format: new precision - character for zero.

* stream.c (formatv): The - precision option character
produces a "sign" that is a zero. If this is used with leading
zeros, it will avoid generating a space. The requirements can
use improvement here, but one step at a time.

* tests/018/format.tl: Some tests.

* txr.1: Documented.

3 files changed, 20 insertions, 0 deletions

diff --git a/stream.c b/stream.c
index 248a51b2..77af0b37 100644
--- a/stream.c
+++ b/stream.c
@@ -3351,6 +3351,9 @@ val formatv(val stream_in, val fmtstr, struct args *al)
         case '+': case ' ':
           sign = ch;
           continue;
+        case '-':
+          sign = '0';
+          continue;
         case '*':
           saved_state = state;
           state = vf_star;
diff --git a/tests/018/format.tl b/tests/018/format.tl
index b8cf7445..6c58d45b 100644
--- a/tests/018/format.tl
+++ b/tests/018/format.tl
@@ -24,6 +24,11 @@
   (fmt "~4,03x" #xaf) " 0af"
   (fmt "~-4,03X" #xaf) "0AF ")
 
+(mtest
+  (fmt "~8,02f" 12) " 0012.00"
+  (fmt "~8,+02f" 12) "+0012.00"
+  (fmt "~8,-02f" 12) "00012.00")
+
 (test
   (fmt "~x" (sha256 "abc"))
   "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad")
diff --git a/txr.1 b/txr.1
index 3bbcf973..9ef21238 100644
--- a/txr.1
+++ b/txr.1
@@ -54526,9 +54526,21 @@ The precision specifier may begin with these optional characters:
 (the "leading zero indicator"),
 .coIP +
 (print a sign for positive values")
+.coIP -
+(print a zero in place of a positive sign).
 .IP space
 (print a space in place of a positive sign).
 .RE
+If the
+.codn + ,
+.code -
+or
+space are multiply specified, the rightmost one takes precedence. The
+prefix character that these three options produce (plus sign, zero or space) is
+produced unconditionally, even if it overflows the width of the field.
+Leading zeros produced by the
+.code 0
+option are not generated if they would overflow the field.
 
 The precision specifier itself follows: it must be either a decimal integer
 or the