Solve Exercise 5-13: tail

This exercise was somewhat irritating until I caved and used the code in
section 5.6 like the exercise suggested. The solution is not what I
think is clean, but given that [mc]*alloc haven't been covered yet, it's
probably the best one can muster.

Once I used the code in §5.6, the program fell into place. Still, it was
a neat exercise.

1 files changed, 106 insertions, 0 deletions

diff --git a/ch5/5-13_tail.c b/ch5/5-13_tail.c
new file mode 100644
index 0000000..3a8ea30
--- /dev/null
+++ b/ch5/5-13_tail.c
@@ -0,0 +1,106 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 5-13: Write the program `tail`, which prints the last n lines of
+ * its input. By default, n is 10, let us say, but it can be changed by an
+ * optional argument, so that `tail -n` prints the last n lines. The program
+ * should behave rationally no matter how unreasonable the input or the value
+ * of n. Write the program so it make the best use of available storage; lines
+ * should be stored as in the sorting program of Section 5.6, not in a
+ * two-dimensional array of fixed size.
+ */
+
+#define MAXLINES 50000
+#define MAXLEN 1000
+#define DEFAULT_LINES 10
+#define ALLOCSIZE 10000000
+
+static char allocbuf[ALLOCSIZE];
+static char *allocp = allocbuf;
+
+char *lineptr[MAXLINES];
+int numlines;
+
+int mygetline(char *line, int lim) {
+	int c, i;
+	for (i = 0; i < lim && (c = getchar()) != EOF && c != '\n'; i++) {
+		*(line++) = c;
+	}
+	if (c == '\n') {
+		i++; /* I count newlines; some don't */
+		*(line++) = c;
+	}
+	*line = '\0';
+	return i;
+}
+
+char *alloc(int n) {
+	if (allocbuf + ALLOCSIZE - allocp >= n) {
+		allocp += n;
+		return allocp - n;
+	} else {
+		return 0;
+	}
+}
+
+int readlines(char *lineptr[], int maxlines) {
+	int len, nlines;
+	char *p, line[MAXLEN];
+	nlines = 0;
+	while ((len = mygetline(line, MAXLEN)) > 0) {
+		if (nlines >= maxlines || (p = alloc(len)) == NULL) {
+			return -1;
+		} else {
+			line[len-1] = '\0';
+			strcpy(p, line);
+			lineptr[nlines++] = p;
+		}
+	}
+	return nlines;
+}
+
+void writelines(char *lineptr[], int sline, int nlines) {
+	int i;
+	if (sline > 0) {
+		i = sline;
+	} else {
+		i = 0;
+	}
+	for (; i < nlines && lineptr[i] != NULL; i++) {
+		printf("%s\n", lineptr[i]);
+	}
+}
+
+void parse_args(int argc, char *argv[]) {
+	if (argc == 2 && *(++argv)[0] == '-') {
+		int i = 1;
+		char arg[10];
+		for (i = 1; i < 10 && (*argv)[i] != '\0'; i++) {
+			arg[i - 1] = (*argv)[i];
+		}
+		numlines = atoi(arg);
+	} else {
+		numlines = DEFAULT_LINES;
+	}
+}
+
+int main(int argc, char *argv[]) {
+	int nlines;
+	parse_args(argc, argv);
+
+	if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
+		writelines(lineptr, nlines - numlines, nlines);
+		return 0;
+	} else {
+		/* we should repeat ourselves here so tail fails, but gracefully. We
+		 * _did_ gather some input, and should print it! */
+		nlines = MAXLINES;
+		writelines(lineptr, nlines - numlines, nlines);
+		printf("-----------------------\n");
+		printf("ERROR: Input too large.\n");
+		return 1;
+	}
+}