Arthur Whitney's interpreter, decompressed

gistfile1.c

typedef char C;

typedef long I;

typedef struct a {
  I t,r,d[3],p[2];
}* A;

#define P printf

#define R return

#define V1(f) A f(w)A w;

#define V2(f) A f(a,w)A a,w;

#define DO(n,x) {I i=0,_n=(n);for(;i<_n;++i){x;}}

I *ma(n) {
  R(I*)malloc(n*4);
}

mv(d,s,n) I *d,*s; {
  DO(n,d[i]=s[i]);
}

tr(r,d) I *d; {
  I z=1;
  DO(r,z=z*d[i]);
  R z;
}

A ga(t,r,d) I *d; {
  A z=(A)ma(5+tr(r,d));
  z->t=t,z->r=r,mv(z->d,d,r);
  R z;
}

V1(iota) {
  I n=*w->p;
  A z=ga(0,1,&n);
  DO(n,z->p[i]=i);
  R z;
}

V2(plus) {
  I r=w->r,*d=w->d,n=tr(r,d);
  A z=ga(0,r,d);
  DO(n,z->p[i]=a->p[i]+w->p[i]);
  R z;
}

V2(from) {
  I r=w->r-1,*d=w->d+1,n=tr(r,d);
  A z=ga(w->t,r,d);
  mv(z->p,w->p+(n**a->p),n);
  R z;
}

V1(box) {
  A z=ga(1,0,0);
  *z->p=(I)w;
  R z;
}

V2(cat) {
  I an=tr(a->r,a->d),wn=tr(w->r,w->d),n=an+wn;
  A z=ga(w->t,1,&n);
  mv(z->p,a->p,an);
  mv(z->p+an,w->p,wn);
  R z;
}

V2(find) {
}

V2(rsh) {
  I r=a->r?*a->d:1,n=tr(r,a->p),wn=tr(w->r,w->d);
  A z=ga(w->t,r,a->p);
  mv(z->p,w->p,wn=n>wn?wn:n);
  if(n-=wn)
    mv(z->p+wn,z->p,n);
  R z;
}

V1(sha) {
  A z=ga(0,1,&w->r);
  mv(z->p,w->d,w->r);
  R z;
}

V1(id) {
  R w;
}

V1(size) {
  A z=ga(0,0,0);
  *z->p=w->r ? *w->d : 1;
  R z;
}

pi(i) {
  P("%d ",i);
}

nl() {
  P("\n");
}

pr(w) A w; {
  I r=w->r,*d=w->d,n=tr(r,d);
  DO(r,pi(d[i]));
  nl();
  if(w->t)
    DO(n,P("< ");pr(w->p[i]))
  else
    DO(n,pi(w->p[i]));nl();
}

C vt[]="+{~<#,";

A(*vd[])()={0,plus,from,find,0,rsh,cat},
 (*vm[])()={0,id,size,iota,box,sha,0};

I st[26];

qp(a) { R a >= 'a' && a<='z'; }

qv(a) { R a<'a'; }

A ex(e) I *e; {
  I a=*e;
  if(qp(a)) {
    if(e[1]=='=')
      R st[a-'a']=ex(e+2);
    a=st[ a-'a'];
  }
  R qv(a) ? (*vm[a])(ex(e+1)) : e[1] ? (*vd[e[1]])(a,ex(e+2)) : (A)a;
}

noun(c) {
  A z;
  if(c<'0'||c>'9')
    R 0;
  z=ga(0,0,0);
  *z->p=c-'0';
  R z;
}

verb(c) {
  I i=0;
  for(;vt[i];)
    if(vt[i++]==c)
      R i;
  R 0;
}

I *wd(s) C *s; {
  I a,n=strlen(s),*e=ma(n+1);
  C c;
  DO(n,e[i]=(a=noun(c=s[i]))?a:(a=verb(c))?a:c);
  e[n]=0;
  R e;
}

main() {
  C s[99];
  while(gets(s))
    pr(ex(wd(s)));
}

Link to the original: http://www.jsoftware.com/jwiki/Essays/Incunabulum with commentary at https://news.ycombinator.com/item?id=8534989