F longest_common_substring(s1, s2)
   V ir = 0
   V jr = -1
   L(i1) 0 .< s1.len
      V? i2 = s2.find(s1[i1])
      L i2 != N
         V (j1, j2) = (i1, i2)
         L j1 < s1.len & j2 < s2.len & s2[j2] == s1[j1]
            I j1 - i1 >= jr - ir
               (ir, jr) = (i1, j1)
            j1++
            j2++
         i2 = s2.find(s1[i1], i2 + 1)
   R s1[ir..jr]

print(longest_common_substring(‘thisisatest’, ‘testing123testing’))