Solve Exercise 5-12: flexible `detab` and `entab`

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diff --git a/ch5/5-12_entab-spec.c b/ch5/5-12_entab-spec.c
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+++ b/ch5/5-12_entab-spec.c
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+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 5-12: Extend `entab` and `detab` to accept the shorthand
+ * `entab -m +n` to mean tab stops every n columns, starting at column m.
+ * Choose convenient (for the user) default behavior.
+ */
+
+#define DEFAULTTAB 8
+
+int main(int argc, char *argv[]) {
+	int column, c, spaces;
+	int tabstart = (-1);
+	int tabevery = (-1);
+	char arg[4];
+	spaces = column = 0;
+
+	if (argc > 1) {
+		int i = 1;
+		int j = 0;
+		while (--argc > 0) {
+			c = argv[i][j++];
+			if (c == '-' || c == '+') {
+				while (isdigit(argv[i][j])) {
+					arg[j - 1] = argv[i][j];
+					j++;
+				}
+				arg[3] = '\0';
+				if (c == '-') {
+					tabstart = atoi(arg);
+				} else {
+					tabevery = atoi(arg);
+				}
+				j = 0;
+			} else {
+				printf("entab: unrecognized argument %s\n", *argv);
+			}
+			i++;
+		}
+	}
+	/* Clean up the mess if either argument hasn't been set to something sane */
+	if (tabstart == -1) {
+		tabstart = 0;
+	}
+	if (tabevery == -1) {
+		tabevery = DEFAULTTAB;
+	}
+
+	while ((c = getchar()) != EOF) {
+		column++;
+
+		if (c == ' ') {
+			/* No point in starting our tabs until we're after the offset */
+			if (column > tabstart) {
+				spaces++;
+				if ((column - tabstart) % tabevery == 0) {
+					if (spaces > 1) {
+						putchar('\t');
+						spaces = 0;
+					}
+				}
+			}
+		} else {
+			/* output all extra spaces first */
+			while (spaces > 0) {
+				putchar(' ');
+				spaces--;
+			}
+			/* reset the counts and argument position on a newline */
+			if (c == '\n') {
+				column = 0;
+				spaces = 0;
+			}
+			putchar(c);
+		}
+	}
+	
+	return 0;
+}