/* This program demonstrates c-strings
 * a c-string is an array of chars.
 * chars are still integer types. When we store a character in a variable or as an array
 * element, it stores the equivalent ASCII value of that character, as an integer
 * So, we can do math with characters, since they are internally numbers.
 *
 * This program shows us the 4 main functions for c-string
 * 1. Finding the length of a string: strlen
 * 2. Copying strings: strcpy
 * 3. concatenating strings: strcat
 * 4. Comparing strings: strcmp
 *
 * We also implement strcpy and strcmp ourselves, to understand how they work
 *
 * A note on ASCII:
 * In C++, char is both an integer type and a character type. This is because
 * C++ stores a char as an integer but interprets it as a character when it is
 * printed.
 * Since the days of telegraph, we have had the American Standard Code for
 * Information Interchange (ASCII), which assigns a numeric (integer) value for
 * every character that can be typed in. We have a numeric equivalent for every
 * uppercase letter, lowercase letter, numeral character, special character,
 * whitespace, control characters, etc.
 * When we store a character in a char variable (or a spot in a C-String), we
 * actually store the ASCII value of the character.
 */
#include<iostream>
#include<cstring>

using namespace std;

void stringCopy(char st1[], char st2[]);
int stringCompare (char st1[], char st2[]);

int main()
{
    char str[200];
    char copy[200];

    cout<<"Enter a string: ";
    cin.getline(str, 200, '#');

    cout<<"Enter another string: ";
    cin.getline(copy, 200);

    cout<<"You entered: "<<copy<<endl;

    stringCopy(copy, str);
    cout<<"Copy is now: "<<copy<<"\nstr is now "<<str<<endl;

    cout<<"\nString concatenation:\n";
    cout<<"Enter 2 string: ";
    cin.getline(str, 200);
    cin.getline(copy, 200);

    /* Concatenation is the process of attaching one string at the end of another string
    * The cstring library contains the strcat function that will concatenate strings
    * strcat(target string, source string)
    * The source string is attache dto the end of the target string. The source
    * string is left unchanged. We need to make sure the target string has enough
    * room for both strings. C++ does not do this check for us.
    * We cannot concatenate multiple strings at one go. strcat only takes 2 parameters.
    * If we need multiple concatenations, we should do it one at a time.
    */

    strcat(str, copy);      // attaches the contents of the second string to the end of the first
    // second string left unchanged

    cout<<"str: "<<str<<"\ncopy: "<<copy<<endl;

    /* Lexicographic ordering:
     * space < numeral (0-9) < uppercase letters (A-Z) < lowercase letters (a-z)
     * Spongebob, Patrick -> Patrick is "first" P < S
     * Spongebob , patirck -> Spongebob is "first", S < p
     * Spongebob, 4atrick -> 4atrick is "first", numeral < letters
     * Spongebob, Sqiudward -> Spongebob is "first" S==S, p<q
     * Squid, Square -> Square is first, first 3 letters are the same, a < i
     * Squid, Squidward -> Squid is "first", all letters matched until "Squid" ran out of letters
     * Squid Squid -> strings are equal
     */

    cout<<"Demonstrating strcmp for string comparison:\n";
    char str2[100];
    cout<<"Enter a string to compare: ";
    cin.getline(str2,100);
    int diff = strcmp(str, str2);
    if (diff < 0)
        cout<<str<< " was first"<<endl;
    else if (diff > 0)
        cout<<str2<<" was first"<<endl;
    else cout<<"The two strings were equal"<<endl;

    cout<<"Demonstrating our version of strcmp:\n";
    diff = stringCompare(str, str2);
    if (diff < 0)
        cout<<str<< " was first"<<endl;
    else if (diff > 0)
        cout<<str2<<" was first"<<endl;
    else cout<<"The two strings were equal"<<endl;

    return 0;
}

/* function stringCopy
 * parameters: 2 - 2 strings (c-style)
 * returns nothing
 *
 * algorithm: iterate through the second string, copy out individual characters
 * into the same position on the first string
 * Note- remember to terminate the string
 */
void stringCopy(char st1[], char st2[])
{
    int i;
    for( i=0; i<strlen(st2); i++)
    {
        st1[i] = st2[i];
    }
    st1[i] = '\0';      // string termination

}

/* Function stringCompare - compares 2 strings in lexicographic order
 * 2 parameters - cstrings to be compared.
 * Returns int - first non-zero comparison value. 0 if strings are equal
 * Method: Iterate through strings, until one or both terminate
 * Compare (subtract ASCII values) of the corresponding characters. If non-zero, return it
 * Here, we replicate the strcmp function
 * When we compare strings, we try and figure out which string will be found
 * before the other string in lexicographic ordering.
 * lexicographic ordering is alphabetic ordering except for the following
 * numeral characters -> uppercase characters -> lowercase characters
 * This is because we compare the ASCII values.
 * strcmp(string1, string2)
 * string comparison moves through both strings one character at a time,
 * and subtracts the ASCII value of the character of the second string
 * from the character of the first string.
 * The moment it sees a non-zero result on the subtraction, it returns that
 * So, strcmp returns
 * value < 0 if the first string occurs before the second string
 * value > 0 if the second string occurs before the first string
 * 0 if the strings are exactly equal (including case, spaces, etc.)
 */

int stringCompare(char st1[], char st2[])
{
    int diff = 0;
    int i=0;
    int len1 = strlen(st1), len2 = strlen(st2);

    while( i< len1 && i <len2 && diff == 0)
    {
        diff = st1[i] - st2[i];
        i++;
    }

    if(diff == 0)
    {
        if(i == len2)
        {
            diff = st1[i];
        }
        if(i == len1)
        {
            diff = - st2[i];
        }
    }

    return diff;

}