lopez suffix array extractor with copyrighted david chiang code excised

1 files changed, 113 insertions, 0 deletions

diff --git a/sa-extract/lcp.pyx b/sa-extract/lcp.pyx
new file mode 100644
index 00000000..a992d3ee
--- /dev/null
+++ b/sa-extract/lcp.pyx
@@ -0,0 +1,113 @@
+#!/usr/bin/env python2.4
+
+'''Compute LCP array for a suffix array using the Kasai et al. algorithm'''
+'''Can also be used to compute statistics such
+as k most frequent n-grams'''
+
+import sys
+
+cimport cintlist
+cimport csuf
+cimport cdat
+cimport cveb
+
+cdef class LCP:
+
+	cdef csuf.SuffixArray sa
+	cdef cintlist.CIntList lcp
+
+	def __init__(self, sa):
+		self._construct(sa)
+
+	cdef _construct(self, csuf.SuffixArray sa):
+		cdef int i, k, j, h, n
+		cdef cintlist.CIntList rank
+
+		sys.stderr.write("Constructing LCP array\n")
+		self.sa = sa
+		n = self.sa.sa.len
+		self.lcp = cintlist.CIntList(initial_len=n)
+
+		rank = cintlist.CIntList(initial_len=n)
+		for i from 0 <= i < n:
+			rank.arr[sa.sa.arr[i]] = i
+
+		h = 0
+		for i from 0 <= i < n:
+			k = rank.arr[i]
+			if k == 0:
+				self.lcp.arr[k] = -1
+			else:
+				j = sa.sa.arr[k-1]
+				while i+h < n and j+h < n and sa.darray.data.arr[i+h] == sa.darray.data.arr[j+h]:
+					h = h+1
+				self.lcp.arr[k] = h
+			if h > 0:
+				h = h-1
+		sys.stderr.write("LCP array completed\n")
+
+
+	def compute_stats(self, max_n):
+		self._compute_stats(max_n)
+
+	cdef _compute_stats(self, int max_n):
+		'''Note: the output of this function is not exact.  In
+		particular, the frequency associated with each word is 
+		not guaranteed to be correct.  This is due to a bit of
+		laxness in the design; the function is intended only to
+		obtain a list of the most frequent words; for this 
+		purpose it is perfectly fine'''
+		cdef int i, ii, iii, j, k, h, n, N, rs, freq, valid
+		cdef cintlist.CIntList run_start
+		cdef cintlist.CIntList ngram_start
+		cdef cveb.VEB veb
+		
+		N = self.sa.sa.len
+
+		ngram_starts = []
+		for n from 0 <= n < max_n:
+			ngram_starts.append(cintlist.CIntList(initial_len=N))
+
+		run_start = cintlist.CIntList(initial_len=max_n)
+		veb = cveb.VEB(N)
+
+		for i from 0 <= i < N:
+			h = self.lcp.arr[i]
+			if h < 0:
+				h = 0
+			for n from h <= n < max_n:
+				rs = run_start.arr[n]
+				run_start.arr[n] = i
+				freq = i - rs
+				if freq > 1000: # arbitrary, but see note below
+					veb._insert(freq)
+					ngram_start = ngram_starts[n]
+					while ngram_start.arr[freq] > 0:
+						freq = freq + 1 # cheating a bit, should be ok for sparse histogram
+					ngram_start.arr[freq] = rs
+		i = veb.veb.min_val
+		while i != -1:
+			ii = veb._findsucc(i)
+			for n from 0 <= n < max_n:
+				ngram_start = ngram_starts[n]
+				iii = i
+				rs = ngram_start.arr[iii]
+				while (ii==-1 or iii < ii) and rs != 0:
+					j = self.sa.sa.arr[rs]
+					valid = 1
+					for k from 0 <= k < n+1:
+						if self.sa.darray.data.arr[j+k] < 2:
+							valid = 0
+					if valid:
+						ngram = ""
+						for k from 0 <= k < n+1:
+							ngram= ngram+ self.sa.darray.id2word[self.sa.darray.data.arr[j+k]] + " "
+						print i, n+1, ngram
+					iii = iii + 1
+					rs = ngram_start.arr[iii]
+			i = ii
+
+
+
+
+