First apparently functional solution with suffix arrays

Author: rohantib

Problem - Longest Strand.md

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+
+Given a large number of binary files, write a program that finds the
+longest strand of bytes that is identical between two or more files
+
+Use the test set attached (files sample.*)
+
+The program should display:
+- the length of the strand
+- the file names where the largest strand appears
+- the offset where the strand appears in each file

filelcsDP.py

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+import sys
+import os
+
+# Reference DP Solution
+
+if len(sys.argv) <= 2:
+    print("Usage: python filelcs.py <file> <file> ... <file>")
+    exit()
+
+
+# def get_lcs_offsets(f1_data, f2_data):
+#     """ f2 is the smaller file """
+#     f1_len, f1_name, f1 = f1_data
+#     f2_len, f2_name, f2 = f2_data
+#     matrix = [[0] * (f2_len+1), [0] * (f2_len+2)]
+#     f1.seek(0)
+#     maxlen = 0
+#     of1 = 0
+#     of2 = 0
+#     for i in range(1, f1_len+1):
+#         b1 = f1.read(1)
+#         f2.seek(0)
+#         # print(i)
+#         for j in range(1, f2_len+1):
+#             b2 = f2.read(1)
+#             if b1 == b2:
+#                 num = matrix[(i%2) - 1][j-1] + 1
+#                 matrix[i%2][j] = num
+#                 if num > maxlen:
+#                     maxlen = num
+#                     of1 = i
+#                     of2 = j
+#     offset_1 = (f1_name, of1)
+#     offset_2 = (f2_name, of2)
+#     return maxlen, [min(offset_1, offset_2), max(offset_1, offset_2)]
+
+def get_lcs_offsets(f1_data, f2_data):
+    """ f2 is the smaller file """
+    f1_len, f1_name, f1 = f1_data
+    f2_len, f2_name, f2 = f2_data
+    matrix = [[0] * (f2_len+1), [0] * (f2_len+1)]
+    f1.seek(0)
+    f2.seek(0)
+    f1_str = f1.read()
+    f2_str = f2.read()
+    maxlen = 0
+    of1 = 0
+    of2 = 0
+    for i in range(1, f1_len+1):
+        b1 = f1_str[i-1]
+        # print(i)
+        for j in range(1, f2_len+1):
+            b2 = f2_str[j-1]
+            if b1 == b2:
+                num = matrix[(i-1)%2][j-1] + 1
+                matrix[i%2][j] = num
+                if num > maxlen:
+                    maxlen = num
+                    of1 = i-1
+                    of2 = j-1
+    offset_1 = (f1_name, of1)
+    offset_2 = (f2_name, of2)
+    return maxlen, [min(offset_1, offset_2), max(offset_1, offset_2)]
+
+
+filenames = sys.argv[1:]
+
+maxlen = 0
+offsets = []
+for i in range(len(filenames)-1):
+    name1 = filenames[i]
+    try:
+        f1 = open(name1, "rb")
+    except FileNotFoundError:
+        print("ERROR: FILE '{}' DOES NOT EXIST.".format(name1))
+        exit()
+    f1_len = os.path.getsize(name1)
+    f1_data = (f1_len, name1, f1)
+    for j in range(i+1, len(filenames)):
+        name2 = filenames[j]
+        try:
+            f2 = open(name2, "rb")
+        except FileNotFoundError:
+            print("ERROR: FILE '{}' DOES NOT EXIST.".format(name2))
+        f2_len = os.path.getsize(name2)
+        f2_data = (f2_len, name2, f2)
+        lcs_len, lcs_offset = get_lcs_offsets(max(f1_data, f2_data), min(f1_data, f2_data))
+        if lcs_len > 0:
+            if lcs_len > maxlen:
+                maxlen = lcs_len
+                offsets = lcs_offset
+            elif lcs_len == maxlen and lcs_offset[0][0] == offsets[0][0]:
+                offsets.append(lcs_offset[1])
+        f2.close()
+    f1.close()
+
+print("Length of longest shared strand of bytes: {}".format(maxlen))
+for off in offsets:
+    print("File name: {}, Offset where sequence begins: {}".format(off[0], off[1] - maxlen))