Fix null pointer handling in edit_distance

[THE-Amrit-mahto-05]

In raising this pull request, I confirm the following (please check boxes):

Reason for this PR:

• This PR adds new functionality.
• This PR fixes a bug that I have personally experienced or that a real user has reported and for which a sample exists.
• This PR is porting code from C to Rust.

---

Sanity check:

• I have read and understood the contributors guide.
• I have checked that another pull request for this purpose does not exist.
• If the PR adds new functionality, I've added it to the changelog. If it's just a bug fix, I have NOT added it to the changelog.
• I am NOT adding new C code unless it's to fix an existing, reproducible bug.

---

Problem

The function edit_distance is part of the Rust FFI layer and is exposed to C code. In C usage, NULL pointers are valid inputs, but the current implementation does not safely handle them.

Issue

• Passing NULL word1 or word2 leads to unsafe dereference via CStr::from_ptr
• This results in undefined behavior and potential segmentation faults

Proof

The issue can be reproduced by calling the function with NULL pointers from the FFI boundary.

A crash was observed during test execution due to a NULL pointer dereference (SIGSEGV).

---

Steps to reproduce

1. Call:
   edit_distance(word1 = NULL, word2 = "abc", len1 = 0, len2 = 3)
2. Call:
   edit_distance(word1 = "abc", word2 = NULL, len1 = 3, len2 = 0)

---

Expected behavior

The function should safely handle NULL inputs and return:

• Length of the non-null string
• 0 if both inputs are NULL

---

Actual behavior (before fix)

Unsafe pointer dereference occurs when converting NULL pointers using CStr::from_ptr, leading to undefined behavior or crash.

---

Fix

• Added NULL pointer checks before any CStr::from_ptr usage
• Safe handling logic introduced:
  • both NULL → return 0
  • word1 NULL → return len2
  • word2 NULL → return len1
• Prevents unsafe dereferencing at the FFI boundary

In the C FFI boundary, NULL pointers are treated as empty inputs when passed to edit_distance.
When one input is NULL, the function returns the length of the other string, treating it as a full deletion/insertion cost.
This behavior is consistent with the existing interpretation of edit distance and ensures safe handling without undefined behavior.

---

Testing

• Ran: cargo test --features hardsubx_ocr
• Result: 406 tests passed, 0 failures
• No regressions introduced in edit_distance

---

Impact

This change improves memory safety and prevents undefined behavior in Rust bindings exposed to C via FFI.

[ccextractor-bot]

CCExtractor CI platform finished running the test files on linux. Below is a summary of the test results, when compared to test for commit ad4886e...:

Report Name	Tests Passed
Broken	9/13
CEA-708	1/14
DVB	2/7
DVD	3/3
DVR-MS	2/2
General	22/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	76/86
Teletext	20/21
WTV	13/13
XDS	31/34

Your PR breaks these cases:

• ccextractor --autoprogram --out=ttxt --latin1 --ucla --xds 8e8229b88b...
• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2...
• ccextractor --autoprogram --out=ttxt --latin1 99e5eaafdc...
• ccextractor --autoprogram --out=ttxt --latin1 --ucla 7aad20907e...
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65...
• ccextractor --autoprogram --out=ttxt --latin1 01509e4d27...
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b...
• ccextractor --out=spupng c83f765c66...
• ccextractor --datastreamtype 2 --streamtype 2 c83f765c66...
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --autoprogram --out=ttxt --xds --latin1 --ucla 85058ad37e...
• ccextractor --autoprogram --out=srt --latin1 --ucla b22260d065...
• ccextractor --autoprogram --out=ttxt --latin1 --ucla --xds 7f41299cc7...

NOTE: The following tests have been failing on the master branch as well as the PR:

• ccextractor --out=srt --latin1 --autoprogram 73d9313d64..., Last passed:

  Test 8738

• ccextractor --out=ttxt --latin1 001dd8cdf7..., Last passed:

  Test 8738

• ccextractor --out=srt --latin1 4d4e938ef6..., Last passed:

  Test 8738

• ccextractor --service 1 --out=txt --no-bom --no-rollup ea83ff7bcb..., Last passed:

  Test 8738

• ccextractor --service 1 --out=txt f17524b53f..., Last passed:

  Test 8738

• ccextractor --service 1 --out=txt 80848c45f8..., Last passed:

  Test 8738

• ccextractor --service 1 --out=txt --no-bom --no-rollup b5d6aad89f..., Last passed:

  Test 8738

• ccextractor --service 1[EUC-KR] --out=txt --no-rollup b5d6aad89f..., Last passed:

  Test 8738

• ccextractor --service 1 --out=srt da904de35d..., Last passed:

  Test 8738

• ccextractor --service 1 --out=sami da904de35d..., Last passed:

  Test 8738

• ccextractor --service 1 --out=ttxt da904de35d..., Last passed:

  Test 8926

• ccextractor --service 1[EUC-KR] b5d6aad89f..., Last passed:

  Test 8738

• ccextractor --service 1[EUC-KR] --no-rollup b5d6aad89f..., Last passed:

  Test 8738

• ccextractor --service all da904de35d..., Last passed:

  Test 8738

• ccextractor --service all[EUC-KR] b5d6aad89f..., Last passed:

  Test 8738

• ccextractor --service 1,2[UTF-8],3[EUC-KR],54 --out=txt da904de35d..., Last passed:

  Test 8738

• ccextractor --autoprogram --out=srt --latin1 f1422b8bfe..., Last passed:

  Test 9208

• ccextractor --autoprogram --out=srt --latin1 d41b53b504..., Last passed:

  Test 8738

• ccextractor --stdout --quiet --no-fontcolor 79a51f3500..., Last passed:

  Test 8738

• ccextractor --stdout --quiet --no-fontcolor 767b546f96..., Last passed:

  Test 8738

• ccextractor --service 1 c83f765c66..., Last passed:

  Test 8738

• ccextractor --myth c83f765c66..., Last passed:

  Test 8738

• ccextractor --in=raw fb79021542..., Last passed:

  Test 8738

• ccextractor --mp4vidtrack 5df914ce77..., Last passed:

  Test 8738

• ccextractor --xmltv=3 --out=null 96efd279cf..., Last passed:

  Test 8738

• ccextractor --datapid 2310 --autoprogram --out=srt --latin1 e639e54550..., Last passed:

  Test 8738

Congratulations: Merging this PR would fix the following tests:

• ccextractor --autoprogram --out=ttxt --latin1 132d7df7e9..., Last passed: Never
• ccextractor --autoprogram --out=srt --latin1 b22260d065..., Last passed: Never
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never

It seems that not all tests were passed completely. This is an indication that the output of some files is not as expected (but might be according to you).

Check the result page for more info.

[ccextractor-bot]

CCExtractor CI platform finished running the test files on windows. Below is a summary of the test results, when compared to test for commit ad4886e...:

Report Name	Tests Passed
Broken	9/13
CEA-708	1/14
DVB	3/7
DVD	3/3
DVR-MS	2/2
General	22/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	81/86
Teletext	20/21
WTV	13/13
XDS	31/34

Your PR breaks these cases:

• ccextractor --autoprogram --out=ttxt --latin1 --ucla --xds 8e8229b88b...
• ccextractor --autoprogram --out=ttxt --latin1 132d7df7e9...
• ccextractor --autoprogram --out=ttxt --latin1 99e5eaafdc...
• ccextractor --autoprogram --out=srt --latin1 b22260d065...
• ccextractor --autoprogram --out=ttxt --latin1 --ucla 7aad20907e...
• ccextractor --autoprogram --out=ttxt --latin1 01509e4d27...
• ccextractor --autoprogram --out=ttxt --xds --latin1 --ucla 85058ad37e...
• ccextractor --autoprogram --out=srt --latin1 --ucla b22260d065...
• ccextractor --autoprogram --out=ttxt --latin1 --ucla --xds 7f41299cc7...

NOTE: The following tests have been failing on the master branch as well as the PR:

• ccextractor --out=srt --latin1 --autoprogram 73d9313d64..., Last passed:

  Test 8611

• ccextractor --out=ttxt --latin1 001dd8cdf7..., Last passed:

  Test 8611

• ccextractor --out=srt --latin1 4d4e938ef6..., Last passed:

  Test 8611

• ccextractor --service 1 --out=txt --no-bom --no-rollup ea83ff7bcb..., Last passed:

  Test 8611

• ccextractor --service 1 --out=txt f17524b53f..., Last passed:

  Test 8611

• ccextractor --service 1 --out=txt 80848c45f8..., Last passed:

  Test 8611

• ccextractor --service 1 --out=txt --no-bom --no-rollup b5d6aad89f..., Last passed:

  Test 8611

• ccextractor --service 1[EUC-KR] --out=txt --no-rollup b5d6aad89f..., Last passed:

  Test 8611

• ccextractor --service 1 --out=srt da904de35d..., Last passed:

  Test 8611

• ccextractor --service 1 --out=sami da904de35d..., Last passed:

  Test 8611

• ccextractor --service 1 --out=ttxt da904de35d..., Last passed:

  Test 8943

• ccextractor --service 1[EUC-KR] b5d6aad89f..., Last passed:

  Test 8611

• ccextractor --service 1[EUC-KR] --no-rollup b5d6aad89f..., Last passed:

  Test 8611

• ccextractor --service all da904de35d..., Last passed:

  Test 8611

• ccextractor --service all[EUC-KR] b5d6aad89f..., Last passed:

  Test 8611

• ccextractor --service 1,2[UTF-8],3[EUC-KR],54 --out=txt da904de35d..., Last passed:

  Test 8611

• ccextractor --autoprogram --out=srt --latin1 f1422b8bfe..., Last passed:

  Test 9209

• ccextractor --autoprogram --out=srt --latin1 d41b53b504..., Last passed:

  Test 8611

• ccextractor --stdout --quiet --no-fontcolor 79a51f3500..., Last passed:

  Test 8611

• ccextractor --stdout --quiet --no-fontcolor 767b546f96..., Last passed:

  Test 8611

• ccextractor --service 1 c83f765c66..., Last passed:

  Test 8611

• ccextractor --myth c83f765c66..., Last passed:

  Test 8611

• ccextractor --in=raw fb79021542..., Last passed:

  Test 8611

• ccextractor --mp4vidtrack 5df914ce77..., Last passed:

  Test 8611

• ccextractor --xmltv=3 --out=null 96efd279cf..., Last passed:

  Test 8611

• ccextractor --datapid 2310 --autoprogram --out=srt --latin1 e639e54550..., Last passed:

  Test 8611

Congratulations: Merging this PR would fix the following tests:

• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2..., Last passed: Never
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65..., Last passed: Never
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b..., Last passed: Never
• ccextractor --out=spupng c83f765c66..., Last passed: Never
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never

It seems that not all tests were passed completely. This is an indication that the output of some files is not as expected (but might be according to you).

Check the result page for more info.