now version controlled.

1 files changed, 51 insertions, 0 deletions

diff --git a/rpn.c b/rpn.c
new file mode 100644
index 0000000..010afde
--- /dev/null
+++ b/rpn.c
@@ -0,0 +1,51 @@
+#include <u.h>
+#include <libc.h>
+#include <ctype.h>
+#include <bio.h>
+
+int stk[256], sp;
+Biobuf *bin;
+
+void
+push(int n)
+{
+	if(sp >= sizeof stk)
+		sp = sizeof(stk)-1;
+	fprint(2, "push [%d] %d\n", sp, n);
+	stk[sp++] = n;
+}
+
+int
+pop(void)
+{
+	if(sp <= 0)
+		sp = 1;
+	fprint(2, "pop [%d] %d\n", sp-1, stk[sp-1]);
+	return stk[--sp];
+}
+
+void
+main()
+{
+	int x;
+	char c;
+
+	bin = Bfdopen(0, OREAD);
+	if(bin == nil)
+		sysfatal("Bfdopen: %r");
+	while((c = Bgetc(bin)) != Beof){
+		x = 0;
+		switch(c){
+		case '+': x = pop() + pop(); break;
+		case '*': x = pop() * pop(); break;
+		default:
+			while(isdigit(c)){
+				x = x*10 + c-'0';
+				c = Bgetc(bin);
+			}
+		}
+		push(x);
+	}
+	print("%d\n", pop());
+	exits(nil);
+}