From d1efbdf3bdabe1f400d27c5620301fe7fb672595 Mon Sep 17 00:00:00 2001
From: Imbus <>
Date: Fri, 9 May 2025 10:38:47 +0200
Subject: [PATCH] crc32 user program

---
 Makefile     |  1 +
 user/crc32.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+)
 create mode 100644 user/crc32.c

diff --git a/Makefile b/Makefile
index 186c563..76bba31 100644
--- a/Makefile
+++ b/Makefile
@@ -138,6 +138,7 @@ UPROGS=\
 	$U/_clear\
 	$U/_halt\
 	$U/_reset\
+	$U/_crc32\
 	$U/_traced\
 
 fs.img: mkfs/mkfs README.md $(UPROGS)
diff --git a/user/crc32.c b/user/crc32.c
new file mode 100644
index 0000000..77cf3c3
--- /dev/null
+++ b/user/crc32.c
@@ -0,0 +1,64 @@
+#include <stddef.h>
+#include <stdint.h>
+#include "user.h"
+
+uint32_t
+crc32_for_byte(uint32_t r)
+{
+  for(int j = 0; j < 8; ++j)
+    r = (r & 1 ? 0xEDB88320L ^ (r >> 1) : r >> 1);
+  return r;
+}
+
+void
+init_crc32_table(uint32_t table[256])
+{
+  for(int i = 0; i < 256; ++i)
+    table[i] = crc32_for_byte(i);
+}
+
+uint32_t
+crc32(uint8_t *data, size_t len, uint32_t crc, uint32_t table[256])
+{
+  for(size_t i = 0; i < len; ++i)
+    crc = table[(crc ^ data[i]) & 0xFF] ^ (crc >> 8);
+  return crc;
+}
+
+int
+main(int argc, char *argv[])
+{
+  int      fd;
+  uint8_t  buf[1024];
+  size_t   n;
+  uint32_t crc = 0xFFFFFFFF;
+  uint32_t table[256];
+
+  if(argc != 2) {
+    fprintf(stderr, "Usage: %s <file>\n", argv[0]);
+    exit(1);
+  }
+
+  if((fd = open(argv[1], 0)) < 0) {
+    fprintf(stderr, "Couldnt open: %s\n", argv[1]);
+    exit(1);
+  }
+
+  init_crc32_table(table);
+
+  while((n = read(fd, buf, sizeof(buf))) > 0)
+    crc = crc32(buf, n, crc, table);
+
+  if(n < 0) {
+    fprintf(stderr, "Couldnt read file...\n");
+    close(fd);
+    exit(1);
+  }
+
+  close(fd);
+
+  crc ^= 0xFFFFFFFF;
+  printf("%x\n", crc);
+
+  return 0;
+}