--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/checksum/dosum.c	Sat Dec 22 17:52:45 2007 -0500
@@ -0,0 +1,102 @@
+/*
+ * AnaGram, A System for Syntax Directed Programming
+ * Copyright 1993 Parsifal Software. All Rights Reserved.
+ * Copyright 2006 David A. Holland. All Rights Reserved.
+ * See the file COPYING for license and usage terms.
+ *
+ * dosum.c - checksums.
+ */
+
+/*
+ * Fletcher's check-sum routines for AnaGram
+ * 
+ * Freely adapted from routines published in Dr. Dobbs Journal
+ * May 1992, p. 64
+ *
+ * Revised as portable C in June 2006.
+ */
+
+#include <stdint.h>
+#include <string.h>
+#include <assert.h>
+
+#include "bits.h"
+
+struct sumstate {
+  uint16_t k1, k2;
+};
+
+static void sumstate_init(struct sumstate *s, size_t length) {
+  s->k1 = (uint16_t) (length >> 16);
+  s->k2 = (uint16_t) length;
+
+  if (s->k1 == 0) s->k1++;
+  if (s->k2 == 0) s->k2++;
+}
+
+static void sumstate_sum(struct sumstate *s, const void *buf, size_t len) {
+  const uint8_t *cb;
+  unsigned i, n;
+  uint16_t x;
+
+  cb = buf;
+  n = len/sizeof(uint16_t);
+  for (i=0; i<n; i++) {
+
+    // read one octet at a time so the byte order is deterministic
+    x = *cb++;
+    x += ((uint16_t)*cb++) << 8;
+
+    s->k1 += x;
+    if (s->k1 < x) s->k1++;
+    s->k2 += s->k1;
+    if (s->k2 < s->k1) s->k2++;
+  }
+}
+
+static uint32_t sumstate_finish(struct sumstate *s) {
+  uint32_t sum;
+
+  s->k1 ^= 0xffff;
+  s->k2 ^= 0xffff;
+  sum = 65535UL*s->k2 + s->k1;
+
+  return sum;
+}
+
+uint32_t dosum(const char *buf, size_t len, size_t skipstart, size_t skiplen) {
+  struct sumstate s;
+  unsigned fudge;
+  size_t numwords;
+
+  assert(skiplen%2==0);
+  assert(skipstart==0 || skipstart+skiplen <= len);
+  sumstate_init(&s, len);
+
+  /*
+   * It would be nice if we could count on skipstart being aligned.
+   * But we can't. Nor is the filesize necessarily even.
+   *
+   * The followed mangled logic that rounds some things up matches
+   * what used to be done in an even uglier way.
+   */
+
+  assert(sizeof(uint16_t)==2);
+  numwords = (len+1)/2;
+
+  if (skipstart > 0) {
+    size_t p0 = skipstart+skiplen;
+    size_t l0 = (numwords - p0/2) * 2;
+    size_t p1 = 0;
+    size_t l1 = (skipstart/2) * 2;
+
+    fudge = skipstart%2;
+    sumstate_sum(&s, buf+p0, l0);
+    sumstate_sum(&s, buf+p1, l1);
+  }
+  else {
+    sumstate_sum(&s, buf, numwords * 2);
+  }
+
+  return sumstate_finish(&s);
+}