Sassy Audio Spreadsheet

Sassy is a virtual modular synthesizer with the interface of a spreadsheet.

Binaries can be found at https://sol-hsa.itch.io/sassy

The project is largely split into two parts: eval, which deals with evaluating the formulas in the cells, and sassy, the user interface. There's some overlap where eval handles things that are "pure math" whereas the interface can access data files and user interface elements.

Eval

Eval contains code to evaluate the equations the user enters. The input strings are parsed into tokens, the tokens run through lexical analysis to make sure they make sense, then they're run through a simple constant folder, converted to postfix format and finally either interpreted or converted to x64 binary.

input -> parse -> lex -> opt -> postfix -> compute/jit

Note that the code is never converted to syntax tree or anything.

eval.cpp

eval.cpp contains unit tests for the formula evaluator. Most of the functions are stubbed, because eval itself does not know what they do. All of the functions are called both through both interpreted and JITted code paths and the results are compared.

There's also a simple fuzzer.

eval.h

eval.h contains definitions for all of the functions that can be used in the formulas. Each function has a FUNC_ enum and a line in the gFunc[] array. Each line of the array is in the exact order of the enum. For every function there's the function name, parameter list, const flag, how much memory is needed relating to sample rate and how much memory is needed on top of that.

Several functions may have the same name as long as the parameter list differs. These variants will have a different FUNC_ enum.

Parameter list consists of zero or more characters which may be C, A, V, T or L.

{ "rowof",     "V",    1,  0,              0 },
{ "columnof",  "V",    1,  0,              0 },
{ "dt",        "",     1,  0,              0 },
{ "step",      "C",    0,  0,              sizeof(double) },
{ "allpass",   "CCC",  0,  sizeof(double), sizeof(int) },

C is the most common argument, which is anything that eventually collapses into a numeric value. So it can be a number or it can be an equation itself.

A is an area, meaning that the parameter must evaluate into something like A3:C9. Example: average()

V is a variable, meaning that the parameter must evaluate into something like C9. Example: rowof()

T is text, that will not be used for math. It can be used as a filename or comment. Example: loadwav()

L is a literal number, and must not be an equation. In some cases having a parameter be an equation would be really problematic, so it's possible to force them to be just literal numbers. Example: buffer()

If a function is declared as const, it may get constant folded (if all its parameters are also const) and thus will not acually get called at sample rate. Functions that have state (i.e, they have allocated memory associated with them) can never be const.

Functions may need buffers to store their state. Some memory allocations are related to sample rate. For example, if a buffer is needed for echo, the length of the buffer depends on the sample rate. Other functions may need the same amount of memory regardless of the sample rate.

eval_parse.cpp

eval_parse.cpp takes the input string from a cell and parses it into an array of tokens. Each token is stored as an Op. Each op contains opcode, which may be one of C, A, V, T, L, F or some math operator */+-><=(). The opcodes are the same as with the parameter list, with the exception of the new F which stands for function.

eval_impl_lex.cpp

eval_impl_lex.cpp takes the parsed tokens and peforms lexical analysis so we know the input can be executed. Some small transformations are done here, like converting literals to numbers (since eventually they are treated equally) and adding parentheses to function parameters.

eval_impl_opt.cpp

eval_impl_opt.cpp contains a very simple and limited constant folder. If a function is constant and all its parameters are constant, the value is folded. The folder does not know how to reorder items, so things like 5+v+3 are not folded to 8+v, whereas 5+3+v is.

eval_impl_postfix.cpp

eval_impl_postfix.cpp performs postfix transform for the operations and eliminates parentheses.

Examples of the above transforms:

eval:    "((1+2)*3)+1"
Parsed: ( ( L + L ) * L ) + L
Lexed:  ( ( 1.000 + 2.000 ) * 3.000 ) + 1.000
Postfix:1.000 2.000 + 3.000 * 1.000 +


eval:    "((1+2)*3)+1"
Parsed: ( ( L + L ) * L ) + L
Lexed:  ( ( 1.000 + 2.000 ) * 3.000 ) + 1.000
Opt'd:  10.000
Postfix:10.000

eval_impl_compute.cpp

eval_impl_compute.cpp interprets the postfix-form operations and returns the resulting value. It's basically one huge switch-case function.

eval_impl_jit.cpp

eval_impl_jit.cpp takes the postfix-form operations and generates x64 code using Xbyak. Functions are generally not inlined, but called. The virtual stack from eval_impl_compute.cpp turns into actual stack, with xmm0 containing the topmost item of the virtual stack.

All of the cells are jit:ted into a single code blob to be executed once. Even though the level of optimization while jit:ing is minimal, the performance difference is massive, just by getting rid of the repeated switch-case evaluation.

Adding new functions

The process of adding new functions to sassy - one of the most fun things to do in sassy code base - goes something like this:

1. Add new enum, function definition and prototype (eval.h)
2. Add stub and tests (eval.cpp)
3. Find existing function with similar fingerprint (i.e, same number of parameters) and duplicate its code in eval_impl_compute.cpp, eval_impl_jit.cpp and, if the function is const, also in eval_impl_opt.cpp
4. Implement actual function in sassy_func.cpp, or eval_impl_func.cpp if const.
5. Write help text in sassy_help.cpp

Interface

The user interface is implemented with Dear Imgui, SDL2 and OpenGL. Native file i/o dialogs use tinyfiledialogs.

sassy.h

sassy.h contains todo list, structure definitions and prototypes for global data. All globals are prefixed with g, like gSamplerate. Alternative for globals would be to pass around some structure or (shiver) using a singleton, so deal with it.

sassy.cpp

sassy.cpp contains the main user interface logic plus a bunch of miscellaneous stuff and is a bit of a dumpster. Some of the stuff should be split to a separate file just to make things cleaner. Heck, main() itself is about a thousand lines, which could use some cleanup..

sassy_about.cpp

sassy_about.cpp has the about dialog implementation.

sassy_asio.cpp

sassy_asio.cpp has stubs for the ASIO interface. The code that was written towards ASIO support was removed because ASIO isn't open source friendly. It wasn't ready in any case, so no big loss here.

sassy_config.cpp

sassy_config.cpp hosts the config dialog implementation as well as config file i/o.

sassy_contextmenu.cpp

sassy_contextmenu.cpp deals with the right-click menu.

sassy_data.cpp

sassy_data.cpp has the implementation of global data.

sassy_func.cpp

sassy_func.cpp stores the function implementations for functions in the equations. Except the const ones, that are in eval_func.cpp.

sassy_gear_launchpad.cpp

sassy_gear_launchpad.cpp has the beginnings of novation launchpad support code. The idea in long term was to support different kinds of gear, but this didn't get too far.

sassy_help.cpp

sassy_help.cpp is where the help dialog can be found. Should probably be changed to be data file based..

sassy_keyboard.cpp

sassy_keyboard.cpp has the virtual MIDI keyboard implementation.

sassy_kludge.cpp

sassy_kludge.cpp has some kludges over Dear Imgui's text editor to enable syntax hilighting.

sassy_midi.cpp

sassy_midi.cpp has all the MIDI i/o code, including widgets.

sassy_resource.cpp

sassy_resource.cpp has all external resource related code, such as wav and image loading.

sassy_scope.cpp

sassy_scope.cpp has the virtual oscilloscope implementation.

sassy_sid.cpp

sassy_sid.cpp has c64 SID related stuff, because they're so huge that dropping them into sassy_func.cpp would have been stupid.

sassy_smf.cpp

sassy_smf.cpp has the MIDI file player.

sassy_uibar.cpp

sassy_uibar.cpp has some custom IMGUI widgets.