py/mpprint: Correctly format leading zeros with separators.

Correctly format integers when there are leading zeros with a grouping
character, such as "{:04,d}".format(0x100) -> "0,256".

The new padding patterns for commas-and-zeroes and underscores-and-zeroes
are smooshed together into the existing pad_zeroes to save space.

Only the two combinations of (decimal + commas) and (other bases +
underscores) are properly supported.

Also add a test for it.

Fixes issue #18082.

Signed-off-by: Jeff Epler <jepler@unpythonic.net>

py/mpprint.c

@@ -40,8 +40,20 @@
 #include "py/formatfloat.h"
 #endif
 
-static const char pad_spaces[] = "                ";
-static const char pad_zeroes[] = "0000000000000000";
+static const char pad_spaces[16] = {' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' '};
+#define pad_spaces_size  (sizeof(pad_spaces))
+static const char pad_common[23] = {'0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '_', '0', '0', '0', ',', '0', '0'};
+// The contents of pad_common is arranged to provide the following padding
+// strings with minimal flash size:
+//     0000000000000000 <- pad_zeroes
+//                 0000_000 <- pad_zeroes_underscore (offset: 12, size 5)
+//                      000,00 <- pad_zeroes_comma (offset: 17, size 4)
+#define pad_zeroes       (pad_common + 0)
+#define pad_zeroes_size  (16)
+#define pad_zeroes_underscore (pad_common + 12)
+#define pad_zeroes_underscore_size  (5)
+#define pad_zeroes_comma (pad_common + 17)
+#define pad_zeroes_comma_size  (4)
 
 static void plat_print_strn(void *env, const char *str, size_t len) {
     (void)env;
@@ -65,13 +77,30 @@ int mp_print_strn(const mp_print_t *print, const char *str, size_t len, unsigned
     int pad_size;
     int total_chars_printed = 0;
     const char *pad_chars;
+    char grouping = flags >> PF_FLAG_SEP_POS;
 
     if (!fill || fill == ' ') {
         pad_chars = pad_spaces;
-        pad_size = sizeof(pad_spaces) - 1;
-    } else if (fill == '0') {
+        pad_size = pad_spaces_size;
+    } else if (fill == '0' && !grouping) {
         pad_chars = pad_zeroes;
-        pad_size = sizeof(pad_zeroes) - 1;
+        pad_size = pad_zeroes_size;
+    } else if (fill == '0') {
+        if (grouping == '_') {
+            pad_chars = pad_zeroes_underscore;
+            pad_size = pad_zeroes_underscore_size;
+        } else {
+            pad_chars = pad_zeroes_comma;
+            pad_size = pad_zeroes_comma_size;
+        }
+        // The result will never start with a grouping character. An extra leading zero is added.
+        // width is dead after this so we can use it in calculation
+        if (width % pad_size == 0) {
+            pad++;
+            width++;
+        }
+        // position the grouping character correctly within the pad repetition
+        pad_chars += pad_size - 1 - width % pad_size;
     } else {
         // Other pad characters are fairly unusual, so we'll take the hit
         // and output them 1 at a time.

tests/basics/string_format_sep.py

@@ -0,0 +1,9 @@
+try:
+    "%d" % 1
+except TypeError:
+    print("SKIP")
+    raise SystemExit
+
+for v in (0, 0x10, 0x1000, -0x10, -0x1000):
+    for sz in range(1, 12): print(("{:0%d,d}" % sz).format(v))
+    for sz in range(1, 12): print(("{:0%d_x}" % sz).format(v))