Add matrix effect.

src/matrix.c

@@ -0,0 +1,58 @@
+#include "matrix.h"
+#include "compiler.h"
+#include "crt.h"
+#include "lfsr.h"
+#include "video.h"
+
+#include <stddef.h>
+
+#define MATRIX_LETTER_TRAIL 4
+
+typedef struct MatrixLetter {
+    Lfsr *lfsr;
+    int8_t row;
+    uint8_t column;
+} MatrixLetter;
+
+typedef struct MatrixContext {
+    Lfsr lfsr;
+    MatrixLetter letters[80];
+} MatrixContext;
+
+static MatrixContext ctx;
+
+static void matrixLetter_init(MatrixLetter *ctx, Lfsr *lfsr) {
+    ctx->lfsr = lfsr;
+    ctx->column = lfsr_next(ctx->lfsr) % 80;
+    ctx->row = lfsr_next(ctx->lfsr) % 50 - 50;
+}
+
+static void matrixLetter_poll(MatrixLetter *ctx) {
+    ctx->row++;
+    if (ctx->row < 0)
+        return;
+
+    const uint8_t c = lfsr_next(ctx->lfsr) % 256;
+    video_put(ctx->column, ctx->row, c);
+
+    if (ctx->row < MATRIX_LETTER_TRAIL)
+        return;
+    video_put(ctx->column, ctx->row - MATRIX_LETTER_TRAIL, ' ');
+}
+
+void matrix_init(void) {
+    lfsr_init(&ctx.lfsr, 1);
+
+    // using the same LFSR for different letters makes the randomness
+    // not reproducible in case we decide to refactor the number of letters,
+    // but that's not an issue we care about at the moment
+    for (size_t i = 0; i < countof(ctx.letters); ++i) {
+        matrixLetter_init(&ctx.letters[i], &ctx.lfsr);
+    }
+}
+
+void matrix_poll(void) {
+    for (size_t i = 0; i < countof(ctx.letters); ++i) {
+        matrixLetter_poll(&ctx.letters[i]);
+    }
+}

src/matrix.h

@@ -0,0 +1,7 @@
+#ifndef CEDA_MATRIX_H
+#define CEDA_MATRIX_H
+
+void matrix_init(void);
+void matrix_poll(void);
+
+#endif // CEDA_MATRIX_H