Longest Common Substring

# largest q-gram (longest common substring) in AWK: Michael Sanders - 2024
# A q-gram is a contiguous sequence of characters of fixed length
# see also: https://en.wikipedia.org/wiki/N-gram

BEGIN {
  str1 = "MAWK"
  str2 = "gawk"
  q = largest_qgram(str1, str2)
  l = length(q)

  print str1 " : " str2

  if (length(q) > 0) {
    printf("largest q-gram (longest common substring): \"%s\" size: %d\n", q, l)
  } else {
    printf("no common substring found\n")
  }

}

# find largest q-gram (longest common substring) between 2 words
function largest_qgram(str1, str2, i, j, maxlen, largest, tmp) {
  maxlen = 0
  largest = ""

  # for convenience's sake
  str1 = tolower(str1)
  str2 = tolower(str2)

  # loop over each character of 1st string
  for (i = 1; i <= length(str1); i++) {
     # loop over each character of 2nd string
     for (j = 1; j <= length(str2); j++) {
        tmp = ""
        # check for matching substrings at positions i and j
        while (substr(str1, i + length(tmp), 1) == substr(str2,
              j + length(tmp), 1) && i + length(tmp) <=
              length(str1) && j + length(tmp) <= length(str2)) {
           tmp = tmp substr(str1, i + length(tmp), 1)
        }

        # if matching substring is longer than previous maximum
        if (length(tmp) > maxlen) {
           maxlen = length(tmp)
           largest = tmp
        }
     }
  }
  return largest
}

# eof

--
:wq
Mike Sanders