commit 04bdedd131129bc14345251003f960ca6da19f10
Author: Camden Dixie O'Brien <cobrien@witekio.com>
Date:   Sat Sep 20 13:31:22 2025 +0100

    Write simple farbfeld-writing module

diff --git a/include/ff.h b/include/ff.h
new file mode 100644
index 0000000..d80063b
--- /dev/null
+++ b/include/ff.h
@@ -0,0 +1,8 @@
+#ifndef FF_H
+#define FF_H
+
+#include "img.h"
+
+void ff_write(int fd, img_t img);
+
+#endif
diff --git a/include/img.h b/include/img.h
new file mode 100644
index 0000000..d4d9619
--- /dev/null
+++ b/include/img.h
@@ -0,0 +1,15 @@
+#ifndef IMG_H
+#define IMG_H
+
+#include <stdint.h>
+
+typedef struct {
+	uint16_t r, g, b, a;
+} pix_t;
+
+typedef struct {
+	uint32_t w, h;
+	pix_t *pix;
+} img_t;
+
+#endif
diff --git a/src/ff.c b/src/ff.c
new file mode 100644
index 0000000..0cc9c77
--- /dev/null
+++ b/src/ff.c
@@ -0,0 +1,84 @@
+#include "ff.h"
+
+#include <arpa/inet.h>
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define MAGIC "farbfeld"
+#define MAGICLEN 8
+
+#define BUFSIZE 4096
+
+#define MIN(x, y) ((x) < (y) ? (x) : (y))
+
+static uint8_t buf[BUFSIZE];
+
+static uint8_t *loadpix(uint8_t *p, const pix_t *pix, unsigned n)
+{
+	for (unsigned i = 0; i < n; ++i) {
+		*((uint16_t *)p) = htons(pix[i].r);
+		p += sizeof(uint16_t);
+		*((uint16_t *)p) = htons(pix[i].g);
+		p += sizeof(uint16_t);
+		*((uint16_t *)p) = htons(pix[i].b);
+		p += sizeof(uint16_t);
+		*((uint16_t *)p) = htons(pix[i].a);
+		p += sizeof(uint16_t);
+	}
+	return p;
+}
+
+static void writebuf(int fd, size_t size)
+{
+	while (1) {
+		errno = 0;
+		const ssize_t res = write(fd, buf, size);
+
+		if (res == (ssize_t)size) {
+			return;
+		} else if (res > 0) {
+			fputs(
+			    "Error writing image: Couldn't write entire buffer"
+			    " (storage may be full)\n",
+			    stderr);
+			exit(1);
+		} else {
+			if (errno == EINTR) {
+				continue;
+			} else {
+				perror("Error writing image");
+				exit(1);
+			}
+		}
+	}
+}
+
+void ff_write(int fd, img_t img)
+{
+	uint8_t *p = buf;
+
+	memcpy(p, MAGIC, MAGICLEN);
+	p += MAGICLEN;
+	*((uint32_t *)p) = htonl(img.w);
+	p += sizeof(uint32_t);
+	*((uint32_t *)p) = htonl(img.h);
+	p += sizeof(uint32_t);
+
+	const pix_t *pix = img.pix;
+	unsigned rem = img.w * img.h;
+	while (rem > 0) {
+		const unsigned cap = (BUFSIZE - (p - buf)) / sizeof(pix_t);
+		const unsigned n = MIN(rem, cap);
+
+		p = loadpix(p, pix, n);
+		pix += n;
+		rem -= n;
+
+		writebuf(fd, p - buf);
+		p = buf;
+	}
+}