/*
CSE 303, Spring 2009, Marty Stepp
This program reads student data from a file in the following format:

section,  studentID,  year (1=frosh 4=senior),  credits
    section    is a 1-letter string
    studentID  is a 7-digit number
    year       is an integer from 1 through 4
    credits    is an integer from 0 through 27

example of input:
C 9087231 2 18
B 7843254 3 10
*/

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>

// Shifts the given pointer by the given number of bytes and returns it.
void* shift(void* p, int bytes) {
    return (void*) (((char*) p) + bytes);
}

// A struct type for students that occupies only 32 bits (4 bytes).
typedef struct Student {
    unsigned studentID : 20;   // 5-digit number:  2^20 > 1m
    unsigned year      : 2;    // 1-4
    unsigned credits   : 5;    // 0-27
    unsigned section   : 5;    // A-Z
} Student;

int main(void) {
    int i;
    int num_students;
    
    // a block of memory that we'll use to store 6 students;
    // C can store anything in any memory if we cast the pointers
                    // stud1 stud2 ...
                    // 0     4     8     12    16    20
    char blob[24];  // [ 4 ] [ 4 ] [ 4 ] [ 4 ] [ 4 ] [ 4 ]
    
    FILE* f = fopen("students.txt", "r");
    fscanf(f, "%d\n", &num_students);
    
    for (i = 0; i < num_students; i++) {
        // read a student:    C 9087231 2 18
        Student* s;
        char section;
        int id;
        int year;
        int credits;
        fscanf(f, "%c %d %d %d\n", &section, &id, &year, &credits);
        s = (Student*) shift(blob, i * sizeof(Student));
        
        // store data into fields of struct  (pack those bits!)
        s->section = section - 'A';   // A-Z --> 0-25
        s->studentID = id;
        s->year = year - 1;           // 1-4 --> 0-3
        s->credits = credits;
    }
    fclose(f);
    
    // print info about all students
    for (i = 0; i < num_students; i++) {
        Student* s = (Student*) (blob + (i * sizeof(Student)));
        printf("Student %d: section %c, id #%d, year %d, credits %d\n",
            i,
            s->section + 'A',         // 0-25 --> A-Z
            s->studentID,
            s->year + 1,              // 0-3  --> 1-4
            s->credits
        );
    }
    
    return 0;
}