"""
Operations on strings:
- calculate distance between two strings
- transform strings with transformation codes
"""
#### Ngrams
def getNgrams (sWord, n=2):
"return a list of Ngrams strings"
return [ sWord[i:i+n] for i in range(len(sWord)-n+1) ]
#### DISTANCE CALCULATIONS
def longestCommonSubstring (s1, s2):
"longest common substring"
# http://en.wikipedia.org/wiki/Longest_common_substring_problem
# http://en.wikibooks.org/wiki/Algorithm_implementation/Strings/Longest_common_substring
lMatrix = [ [0]*(1+len(s2)) for i in range(1+len(s1)) ]
nLongest, nLongestX = 0, 0
for x in range(1, 1+len(s1)):
for y in range(1, 1+len(s2)):
if s1[x-1] == s2[y-1]:
lMatrix[x][y] = lMatrix[x-1][y-1] + 1
if lMatrix[x][y] > nLongest:
nLongest = lMatrix[x][y]
nLongestX = x
else:
lMatrix[x][y] = 0
return s1[nLongestX-nLongest : nLongestX]
def distanceDamerauLevenshtein (s1, s2):
"distance of Damerau-Levenshtein between <s1> and <s2>"
# https://fr.wikipedia.org/wiki/Distance_de_Damerau-Levenshtein
d = {}
nLen1 = len(s1)
nLen2 = len(s2)
for i in range(-1, nLen1+1):
d[i, -1] = i + 1
for j in range(-1, nLen2+1):
d[-1, j] = j + 1
for i in range(nLen1):
for j in range(nLen2):
nCost = 0 if s1[i] == s2[j] else 1
d[i, j] = min(
d[i-1, j] + 1, # Deletion
d[i, j-1] + 1, # Insertion
d[i-1, j-1] + nCost, # Substitution
)
if i and j and s1[i] == s2[j-1] and s1[i-1] == s2[j]:
d[i, j] = min(d[i, j], d[i-2, j-2] + nCost) # Transposition
return d[nLen1-1, nLen2-1]
def distanceSift4 (s1, s2, nMaxOffset=5):
"implementation of general Sift4."
# https://siderite.blogspot.com/2014/11/super-fast-and-accurate-string-distance.html
if not s1:
return len(s2)
if not s2:
return len(s1)
nLen1, nLen2 = len(s1), len(s2)
i1, i2 = 0, 0 # Cursors for each string
nLargestCS = 0 # Largest common substring
nLocalCS = 0 # Local common substring
nTrans = 0 # Number of transpositions ('ab' vs 'ba')
lOffset = [] # Offset pair array, for computing the transpositions
while i1 < nLen1 and i2 < nLen2:
if s1[i1] == s2[i2]:
nLocalCS += 1
# Check if current match is a transposition
bTrans = False
i = 0
while i < len(lOffset):
t = lOffset[i]
if i1 <= t[0] or i2 <= t[1]:
bTrans = abs(i2-i1) >= abs(t[1] - t[0])
if bTrans:
nTrans += 1
elif not t[2]:
t[2] = True
nTrans += 1
break
elif i1 > t[1] and i2 > t[0]:
del lOffset[i]
else:
i += 1
lOffset.append([i1, i2, bTrans])
else:
nLargestCS += nLocalCS
nLocalCS = 0
if i1 != i2:
i1 = i2 = min(i1, i2)
for i in range(nMaxOffset):
if i1 + i >= nLen1 and i2 + i >= nLen2:
break
elif i1 + i < nLen1 and s1[i1+i] == s2[i2]:
i1 += i - 1
i2 -= 1
break
elif i2 + i < nLen2 and s1[i1] == s2[i2+i]:
i2 += i - 1
i1 -= 1
break
i1 += 1
i2 += 1
if i1 >= nLen1 or i2 >= nLen2:
nLargestCS += nLocalCS
nLocalCS = 0
i1 = i2 = min(i1, i2)
nLargestCS += nLocalCS
return round(max(nLen1, nLen2) - nLargestCS + nTrans)
def showDistance (s1, s2):
"display Damerau-Levenshtein distance and Sift4 distance between <s1> and <s2>"
print("Damerau-Levenshtein: " + s1 + "/" + s2 + " = " + distanceDamerauLevenshtein(s1, s2))
print("Sift4:" + s1 + "/" + s2 + " = " + distanceSift4(s1, s2))
#### STEMMING OPERATIONS
## No stemming
def noStemming (sFlex, sStem):
"return <sStem>"
return sStem
def rebuildWord (sFlex, sCode1, sCode2):
""" Change <sFlex> with codes (each inserts a char at a defined possition).
<I forgot what purpose it has…>
"""
if sCode1 == "_":
return sFlex
n, c = sCode1.split(":")
sFlex = sFlex[:n] + c + sFlex[n:]
if sCode2 == "_":
return sFlex
n, c = sCode2.split(":")
return sFlex[:n] + c + sFlex[n:]
## Define affixes for stemming
# Note: 48 is the ASCII code for "0"
# Suffix only
def defineSuffixCode (sFlex, sStem):
""" Returns a string defining how to get stem from flexion
"n(sfx)"
with n: a char with numeric meaning, "0" = 0, "1" = 1, ... ":" = 10, etc. (See ASCII table.) Says how many letters to strip from flexion.
sfx [optional]: string to add on flexion
Examples:
"0": strips nothing, adds nothing
"1er": strips 1 letter, adds "er"
"2": strips 2 letters, adds nothing
"""
if sFlex == sStem:
return "0"
jSfx = 0
for i in range(min(len(sFlex), len(sStem))):
if sFlex[i] != sStem[i]:
break
jSfx += 1
return chr(len(sFlex)-jSfx+48) + sStem[jSfx:]
def changeWordWithSuffixCode (sWord, sSfxCode):
"apply transformation code <sSfxCode> on <sWord> and return the result string"
if sSfxCode == "0":
return sWord
return sWord[:-(ord(sSfxCode[0])-48)] + sSfxCode[1:] if sSfxCode[0] != '0' else sWord + sSfxCode[1:]
# Prefix and suffix
def defineAffixCode (sFlex, sStem):
""" Returns a string defining how to get stem from flexion. Examples:
"0" if stem = flexion
"stem" if no common substring
"n(pfx)/m(sfx)"
with n and m: chars with numeric meaning, "0" = 0, "1" = 1, ... ":" = 10, etc. (See ASCII table.) Says how many letters to strip from flexion.
pfx [optional]: string to add before the flexion
sfx [optional]: string to add after the flexion
"""
if sFlex == sStem:
return "0"
# is stem a substring of flexion?
n = sFlex.find(sStem)
if n >= 0:
return "{}/{}".format(chr(n+48), chr(len(sFlex)-(len(sStem)+n)+48))
# no, so we are looking for common substring
sSubs = longestCommonSubstring(sFlex, sStem)
if len(sSubs) > 1:
iPos = sStem.find(sSubs)
sPfx = sStem[:iPos]
sSfx = sStem[iPos+len(sSubs):]
n = sFlex.find(sSubs)
m = len(sFlex) - (len(sSubs)+n)
return chr(n+48) + sPfx + "/" + chr(m+48) + sSfx
return sStem
def changeWordWithAffixCode (sWord, sAffCode):
"apply transformation code <sAffCode> on <sWord> and return the result string"
if sAffCode == "0":
return sWord
if '/' not in sAffCode:
return sAffCode
sPfxCode, sSfxCode = sAffCode.split('/')
sWord = sPfxCode[1:] + sWord[(ord(sPfxCode[0])-48):]
return sWord[:-(ord(sSfxCode[0])-48)] + sSfxCode[1:] if sSfxCode[0] != '0' else sWord + sSfxCode[1:]