XLSXCellPos::GetColumnName off-by-one error

[genericallyterrible]

Environment

Attribute	Value
library_version	v1.3.1
source_id	2063dda3e6
platform	windows_amd64
excel extension_version	7e97933

Issue

When querying sheets with many columns and providing header=false, the generated column names experience an off-by-one error at every rollover from "Z" to "AA", or "ZZ" to "AAA", etc. Currently "Z" is followed by "BA" instead of "AA". I created a sheet column_name.xlsx to test this behavior.

Excerpt:

1	2	...	704	705
A	B	...	AAB	AAC

From my testing this erroneous behavior only applies to read_xlsx and not write_xlsx.

Z -> AA Rollover Error

FROM read_xlsx(
	'column_name.xlsx',
	all_varchar=true,
	header=false,
	range="Z1:AA2"
);

Current behavior

Z varchar	BA varchar
26	27
Z	AA

Expected behavior

Z varchar	AA varchar
26	27
Z	AA

ZZ -> AAA Rollover Error

FROM read_xlsx(
	'column_name.xlsx',
	all_varchar=true,
	header=false,
	range="ZZ1:AAA2"
);

Current behavior

BAZ varchar	BBA varchar
702	703
ZZ	AAA

Expected behavior

ZZ varchar	AAA varchar
702	703
ZZ	AAA

Possible Resolution

As I'm unable to compile the project locally, I didn't feel comfortable issuing a pull request but I believe I've found the source of the inconsistency.

Suspected Code

duckdb-excel/src/excel/include/xlsx/xlsx_parts.hpp

Lines 86 to 95 in 9189a80

	inline string XLSXCellPos::GetColumnName() const {
	D_ASSERT(col != 0);
	string result;
	idx_t col = this->col - 1;
	do {
	result = static_cast<char>('A' + col % 26) + result;
	col /= 26;
	} while (col > 0);
	return result;
	}

Suggestion

From some testing outside this project, I believe moving the decrement of col inside the loop body resolves this issue.

inline string XLSXCellPos::GetColumnName() const {
	D_ASSERT(col != 0);
	string result;
	idx_t col = this->col;
	do {
		col -= 1
		result = static_cast<char>('A' + col % 26) + result;
		col /= 26;
	} while (col > 0);
	return result;
}

With the decrement of col moved inside the loop body, the do-while could be rewritten as just a while loop.

inline string XLSXCellPos::GetColumnName() const {
	D_ASSERT(col != 0);
	string result;
	idx_t col = this->col;
	while (col > 0) {
		col -= 1
		result = static_cast<char>('A' + col % 26) + result;
		col /= 26;
	}
	return result;
}

Thank you so much for your time and effort on this awesome extension!