This seems like the most efficient solution. I have a neighboring comment on this post which suggests using bit arithmetic, but the above solution is more efficient than that. Here’s what the assembly code for the body of the first loop compiles down to (I had to use ChatGPT-4 as godbolt unfortunately doesn’t work on mobile):

    cmp dl, 's'    ; Compare the character with 's'
    sete dl        ; If the character is 's', set dl to 1. Otherwise, set dl to 0.
    sub al, dl     ; Subtract the result from res

    cmp dl, 'p'    ; Compare the character with 'p'
    sete dl        ; If the character is 'p', set dl to 1. Otherwise, set dl to 0.
    add al, dl     ; Add the result to res