add initial files

README.md

@@ -1,3 +1,10 @@
-# lcs-diff
+# Toy LCS-based Diff program
 
-Toy Longest-Common-Subsequence based diff and patch program, minimizing the edit script's sum of line-addition and line-deletions.
+Toy Longest-Common-Subsequence based diff and patch program, minimizing the edit script's sum of line-addition and line-deletions.
+
+Example usage:
+```bash
+python diff.py a1 b1 > patch1
+python patch.py a1 patch1 > c1 # patch is not in-place
+python diff.py a1 c1 # verify that a1 and c1 has the same content
+```

diff.py

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+import argparse
+parser = argparse.ArgumentParser(description="Compare two documents line by line")
+parser.add_argument('a', help="reference document")
+parser.add_argument('b', help="target document")
+args = parser.parse_args()
+
+fa = open(args.a,'r')
+la = fa.read().split('\n')
+fb = open(args.b,'r')
+lb = fb.read().split('\n')
+
+# Solve the Longest Common Subsequence (LCS) subproblem with dynamic programming
+dp = [[0] * (len(lb)+1) for _ in range(len(la)+1)]
+# dp[i+1][j+1] stores the LCS length between la[i] and lb[j]
+for i in range(len(la)):
+    for j in range(len(lb)):
+        if la[i]==lb[j]:
+            dp[i+1][j+1] = dp[i][j] + 1
+        else:
+            dp[i+1][j+1] = max(dp[i][j+1], dp[i+1][j])
+
+# Backtrack to find one scheme to reduce both a and b to their LCS
+diffs = []
+i,j = len(la)-1, len(lb)-1
+while i>=0 or j>=0: # current position (i+1, j+1), try to move to (0, 0)
+    oi, oj = i, j
+    while j >= 0 and dp[i+1][j+1]==dp[i+1][j]: # can safely delete lb[j]
+        j -= 1
+    while i >= 0 and dp[i+1][j+1]==dp[i][j+1]: # can safely delete la[i]
+        i -= 1
+    if i==oi and j==oj:
+        assert dp[i+1][j+1]==dp[i][j]+1 and la[i]==lb[j]
+        i,j = i-1,j-1
+    else:
+        diffs.append((oi,i,oj,j))
+diffs.reverse()
+
+def describe(oi,i,oj,j):
+    # by diff convension line numbering starts from 1
+    oi,i,oj,j = oi+1,i+1,oj+1,j+1
+    def intv(x,y): # simplify expression if interval is one line
+        return str(x) if x==y else f"{x},{y}"
+    if i==oi:
+        print(f"{i}a" + intv(j+1,oj))
+    elif j==oj:
+        print(intv(i+1,oi) + f"d{j}")
+    else:
+        print(intv(i+1,oi) + "c" + intv(j+1,oj))
+
+for oi,i,oj,j in diffs:
+    # delete (i,oi], add (j,oj]
+    describe(oi,i,oj,j)
+    for p in range(i+1,oi+1):
+        print("< " + la[p])
+    if oi>i and oj>j:
+        print("---")
+    for p in range(j+1,oj+1):
+        print("> " + lb[p])

patch.py

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+import argparse
+parser = argparse.ArgumentParser(description="Prints to stdout the patched document")
+parser.add_argument('original', help='original document')
+parser.add_argument('patch', help='patch file')
+args = parser.parse_args()
+
+fa = open(args.original,'r')
+la = fa.read().split('\n')
+# a workspace for us to edit, each element being a list that 1) originally contains a corresponding line
+# in a, 2) may be emptied if that line is removed, 3) may be where we insert new content
+ws = [[l] for l in la]
+fp = open(args.patch,'r')
+lp = fp.read().split('\n')
+
+def str2intv(s):
+    if "," in s:
+        x, y = s.split(',')
+    else:
+        x, y = s, s
+    return int(x)-1, int(y)
+
+def ws_rm(desc, lp, nb):
+    l,r = str2intv(desc)
+    for p in range(l,r):
+        nb += 1
+        assert lp[nb][:2] == "< "
+        assert ws[p][0] == lp[nb][2:],\
+            f"Patch file wants to delete '{lp[nb][2:]}', but originally line {p} is '{ws[p][0]}'"
+        ws[p] = list()
+    return nb + 1, l
+
+def ws_insert(p, desc, lp, nb):
+    l,r = str2intv(desc)
+    for _ in range(l,r):
+        nb += 1
+        assert lp[nb][:2] == "> "
+        ws[p].append(lp[nb][2:])
+    return nb + 1
+
+nb = 0
+while nb < len(lp):
+    line = lp[nb]
+    if line:
+        assert line[0] != '<' and line[0] != '>', f"Expected description at line {nb}, but got {line}"
+    else:
+        nb += 1
+        continue
+    idx_d = line.find('d')
+    if idx_d != -1:
+        nb, _ = ws_rm(line[:idx_d], lp, nb)
+        continue
+    idx_a = line.find('a')
+    if idx_a != -1:
+        p = int(line[:idx_a]) - 1
+        nb = ws_insert(p, line[idx_a+1:], lp, nb)
+        continue
+    idx_c = line.find('c')
+    if idx_c != -1:
+        nb, p = ws_rm(line[:idx_c], lp, nb)
+        nb = ws_insert(p, line[idx_c+1:], lp, nb)
+        continue
+    assert False, f"Unreachable. Cannot parse line {nb}: {line}"
+
+flatws = [line for _ in ws for line in _]
+print('\n'.join(flatws), end="") # Don't want to print additional new line