Duvet
Duvet is a spreadsheet add-in for Casio fx-9860G or later (including Slim), both mono and color calculators, as an alternative to Casio's built in spreadsheet/S-SHT app.



Motivation
The initial idea for writing a spreadsheet add-in came because, at the time, the Math+ didn't provide a spreadsheet app. I was also interested to see whether a spreadsheet could be made to be Turing-complete with a SET(column, row, value) formula. Additionally, S-SHT was very slow and I wondered if I could do better. However, Casio announced a spreadsheet for Math+ and other projects were more interesting, so it got shelved. The project was revived when I received my fx-CG50 and my CPC game entry would load, but could not be edited without a math error, despite working (albeit very slowly) on the fx-9750GIII and even working on the fx-9860G Slim. In order to get the game working on the fx-CG50, I had to delete over 500 cells. And, even then, it was still quite slow. I wanted my game to be playable on all calculators, so I challenged myself to write a spreadsheet add-in before CPC 31.

Goals
My goals for Duvet are:

1. Faster calculation than S-SHT. Duvet uses integer math, when possible, and the plan is to examine floating point results and return them back to integers, when possible. For example, 10 / 2 = 5.0, but 5.0 == 5, so it can remain an integer, and cos(0) = 1. Currently, Duvet calculates my CPC entry, a sheet containing around 1000 formula cells, in a fraction of a second, compared to approximately 7 seconds in S-SHT on an fx-9750GIII. Video from an earlier build (fx-9750GIII and S-SHT on the left, fx-9860G Slim and Duvet on the right):

2. Provide a larger sheet area than S-SHT, using sparse cells. Currently, Duvet allows 26 columns (A-Z) and 2048 rows.

3. Be compliant with some subset of ODF types, operators and formulas. In particular, I want to allow formulas involving TEXT fields, which isn't possible in S-SHT.

4. Turing-completeness, by way of a SET(column,row,value) formula.

5. File import/export. Formats to be determined, but at least CSV import/export.

6. Best effort: due to spreadsheets often being used for financial calculations, I'd like to use decimal64 floats (or an equivalent) rather than using double. The decimal library I was planning on using (libdfp)) is not available for SuperH, and I need to investigate the feasibility of porting it.

Notably, it is not a goal to perfectly replicate the UI or feature set of Casio's S-SHT. For example, the UI has already diverged slightly, the menus and key input are going to be different, and I don't currently plan on implementing graphing, regressions, list import/export, etc. Nor is it a goal to be 100% Open Document Format or OpenFormula compliant. However, I'll strive (best effort) for subset compliance as far as being able to export a LibreOffice sheet to CSV and have some subset of OpenFormula formulas, operators, and data types work without changes.

Formulas
All formulas are currently limited to 64-bit integer math:

• ABS(integer) → integer
• BITAND(integer,integer) → integer
• BITLSHIFT(integer,integer) → integer
• BITOR(integer,integer) → integer
• BITRSHIFT(integer,integer) → integer
• BITXOR(integer,integer) → integer
• FALSE() → integer
• IF(integer,integer,integer) → integer
• ISEVEN(integer) → integer
• ISODD(integer) → integer
• MOD(integer,integer) → integer
• NOT(integer) → integer
• SIGN(integer) → integer
• SUM(range) → integer
• TRUE() → integer

Formulas Planned
This is just an initial list of formulas where integer math can be used; there will be more once I implement the NUMBER type and determine which TEXT operations are feasible:

AND(range), AVERAGE(range), DELTA(a,b), FACT(integer), GCD(range), LCM(range), MAX(range), MIN(range), OR(range), POWER(integer,integer), PRODUCT(range), QUOTIENT(integer,integer), RANDBETWEEN(integer,integer), SET(column,row,value), XOR(range)

Operators
Listed in order of precedence, highest first:

• (integer) → integer
• -integer → integer, +integer → integer
• integer^integer → integer
• integer*integer → integer, integer/integer → integer
• integer+integer → integer, integer-integer → integer
• integer=integer → integer, integer<>integer → integer, integer<integer → integer, integer<=integer → integer, integer>integer → integer, integer>=integer → integer

Limitations
Currently Duvet is extremely limited. It can play my CPC #31 entry, but that's about it:

• Cannot create new sheets.
• Sheets cannot be edited, except for cells containing integers (and also 64-bit integers cannot currently be edited).
• Cells cannot be created, deleted, edited to become empty, copied, moved, etc.
• Only loads and saves spreadsheets in a proprietary format.
• Floating point math is not available (needed for Video Poker)
• A Ran# substitute is not available (also needed for Video Poker)

Name
The name is meant to be silly. In British English, a duvet (or "duvet cover" here in USA) is a comforter, often used instead of a bedsheet ("sheet"). So, the name represents the idea of Duvet replacing S-SHT. And the name also appeals to me because it's a word common to both French and English.