Fix bugs in frexp: when scaling up or down, was not making sure that

state variable did not overflow or underflow.

diff --git a/v7/src/microcode/missing.c b/v7/src/microcode/missing.c
index 19c6af4e462337ea77a73e1892748f9d2f736dcd..b53286ac466fe635eb8f863765d27548cd9e3a75 100644 (file)
--- a/v7/src/microcode/missing.c
+++ b/v7/src/microcode/missing.c
@@ -1,8 +1,8 @@
 /* -*-C-*-
 
-$Header: /Users/cph/tmp/foo/mit-scheme/mit-scheme/v7/src/microcode/missing.c,v 9.26 1989/10/28 11:00:32 cph Rel $
+$Header: /Users/cph/tmp/foo/mit-scheme/mit-scheme/v7/src/microcode/missing.c,v 9.27 1991/12/07 01:55:51 cph Exp $
 
-Copyright (c) 1987, 1988, 1989 Massachusetts Institute of Technology
+Copyright (c) 1987-91 Massachusetts Institute of Technology
 
 This material was developed by the Scheme project at the Massachusetts
 Institute of Technology, Department of Electrical Engineering and
@@ -49,62 +49,67 @@ frexp (value, eptr)
     {
       while (1)
        {
-         x /= 2;
-         if (x > 1)
+         if (x > 2)
            {
              register double xr = (x / 2);
-             register double r = 4;
+             register double r = 2;
              register int n = 1;
-             while (xr > 1)
+             while (xr >= r)
                {
-                 x = xr;
+                 /* ((xr == (x / r)) && (xr >= r) && (x >= (r * r))) */
                  xr /= r;
+                 /* ((xr == (x / (r * r))) && (xr >= 1)) */
                  r *= r;
+                 /* ((xr == (x / r)) && (x >= r)) */
                  n += n;
                }
-             if (xr < 1)
-               e += n;
-             else
-               {
-                 x = (xr / 2);
-                 e += (n + n + 1);
-                 break;
-               }
+             /* ((xr >= 1) && (xr < r)) */
+             x = xr;
+             e += n;
            }
-         else if (x < 1)
+         else if (x < 2)
            {
+             x /= 2;
              e += 1;
              break;
            }
          else
            {
-             x /= 2;
+             x /= 4;
              e += 2;
              break;
            }
        }
     }
-  else if (x > 0)
+  else if ((x > 0) && (x < 0.5))
     {
       while (1)
        {
-         if (x < 0.5)
+         if (x < 0.25)
            {
-             register double xr = (x * 4);
-             register double r = 4;
+             register double xr = (x * 2);
+             register double r = 0.5;
              register int n = 1;
-             x *= 2;
-             while (xr < 1)
+             /* ((xr == (x / r)) && (xr < 0.5) && (x < (r / 2))) */
+             while (xr < (r / 2))
                {
-                 x = xr;
-                 xr *= r;
+                 /* ((xr < (r / 2)) && (x < ((r * r) / 2))) */
+                 xr /= r;
+                 /* ((xr == (x / (r * r))) && (xr < 0.5)) */
                  r *= r;
+                 /* ((xr == (x / r)) && (x < (r / 2))) */
                  n += n;
                }
+             /* ((xr >= (r / 2)) && (xr < 0.5)) */
+             x = xr;
              e -= n;
            }
          else
-           break;
+           {
+             x *= 2;
+             e -= 1;
+             break;
+           }
        }
     }
   (*eptr) = e;