-pairlike
+
+ drop pair-type 2dup
+
+ car recurse
+ cdr recurse
+;
+
+: gc-sweep
+ scheme-memsize nextfree !
+ 0 scheme-memsize 1- do
+ nextfrees i + @ 0<> if
+ nextfree @ nextfrees i + !
+ i nextfree !
+ then
+ -1 +loop
+;
+
+\ Following a GC, this gives the amount of free memory
+: gc-count-marked
+ 0
+ scheme-memsize 0 do
+ nextfrees i + @ 0= if 1+ then
+ loop
+;
+
+\ Debugging word - helps spot memory that is retained
+: gc-zero-unmarked
+ scheme-memsize 0 do
+ nextfrees i + @ 0<> if
+ 0 car-cells i + !
+ 0 cdr-cells i + !
+ then
+ loop
+;
+
+:noname
+ \ ." GC! "
+
+ gc-unmark
+
+ symbol-table obj@ gc-mark-obj
+ global-env obj@ gc-mark-obj
+
+ depth gc-stack-depth @ do
+ PSP0 i + 1 + @
+ PSP0 i + 2 + @
+
+ gc-mark-obj
+ 2 +loop
+
+ gc-sweep
+
+ \ ." (" gc-count-marked . ." pairs marked as used.)" cr
+; is collect-garbage
+
+\ }}}
+