[vim/vim] refactor: convert json_encode_item() from recursive to iterative (PR #19839)

[mattn]

Convert json_encode_item() from a recursive implementation to an iterative one using an explicit stack (garray_T of json_enc_frame_T frames).

The recursive version uses C call stack proportional to nesting depth, which can cause a stack overflow (segfault) on deeply nested structures even when maxfuncdepth is set high enough to allow it. The iterative version uses heap-allocated frames, so nesting depth is limited only by available memory.

Benchmark comparison (average of 3 runs, ops/sec):

Test case	master	this branch	change
string	456,603	459,764	+0.7%
number	446,534	453,610	+1.6%
small_dict	424,878	414,664	-2.4%
nested_list	341,987	337,716	-1.2%
deep_100	325,402	327,886	+0.8%
wide_1000	12,940	12,755	-1.4%
dict_100	79,998	83,109	+3.9%
mixed	68,109	74,406	+9.2%
blob	443,559	446,154	+0.6%

No performance regression. Complex structures (mixed, dict_100) show improvement.

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You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/19839

Commit Summary

• 792c107 refactor: convert json_encode_item() from recursive to iterative

File Changes

(1 file)

• M src/json.c (371)

Patch Links:

• https://github.com/vim/vim/pull/19839.patch
• https://github.com/vim/vim/pull/19839.diff

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