mpprint: Correctly format leading zeros with separators.

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.

Add a test for it.

Closes micropython#18082

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

Author: jepler

py/mpprint.c

@@ -40,8 +40,13 @@
 #include "py/formatfloat.h"
 #endif
 
-static const char pad_spaces[] = "                ";
-static const char pad_zeroes[] = "0000000000000000";
+static const char pad_spaces[16] = "                ";
+static const char pad_zeroes[23] = "0000000000000000_000,00";
+//                                  0000000000000000 <- pad_zeros
+//                                              0000_000 <- pad_zeros_comma (offset: 12)
+//                                                   000,00 <- pad_zeros_comma (offset: 17)
+#define pad_zeroes_comma (pad_zeroes + 17)
+#define pad_zeroes_underscore (pad_zeroes + 12)
 
 static void plat_print_strn(void *env, const char *str, size_t len) {
     (void)env;
@@ -65,13 +70,29 @@ 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 comma = flags >> PF_FLAG_SEP_POS;
 
     if (!fill || fill == ' ') {
         pad_chars = pad_spaces;
-        pad_size = sizeof(pad_spaces) - 1;
-    } else if (fill == '0') {
+        pad_size = sizeof(pad_spaces);
+    } else if (fill == '0' && !comma) {
         pad_chars = pad_zeroes;
-        pad_size = sizeof(pad_zeroes) - 1;
+        pad_size = 16;
+    } else if (fill == '0') {
+        if (comma == '_') {
+            pad_chars = pad_zeroes_underscore;
+            pad_size = 5;
+        } else {
+            pad_chars = pad_zeroes_comma;
+            pad_size = 4;
+        }
+        // The result will never start with a comma. An extra leading zero is added.
+        if (width % pad_size == 0) {
+            pad++;
+            width++;
+        }
+        // position the comma 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 +1,4 @@
+for v in (0, 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))
+