arith: logtrunc, lognot, bit, sign-extend: handle bits <= 0.

This fixes an exception in (int-buf #b'').

* arith.c (comp_trunc, lognot): Do not throw on negative bits,
just return zero.
(bit): Return nil if bit is negative rather than throwing.
(sign_extend): Fix instance of bad indentation. Renumber
goto labels.
Otherwise, nothing needs to be done here since bit
is where it was blowing up on a zero or negative bits value.

* txr.1: Document that logtrunc, lognot, sign-extend and bit
handle nonpositive bit widths, and negative bit positions.

2 files changed, 29 insertions, 22 deletions

diff --git a/arith.c b/arith.c
index eeb84c00..ad7e3e69 100644
--- a/arith.c
+++ b/arith.c
@@ -3802,7 +3802,7 @@ static val comp_trunc(val a, val bits)
   bn = c_n(bits);
 
   if (bn < 0)
-    goto bad4;
+    return zero;
 
   switch (type(a)) {
   case NUM:
@@ -3836,9 +3836,6 @@ bad2:
 
 bad3:
   uw_throwf(error_s, lit("~a: non-integral operand ~s"), self, a, nao);
-
-bad4:
-  uw_throwf(error_s, lit("~a: negative bits value ~s"), self, bits, nao);
 }
 
 val lognot(val a, val bits)
@@ -3880,7 +3877,7 @@ val logtrunc(val a, val bits)
   bn = c_n(bits);
 
   if (bn < 0)
-    goto bad4;
+    return zero;
 
   switch (type(a)) {
     mp_err mpe;
@@ -3912,9 +3909,6 @@ bad2:
 
 bad3:
   uw_throwf(error_s, lit("~a: non-integral operand ~s"), self, a, nao);
-
-bad4:;
-  uw_throwf(error_s, lit("~a: negative bits value ~s"), self, bits, nao);
 }
 
 val sign_extend(val n, val nbits)
@@ -3922,7 +3916,7 @@ val sign_extend(val n, val nbits)
   val self = sign_extend_s;
 
   if (cobjp(n)) {
-      return do_binary_method(self, self, n, nbits);
+    return do_binary_method(self, self, n, nbits);
   } else {
     val msb = minus(nbits, one);
     val ntrunc = logtrunc(n, nbits);
@@ -4065,7 +4059,7 @@ val bit(val a, val bit)
   bn = c_n(bit);
 
   if (bn < 0)
-    goto bad2;
+    return nil;
 
   switch (ta) {
   case NUM:
@@ -4073,14 +4067,14 @@ val bit(val a, val bit)
       cnum an = c_n(a);
       if (bn < (SIZEOF_PTR * CHAR_BIT))
         return (an & (convert(cnum, 1) << bn)) ? t : nil;
-      return an < 0 ? t : nil;
+      return tnil(an < 0);
     }
   case CHR:
     {
       cnum an = c_ch(a);
       if (bn < (SIZEOF_PTR * CHAR_BIT))
         return (an & (convert(cnum, 1) << bn)) ? t : nil;
-      return an < 0 ? t : nil;
+      return tnil(an < 0);
     }
   case BGNUM:
     {
@@ -4092,23 +4086,20 @@ val bit(val a, val bit)
       case MP_NO:
         return nil;
       default:
-        goto bad4;
+        goto bad3;
       }
     }
   default:
-    goto bad3;
+    goto bad2;
   }
 
 bad:
   uw_throwf(error_s, lit("~a: bit position ~s is not a fixnum"), self, bit, nao);
 
 bad2:
-  uw_throwf(error_s, lit("~a: bit position ~s is negative"), self, bit, nao);
-
-bad3:
   uw_throwf(error_s, lit("~a: non-integral operand ~s"), self, a, nao);
 
-bad4:
+bad3:
   do_mp_error(self, mpe);
 }
 
diff --git a/txr.1 b/txr.1
index b6bdaa04..e6983904 100644
--- a/txr.1
+++ b/txr.1
@@ -53692,7 +53692,14 @@ to the  specified number
 of bits. If
 .meta value
 is negative, then the two's complement representation
-is truncated. The return value of
+is truncated.
+
+If
+.meta bits
+is zero or negative, the value returned by both functions
+is zero.
+
+The return value of
 .code logtrunc
 is always a nonnegative integer.
 
@@ -53713,6 +53720,10 @@ function. Then, this truncated value is regarded as a
 two's complement integer. The value of this integer is
 calculated and returned.
 
+If the bits
+.meta bits
+argument is zero or negative the value returned is zero.
+
 .TP* Examples:
 
 .verb
@@ -53778,13 +53789,18 @@ function tests whether the integer or character
 .meta value
 has a 1 in bit position
 .metn bit .
-The
-.meta bit
-argument must be a nonnegative integer. A value of
+
+A value of
 .meta bit
 of zero indicates the least-significant-bit position of
 .metn value .
 
+If
+.meta bit
+is negative,
+.code nil
+is returned.
+
 The
 .code bit
 function has a Boolean result, returning the symbol