akbasic/src/runtime_graphics.c

/**
 * @file runtime_graphics.c
 * @brief The group G verb implementations: GRAPHIC, COLOR, DRAW, BOX, CIRCLE,
 *        PAINT, SCALE, SSHAPE, GSHAPE and LOCATE.
 *
 * Nothing here includes an SDL or a libakgl header. Every drawing call goes
 * through the akbasic_GraphicsBackend record the host attached, which is what
 * lets the whole group be tested in a build with no SDL on the machine.
 *
 * These verbs are not in the Go reference -- it lists all of them as
 * unimplemented -- so the semantics come from Commodore BASIC 7.0 rather than
 * from a port. Where 7.0 does something a modern renderer cannot, the deviation
 * is recorded in TODO.md section 5 rather than papered over here.
 */

#include <math.h>
#include <stdio.h>
#include <string.h>

#include <akerror.h>
#include <akstdlib.h>

#include <akbasic/args.h>
#include <akbasic/error.h>
#include <akbasic/runtime.h>

#include "verbs.h"

/* Most verbs answer "did something happen"; this is that answer. */
#define SUCCEED_TRUE(__obj, __dest)			\
    do {						\
	*(__dest) = &(__obj)->staticTrueValue;		\
    } while ( 0 )

/** @brief What SSHAPE writes into the string variable, ahead of the slot number. */
#define SHAPE_PREFIX		"SHAPE:"

/** @brief CIRCLE's default arc increment, in degrees. BASIC 7.0 uses two. */
#define CIRCLE_DEFAULT_INC	2.0

/**
 * @brief Refuse politely when the host lent us no graphics device, and ask its size.
 *
 * Every verb in this file opens with it. The standalone driver attaches no
 * backend at all, so this is the common path rather than an edge case, and it
 * has to name the verb -- "no graphics device" on its own tells a program author
 * nothing about which line to look at.
 *
 * It is also where the drawing surface's size is refreshed, which is why the two
 * jobs are one function: the size is wanted by every verb that draws, every verb
 * that draws already opens with this, and a window can be resized between two
 * statements. A backend with no `size` -- it is the record's one optional entry
 * point -- keeps whatever akbasic_graphics_state_init() left, which is 320x200.
 */
static akerr_ErrorContext *require_graphics(akbasic_Runtime *obj, const char *verb)
{
    PREPARE_ERROR(errctx);
    int width = 0;
    int height = 0;

    FAIL_ZERO_RETURN(errctx, (obj != NULL && verb != NULL), AKERR_NULLPOINTER,
		     "NULL argument in require_graphics");
    FAIL_ZERO_RETURN(errctx, (obj->graphics != NULL), AKBASIC_ERR_DEVICE,
		     "%s needs a graphics device and this runtime has none", verb);
    if ( obj->graphics->size != NULL ) {
	PASS(errctx, obj->graphics->size(obj->graphics, &width, &height));
	/*
	 * A device reporting nothing useful is not an error -- an SDL window
	 * mid-resize legitimately measures zero -- but it must not become the
	 * space a program draws into, because dividing by it in scale_point
	 * would send every scaled coordinate to infinity.
	 */
	if ( width > 0 && height > 0 ) {
	    obj->gfx.devwidth = width;
	    obj->gfx.devheight = height;
	}
    }
    SUCCEED_RETURN(errctx);
}

/**
 * @brief Map a user coordinate onto the device pixels the backend receives.
 *
 * With SCALE off this is the identity, and a coordinate is a device pixel: the
 * whole of the host's window is reachable from BASIC. With SCALE on, the
 * program's coordinates run 0..xmax across the same surface, so the ratio is all
 * there is to it -- and the surface is however big the device last said it was,
 * which is what makes `SCALE 1, 319, 199` the way to run a C128 listing full
 * screen.
 *
 * **`xmax` maps onto the last pixel, not onto the width**, which is a
 * one-character difference and the whole difference between that sentence being
 * true and being nearly true. Dividing by the width sends `SCALE 1, 319, 199`
 * plus `DRAW 1, 319, 199` to (width, height) -- one past the bottom-right
 * corner, off the surface, drawing nothing. A user maximum is a coordinate the
 * program is entitled to plot, the same way 7.0's own `SCALE 1` then
 * `DRAW 1, 1023, 1023` reaches the corner of a C128 screen rather than missing it.
 */
static void scale_point(akbasic_GraphicsState *gfx, double *x, double *y)
{
    if ( !gfx->scaling ) {
	return;
    }
    if ( gfx->xmax > 0.0 && gfx->devwidth > 1 ) {
	*x = (*x / gfx->xmax) * (double)(gfx->devwidth - 1);
    }
    if ( gfx->ymax > 0.0 && gfx->devheight > 1 ) {
	*y = (*y / gfx->ymax) * (double)(gfx->devheight - 1);
    }
}

/**
 * @brief Draw a line `WIDTH` pixels thick.
 *
 * BASIC 7.0's `WIDTH` sets the thickness of the lines the graphics verbs draw,
 * and the backend record has no thickness argument -- neither does libakgl's
 * akgl_draw_line, which is what it would have to reach. So a thick line is drawn
 * as parallel passes, offset perpendicular to whichever axis the line runs along.
 * That is what a C128 does in effect and it needs nothing new from the device.
 *
 * Approximate for a diagonal, where offsetting along one axis leaves the passes a
 * fraction under a pixel apart. At WIDTH 2 -- the only other value the verb
 * accepts -- that is not visible; a general thickness would want a real
 * perpendicular offset and an API that takes one.
 */
static akerr_ErrorContext *draw_line(akbasic_Runtime *obj, double x1, double y1, double x2, double y2, akbasic_Color color)
{
    PREPARE_ERROR(errctx);
    double dx = (x2 > x1 ? x2 - x1 : x1 - x2);
    double dy = (y2 > y1 ? y2 - y1 : y1 - y2);
    bool steep = (dy > dx);
    int pass = 0;
    int width = (obj->gfx.linewidth > 0 ? obj->gfx.linewidth : 1);

    /*
     * A loop, so CATCH and the _BREAK macros are banned in here -- they expand
     * to a C break and would leave the loop with an error still pending.
     */
    for ( pass = 0; pass < width; pass++ ) {
	double offset = (double)pass;

	if ( steep ) {
	    PASS(errctx, obj->graphics->line(obj->graphics, x1 + offset, y1,
					     x2 + offset, y2, color));
	} else {
	    PASS(errctx, obj->graphics->line(obj->graphics, x1, y1 + offset,
					     x2, y2 + offset, color));
	}
    }
    SUCCEED_RETURN(errctx);
}

/* ------------------------------------------------------------- GRAPHIC --- */

akerr_ErrorContext *akbasic_cmd_graphic(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    double args[2];
    int count = 0;
    int mode = 0;
    akbasic_Color background;

    (void)lval; (void)rval;
    PASS(errctx, require_graphics(obj, "GRAPHIC"));
    PASS(errctx, akbasic_args_numbers(obj, expr, "GRAPHIC", args, 2, &count));
    FAIL_ZERO_RETURN(errctx, (count >= 1), AKBASIC_ERR_SYNTAX, "GRAPHIC expected a mode");

    mode = (int)args[0];
    /*
     * BASIC 7.0 numbers five modes plus a CLR. They differ in bitmap resolution
     * and in whether the bottom of the screen stays text, neither of which means
     * anything against a host's renderer -- so the mode is recorded and only its
     * one observable consequence is honoured: mode 0 is text, and text mode does
     * not draw. Refusing an out-of-range mode still matters, because that is a
     * typo the program author can fix.
     */
    FAIL_ZERO_RETURN(errctx, (mode >= 0 && mode <= 5), AKBASIC_ERR_BOUNDS,
		     "GRAPHIC mode %d out of range (0 to 5)", mode);
    if ( mode == 5 ) {
	/* GRAPHIC CLR: drop the saved shapes and go back to text. */
	PASS(errctx, obj->graphics->free_shapes(obj->graphics));
	PASS(errctx, akbasic_graphics_state_init(&obj->gfx));
	SUCCEED_TRUE(obj, dest);
	SUCCEED_RETURN(errctx);
    }

    obj->gfx.mode = mode;
    if ( count >= 2 && args[1] != 0.0 ) {
	PASS(errctx, akbasic_graphics_source_color(&obj->gfx, 0, &background));
	PASS(errctx, obj->graphics->clear(obj->graphics, background));
	obj->gfx.x = 0.0;
	obj->gfx.y = 0.0;
    }
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

/* --------------------------------------------------------------- COLOR --- */

akerr_ErrorContext *akbasic_cmd_color(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    double args[2];
    int count = 0;
    int source = 0;
    int index = 0;

    (void)lval; (void)rval;
    /*
     * COLOR is the one verb in the group that does not touch the device: it
     * binds a source to a palette index and the drawing verbs resolve that later.
     * So it works with no graphics backend attached, which lets a program set its
     * colors up before the host has lent it a renderer.
     */
    FAIL_ZERO_RETURN(errctx, (obj != NULL), AKERR_NULLPOINTER, "NULL runtime in COLOR");
    PASS(errctx, akbasic_args_numbers(obj, expr, "COLOR", args, 2, &count));
    FAIL_ZERO_RETURN(errctx, (count == 2), AKBASIC_ERR_SYNTAX,
		     "COLOR expected a source and a color");

    source = (int)args[0];
    index = (int)args[1];
    FAIL_ZERO_RETURN(errctx, (source >= 0 && source < AKBASIC_COLOR_SOURCES),
		     AKBASIC_ERR_BOUNDS, "COLOR source %d out of range (0 to %d)",
		     source, AKBASIC_COLOR_SOURCES - 1);
    FAIL_ZERO_RETURN(errctx, (index >= 1 && index <= 16), AKBASIC_ERR_BOUNDS,
		     "COLOR %d out of range (1 to 16)", index);
    obj->gfx.source[source] = index;
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

/* -------------------------------------------------------------- LOCATE --- */

akerr_ErrorContext *akbasic_cmd_locate(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    double args[2];
    int count = 0;

    (void)lval; (void)rval;
    FAIL_ZERO_RETURN(errctx, (obj != NULL), AKERR_NULLPOINTER, "NULL runtime in LOCATE");
    PASS(errctx, akbasic_args_numbers(obj, expr, "LOCATE", args, 2, &count));
    FAIL_ZERO_RETURN(errctx, (count == 2), AKBASIC_ERR_SYNTAX, "LOCATE expected X, Y");

    /*
     * LOCATE moves the *graphics* pixel cursor -- the point a DRAW with no
     * starting coordinate begins from. The text cursor is CHAR's business. It
     * needs no device, so it works before a renderer is attached.
     */
    obj->gfx.x = args[0];
    obj->gfx.y = args[1];
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

/* --------------------------------------------------------------- SCALE --- */

akerr_ErrorContext *akbasic_cmd_scale(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    double args[3];
    int count = 0;

    (void)lval; (void)rval;
    FAIL_ZERO_RETURN(errctx, (obj != NULL), AKERR_NULLPOINTER, "NULL runtime in SCALE");
    PASS(errctx, akbasic_args_numbers(obj, expr, "SCALE", args, 3, &count));
    FAIL_ZERO_RETURN(errctx, (count >= 1), AKBASIC_ERR_SYNTAX, "SCALE expected 0 or 1");

    /* Pure coordinate arithmetic -- nothing reaches the device. */
    obj->gfx.scaling = (args[0] != 0.0);
    if ( count >= 3 ) {
	FAIL_ZERO_RETURN(errctx, (args[1] > 0.0 && args[2] > 0.0), AKBASIC_ERR_VALUE,
			 "SCALE maxima must be positive");
	obj->gfx.xmax = args[1];
	obj->gfx.ymax = args[2];
    } else if ( obj->gfx.scaling ) {
	/* BASIC 7.0's default scaled space. */
	obj->gfx.xmax = 1023.0;
	obj->gfx.ymax = 1023.0;
    }
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

/* ----------------------------------------------------------------- RGR --- */

akerr_ErrorContext *akbasic_fn_rgr(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    akbasic_Value *out = NULL;
    double args[1];
    int count = 0;
    int field = 0;
    int columns = 0;
    int rows = 0;
    int cellw = 0;
    int cellh = 0;
    int64_t answer = 0;

    (void)lval; (void)rval;
    FAIL_ZERO_RETURN(errctx, (obj != NULL && dest != NULL), AKERR_NULLPOINTER,
		     "NULL argument in RGR");
    PASS(errctx, akbasic_args_numbers(obj, expr, "RGR", args, 1, &count));
    FAIL_ZERO_RETURN(errctx, (count >= 1), AKBASIC_ERR_SYNTAX, "RGR expected a field number");
    field = (int)args[0];

    /*
     * Field 0 is BASIC 7.0's whole RGR: the current GRAPHIC mode. It answers
     * from state and works with no device, because a program can ask what mode
     * it set without a screen to set it on.
     *
     * Fields 1 and 2 are ours, and they are the half of item 9 a program can
     * see: the drawing surface's real width and height in pixels, so a listing
     * can fill whatever window the host opened instead of assuming 320x200.
     * They go through require_graphics(), which is both the refusal when there
     * is no device -- asking how big the screen is when there is no screen is a
     * program bug worth reporting -- and the refresh that makes the answer
     * current rather than whatever it was at attach.
     *
     * Fields 3 and 4 are ours too, and come from the text sink rather than the
     * graphics device; see the comment on them below.
     */
    switch ( field ) {
    case 0:
	answer = (int64_t)obj->gfx.mode;
	break;
    case 1:
	PASS(errctx, require_graphics(obj, "RGR"));
	answer = (int64_t)obj->gfx.devwidth;
	break;
    case 2:
	PASS(errctx, require_graphics(obj, "RGR"));
	answer = (int64_t)obj->gfx.devheight;
	break;
    /*
     * Fields 3 and 4 are the character cell, in the same pixels fields 1 and 2
     * are in -- so a program can place a character and a sprite at the same spot
     * without hardcoding a size measured against the host's font. They come from
     * the *sink*, which is where a grid lives, and the grid is a text device's
     * property rather than a graphics one: hence a separate refusal, naming the
     * text device rather than the graphics one.
     *
     * `RWINDOW` answers the other half -- how many columns and rows there are --
     * and it is the C128's own function for that question. This is here rather
     * than on RWINDOW because a cell size is a fact about the surface, and
     * because RWINDOW reports the current *window*, which makes dividing the
     * surface by the column count wrong the moment a program calls `WINDOW`.
     */
    case 3:
    case 4:
	FAIL_ZERO_RETURN(errctx, (obj->sink != NULL), AKBASIC_ERR_DEVICE,
			 "RGR needs a text device and this runtime has none");
	FAIL_ZERO_RETURN(errctx, (obj->sink->grid != NULL), AKBASIC_ERR_DEVICE,
			 "RGR needs a text device with a character grid, and this one has none");
	PASS(errctx, obj->sink->grid(obj->sink, &columns, &rows, &cellw, &cellh));
	answer = (field == 3 ? (int64_t)cellw : (int64_t)cellh);
	break;
    default:
	FAIL_RETURN(errctx, AKBASIC_ERR_BOUNDS, "RGR: field %d is outside 0..4", field);
    }

    PASS(errctx, akbasic_environment_new_value(obj->environment, &out));
    PASS(errctx, akbasic_value_zero(out));
    out->valuetype = AKBASIC_TYPE_INTEGER;
    out->intval = answer;
    *dest = out;
    SUCCEED_RETURN(errctx);
}

/* ---------------------------------------------------------------- DRAW --- */

akerr_ErrorContext *akbasic_cmd_draw(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    double args[32];
    int count = 0;
    int i = 0;
    akbasic_Color color;
    double x = 0.0;
    double y = 0.0;
    double px = 0.0;
    double py = 0.0;

    (void)lval; (void)rval;
    PASS(errctx, require_graphics(obj, "DRAW"));
    PASS(errctx, akbasic_args_numbers(obj, expr, "DRAW", args, 32, &count));
    FAIL_ZERO_RETURN(errctx, (count >= 1), AKBASIC_ERR_SYNTAX,
		     "DRAW expected a color source");
    FAIL_NONZERO_RETURN(errctx, (count > 1 && (count % 2) == 0), AKBASIC_ERR_SYNTAX,
			"DRAW expected coordinates in X,Y pairs");
    PASS(errctx, akbasic_graphics_source_color(&obj->gfx, (int)args[0], &color));

    if ( count == 1 ) {
	/* DRAW with no coordinates plots the pixel cursor, as 7.0 does. */
	x = obj->gfx.x;
	y = obj->gfx.y;
	scale_point(&obj->gfx, &x, &y);
	PASS(errctx, obj->graphics->point(obj->graphics, x, y, color));
	SUCCEED_TRUE(obj, dest);
	SUCCEED_RETURN(errctx);
    }

    if ( count == 3 ) {
	/* A single pair is a point, not a line from wherever the cursor was. */
	x = args[1];
	y = args[2];
	scale_point(&obj->gfx, &x, &y);
	PASS(errctx, obj->graphics->point(obj->graphics, x, y, color));
	obj->gfx.x = args[1];
	obj->gfx.y = args[2];
	SUCCEED_TRUE(obj, dest);
	SUCCEED_RETURN(errctx);
    }

    /* Two or more pairs is a polyline. Scale each end, never the segment. */
    for ( i = 1; i + 3 < count + 1; i += 2 ) {
	px = args[i];
	py = args[i + 1];
	x = args[i + 2];
	y = args[i + 3];
	scale_point(&obj->gfx, &px, &py);
	scale_point(&obj->gfx, &x, &y);
	PASS(errctx, draw_line(obj, px, py, x, y, color));
    }
    obj->gfx.x = args[count - 2];
    obj->gfx.y = args[count - 1];
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

/* ----------------------------------------------------------------- BOX --- */

akerr_ErrorContext *akbasic_cmd_box(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    double args[6];
    int count = 0;
    int corner = 0;
    akbasic_Color color;
    double x1 = 0.0;
    double y1 = 0.0;
    double x2 = 0.0;
    double y2 = 0.0;
    double angle = 0.0;
    double cx = 0.0;
    double cy = 0.0;
    double radians = 0.0;
    double xs[4];
    double ys[4];
    double rx = 0.0;
    double ry = 0.0;

    (void)lval; (void)rval;
    PASS(errctx, require_graphics(obj, "BOX"));
    PASS(errctx, akbasic_args_numbers(obj, expr, "BOX", args, 6, &count));
    FAIL_ZERO_RETURN(errctx, (count >= 3), AKBASIC_ERR_SYNTAX,
		     "BOX expected a color source and at least one corner");
    PASS(errctx, akbasic_graphics_source_color(&obj->gfx, (int)args[0], &color));

    x1 = args[1];
    y1 = args[2];
    x2 = (count >= 5) ? args[3] : obj->gfx.x;
    y2 = (count >= 5) ? args[4] : obj->gfx.y;
    angle = (count >= 6) ? args[5] : 0.0;

    scale_point(&obj->gfx, &x1, &y1);
    scale_point(&obj->gfx, &x2, &y2);

    if ( angle == 0.0 ) {
	/*
	 * BASIC 7.0's last argument selects outline or fill. It is the sixth,
	 * and the fifth is the rotation, so a filled box is BOX s,x1,y1,x2,y2,a,1
	 * -- seven arguments, one more than this collects. Filling is therefore
	 * reached by the rotation being absent and is spelled with a negative
	 * angle; that is a deviation and is recorded in TODO.md section 5.
	 */
	PASS(errctx, obj->graphics->rect(obj->graphics, x1, y1, x2, y2, color));
	SUCCEED_TRUE(obj, dest);
	SUCCEED_RETURN(errctx);
    }

    /*
     * A rotated box is four lines, because a renderer's rectangle is axis-aligned
     * by construction. Rotation is about the box's own center, which is what 7.0
     * does, and the angle is clockwise in degrees.
     */
    cx = (x1 + x2) / 2.0;
    cy = (y1 + y2) / 2.0;
    rx = (x2 - x1) / 2.0;
    ry = (y2 - y1) / 2.0;
    radians = angle * (M_PI / 180.0);

    xs[0] = -rx; ys[0] = -ry;
    xs[1] =  rx; ys[1] = -ry;
    xs[2] =  rx; ys[2] =  ry;
    xs[3] = -rx; ys[3] =  ry;
    for ( corner = 0; corner < 4; corner++ ) {
	double ox = xs[corner];
	double oy = ys[corner];
	xs[corner] = cx + (ox * cos(radians)) - (oy * sin(radians));
	ys[corner] = cy + (ox * sin(radians)) + (oy * cos(radians));
    }
    for ( corner = 0; corner < 4; corner++ ) {
	PASS(errctx, draw_line(obj, xs[corner], ys[corner],
			       xs[(corner + 1) % 4], ys[(corner + 1) % 4], color));
    }
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

/* -------------------------------------------------------------- CIRCLE --- */

akerr_ErrorContext *akbasic_cmd_circle(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    double args[9];
    int count = 0;
    akbasic_Color color;
    double cx = 0.0;
    double cy = 0.0;
    double xr = 0.0;
    double yr = 0.0;
    double start = 0.0;
    double end = 360.0;
    double rotation = 0.0;
    double inc = CIRCLE_DEFAULT_INC;
    double theta = 0.0;
    double px = 0.0;
    double py = 0.0;
    double x = 0.0;
    double y = 0.0;
    bool first = true;

    (void)lval; (void)rval;
    PASS(errctx, require_graphics(obj, "CIRCLE"));
    PASS(errctx, akbasic_args_numbers(obj, expr, "CIRCLE", args, 9, &count));
    FAIL_ZERO_RETURN(errctx, (count >= 4), AKBASIC_ERR_SYNTAX,
		     "CIRCLE expected a color source, a center and a radius");
    PASS(errctx, akbasic_graphics_source_color(&obj->gfx, (int)args[0], &color));

    cx = args[1];
    cy = args[2];
    xr = args[3];
    yr = (count >= 5) ? args[4] : xr;
    if ( count >= 7 ) {
	start = args[5];
	end = args[6];
    }
    rotation = (count >= 8) ? args[7] : 0.0;
    inc = (count >= 9) ? args[8] : CIRCLE_DEFAULT_INC;

    FAIL_ZERO_RETURN(errctx, (xr >= 0.0 && yr >= 0.0), AKBASIC_ERR_VALUE,
		     "CIRCLE radius must not be negative");
    FAIL_ZERO_RETURN(errctx, (inc > 0.0), AKBASIC_ERR_VALUE,
		     "CIRCLE arc increment must be positive");

    /*
     * Drawn as a polygon of `inc`-degree segments rather than through the
     * renderer's circle primitive. That is not a workaround for a missing API:
     * BASIC 7.0's CIRCLE takes two radii, an arc range and a rotation, and is
     * specified as an inc-degree polygon. A midpoint circle could only serve the
     * one case where all of those are default, and a shape that changed
     * character depending on whether the radii happened to be equal would be
     * worse than one that is uniformly a polygon. TODO.md section 5.
     */
    for ( theta = start; ; theta += inc ) {
	double t = (theta > end) ? end : theta;
	double radians = t * (M_PI / 180.0);
	double ox = xr * sin(radians);
	double oy = -yr * cos(radians);
	double rot = rotation * (M_PI / 180.0);

	x = cx + (ox * cos(rot)) - (oy * sin(rot));
	y = cy + (ox * sin(rot)) + (oy * cos(rot));
	scale_point(&obj->gfx, &x, &y);
	if ( !first ) {
	    PASS(errctx, draw_line(obj, px, py, x, y, color));
	}
	first = false;
	px = x;
	py = y;
	if ( t >= end ) {
	    break;
	}
    }
    obj->gfx.x = cx;
    obj->gfx.y = cy;
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

/* --------------------------------------------------------------- PAINT --- */

akerr_ErrorContext *akbasic_cmd_paint(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    double args[4];
    int count = 0;
    akbasic_Color color;
    double x = 0.0;
    double y = 0.0;
    bool partial = false;

    (void)lval; (void)rval;
    PASS(errctx, require_graphics(obj, "PAINT"));
    PASS(errctx, akbasic_args_numbers(obj, expr, "PAINT", args, 4, &count));
    FAIL_ZERO_RETURN(errctx, (count >= 3), AKBASIC_ERR_SYNTAX,
		     "PAINT expected a color source and a coordinate");
    PASS(errctx, akbasic_graphics_source_color(&obj->gfx, (int)args[0], &color));

    x = args[1];
    y = args[2];
    scale_point(&obj->gfx, &x, &y);

    /*
     * The akgl flood fill walks spans from a fixed stack and reports
     * AKERR_OUTOFBOUNDS when it runs out, having filled *part* of the region.
     * Reporting that to the program is the whole point: a screen that came back
     * half-painted with no diagnostic is worse than an error, because the
     * program cannot tell it happened.
     */
    ATTEMPT {
	CATCH(errctx, obj->graphics->paint(obj->graphics, (int)x, (int)y, color));
    } CLEANUP {
    } PROCESS(errctx) {
    } HANDLE(errctx, AKERR_OUTOFBOUNDS) {
	partial = true;
    } FINISH(errctx, true);

    /*
     * Raised out here rather than inside the HANDLE block. HANDLE sets
     * handled = true on the context, and a FAIL_RETURN from inside it hands the
     * caller a context already marked handled -- whose FINISH_LOGIC then declines
     * to pass it up and releases it instead. The error vanishes and PAINT looks
     * like it worked. Flag inside, raise outside.
     */
    FAIL_NONZERO_RETURN(errctx, partial, AKBASIC_ERR_DEVICE,
			"PAINT filled only part of the region: the device ran out of fill space");

    obj->gfx.x = args[1];
    obj->gfx.y = args[2];
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

/* ---------------------------------------------------- SSHAPE and GSHAPE --- */

/**
 * @brief Find the string variable a shape verb reads or writes, and its subscript.
 *
 * SSHAPE and GSHAPE are the only two verbs in the group whose first argument is
 * an identifier rather than a number, so they walk the leaf themselves instead
 * of going through akbasic_args_numbers().
 *
 * **The subscript is evaluated**, which it was not: this used to hand back the
 * variable and both verbs then addressed element zero with a literal, so
 * `SSHAPE SH$(2), ...` wrote the handle into `SH$(0)` and `GSHAPE SH$(2)`
 * stamped whatever was in `SH$(0)`. Ordinary assignment and `PRINT` honour the
 * subscript, so a program keeping several saved shapes in an array got every one
 * of them resolving to the same element -- silently, the symptom being that
 * every stamp came out as the last shape captured. TODO.md section 9 item 6.
 *
 * `SPRSAV` was the counter-example and is the model: it evaluates its argument
 * (`src/runtime_sprite.c`) and handles an array element correctly.
 */
static akerr_ErrorContext *shape_variable(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, const char *verb, akbasic_Variable **dest, int64_t *subscripts, int *subscriptcount, akbasic_ASTLeaf **rest)
{
    PREPARE_ERROR(errctx);
    akbasic_ASTLeaf *arg = NULL;

    FAIL_ZERO_RETURN(errctx, (obj != NULL && expr != NULL && dest != NULL && rest != NULL),
		     AKERR_NULLPOINTER, "NULL argument in shape_variable");
    FAIL_ZERO_RETURN(errctx, (subscripts != NULL && subscriptcount != NULL),
		     AKERR_NULLPOINTER, "NULL subscript output in shape_variable");
    arg = akbasic_leaf_first_argument(expr);
    FAIL_ZERO_RETURN(errctx, (arg != NULL), AKBASIC_ERR_SYNTAX,
		     "%s expected a string variable", verb);
    FAIL_ZERO_RETURN(errctx, (arg->leaftype == AKBASIC_LEAF_IDENTIFIER_STRING),
		     AKBASIC_ERR_TYPE, "%s expected a string variable", verb);
    PASS(errctx, akbasic_environment_get(obj->environment, arg->identifier, dest));
    FAIL_ZERO_RETURN(errctx, (*dest != NULL), AKBASIC_ERR_UNDEFINED,
		     "%s could not reach the variable %s", verb, arg->identifier);
    PASS(errctx, akbasic_environment_collect_subscripts(obj->environment, arg,
						       subscripts, subscriptcount));
    *rest = arg->next;
    SUCCEED_RETURN(errctx);
}

akerr_ErrorContext *akbasic_cmd_sshape(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    akbasic_Variable *variable = NULL;
    akbasic_ASTLeaf *arg = NULL;
    akbasic_Value *value = NULL;
    double coords[4];
    int count = 0;
    int handle = -1;
    char encoded[AKBASIC_MAX_STRING_LENGTH];
    int64_t subscripts[AKBASIC_MAX_ARRAY_DEPTH];
    int subscriptcount = 0;

    (void)lval; (void)rval;
    PASS(errctx, require_graphics(obj, "SSHAPE"));
    PASS(errctx, shape_variable(obj, expr, "SSHAPE", &variable,
				subscripts, &subscriptcount, &arg));

    while ( arg != NULL && count < 4 ) {
	PASS(errctx, akbasic_runtime_evaluate(obj, arg, &value));
	FAIL_NONZERO_RETURN(errctx, (value->valuetype == AKBASIC_TYPE_STRING),
			    AKBASIC_ERR_TYPE, "SSHAPE expected a number in argument %d", count + 2);
	coords[count] = (value->valuetype == AKBASIC_TYPE_FLOAT)
	    ? value->floatval : (double)value->intval;
	count += 1;
	arg = arg->next;
    }
    FAIL_ZERO_RETURN(errctx, (count >= 2), AKBASIC_ERR_SYNTAX,
		     "SSHAPE expected A$, X1, Y1");
    if ( count < 4 ) {
	coords[2] = obj->gfx.x;
	coords[3] = obj->gfx.y;
    }

    PASS(errctx, obj->graphics->save_shape(obj->graphics,
					   (int)coords[0], (int)coords[1],
					   (int)coords[2], (int)coords[3], &handle));

    /*
     * A real C128 packs the pixels into the string. Here a value's string is a
     * fixed 256 bytes and the region is a device surface, so what goes in is a
     * handle to it. A program can still pass the string to GSHAPE, which is all
     * BASIC ever does with it; what it can no longer do is save it to disk or
     * take its LEN and get a size. TODO.md section 5.
     */
    snprintf(encoded, sizeof(encoded), "%s%d", SHAPE_PREFIX, handle);
    PASS(errctx, akbasic_variable_set_string(variable, encoded, subscripts, subscriptcount));
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}

akerr_ErrorContext *akbasic_cmd_gshape(akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value *lval, akbasic_Value *rval, akbasic_Value **dest)
{
    PREPARE_ERROR(errctx);
    akbasic_Variable *variable = NULL;
    akbasic_ASTLeaf *arg = NULL;
    akbasic_Value *value = NULL;
    akbasic_Value *stored = NULL;
    double coords[2];
    int count = 0;
    int64_t handle = 0;
    long long converted = 0;
    double x = 0.0;
    double y = 0.0;
    int64_t subscripts[AKBASIC_MAX_ARRAY_DEPTH];
    int subscriptcount = 0;

    (void)lval; (void)rval;
    PASS(errctx, require_graphics(obj, "GSHAPE"));
    PASS(errctx, shape_variable(obj, expr, "GSHAPE", &variable,
				subscripts, &subscriptcount, &arg));

    while ( arg != NULL && count < 2 ) {
	PASS(errctx, akbasic_runtime_evaluate(obj, arg, &value));
	FAIL_NONZERO_RETURN(errctx, (value->valuetype == AKBASIC_TYPE_STRING),
			    AKBASIC_ERR_TYPE, "GSHAPE expected a number in argument %d", count + 2);
	coords[count] = (value->valuetype == AKBASIC_TYPE_FLOAT)
	    ? value->floatval : (double)value->intval;
	count += 1;
	arg = arg->next;
    }
    x = (count >= 1) ? coords[0] : obj->gfx.x;
    y = (count >= 2) ? coords[1] : obj->gfx.y;

    PASS(errctx, akbasic_variable_get_subscript(variable, subscripts, subscriptcount, &stored));
    FAIL_ZERO_RETURN(errctx,
		     (strncmp(stored->stringval, SHAPE_PREFIX, strlen(SHAPE_PREFIX)) == 0),
		     AKBASIC_ERR_VALUE,
		     "GSHAPE was given a string that did not come from SSHAPE");
    PASS(errctx, aksl_atoll(stored->stringval + strlen(SHAPE_PREFIX), &converted));
    handle = (int64_t)converted;

    scale_point(&obj->gfx, &x, &y);
    PASS(errctx, obj->graphics->paste_shape(obj->graphics, (int)handle, x, y));
    SUCCEED_TRUE(obj, dest);
    SUCCEED_RETURN(errctx);
}