From bfce8f0d0d828209ec0bec71371ee94a7ad62d3e Mon Sep 17 00:00:00 2001
From: Michael Hunteman <michael@huntm.net>
Date: Tue, 4 Jul 2023 17:03:53 -0500
Subject: Initial commit

---
 four/calc.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 four/calc.c

(limited to 'four/calc.c')

diff --git a/four/calc.c b/four/calc.c
new file mode 100644
index 0000000..cb484ad
--- /dev/null
+++ b/four/calc.c
@@ -0,0 +1,99 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+
+#define MAXOP 100
+#define NUMBER '0'
+
+int getop(char[]);
+void push(double);
+double pop(void);
+double peek();
+void ungets();
+
+int
+main()
+{
+	int type, var;
+	double op1, op2, r;
+	char s[MAXOP];
+	char v[26];
+
+	/* reverse Polish calculator */
+	while ((type = getop(s)) != EOF) {
+		switch (type) {
+		case NUMBER:
+			push(atof(s));
+			break;
+		case '+':
+			push(pop() + pop());
+			break;
+		case '*':
+			push(pop() * pop());
+			break;
+		/* to guarantee the right order, the first value must be stored */
+		case '-':
+			op2 = pop();
+			push(pop() - op2);
+			break;
+		case '/':
+			op2 = pop();
+			if (op2 != 0.0) {
+				push(pop() / op2);
+			} else {
+				printf("error: zero divisor\n");
+			}
+			break;
+		case '%':
+			op2 = pop();
+			op1 = pop();
+			if (op1 >= 0.0 && op2 >= 0.0) {
+				push((int)op1 % (int)op2);
+			} else if (op1 < 0.0 || op2 < 0.0) {
+				push(0);
+			} else {
+				printf("error: zero divisor\n");
+			}
+			break;
+		/* print without popping */
+		case 'p':
+			printf("%f\n", peek());
+			break;
+		/* duplicate */
+		case 'd':
+			op2 = peek();
+			push(op2);
+			break;
+		/* swap top two elements */
+		case 's':
+			op2 = pop();
+			op1 = pop();
+			push(op2);
+			push(op1);
+			break;
+		case '=':
+			pop();
+			if (var > 'a' && var < 'z') {
+				v[var - 'a'] = pop();
+			}
+		case '\n':
+			r = pop();
+			printf("\t%.8g\n", r);
+			break;
+		case 'u':
+			ungets();
+			break;
+		default:
+			if (type == 'R') {
+				push(r);
+			} else if (type >= 'a' && type <= 'z') {
+				push(v[type - 'a']);
+			} else {
+				printf("error: unknown command %s\n", s);
+			}
+			break;
+		}
+		var = type;
+	}
+	return 0;
+}
-- 
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