[Bugfix]Fix search start_index of stop_checker

[xu-song]

Fix search start_index of stop_checker

test script

For example
output_text="abcdef", new_char="cdef", stop_str="de", start_index should be 1.

Below is a comprehensive test script:

def find_test(output_text, stop_str, new_char_count):
    print(
        f"output_text: {output_text} (old_char|new_char: {output_text[:-new_char_count]}|{output_text[-new_char_count:]})"
    )
    print(f"stop_str: {stop_str}")
    stop_string_len = len(stop_str)
    search_start = len(output_text) - new_char_count - stop_string_len + 1
    # search_start = 1 - new_char_count - stop_string_len
    print(f"should search start at: {search_start}")

    stop_index = output_text.find(stop_str, search_start)

    print(f"searched_index: {stop_index}")
    if stop_index >= 0:
        print(output_text[:stop_index])
    print("")


examples = [
    ("abcdef", "de", 5),
    ("abcdef", "de", 4),
    ("abcdef", "de", 3),
    ("abcdef", "de", 2),
]
for example in examples:
    find_test(*example)

log

output_text: abcdef (old_char|new_char: a|bcdef)
stop_str: de
should search start at: 0
searched_index: 3
abc

output_text: abcdef (old_char|new_char: ab|cdef)
stop_str: de
should search start at: 1
searched_index: 3
abc

output_text: abcdef (old_char|new_char: abc|def)
stop_str: de
should search start at: 2
searched_index: 3
abc

output_text: abcdef (old_char|new_char: abcd|ef)
stop_str: de
should search start at: 3
searched_index: 3
abc

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[njhill]

Thanks @xu-song ... just to clarify, you're saying that the substring checked each time is one char longer than it needs to be?

[njhill]

Also please update your commit with a sign-off 🙏 : https://github.com/vllm-project/vllm/pull/13280/checks?check_run_id=37223705852

[njhill]

Thanks @xu-song, nice catch. Please make the small simplification and address the DCO thing!

[njhill]

This is safe since both new_char_count and stop_string_len must be >= 1.

Suggested change

	stop_index = output_text.find(stop_str,
	len(output_text) - new_char_count - stop_string_len + 1)
	stop_index = output_text.find(stop_str, 1 - new_char_count - stop_string_len)

[xu-song]

Thanks for your suggestion. A new commit has been pushed which resolves DCO and code simplification.