Add flip-flap display effect.

src/flipflap.c

@@ -0,0 +1,56 @@
+#include "flipflap.h"
+
+#include "lfsr.h"
+#include "video.h"
+
+typedef struct FlipFlapContext {
+    const char *image;
+    uint8_t width;
+    uint8_t height;
+    uint8_t row;
+    uint8_t column;
+    bool finished;
+} FlipFlapContext;
+
+static FlipFlapContext ctx;
+
+void flipflap_init(const uint8_t *image, uint8_t width, uint8_t height,
+                   uint8_t row, uint8_t column) {
+    ctx.image = image;
+    ctx.width = width;
+    ctx.height = height;
+    ctx.row = row;
+    ctx.column = column;
+    ctx.finished = false;
+
+    Lfsr lfsr;
+    lfsr_init(&lfsr, 0xceda);
+
+    for (uint8_t y = 0; y < ctx.height; ++y) {
+        for (uint8_t x = 0; x < ctx.width; ++x) {
+            if (*(ctx.image + y * ctx.width + x) == 1) {
+                *(VIDEO_MEMORY + 80 * (ctx.row + y) + ctx.column + x) =
+                    lfsr_next(&lfsr) % 256;
+            }
+        }
+    }
+}
+
+void flipflap_poll(void) {
+    ctx.finished = true;
+    for (uint8_t y = 0; y < ctx.height; ++y) {
+        for (uint8_t x = 0; x < ctx.width; ++x) {
+            if (*(ctx.image + y * ctx.width + x) == 1) {
+                char *p = VIDEO_MEMORY + 80 * (ctx.row + y) + ctx.column + x;
+                if (*p != 0x86) {
+                    *p += 1;
+                    ctx.finished = false;
+                }
+            }
+        }
+    }
+}
+
+bool flipflap_finished(void) {
+    return ctx.finished;
+}

src/flipflap.h

@@ -0,0 +1,12 @@
+#ifndef CEDA_FLIPFLAP_H
+#define CEDA_FLIPFLAH_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+void flipflap_init(const uint8_t *image, uint8_t width, uint8_t height,
+                   uint8_t row, uint8_t column);
+void flipflap_poll(void);
+bool flipflap_finished(void);
+
+#endif