Take the last N lines

Challenge

Reproduce tail -n: copy the last N lines of a block of text, found by counting newlines from the end.

Task

Implement int tail_lines(const char *text, int n, char *out, size_t cap) that copies the last n lines of text into out (NUL-terminated), keeping their trailing newlines. If text has n or fewer lines, copy all of it.

Input

• text: NUL-terminated text, lines separated by '\n'.
• n: how many trailing lines to keep (n >= 0).
• out / cap: destination buffer and its size.

Output

Return the number of bytes written to out. Return -1 if text/out is NULL, n < 0, or the result doesn't fit in cap.

Example

text = "l1\nl2\nl3\nl4\nl5\n"
tail_lines(text, 2, out, 128)    ->   out = "l4\nl5\n"
tail_lines(text, 10, out, 128)   ->   out = text (fewer lines than n)
tail_lines(text, 1, out, 128)    ->   out = "l5\n"

Edge cases

• n larger than the line count: copy the whole text.
• Text that doesn't end in a newline (final partial line still counts).
• Result too big for cap: -1.

Rules

• Count lines, skip max(0, total - n) from the front, then copy the rest. Bound the copy by cap.

Why this matters

tail -n is a daily tool; reproducing it means finding the right starting offset by counting newlines from the end.

Input format

text: NUL-terminated lines split by '\n'. n: trailing lines to keep. out/cap: buffer and size.

Output format

int bytes written to out, or -1 on NULL / overflow / n<0.

Constraints

Preserve trailing newlines; if there are <= n lines, copy everything.

Starter code

#include <stddef.h>
int tail_lines(const char *text, int n, char *out, size_t cap) {
    /* TODO */
    (void)text; (void)n; (void)out; (void)cap;
    return -1;
}

Common mistakes

Off-by-one when the text doesn't end in a newline. Not bounding the copy by cap.

Edge cases to handle

n larger than the line count → whole text. n=1 → just the last line.

Complexity

O(text length).

Background lessons

• Log parsing — line iteration + classification

Up next

• Convert CRLF to LF