RISC OS Font control parsing in Python

This repository contains a class (FontControlParser) for parsing font control codes from a byte squence, in Python.

It is part of the RISC OS Pyromaniac font system, and provides support for the SWIs which operate on the font strings:

Font_Paint - uses the controls to determine what should be rendered.
Font_ScanString - uses the controls to determine the size or break points of a string.

And all the other calls which call on to Font_ScanString:

Font_StringWidth - reads the width of a string.
Font_StringBBox - reads the coverage of a string.
Font_FindCaret - finds the position of the caret within a string.
Font_FindCaretJ - a variant of the Font_FindCaret call.

The FontContext provides information which allows the operation of the following calls:

Font_CurrentFont - reads the current font.
Font_FutureFont - reads what the font would be after one of the sizing calls.
Font_CurrentRGB - reads the current colour.
Font_FutureRGB - reads what the colour would be after one of the sizing calls.

Usage

Inside RISC OS Pyromaniac...

The FontControlParser is subclassed to allow the memory access to occur within the emulated memory, not using the bytes.
The FontSpacing is subclassed to create spacing from the memory blocks.
The FontContext is subclassed to allow the GCOL/RGB operations, font lookups, sizing and rendering operations to be performed on the RISC OS graphics system.

A font context is created on initialisation, and will be updated by different operations:

self.context = FontContextPyromaniac(self.ro, self.fonts)

The font parser is constructed and supplied the memory buffers to parse:

memstring = self.ro.memory[regs[1]]
fc = FontControlParserPyromaniac(self.ro)
fc.debug_enable = self.debug_fontparser
fc.parse(memstring, string_length)

Once the other parameters for Font_Paint have been decoded and spacing and transformed written to the FontContext, the paint operation is called.

self.context.x = xmilli
self.context.y = ymilli
self.context.transform = transform

with self.ro.kernel.graphics.vducursor.disable():
    self.context.paint(fc.sequence, spacing)

    # Update OS_ChangedBox
    x0 = int(self.context.bounds.x0 / riscos.graphics.font.FontConstants.Font_OSUnit) >> xeig
    y0 = int(self.context.bounds.y0 / riscos.graphics.font.FontConstants.Font_OSUnit) >> xeig
    x1 = int(self.context.bounds.x1 / riscos.graphics.font.FontConstants.Font_OSUnit) >> yeig
    y1 = int(self.context.bounds.y1 / riscos.graphics.font.FontConstants.Font_OSUnit) >> yeig
    self.ro.kernel.graphics.changedbox_update(x0, y0, x1, y1)

Font_ScanString is similar, but instead of operating on the current context, the future context is updated:

self.context.copy(to=self.future_context)
...
memstring = self.ro.memory[regs[1]]
fc = FontControlParserPyromaniac(self.ro)
fc.debug_enable = self.debug_fontparser
fc.parse(memstring, string_length)
...
(split_offset, splits) = self.future_context.size(fc.sequence, spacing=spacing, limits=(xmilli, ymilli),
                                                  split_char=split_char)

This allows all the Font_Future* calls to query the self.future_context