format: support leading zero printing for floats.

For a floating-point argument, the following behavior
is supported by the ~a, ~s and ~f format directives:
if the precision has a leading zero, then leading zeros
are added, up to one less than the field width.

* stream.c (formatv): Set the precision to one less than
the field width when rendering floats, rather than to
zero. The output_num code takes the precision as the cue
for padding zeros.

* txr.1: Updated format documentation, and clarified
a few things also.

1 files changed, 2 insertions, 2 deletions

diff --git a/stream.c b/stream.c
index 48738bb7..d5886336 100644
--- a/stream.c
+++ b/stream.c
@@ -3259,7 +3259,7 @@ val formatv(val stream_in, val fmtstr, struct args *al)
               vformat_str(stream, lit("#<bad-float>"), width, align, 0);
               continue;
             }
-            precision = 0;
+            precision = (width ? width - 1 : 0);
             goto output_num;
           }
         case 'd':
@@ -3348,7 +3348,7 @@ val formatv(val stream_in, val fmtstr, struct args *al)
               continue;
             }
 
-            precision = 0;
+            precision = (width ? width - 1 : 0);
             goto output_num;
           default:
             if (width != 0 || precision_p) {