3.0 KiB
given a string A, we want it to be B.
A: hello world and thanks for the fish. B: hello cruel o_o world and thanks for the fish!
however, the instructions to reconstruct B must be as small as possible, to minimize transmission costs. this is why we dont send B entirely.
different algorithms as follow:
ALGORITHM 1
-
identify the first mismatching character (S).
v-- 6(S)
A: hello world and thanks for the fish. B: hello cruel o_o world and thanks for the fish!
-
identify the last mismatch character (E).
v-- 0(E)
A: hello world and thanks for the fish. B: hello cruel o_o world and thanks for the fish!
- we construct the patch now with 3 numbers:
- number of bytes to copy from beginning(A) till S(A)
- number of bytes to copy from S(B) till E(B). plus the substring that is get copied.
- number of bytes to copy from E(A) till end of string.
6 40 "cruel o_o world and thanks for the fish!" 0
- total patch size is 3 control bytes + 40 (string) = 43 bytes
ALGORITHM 2
- We delta every character in both strings, from beginning to end, and from end to beginning.
A: hello world and thanks for the fish.0000000000 B: hello cruel o_o world and thanks for the fish! C: 000000XXXXX0XXX0XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX (-)
A: 0000000000hello world and thanks for the fish. B: hello cruel o_o world and thanks for the fish! C: XXXXXXXXXXXXXX0000000000000000000000000000000X (-)
- Pick the option with most zeros (2nd). Aka, select the option with less Xs.
do { Encode every XXXX island in C as a positive operation indicating how many bytes to copy from. Run-length the number of zeros into a negative operation. } repeat till lane is exhausted.
XXXXXXXXXXXXXXX: +14 "hello cruel o_" 0000000000000000000000000000000: -29 X: +1 "!"
- Patch size is 3 control bytes + 14 (string) + 1 (string) = 18 bytes
ALGORITHM 3
- start source (A) and target (B) strings. start with A:
- select how many bytes to copy (positive), how many to skip (negative), or switch A<-->B lanes (zero).
- repeat previous step over and over until both lanes do reach their respective string ends.
A: hello world and thanks for the fish. B: hello cruel o_o world and thanks for the fish!
A: +6 0 >> hello B: -6 +10 0 >> hello cruel o_o A: +24 -1 0 >> hello cruel o_o world and thanks for the fish B: -29 +1 >> hello cruel o_o world and thanks for the fish! (10 bytes)
what if we always specify a dual +/- operation per lane? lanes changed automatically after every tuple. in this case a 0 shall indicate a no op.
A: +6 0 >> hello B: -6 +10 >> hello cruel o_o A: +24 -1 >> hello cruel o_o world and thanks for the fish B: -29 +1 >> hello cruel o_o world and thanks for the fish! (8 bytes)
- we construct the patch now. a substring excerpt must come after any positive operations on lane B.
+6 0 -6 +10 "cruel o_o " +24 -1 -29 +1 "!"
- total patch size is 8 control bytes + 10 (string) + 1 (string) = 19 bytes