strstri.c

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#include <ctype.h>
#include <stddef.h>
 
#include "strstri.h"
 
 
    //
    // Function   : strstri
    //
    // Scope      : Public
    //
    // Arguments  :  str1              = pointer to string to search in
    //               str2              = pointer to string to search for
    //
    // Returns    : Pointer into str1 to first character of found str2 or
    //              nullptr if str2 is not found in str1.
    //
    // Description: Find string str2 in string str1, returning pointer to first
    //              occurrence of str2.  Returns nullptr if str2 not found in str1.
    //
    // Notes      : This is functionally identical to the C Library's strstr()
    //              except for the use of toupper().
    //
 
char* strstri(const char* s1, const char* s2)
{
    // Guard against null pointers (strstr() does not do this).
    if (s1 == NULL || s2 == NULL)
    {
        return (NULL);
    }
 
    // If string to search for is empty, return beginning of str1.
    if (*s2 == '\0')
    {
        return ((char*)s1);
    }
 
    // Loop over all characters in string to search
    while (*s1 != '\0')
    {
        // Find first occurrence of second string in string 1
        while (*s1 != '\0' && toupper(*s1) != toupper(*s2))
        {
            s1++;
        }
 
        if (*s1 == '\0')
        {
            break;
        }
        const char* sc1;
        const char* sc2;
 
        // Now that the first character of string 2 might have been
        // found, compare subsequent characters of string 2 to see
        // if we have an exact match.
        for (sc1 = s1, sc2 = s2; ; )
        {
            if (*++sc2 == '\0')
            {
                // We have reached the end of the string to search for
                // so we must have a match.  Return pointer into s1
                // to the found string.
                return ((char*)s1);
            }
            else
            {
                ++sc1;
 
                if (toupper(*sc1) != toupper(*sc2))
                {
                    // a character in string 2 doesn't match so start
                    // the whole thing over with the next character
                    // in string 1.
                    break;
                }
            }
        }
        ++s1;
    }
    // If we get here, there is no match so return nullptr.
    return (NULL);
}