human-readable: calculate numbers in a loop

This drops a lot of `else if` blocks and extends units by "E" (Exa),
which is 2^60 and thus the last to fit into int64.

Co-authored-by: Zen Dodd <mail@steadytao.com>

Author: eworm-de

lib/compat.c

@@ -173,34 +173,34 @@ char *do_big_num(int64 num, int human_flag, const char *fract)
 	static unsigned int n;
 	char *s;
 	int len, negated;
+	uint64_t abs_num;
 
 	if (human_flag && !number_separator)
 		(void)get_number_separator();
 
 	n = (n + 1) % (sizeof bufs / sizeof bufs[0]);
+	abs_num = num < 0 ? 0 - (uint64_t)num : (uint64_t)num;
 
 	if (human_flag > 1) {
-		int mult = human_flag == 2 ? 1000 : 1024;
-		if (num >= mult || num <= -mult) {
-			double dnum = (double)num / mult;
-			char units;
-			if (num < 0)
-				dnum = -dnum;
-			if (dnum < mult)
-				units = 'K';
-			else if ((dnum /= mult) < mult)
-				units = 'M';
-			else if ((dnum /= mult) < mult)
-				units = 'G';
-			else if ((dnum /= mult) < mult)
-				units = 'T';
-			else {
-				dnum /= mult;
-				units = 'P';
+		unsigned int mult = human_flag == 2 ? 1000 : 1024;
+
+		if (abs_num >= mult) {
+			const char* units = " KMGTPE";
+			uint64_t powi = 1;
+
+			for (;;) {
+				if (abs_num / mult < powi)
+					break;
+
+				if (units[1] == '\0')
+					break;
+
+				powi *= mult;
+				++units;
 			}
-			if (num < 0)
-				dnum = -dnum;
-			snprintf(bufs[n], sizeof bufs[0], "%.2f%c", dnum, units);
+
+			snprintf(bufs[n], sizeof bufs[0], "%.2f%c",
+				(double) num / powi, *units);
 			return bufs[n];
 		}
 	}

rsync.1.md

@@ -3362,9 +3362,9 @@ expand it.
     digits) by specifying the `--no-human-readable` (`--no-h`) option.
 
     The unit letters that are appended in levels 2 and 3 are: `K` (kilo), `M`
-    (mega), `G` (giga), `T` (tera), or `P` (peta).  For example, a 1234567-byte
-    file would output as 1.23M in level-2 (assuming that a period is your local
-    decimal point).
+    (mega), `G` (giga), `T` (tera), `P` (peta) or `E` (exa). For example, a
+    1234567-byte file would output as 1.23M in level-2 (assuming that a
+    period is your local decimal point).
 
     Backward compatibility note: versions of rsync prior to 3.1.0 do not
     support human-readable level 1, and they default to level 0.  Thus,