For "last read" in parse errors, the lexer keeps the text of the token it reads. The token start moves only at strings and numbers, so everything after a string or number belongs to it until the next one starts. Input that cannot be read again copies that text while parsing (the istream and FILE* adapters in their carry buffer, the lexer in token_string for iterators), so valid input of 1 followed by 100,000,000 newlines peaked at about 225 MB, and a long string was kept a second time. Replace the carry buffer and token_string by token_text_buffer, which keeps the first 4096 and at least the last 4112 characters of a token and drops the characters in between where it would otherwise grow, so appending stays as cheap as before. get_token_string() shows a token of more than 8192 characters as its first and last 4096 characters around "...", the same for every kind of input; contiguous input copies only these parts. The example input now peaks at 1.8 MB. Instruction counts of parsing from std::string, std::istream and FILE* are unchanged (clang 21 and GCC 15); parsing from std::list<char> needs 1 to 17 % fewer. Fixes #6120. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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