Practical Reverse Engineering p. 79 #9

Question number 9 on page 79 of Practical Reverse Engineering is as follows:

What does the function shown in Figure 2-15 do?

Here is the function's disassembly:

Figure 2-15. Practical Reverse Engineering. © 2014 by Bruce Dang

The ARM processor is in Thumb state. This is essentially the same functionality as Figure 2-14, except the count variable is gone.

int32_t comparison(char *str1, char *str2)
{
    /* LDR R5, =byteArray */
    static BYTE byteArray[] = {0, 1, ..., 0xff};

    while(1)
    {
        /* CMP R4, #0 */
        if (*str1 == '\0')
            break;

        /* LDRB R3, [R1] */
        /* LDRB R4, [R3,R6] */
        /* LDRB R3, [R5,R6] */
        /* CMP R3, R4 */
        if (byteArray[*str1] != byteArray[*str2])
            break;

        ++str1;         /* ADDS R0, #1 */
        ++str2;         /* ADDS R1, #1 */
    }

    /* LDRB R2, [R3,R5] */   
    /* LDRB R3, [R3,R5] */
    /* SUBS R0, R3, R2 */
    return byteArray[*str1] - byteArray[*str2];   
}