Introduce countDigits() function for PID and UID field widths

countDigits() is designed with 64-bit integer input and may be reused
for computing field widths of, for example, memory addresses.

Signed-off-by: Kang-Che Sung <[email protected]>
Co-authored-by: Benny Baumann <[email protected]>
Co-authored-by: Odric Roux-Paris <[email protected]>

Row.c

@@ -89,7 +89,7 @@ void Row_setPidColumnWidth(pid_t maxPid) {
       return;
    }
 
-   Row_pidDigits = (int)log10(maxPid) + 1;
+   Row_pidDigits = countDigits((size_t)maxPid, 10);
    assert(Row_pidDigits <= ROW_MAX_PID_DIGITS);
 }
 
@@ -99,7 +99,7 @@ void Row_setUidColumnWidth(uid_t maxUid) {
       return;
    }
 
-   Row_uidDigits = (int)log10(maxUid) + 1;
+   Row_uidDigits = countDigits((size_t)maxUid, 10);
    assert(Row_uidDigits <= ROW_MAX_UID_DIGITS);
 }
 

XUtils.c

@@ -374,6 +374,21 @@ double sumPositiveValues(const double* array, size_t count) {
    return sum;
 }
 
+/* Counts the number of digits needed to print "n" with a given base.
+   If "n" is zero, returns 1. This function expects small numbers to
+   appear often, hence it uses a O(log(n)) time algorithm. */
+size_t countDigits(size_t n, size_t base) {
+   assert(base > 1);
+   size_t res = 1;
+   for (size_t limit = base; n >= limit; limit *= base) {
+      res++;
+      if (base && limit > SIZE_MAX / base) {
+         break;
+      }
+   }
+   return res;
+}
+
 #if !defined(HAVE_BUILTIN_CTZ)
 // map a bit value mod 37 to its position
 static const uint8_t mod37BitPosition[] = {

XUtils.h

@@ -141,6 +141,11 @@ int compareRealNumbers(double a, double b);
 ATTR_NONNULL ATTR_ACCESS3_R(1, 2)
 double sumPositiveValues(const double* array, size_t count);
 
+/* Counts the number of digits needed to print "n" with a given base.
+   If "n" is zero, returns 1. This function expects small numbers to
+   appear often, hence it uses a O(log(n)) time algorithm. */
+size_t countDigits(size_t n, size_t base);
+
 /* Returns the number of trailing zero bits */
 #if defined(HAVE_BUILTIN_CTZ)
 static inline unsigned int countTrailingZeros(unsigned int x) {