Solve Exercise 1-21: entab

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+#include <stdio.h>
+
+/* The C Programming Language: 2nd Edition
+ * Exercise 1-21:
+ * "Write a program `entab` that replaces strings of blanks by the minimum
+ * number of tabs and blanks to achieve the same spacing. Use the same tab
+ * stops as for `detab`. When either a tab or a single blank would suffice to
+ * reach a tab stop, which should be given preference?"
+ *
+ * Answer: A blank. A tab character that comes after (tabstop - 1) blanks makes
+ * little-to-no sense and could mess up alignment in some environments.
+ *
+ */
+
+#define TABWIDTH 8
+
+int main(void) {
+	int c, spaces;
+	spaces = 0;
+	while ((c = getchar()) != EOF) {
+		// Make sure the character is a space...
+		if (c == ' ') {
+			++spaces;
+			// When spaces is equal to TABWIDTH, we can add a tab
+			if (spaces == TABWIDTH) {
+				putchar('\t');
+				spaces = 0;
+			}
+		} else {
+			/* As soon as we hit a non-space character, we need to make sure
+			 * there aren't 1-7 spaces leftover. These need to be output before
+			 * we output the non-space character itself! This little loop is
+			 * interesting because it solves the problem of leftover spaces
+			 * _and_ gets the 'spaces' back to zero, which it needs to be once
+			 * we hit a non-space character.
+			 */
+			while (spaces != 0) {
+				putchar(' ');
+				--spaces;
+			}
+			// Output the non-space character.
+			putchar(c);
+		}
+	}
+}