Count Lines in a Text File with Filters (LF/CRLF Safe)

Title: Count Text Lines with Optional Blank/Comment Filters
Level: Easy
Concepts: Text File I/O (fopen, fgets, fclose), newline handling (\n, \r\n), line trimming, input validation, error codes

Scenario

You need a utility that counts how many lines a text file has, with options to ignore blank lines and ignore comment lines (lines whose first non-space character is #). The file may contain LF (\n) or CRLF (\r\n) newlines. You must not load the entire file into memory—process line by line.

Problem Statement

Implement a function that opens a file by path, reads it line by line, and returns the total line count after applying the chosen filters. It must be robust to mixed \n/\r\n endings and very long lines (by reading them in chunks and treating a logical line as continuing until a newline is encountered).

Requirements

• Allowed types only: int, long, double, char, bool, enum.
• Inputs:
  • const char *path — file path.
  • bool ignore_blank — skip lines that are blank or only spaces/tabs.
  • bool ignore_comment_hash — skip lines where the first non-space is #.
• Outputs:
  • int *out_count — number of lines after filtering.
• Behavior:
  • Open file with "rb" (binary) to handle \r\n safely; parse text manually.
  • Treat a logical line as bytes up to \n. Strip a trailing \r if present before checks.
  • If ignore_blank, a line with only spaces or tabs is skipped.
  • If ignore_comment_hash, a line whose first non-space char is # is skipped.
  • Count every other logical line.
• Error handling:
  • On NULL pointers or open/read errors, return -1 and do not modify outputs.
• Performance: O(file size), fixed-size buffer (e.g., 4 KB), no dynamic allocation required.

Function Details

• Name: count_lines_filtered
• Arguments:
  • const char *path
  • bool ignore_blank
  • bool ignore_comment_hash
  • int *out_count
• Return Value:
  • int — 0 on success; -1 on invalid input or I/O failure.
• Description:
  Open the file; read in chunks, constructing logical lines (accumulate until \n). For each logical line, trim a single trailing \r if present, then decide whether to count it based on ignore_blank/ignore_comment_hash. Close the file before returning.

Solution Approach

• Validate pointers.
• FILE *f = fopen(path, "rb"); on failure → -1.
• Use a small buffer (e.g., char buf[4096];) and a secondary “line” buffer to accumulate until newline.
• For each logical line:
  • Remove final \r if preceded by \n.
  • Check blank/comment rules.
  • Increment count if not filtered.
• Set *out_count and return 0. Ensure fclose(f) on all paths.

Tasks to Perform

1. Validate path and out_count are not NULL.
2. Open file with "rb". Handle failure.
3. Stream-read and assemble logical lines until EOF.
4. For each line:
   • Strip trailing \r if present.
   • If ignore_blank, skip lines with only spaces/tabs.
   • If ignore_comment_hash, skip lines whose first non-space is #.
   • Otherwise, increment count.
5. Close the file and set *out_count.
6. Return 0 on success; -1 on any error (without writing outputs).

Test Cases