Add code and readme

README.md

@@ -1,3 +1,62 @@
-# baud
+---
+title: baud.c
+---
 
-Modem simulator
+[baud.c](baud.c) is a program to simulate going through a modem.
+It reads stdin, and prints it to stdout at the provided baud rate (bits per second).
+
+
+Building
+--------
+
+Download [baud.c](baud.c). Change to that directory, and type
+
+    make baud
+
+That will generate a file named `baud` in whatever directory you're in.
+If you like,
+you can put it in `~/bin` or `~/.local/bin` or `/usr/games/bin` or
+wherever you prefer.
+
+
+Running
+-------
+
+If you'd like to see what life was like when I got my first modem, try
+
+    cat file | ./baud 110 40
+
+Run that for a few hours, then try
+
+    cat file | ./baud 1200 20
+
+And you will understand how mind-blowingly fast 1200 baud modems were at the time.
+
+    frotz zork1.zip | ./baud 4800
+
+Gives Zork a retro vibe,
+without being so slow that it's aggravating.
+I don't think more than a dozen or so people every played Zork over a modem link,
+but whatever.
+
+
+Why
+---
+
+Every 10 years or so, somebody asks me for this program.
+I don't know why.
+
+The other day it happened again, and I couldn't find it soon enough,
+so I just wrote it again from scratch.
+I'm putting it on my web site so I don't have to write it again.
+
+
+License
+-------
+
+I'm placing this in the public domain.
+So there's no license.
+
+Some jurisdictions don't recognize the existence of a public domain.
+If you live in one of those,
+you can use the terms of [Creative Commons 0](COPYING.txt).

baud.c

@@ -0,0 +1,80 @@
+/*
+ * Pretend to be a modem by throttling output.
+ *
+ * 2023-09-12 
+ * Neale Pickett <neale@woozle.org>
+ *
+ * To the extent possible under law, 
+ * I waive all copyright and related or neighboring rights to this software.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <sysexits.h>
+#include <unistd.h>
+
+const int MICROSECOND = 1;
+const int MILLISECOND = 1000 * MICROSECOND;
+const int SECOND = 1000 * MILLISECOND;
+
+const int NOISINESS_DENOMINATOR = 36000;
+const int NOISELEN_DEFAULT = 16;
+
+int
+usage(char *me, char *error) {
+    if (error) {
+        fprintf(stderr, "ERROR: %s\n", error);
+    }
+    fprintf(stderr, "Usage: %s BAUD [NOISINESS] [NOISELEN]\n", me);
+    fprintf(stderr, "\n");
+    fprintf(stderr, "Reads stdin, and writes it back to stdout at BAUD bits per second.\n");
+    fprintf(stderr, "\n");
+    fprintf(stderr, "NOISINESS\tEvery bit can begin line noise, with probability NOISINESS/%d\n", NOISINESS_DENOMINATOR);
+    fprintf(stderr, "NOISELEN\tUp to this many bits of noise happens per burst (default: %d)\n", NOISELEN_DEFAULT);
+    return EX_USAGE;
+}
+
+int
+main(int argc, char *argv[]) {
+    if (argc < 2) {
+        return usage(argv[0], NULL);
+    }
+
+    int baud = atoi(argv[1]);
+    if (baud < 1) {
+        return usage(argv[0], "Invalid baud rate");
+    }
+
+    int noisiness = 0;
+    if (argc > 2) {
+        noisiness = atoi(argv[2]);
+    }
+    int noiselen = NOISELEN_DEFAULT;
+    if (argc > 3) {
+        noiselen = atoi(argv[3]);
+    }
+
+    int cps = baud / 10; // 8N1 has 10 bits per octet: 8 data, 1 start, 1 parity
+    int delay = SECOND / cps;
+    int noisybits = 0;
+    int c;
+    while ((c = getchar()) != EOF) {
+        usleep(delay);
+        for (int bit = 0; bit < 8; bit++) {
+            int r = rand() % NOISINESS_DENOMINATOR;
+            if (r < noisiness) {
+                noisybits = rand() % noiselen;
+            }
+
+            if (noisybits) {
+                c ^= (rand() & 1) << bit;
+                noisybits -= 1;
+            }
+        }
+        putchar(c);
+        fflush(stdout);
+    }
+    
+    return EX_OK;
+}