Add reverse Polish notation

lib/rpn.c

@@ -0,0 +1,29 @@
+// Licensed under the MIT License.
+
+#include "rpn.h"
+#include "rpn_operator.h"
+
+int rpn_evaluate(String tokens[], size_t length)
+{
+    size_t sp = 0;
+    int* stack = malloc(length * sizeof * stack);
+
+    for (size_t i = 0; i < length; i++)
+    {
+        if (tokens[i][1] != '\0' || isdigit(tokens[i][0]))
+        {
+            stack[sp] = atoi(tokens[i]);
+            sp++;
+
+            continue;
+        }
+
+        stack[sp - 2] = rpn_operator_apply(
+            tokens[i][0], 
+            stack[sp - 2], 
+            stack[sp - 1]);
+        sp--;
+    }
+
+    return stack[0];
+}

lib/rpn.h

@@ -0,0 +1,5 @@
+// Licensed under the MIT License.
+
+#include "euler.h"
+
+int rpn_evaluate(String tokens[], size_t length);

lib/rpn_operator.c

@@ -0,0 +1,16 @@
+// Licensed under the MIT License.
+
+#include "rpn_operator.h"
+
+int rpn_operator_apply(RpnOperator value, int left, int right)
+{
+    switch (value)
+    {
+        case RPN_OPERATOR_ADD: return left + right;
+        case RPN_OPERATOR_SUBTRACT: return left - right;
+        case RPN_OPERATOR_MULTIPLY: return left * right;
+        case RPN_OPERATOR_DIVIDE: return left / right;
+    }
+
+    return 0;
+}

lib/rpn_operator.h

@@ -0,0 +1,13 @@
+// Licensed under the MIT License.
+
+enum RpnOperator
+{
+    RPN_OPERATOR_ADD = '+',
+    RPN_OPERATOR_SUBTRACT = '-',
+    RPN_OPERATOR_MULTIPLY = '*',
+    RPN_OPERATOR_DIVIDE = '/'
+};
+
+typedef enum RpnOperator RpnOperator;
+
+int rpn_operator_apply(RpnOperator value, int left, int right);