Finding Palindromes

• A palindrome is a phrase that has "reads" the same forward and backword
• Palindromes normally only consider alphabetic characters.
• And usually ignores the case of the letter.
• Here is a c++ solution
• It looks to me like we will need
  • A routine to read in the phrase
  • isalpha
  • toupper
  • string.length
• We already have toUpper
• isAlpha
  • Character in $a0
  • return a 1 in $a0 is it is between 'A' and 'Z' or between 'a' and 'z'
  • Just a series of jumps.
  • It is a leaf
  • And it does not use any registers.
  • So there is no need to buid an activation frame.

    IsAlpha:
            # input: $a0 holds the character to test
            # output: $a0 holds a 1 if the character is a letter and 0 otherwise.
    
            blt     $a0, 65, NotAlpha       # if letter < 'A'
            bgt     $a0, 122, NotAlpha      # if letter > 'z'
            bge     $a0, 97, YesAlpha       # if letter >= 'a'
            ble     $a0, 90, YesAlpha       # if letter &alt;= 'Z'
                                            # fall through, 'Z' < letter < 'a'
    NotAlpha:
            li      $a0, 0
            b       DoneIsAlpha
    YesAlpha:
            li      $a0, 1
    DoneIsAlpha:
            jr      $ra
    	 

• StrLen is not too hard either.
  • The algorithm is

    for(i=&word; *i!=NULL;i++);
    return i
    

  • So this isn't really a strlen, it is more of a find end.
  • It will return a pointer to the last character.
  • I will use $s1 and $s2 to do computations
    • I could use the temps and not need to build an activation record
    • but I want to show how it is done.
  • So I will store the following in the activation record
    • fp at -4(sp)
    • s1 at -4(fp)
    • s2 at -8(fp)
  • To accomplish this

            sw      $fp, -4($sp)
            addiu   $fp, $sp, -4
            sw      $s1, -4($fp)
            sw      $s2, -8($fp)
            addiu   $sp, $sp, -8
    

  • The code for the for loop is very short

            # for(i=&word; 
            move    $s1, $a0
    StrLenTop:
            # *i!= NULL;
            lb      $s2, 0($s1)
            beq     $s2, 0, FoundNull
            #  i++) 
            addi    $s1, $s1, 1
            b       StrLenTop
    FoundNull:
            # return i
            move $a0, $s1
    

  • Finally we need to restore the variables and the stack

            lw      $s1, -4($fp)
            lw      $s3, -8($fp)
            addiu   $sp, $fp, 4
            lw      $fp, -4($sp)
            jr      $ra