またもやミス発見

video = (length == 1) ? trim(alt, 0, -1) : trim(alt, start_alt, start_alt + length -1)　は
↓
video = (start_alt == 0) && (length == 1) ? trim(alt, 0, -1) : trim(alt, start_alt, start_alt + length -1)

に変更したほうがいいな