Reach hardware through backend records, and take the time from the host

Groups G, I and E are unblocked but cannot be written yet: the core library is
free of SDL and builds with no libakgl present, so a graphics verb cannot call
akgl_draw_* and a sound verb cannot call akgl_audio_*. This adds what they call
instead.

Three records of function pointers -- akbasic_GraphicsBackend, _AudioBackend and
_InputBackend -- in the same shape as akbasic_TextSink, and the same shape
libakgl uses for akgl_RenderBackend. akbasic_runtime_set_devices() attaches any
subset; all three may be NULL and that is the standalone driver's normal state,
so a runtime with no backends still comes up and still prints. A verb that needs
one it was not given raises the new AKBASIC_ERR_DEVICE rather than dereferencing
a NULL vtable.

Two decisions worth stating. The graphics record has no circle entry point:
BASIC 7.0's CIRCLE takes two radii, an arc range, a rotation and a degree
increment, which makes it a polygon by definition, so it will be built from line
calls rather than from akgl_draw_circle. And coordinates are double rather than
an integer pixel address, because SCALE makes them fractional and rounding at
each verb rather than once at the backend accumulates drift along a polyline.

akbasic_runtime_settime() is how SOUND, PLAY and TEMPO get a clock without the
library reading one. Section 1.6 forbids blocking or owning a loop, so the caller
that owns the frame owns the time -- which is what libakgl already does, since
akgl_actor_logic_changeframe takes curtimems as an argument. Left unset it is
zero and every duration expires immediately: audible, but never a hang.

AKBASIC_ERR_LAST is a sentinel rather than a status, so tests/error_codes.c walks
every code looking for an unnamed one without anybody remembering to widen the
loop when a code is added.

tests/mockdevice.h records every backend call as a formatted line. The graphics
and audio verbs emit nothing a golden file can compare, so that log is where
their assertions have to live -- and since it needs no SDL, the whole of groups
G, I and E stays testable in the default build.

62/62 ctest, clean under -Wall -Wextra, doxygen clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/devices.c

@@ -0,0 +1,171 @@
+/**
+ * @file devices.c
+ * @brief Tests the device backend records, the palette table and the host clock.
+ *
+ * These are the pieces groups G, I and E are built on. Nothing here runs a BASIC
+ * verb -- that is what the per-group suites do -- it establishes that a runtime
+ * can be given backends, can be given none, and reports the time it was handed.
+ */
+
+#include <string.h>
+
+#include <akbasic/error.h>
+#include <akbasic/graphics.h>
+#include <akbasic/runtime.h>
+
+#include "harness.h"
+#include "mockdevice.h"
+#include "testutil.h"
+
+/**
+ * @brief A runtime with no backends attached must still come up and still print.
+ *
+ * This is the standalone driver's situation and the default for an embedding
+ * host that has not opted in, so it is the case that has to keep working.
+ */
+static void test_no_devices_is_normal(void)
+{
+    TEST_REQUIRE_OK(harness_start(NULL));
+    TEST_REQUIRE(HARNESS_RUNTIME.graphics == NULL, "graphics backend should start NULL");
+    TEST_REQUIRE(HARNESS_RUNTIME.audio == NULL, "audio backend should start NULL");
+    TEST_REQUIRE(HARNESS_RUNTIME.input == NULL, "input backend should start NULL");
+    TEST_REQUIRE_INT(HARNESS_RUNTIME.timems, 0);
+
+    TEST_REQUIRE_OK(akbasic_runtime_load(&HARNESS_RUNTIME, "10 PRINT \"HI\"\n"));
+    TEST_REQUIRE_OK(akbasic_runtime_start(&HARNESS_RUNTIME, AKBASIC_MODE_RUN));
+    TEST_REQUIRE_OK(akbasic_runtime_run(&HARNESS_RUNTIME, 0));
+    TEST_REQUIRE_STR(HARNESS_OUTPUT, "HI\n");
+    harness_stop();
+}
+
+/** @brief Attaching backends stores them; attaching NULLs takes them away again. */
+static void test_set_devices(void)
+{
+    TEST_REQUIRE_OK(harness_start(NULL));
+    mock_devices_init();
+
+    TEST_REQUIRE_OK(akbasic_runtime_set_devices(&HARNESS_RUNTIME, &MOCK_GRAPHICS,
+						&MOCK_AUDIO, &MOCK_INPUT));
+    TEST_REQUIRE(HARNESS_RUNTIME.graphics == &MOCK_GRAPHICS, "graphics backend was not stored");
+    TEST_REQUIRE(HARNESS_RUNTIME.audio == &MOCK_AUDIO, "audio backend was not stored");
+    TEST_REQUIRE(HARNESS_RUNTIME.input == &MOCK_INPUT, "input backend was not stored");
+
+    /* Withholding a capability is spelled NULL, and must actually detach. */
+    TEST_REQUIRE_OK(akbasic_runtime_set_devices(&HARNESS_RUNTIME, &MOCK_GRAPHICS, NULL, NULL));
+    TEST_REQUIRE(HARNESS_RUNTIME.graphics == &MOCK_GRAPHICS, "graphics backend should have stayed");
+    TEST_REQUIRE(HARNESS_RUNTIME.audio == NULL, "audio backend should have been detached");
+    TEST_REQUIRE(HARNESS_RUNTIME.input == NULL, "input backend should have been detached");
+
+    TEST_REQUIRE_STATUS(akbasic_runtime_set_devices(NULL, NULL, NULL, NULL), AKERR_NULLPOINTER);
+    harness_stop();
+}
+
+/** @brief The clock is stored verbatim, including a value that moves backwards. */
+static void test_settime(void)
+{
+    TEST_REQUIRE_OK(harness_start(NULL));
+
+    TEST_REQUIRE_OK(akbasic_runtime_settime(&HARNESS_RUNTIME, 1000));
+    TEST_REQUIRE_INT(HARNESS_RUNTIME.timems, 1000);
+
+    /*
+     * A host driving a paused or scrubbed clock is allowed. Rejecting it would
+     * make the interpreter an authority on the host's own time, which it is not.
+     */
+    TEST_REQUIRE_OK(akbasic_runtime_settime(&HARNESS_RUNTIME, 400));
+    TEST_REQUIRE_INT(HARNESS_RUNTIME.timems, 400);
+
+    TEST_REQUIRE_STATUS(akbasic_runtime_settime(NULL, 0), AKERR_NULLPOINTER);
+    harness_stop();
+}
+
+/** @brief The palette covers 1 to 16 and refuses everything else. */
+static void test_palette(void)
+{
+    akbasic_Color color;
+    int index = 0;
+
+    memset(&color, 0, sizeof(color));
+
+    for ( index = 1; index <= 16; index++ ) {
+	TEST_REQUIRE_OK(akbasic_graphics_palette(index, &color));
+	TEST_REQUIRE_INT(color.a, 0xff);
+    }
+
+    /* Two entries pinned by value, so a shifted table is a failure and not a shrug. */
+    TEST_REQUIRE_OK(akbasic_graphics_palette(1, &color));
+    TEST_REQUIRE(color.r == 0x00 && color.g == 0x00 && color.b == 0x00,
+		 "color 1 should be black, got %02x%02x%02x", color.r, color.g, color.b);
+    TEST_REQUIRE_OK(akbasic_graphics_palette(2, &color));
+    TEST_REQUIRE(color.r == 0xff && color.g == 0xff && color.b == 0xff,
+		 "color 2 should be white, got %02x%02x%02x", color.r, color.g, color.b);
+
+    /*
+     * BASIC counts from 1. Zero is the off-by-one somebody writes when they
+     * forget that, and it has to be refused rather than quietly return slot 0.
+     */
+    TEST_REQUIRE_STATUS(akbasic_graphics_palette(0, &color), AKBASIC_ERR_BOUNDS);
+    TEST_REQUIRE_STATUS(akbasic_graphics_palette(17, &color), AKBASIC_ERR_BOUNDS);
+    TEST_REQUIRE_STATUS(akbasic_graphics_palette(-1, &color), AKBASIC_ERR_BOUNDS);
+    TEST_REQUIRE_STATUS(akbasic_graphics_palette(1, NULL), AKERR_NULLPOINTER);
+}
+
+/** @brief The mock backends themselves record what they were told. */
+static void test_mock_records(void)
+{
+    akbasic_Color red = { 0xff, 0x00, 0x00, 0xff };
+    bool active = false;
+    int handle = -1;
+    int keycode = 0;
+    bool available = false;
+
+    mock_devices_init();
+
+    TEST_REQUIRE_OK(MOCK_GRAPHICS.point(&MOCK_GRAPHICS, 10.0, 20.0, red));
+    TEST_REQUIRE_OK(MOCK_GRAPHICS.line(&MOCK_GRAPHICS, 0.0, 0.0, 5.0, 5.0, red));
+    TEST_REQUIRE_STR(MOCK.log, "point 10.0,20.0 #ff0000\nline 0.0,0.0-5.0,5.0 #ff0000\n");
+
+    /* A saved shape yields a handle, and only a handle that exists can be pasted. */
+    TEST_REQUIRE_OK(MOCK_GRAPHICS.save_shape(&MOCK_GRAPHICS, 0, 0, 8, 8, &handle));
+    TEST_REQUIRE_INT(handle, 0);
+    TEST_REQUIRE_OK(MOCK_GRAPHICS.paste_shape(&MOCK_GRAPHICS, handle, 16.0, 16.0));
+    TEST_REQUIRE_STATUS(MOCK_GRAPHICS.paste_shape(&MOCK_GRAPHICS, 99, 0.0, 0.0), AKBASIC_ERR_BOUNDS);
+
+    /* The exhaustion switch is what lets the PAINT test reach the partial-fill path. */
+    MOCK.paint_exhausts = true;
+    TEST_REQUIRE_STATUS(MOCK_GRAPHICS.paint(&MOCK_GRAPHICS, 1, 1, red), AKERR_OUTOFBOUNDS);
+    MOCK.paint_exhausts = false;
+    TEST_REQUIRE_OK(MOCK_GRAPHICS.paint(&MOCK_GRAPHICS, 1, 1, red));
+
+    TEST_REQUIRE_OK(MOCK_AUDIO.tone(&MOCK_AUDIO, 0, 440.0, 250));
+    TEST_REQUIRE_OK(MOCK_AUDIO.voice_active(&MOCK_AUDIO, 0, &active));
+    TEST_REQUIRE(active, "voice 0 should be sounding after a tone");
+    TEST_REQUIRE_OK(MOCK_AUDIO.stop(&MOCK_AUDIO, 0));
+    TEST_REQUIRE_OK(MOCK_AUDIO.voice_active(&MOCK_AUDIO, 0, &active));
+    TEST_REQUIRE(!active, "voice 0 should be silent after a stop");
+    TEST_REQUIRE_STATUS(MOCK_AUDIO.tone(&MOCK_AUDIO, AKBASIC_AUDIO_VOICES, 440.0, 1),
+			AKBASIC_ERR_BOUNDS);
+
+    /* An empty key buffer is success with available false, never an error. */
+    TEST_REQUIRE_OK(MOCK_INPUT.poll_key(&MOCK_INPUT, &keycode, &available));
+    TEST_REQUIRE(!available, "an empty mock buffer should report nothing available");
+
+    mock_push_keys("AB");
+    TEST_REQUIRE_OK(MOCK_INPUT.poll_key(&MOCK_INPUT, &keycode, &available));
+    TEST_REQUIRE(available, "a primed mock buffer should report a key");
+    TEST_REQUIRE_INT(keycode, 'A');
+    TEST_REQUIRE_OK(MOCK_INPUT.poll_key(&MOCK_INPUT, &keycode, &available));
+    TEST_REQUIRE_INT(keycode, 'B');
+    TEST_REQUIRE_OK(MOCK_INPUT.poll_key(&MOCK_INPUT, &keycode, &available));
+    TEST_REQUIRE(!available, "the mock buffer should drain to empty");
+}
+
+int main(void)
+{
+    test_no_devices_is_normal();
+    test_set_devices();
+    test_settime();
+    test_palette();
+    test_mock_records();
+    return akbasic_test_failures;
+}

tests/error_codes.c

@@ -22,7 +22,7 @@ int main(void)
      * "Unknown Error" in every stack trace that carries it, which nothing else
      * in the suite would notice.
      */
-    for ( code = AKBASIC_ERR_SYNTAX; code <= AKBASIC_ERR_STATE; code++ ) {
+    for ( code = AKBASIC_ERR_SYNTAX; code < AKBASIC_ERR_LAST; code++ ) {
 	const char *name = akerr_name_for_status(code, NULL);
 	TEST_REQUIRE(name != NULL, "status %d has a NULL name", code);
 	TEST_REQUIRE(strcmp(name, "Unknown Error") != 0,
@@ -31,6 +31,16 @@ int main(void)
 
     TEST_REQUIRE_STR(akerr_name_for_status(AKBASIC_ERR_SYNTAX, NULL), "Syntax Error");
     TEST_REQUIRE_STR(akerr_name_for_status(AKBASIC_ERR_STATE, NULL), "State Error");
+    TEST_REQUIRE_STR(akerr_name_for_status(AKBASIC_ERR_DEVICE, NULL), "Device Error");
+
+    /*
+     * The sentinel must stay inside the band it terminates. If somebody ever adds
+     * 250 codes, the loop above walks straight out of our reservation and starts
+     * asserting on names libakgl or nobody owns.
+     */
+    TEST_REQUIRE(AKBASIC_ERR_LAST < AKBASIC_ERR_LIMIT,
+		 "AKBASIC_ERR_LAST %d has run past the reserved band ending at %d",
+		 AKBASIC_ERR_LAST, AKBASIC_ERR_LIMIT);
 
     /*
      * The assertion that proves the range is actually ours rather than merely

tests/mockdevice.h

@@ -0,0 +1,333 @@
+/**
+ * @file mockdevice.h
+ * @brief Recording graphics, audio and input backends for the device tests.
+ *
+ * The graphics and audio verbs produce no output a golden file can compare, so
+ * the assertions have to be about what reached the backend. These record every
+ * call as a formatted line, which a test compares against the sequence it
+ * expected -- what was drawn, in what order, in what color.
+ *
+ * Nothing here needs SDL, which is the point of the backend records in the first
+ * place: the whole of groups G, I and E is testable in the default build.
+ */
+
+#ifndef _AKBASIC_TEST_MOCKDEVICE_H_
+#define _AKBASIC_TEST_MOCKDEVICE_H_
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <akerror.h>
+
+#include <akbasic/audio.h>
+#include <akbasic/error.h>
+#include <akbasic/graphics.h>
+#include <akbasic/input.h>
+
+/** @brief Room for the call log. Longer than any test needs; overflow truncates loudly. */
+#define MOCK_LOG_SIZE		8192
+/** @brief How many keystrokes mock_input can be primed with. */
+#define MOCK_MAX_KEYS		32
+/** @brief How many regions mock_graphics will hand out handles for. */
+#define MOCK_MAX_SHAPES		8
+
+/** @brief State shared by all three mock backends, so one log records the lot. */
+typedef struct
+{
+    char log[MOCK_LOG_SIZE];
+    size_t used;
+    bool overflowed;
+
+    /* Graphics */
+    int shapes;			/* handles handed out so far			*/
+    bool paint_exhausts;	/* when true, paint reports a partial fill	*/
+
+    /* Audio */
+    bool voice_active[AKBASIC_AUDIO_VOICES];
+
+    /* Input */
+    int keys[MOCK_MAX_KEYS];
+    int keycount;
+    int keynext;
+} akbasic_MockDevice;
+
+static akbasic_MockDevice MOCK;
+static akbasic_GraphicsBackend MOCK_GRAPHICS;
+static akbasic_AudioBackend MOCK_AUDIO;
+static akbasic_InputBackend MOCK_INPUT;
+
+/**
+ * @brief Append one formatted call to the log.
+ *
+ * Deliberately not an akerr_ErrorContext * function. It is called from inside
+ * the vtable entry points, and a test whose log overflowed should say so through
+ * the assertion on the log contents rather than by raising an error that the
+ * verb under test would then report as a device failure.
+ */
+__attribute__((format(printf, 1, 2)))
+static void mock_log(const char *format, ...)
+{
+    va_list args;
+    int written = 0;
+
+    if ( MOCK.used >= sizeof(MOCK.log) - 1 ) {
+	MOCK.overflowed = true;
+	return;
+    }
+    va_start(args, format);
+    written = vsnprintf(MOCK.log + MOCK.used, sizeof(MOCK.log) - MOCK.used, format, args);
+    va_end(args);
+    if ( written < 0 || (size_t)written >= sizeof(MOCK.log) - MOCK.used ) {
+	MOCK.overflowed = true;
+	MOCK.used = sizeof(MOCK.log) - 1;
+	return;
+    }
+    MOCK.used += (size_t)written;
+}
+
+/* --------------------------------------------------------------- graphics -- */
+
+static akerr_ErrorContext *mock_point(akbasic_GraphicsBackend *self, double x, double y, akbasic_Color color)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    mock_log("point %.1f,%.1f #%02x%02x%02x\n", x, y, color.r, color.g, color.b);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_line(akbasic_GraphicsBackend *self, double x1, double y1, double x2, double y2, akbasic_Color color)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    mock_log("line %.1f,%.1f-%.1f,%.1f #%02x%02x%02x\n", x1, y1, x2, y2, color.r, color.g, color.b);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_rect(akbasic_GraphicsBackend *self, double x1, double y1, double x2, double y2, akbasic_Color color)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    mock_log("rect %.1f,%.1f-%.1f,%.1f #%02x%02x%02x\n", x1, y1, x2, y2, color.r, color.g, color.b);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_filled_rect(akbasic_GraphicsBackend *self, double x1, double y1, double x2, double y2, akbasic_Color color)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    mock_log("fillrect %.1f,%.1f-%.1f,%.1f #%02x%02x%02x\n", x1, y1, x2, y2, color.r, color.g, color.b);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_paint(akbasic_GraphicsBackend *self, int x, int y, akbasic_Color color)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    mock_log("paint %d,%d #%02x%02x%02x\n", x, y, color.r, color.g, color.b);
+    /*
+     * Stands in for the akgl flood fill running out of its fixed span stack,
+     * which reports AKERR_OUTOFBOUNDS having filled part of the region. PAINT has
+     * to surface that rather than treat it as success.
+     */
+    FAIL_ZERO_RETURN(errctx, (!MOCK.paint_exhausts), AKERR_OUTOFBOUNDS,
+		     "mock span stack exhausted");
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_clear(akbasic_GraphicsBackend *self, akbasic_Color color)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    mock_log("clear #%02x%02x%02x\n", color.r, color.g, color.b);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_save_shape(akbasic_GraphicsBackend *self, int x1, int y1, int x2, int y2, int *handle)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+
+    FAIL_ZERO_RETURN(errctx, (handle != NULL), AKERR_NULLPOINTER, "NULL handle in save_shape");
+    FAIL_ZERO_RETURN(errctx, (MOCK.shapes < MOCK_MAX_SHAPES), AKBASIC_ERR_DEVICE,
+		     "mock shape pool exhausted");
+    *handle = MOCK.shapes;
+    MOCK.shapes += 1;
+    mock_log("save %d,%d-%d,%d -> %d\n", x1, y1, x2, y2, *handle);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_paste_shape(akbasic_GraphicsBackend *self, int handle, double x, double y)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+
+    FAIL_ZERO_RETURN(errctx, (handle >= 0 && handle < MOCK.shapes), AKBASIC_ERR_BOUNDS,
+		     "no such mock shape %d", handle);
+    mock_log("paste %d at %.1f,%.1f\n", handle, x, y);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_free_shapes(akbasic_GraphicsBackend *self)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    MOCK.shapes = 0;
+    mock_log("freeshapes\n");
+    SUCCEED_RETURN(errctx);
+}
+
+/* ------------------------------------------------------------------ audio -- */
+
+static akerr_ErrorContext *mock_tone(akbasic_AudioBackend *self, int voice, double hz, int ms)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+
+    FAIL_ZERO_RETURN(errctx, (voice >= 0 && voice < AKBASIC_AUDIO_VOICES), AKBASIC_ERR_BOUNDS,
+		     "mock voice %d out of range", voice);
+    MOCK.voice_active[voice] = true;
+    mock_log("tone v%d %.1fhz %dms\n", voice, hz, ms);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_audio_stop(akbasic_AudioBackend *self, int voice)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+
+    FAIL_ZERO_RETURN(errctx, (voice >= 0 && voice < AKBASIC_AUDIO_VOICES), AKBASIC_ERR_BOUNDS,
+		     "mock voice %d out of range", voice);
+    MOCK.voice_active[voice] = false;
+    mock_log("stop v%d\n", voice);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_waveform(akbasic_AudioBackend *self, int voice, akbasic_Waveform waveform)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+
+    FAIL_ZERO_RETURN(errctx, (voice >= 0 && voice < AKBASIC_AUDIO_VOICES), AKBASIC_ERR_BOUNDS,
+		     "mock voice %d out of range", voice);
+    mock_log("wave v%d %d\n", voice, (int)waveform);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_envelope(akbasic_AudioBackend *self, int voice, int attack, int decay, double sustain, int release)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+
+    FAIL_ZERO_RETURN(errctx, (voice >= 0 && voice < AKBASIC_AUDIO_VOICES), AKBASIC_ERR_BOUNDS,
+		     "mock voice %d out of range", voice);
+    mock_log("env v%d %d/%d/%.2f/%d\n", voice, attack, decay, sustain, release);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_volume(akbasic_AudioBackend *self, double level)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    mock_log("vol %.2f\n", level);
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_voice_active(akbasic_AudioBackend *self, int voice, bool *active)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+
+    FAIL_ZERO_RETURN(errctx, (active != NULL), AKERR_NULLPOINTER, "NULL active in voice_active");
+    FAIL_ZERO_RETURN(errctx, (voice >= 0 && voice < AKBASIC_AUDIO_VOICES), AKBASIC_ERR_BOUNDS,
+		     "mock voice %d out of range", voice);
+    *active = MOCK.voice_active[voice];
+    SUCCEED_RETURN(errctx);
+}
+
+/* ------------------------------------------------------------------ input -- */
+
+static akerr_ErrorContext *mock_poll_key(akbasic_InputBackend *self, int *keycode, bool *available)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+
+    FAIL_ZERO_RETURN(errctx, (keycode != NULL && available != NULL), AKERR_NULLPOINTER,
+		     "NULL argument in poll_key");
+    if ( MOCK.keynext >= MOCK.keycount ) {
+	/* An empty buffer is success, not an error -- see input.h. */
+	*available = false;
+	*keycode = 0;
+	SUCCEED_RETURN(errctx);
+    }
+    *keycode = MOCK.keys[MOCK.keynext];
+    *available = true;
+    MOCK.keynext += 1;
+    SUCCEED_RETURN(errctx);
+}
+
+static akerr_ErrorContext *mock_flush_keys(akbasic_InputBackend *self)
+{
+    PREPARE_ERROR(errctx);
+    (void)self;
+    MOCK.keynext = MOCK.keycount;
+    mock_log("flushkeys\n");
+    SUCCEED_RETURN(errctx);
+}
+
+/* ---------------------------------------------------------------- fixture -- */
+
+/** @brief Reset the recorder and populate all three vtables. */
+__attribute__((unused))
+static void mock_devices_init(void)
+{
+    memset(&MOCK, 0, sizeof(MOCK));
+
+    MOCK_GRAPHICS.self		= &MOCK;
+    MOCK_GRAPHICS.point		= mock_point;
+    MOCK_GRAPHICS.line		= mock_line;
+    MOCK_GRAPHICS.rect		= mock_rect;
+    MOCK_GRAPHICS.filled_rect	= mock_filled_rect;
+    MOCK_GRAPHICS.paint		= mock_paint;
+    MOCK_GRAPHICS.clear		= mock_clear;
+    MOCK_GRAPHICS.save_shape	= mock_save_shape;
+    MOCK_GRAPHICS.paste_shape	= mock_paste_shape;
+    MOCK_GRAPHICS.free_shapes	= mock_free_shapes;
+
+    MOCK_AUDIO.self		= &MOCK;
+    MOCK_AUDIO.tone		= mock_tone;
+    MOCK_AUDIO.stop		= mock_audio_stop;
+    MOCK_AUDIO.waveform		= mock_waveform;
+    MOCK_AUDIO.envelope		= mock_envelope;
+    MOCK_AUDIO.volume		= mock_volume;
+    MOCK_AUDIO.voice_active	= mock_voice_active;
+
+    MOCK_INPUT.self		= &MOCK;
+    MOCK_INPUT.poll_key		= mock_poll_key;
+    MOCK_INPUT.flush_keys	= mock_flush_keys;
+}
+
+/** @brief Prime the input backend with keystrokes GET will drain in order. */
+__attribute__((unused))
+static void mock_push_keys(const char *keys)
+{
+    size_t i = 0;
+
+    MOCK.keycount = 0;
+    MOCK.keynext = 0;
+    for ( i = 0; keys != NULL && keys[i] != '\0' && MOCK.keycount < MOCK_MAX_KEYS; i++ ) {
+	MOCK.keys[MOCK.keycount] = (int)(unsigned char)keys[i];
+	MOCK.keycount += 1;
+    }
+}
+
+/** @brief Forget everything logged so far, keeping the primed state. */
+__attribute__((unused))
+static void mock_log_reset(void)
+{
+    memset(MOCK.log, 0, sizeof(MOCK.log));
+    MOCK.used = 0;
+    MOCK.overflowed = false;
+}
+
+#endif // _AKBASIC_TEST_MOCKDEVICE_H_