commit 465ee17549ceecdf934cdf865ba4a28ebf3366de
Author: Jon-William Lewis <xilmwa@xenami.net>
Date:   Mon Mar 28 00:36:40 2016 -0500

    Initial Commit

diff --git a/ee.c b/ee.c
new file mode 100644
index 0000000..b84f9fa
--- /dev/null
+++ b/ee.c
@@ -0,0 +1,27 @@
+
+#include "ee.h"
+
+int
+main (int argc, const char* argv[]){
+    STACK ee_stack = NULL;
+    int iter = 1;
+    while (iter < argc){
+        if (isNum(argv[iter])){
+            ee_stack = push(strtod(argv[iter],NULL), ee_stack);
+        }else if (isOperator(argv[iter][0])){
+            ee_stack = push(evaluate(&ee_stack, argv[iter][0]), ee_stack);
+        } else{
+            ERR_INVALID_INPUT = 1;
+        }
+        iter++;
+    }
+    if (ee_stack == NULL){
+        ERR_NS_OPERANDS = 1;
+    } else if (ee_stack->tail!=NULL){
+        ERR_NS_OPERATORS = 1;
+    }
+    if (!reportErrors()){
+        printf("%f\n", peek(ee_stack));
+    }
+    return 0;
+}
diff --git a/ee.h b/ee.h
new file mode 100644
index 0000000..cbb6d5c
--- /dev/null
+++ b/ee.h
@@ -0,0 +1,164 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+#include <ctype.h>
+#include "ee_stack.h"
+
+int ERR_NS_OPERANDS = 0;
+int ERR_NS_OPERATORS = 0;
+int ERR_INVALID_INPUT = 0;
+int ERR_INVALID_FACTORIAL = 0;
+
+// Summary
+int     isNum(const char*);
+int     isOperator(const char);
+
+double  evaluate(STACK*, int);
+double* getOperands(STACK*, int);
+double  fact(int n);
+
+int     reportErrors();
+
+int // Dependency of main()
+isNum(const char* query){
+    /*
+     * If query is a string, isNum(query) iff query represents a number.
+     */
+    int foundDecimal = 0; int foundSign = 0; int noDigits = 1;
+    int ch=0; int issues = 0; int i = 0;
+    while (!issues && ( ( ch = query[i] ) != '\0') ){
+        ch = query[i];
+        if (isdigit(ch)){
+            noDigits = 0;
+        }else {
+            if (ch == '-' || ch == '+'){
+                if (foundSign){ issues = 1; }
+                else {foundSign = 1;}
+            }
+            else if (ch == '.'){
+                if (foundDecimal) { issues = 1; }
+                else { foundDecimal = 1; }
+            }
+            else { issues = 1; }
+        }
+        i++;
+    }
+    if (noDigits) { issues = 1; }
+    return !issues;
+}
+
+int // Dependency of main()
+isOperator(const char query){
+    /*
+     * If query is a string, isOperator iff query is a
+     * supported operator.
+     */
+
+    switch (query){
+        case '+':
+            return 1; break;
+        case '-':
+            return 1; break;
+        case 'x':
+            return 1; break;
+        case '/':
+            return 1; break;
+        case '^':
+            return 1; break;
+        case '!':
+            return 1; break;
+        default:
+            return 0; break;
+    }
+}
+
+double // Dependency of main()
+evaluate(STACK* target_stack, int operator){
+    /*
+     * if target_stack is a pointer to a STACK and operator is
+     * an integer corresponding to the ascii value of a supported
+     * operator, evaluate(target_stack,operator) is the result of
+     * applying the given operator to the appropriate number of
+     * operands from target_stack.
+     */
+    double value = 0;
+    double a,b; a=b=0;
+    double* operands;
+    switch (operator){
+        case '+' :
+            operands = getOperands(target_stack, 2);
+            value = operands[1] + operands[0];
+            break;
+        case '-' :
+            operands = getOperands(target_stack, 2);
+            value = operands[1] - operands[0];
+            break;
+        case 'x' :
+            operands = getOperands(target_stack, 2);
+            value = operands[1] * operands[0];
+            break;
+        case '/' :
+            operands = getOperands(target_stack, 2);
+            value = operands[1] / operands[0];
+            break;
+        case '^' :
+            operands = getOperands(target_stack, 2);
+            value = pow(operands[1],operands[0]);
+            break;
+        case '!' :
+            operands = getOperands(target_stack, 1);
+            if ((operands[0] != floor(operands[0])))
+                { ERR_INVALID_FACTORIAL = 1; }
+            value = fact((int)operands[0]);
+            break;
+        default :
+            break;
+    }
+    return value;
+}
+double* // Dependency of evaluate()
+getOperands(STACK* target_stack, int size){
+    /*
+     * if target_stack is a pointer to a STACK, and size is an integer
+     * corresponding to the number of operands desired,
+     * getOperands(target_stack,size) returns an array of operands and
+     * pops them off *target_stack.
+     */
+    double* operands = (double*)malloc(size*sizeof(double));
+    int i = 0;
+    for (i=0; i<size; i++){
+        if ((*target_stack) == NULL) { ERR_NS_OPERANDS = 1; }
+        operands[i] = peek((*target_stack));
+        (*target_stack) = pop(*target_stack);
+    }
+    return operands;
+}
+
+double // Dependency of evaluate()
+fact(int n){
+    /*
+     * if n is an integer, fact(n) is 'n factorial'.
+     */
+    return (n==0)? 1 : n*fact(n-1);
+}
+
+int // Dependency of main()
+reportErrors(){
+    /*
+     * reportErrors() returns an integer corresponding to
+     * the number of discrete error types noted in global
+     * variables ERR_NS_OPERANDS, ERR_NS_OPERATORS, and
+     * ERR_INVALID_INPUT.
+     */
+    int errors = 0;
+    if (ERR_NS_OPERANDS){
+        printf("Error: insufficient operands.\n"); errors++;
+    } else if (ERR_NS_OPERATORS){
+        printf("Error: insufficient operators.\n"); errors++;
+    } else if (ERR_INVALID_INPUT){
+        printf("Error: Unexpected input.\n"); errors++;
+    } else if (ERR_INVALID_FACTORIAL){
+        printf("Error: only integer factorials are supported.\n"); errors++;
+    }
+    return errors;
+}
diff --git a/ee_stack.h b/ee_stack.h
new file mode 100644
index 0000000..22513ba
--- /dev/null
+++ b/ee_stack.h
@@ -0,0 +1,39 @@
+struct EE_STACK_NODE {
+    double datum;
+    struct EE_STACK_NODE* tail;
+};
+typedef struct EE_STACK_NODE NODE;
+typedef struct EE_STACK_NODE* STACK;
+
+STACK
+push(double datum, STACK tail){
+    /* Given a double value (datum), and a STACK (tail),
+     * push(datum,tail) is the STACK resulting from
+     * prepending datum to the STACK tail.
+     */
+     NODE* new_node = (NODE*)malloc(sizeof(NODE));
+        (*new_node).datum = datum;
+        (*new_node).tail  = tail;
+    return new_node;
+}
+
+STACK
+pop(STACK target_stack){
+    /* Given a STACK (target_stack), pop(target_stack)
+     * is the STACK resulting from the removal of the first
+     * item in target_stack;
+     */
+     if (target_stack == NULL){ return target_stack; }
+     STACK tail = (*target_stack).tail;
+     free(target_stack);
+     return tail;
+}
+
+double
+peek(STACK target_stack){
+    /* Given a STACK (target_stack), peek(STACK)
+     * is the first element's datum.
+     */
+     if (target_stack == NULL){ return 0; }
+     return (*target_stack).datum;
+}