C code for a nicely trimmed working directory string to be used in shell prompt

prompt_dir.c

// Program to display the current working directory but ensure that the string
// does not take up more than half of the current terminal window size
//
// Compile as gcc prompt_dir.c -o prompt_dir_str -Wall -Wextra -pedantic -std=c11 -ltermcap -O3

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <unistd.h>
#include <linux/limits.h>
#include <termcap.h>
#include <error.h>


#ifndef WIDTH_PERCENTAGE_DEFAULT
#define WIDTH_PERCENTAGE_DEFAULT 0.35
#endif

// Function to advance a pointer to the next '/' char in
// a string.
// Returns a pointer to the '/' char if succesfull
// Returns NULL if there is no next slash
static char* next_slash(const char* restrict path) {
  while(path) {
    const char curr = *path;
    if (curr == '/') return (char*)path;
    else ++path;
  }
  return NULL;
}

static char* prev_slash(const char* restrict path) {
  while(path) {
    const char curr = *path;
    if (curr == '/') return (char*)path;
    else --path;
  }
  return NULL;
}

// Rule (1) function:
//   Always print the first and last directories with a ... in the middle
static void rule1(const char* full_cwd, char* to_print) {
  // Get only the first directory name plus the slash itself
  char* first_slash = next_slash(full_cwd);
  strncpy(to_print, full_cwd, (first_slash-full_cwd+1)/sizeof(char));
  // append '.../' and the current local dir
  strcat(to_print, ".../");
  char* local_dir = prev_slash(full_cwd + strlen(full_cwd))+1;
  strcat(to_print, local_dir);
}

int main(int argc, char** argv) {
  // Get the percentage of allowed screen width in the path prompt
  // which defualts to WIDTH_PERCENTAGE_DEFAULT
  double width_percentage = (argc == 2) ? atof(argv[1]) : WIDTH_PERCENTAGE_DEFAULT;

  // Get the full working directory path
  char* cwd = NULL;
  char cwd_buf[PATH_MAX] = {0};
  if (!getcwd(cwd_buf, PATH_MAX)) {
    fprintf(stderr, "ERROR: [prompt_dir_str] cannot get current working directory\n");
    goto exit;
  }
  cwd = cwd_buf;

  // Get the width of the terminal
  char* termtype = getenv("TERM");
  char termbuf[2048] = {0};
  if (tgetent(termbuf, termtype) < 0) {
    fprintf(stderr, "ERROR: [prompt_dir_str] Could not access the termcap database.\n");
    goto exit;
  }
  int term_width = tgetnum("co");

  size_t allowable_width = (size_t)(width_percentage*(double)term_width);

  // Check if cwd is a sub-dir of $HOME
  char* home_dir = getenv("HOME");
  if (!home_dir) {
    fprintf(stderr, "ERROR: [prompt_dir_str] Can not get HOME directory\n");
    goto exit;
  }
  if (strstr(cwd, home_dir)) {
    // then home_dir is substring of cwd so overwrite the last char of the home dir
    const size_t offset = strlen(home_dir)-1;
    cwd[offset] = '~';
    cwd += offset;
  }
  
  // Print a string of appropriate length
  if (strlen(cwd) <= allowable_width) {
    // Whole string can be printed
    puts(cwd);
  }
  else {
    // Print the allowable number of characters to stdout.
    // The the following:
    //      ~/dir/.../dir/cwd
    //      /dir/.../dir/cwd
    //      /.../dir/cwd
    //      /.../cwd

    char to_print[allowable_width];
    memset(to_print, 0, allowable_width*sizeof(char));

    // Rule (1):
    rule1(cwd, to_print);
    puts(to_print);

  }

 exit:
  return EXIT_SUCCESS;
  
}