--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tomsfastmath/src/addsub/fp_sub.c	Wed Nov 23 18:10:20 2011 +0700
@@ -0,0 +1,50 @@
+/* TomsFastMath, a fast ISO C bignum library.
+ * 
+ * This project is meant to fill in where LibTomMath
+ * falls short.  That is speed ;-)
+ *
+ * This project is public domain and free for all purposes.
+ * 
+ * Tom St Denis, [email protected]
+ */
+#include <tfm.h>
+
+/* c = a - b */
+void fp_sub(fp_int *a, fp_int *b, fp_int *c)
+{
+  int     sa, sb;
+
+  sa = a->sign;
+  sb = b->sign;
+
+  if (sa != sb) {
+    /* subtract a negative from a positive, OR */
+    /* subtract a positive from a negative. */
+    /* In either case, ADD their magnitudes, */
+    /* and use the sign of the first number. */
+    c->sign = sa;
+    s_fp_add (a, b, c);
+  } else {
+    /* subtract a positive from a positive, OR */
+    /* subtract a negative from a negative. */
+    /* First, take the difference between their */
+    /* magnitudes, then... */
+    if (fp_cmp_mag (a, b) != FP_LT) {
+      /* Copy the sign from the first */
+      c->sign = sa;
+      /* The first has a larger or equal magnitude */
+      s_fp_sub (a, b, c);
+    } else {
+      /* The result has the *opposite* sign from */
+      /* the first number. */
+      c->sign = (sa == FP_ZPOS) ? FP_NEG : FP_ZPOS;
+      /* The second has a larger magnitude */
+      s_fp_sub (b, a, c);
+    }
+  }
+}
+
+
+/* $Source$ */
+/* $Revision$ */
+/* $Date$ */