xlstream

xlstream is a streaming Excel formula evaluation engine written in Rust, with Python bindings. It reads .xlsx files row-by-row, evaluates formulas in a single (or two-pass) streaming traversal (no dependency graph, no full-workbook buffering), and writes the results to a new .xlsx — all in bounded memory regardless of file size.

We are not building a general-purpose spreadsheet engine. We are building the fastest, leanest Excel formula evaluator — supporting ~465 of Excel's ~500 functions, everything that fits a streaming architecture.

Performance

3x less memory and 1.4x faster than LibreOffice. No install. Embeddable. Full benchmarks

Benchmark: 100k rows x 50 cols (20 data + 30 formula). Intel i9-10910, 128 GB RAM.

Engine	Version	Wall-clock	Peak RSS	Architecture
xlstream (1 worker)	0.2.1	26.5s	643 MB	Streaming (2-pass)
xlstream (4 workers)	0.2.1	23.0s	681 MB	Streaming (2-pass)
LibreOffice	26.2	31.9s	2,081 MB	Graph
Excel (20 threads)	16.108.2	~99s	~430 MB	Graph
formualizer	0.5.6	2h 8m	11,322 MB	Full dependency graph

Supported functions

225 functions + 13 operators across 12 categories. Full list with cross-reference.

Category	Count	Examples
Operators	13	+, -, *, /, ^, &, %, comparisons
Logical	11	IF, IFS, SWITCH, IFERROR, AND, OR, NOT, XOR
Aggregates	15	SUM, SUMIF, SUMIFS, AVERAGE, COUNTIF, MEDIAN
Lookups	7	VLOOKUP, XLOOKUP, INDEX/MATCH, HLOOKUP, CHOOSE
Text	19	LEFT, UPPER, TRIM, CONCAT, TEXT, FIND, SUBSTITUTE
Date	12	TODAY, YEAR, EDATE, EOMONTH, DATEDIF, NETWORKDAYS
Math	23	ROUND, MOD, ABS, SQRT, LOG, SIN, PI, FLOOR
Info	10	ISNUMBER, ISTEXT, ISERROR, ISBLANK, ISREF, TYPE
Financial	6	PMT, PV, FV, NPV, IRR, RATE

Excel has ~500 functions. ~465 are streaming-compatible. We're adding more each release — see the roadmap.

Install

Rust:

cargo add xlstream

Python:

pip install xlstream

Usage

Python

import xlstream

result = xlstream.evaluate("input.xlsx", "output.xlsx")
# {'rows_processed': 100000, 'formulas_evaluated': 2999970, 'duration_ms': 23000}

# Parallel (row-sharded across cores)
result = xlstream.evaluate("input.xlsx", "output.xlsx", workers=8)

Rust

use std::path::Path;

let summary = xlstream::evaluate(
    Path::new("input.xlsx"),
    Path::new("output.xlsx"),
    &xlstream::EvaluateOptions::default(),
)?;

Error handling

try:
    xlstream.evaluate("input.xlsx", "output.xlsx")
except xlstream.UnsupportedFormula as e:
    print(e)  # names the cell, formula, and reason
except xlstream.FormulaParseError as e:
    print(e)
except OSError as e:
    print(e)  # file not found, corrupt xlsx

License

Dual-licensed under Apache-2.0 or MIT, at your option.