#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

#define MAX_PATH_SIZE 512
#define SEGMENT_LENGTH 2
#define LAST_SEGMENT_MAX_LENGTH 16
#define MAX_SEGMENTS_AMOUNT 64

typedef struct {
    size_t seperator;
    size_t length;
} Segment;

/// @returns Amount of segments
size_t get_path_segments(
        Segment* dest,
        size_t dest_size,
        const char* path,
        size_t path_length)
{
    size_t dest_i = 0;
    size_t i = 0;
    while (i < path_length) {
        size_t seperator = i;
        i += 1;
        while (path[i] != '\0' && path[i] != '/') {
            i += 1;
        }
        size_t length = i - seperator - 1;
        dest[dest_i] = (Segment) { seperator, length };
        dest_i += 1;
    }
    return dest_i;
}

bool in_home_directory(
        const char* path,
        const Segment* segments,
        size_t segments_length,
        const char* username)
{
    if (segments_length < 2)
        return false;
    if (strncmp(&path[segments[0].seperator + 1], "home", segments[0].length) != 0) {
        return false;
    }
    if (strncmp(&path[segments[1].seperator + 1], username, strlen(username)) != 0) {
        return false;
    }
    return true;
}

int main(void)
{
    char cwd_path[MAX_PATH_SIZE] = "";
    if (getcwd(cwd_path, sizeof(cwd_path)) == NULL) {
        fprintf(stderr, "error: failed to get current working directory: %s\n", strerror(errno));
        exit(1);
    }
    const size_t cwd_path_length = strlen(cwd_path);

    const char* username = getlogin();

    Segment segments[MAX_SEGMENTS_AMOUNT] = { 0 };
    size_t segments_length = get_path_segments(
            segments,
            sizeof(segments) / sizeof(segments[0]),
            cwd_path,
            cwd_path_length);

    bool in_home_dir = in_home_directory(cwd_path, segments, segments_length, username);

    char short_path[MAX_PATH_SIZE] = "";

    size_t segments_i = 0;
    size_t dest_i = 0;

    if (in_home_dir) {
        short_path[dest_i] = '~';
        dest_i += 1;
        segments_i += 2;
    }

    for (; segments_i <  segments_length - 1; segments_i += 1) {
        short_path[dest_i] = '/';
        dest_i += 1;
        Segment* segment = &segments[segments_i];
        for (size_t i = 0; i < segment->length && i < SEGMENT_LENGTH; i += 1) {
            short_path[dest_i] = cwd_path[segment->seperator + 1 + i];
            dest_i += 1;
        }
    }

    if (!(segments_length == 2 && in_home_dir)) {
        short_path[dest_i] = '/';
        dest_i += 1;
        Segment* last = &segments[segments_i];
        if (last->length >= LAST_SEGMENT_MAX_LENGTH) {
            for (size_t i = 0; i < 5; i += 1) {
                short_path[dest_i] = cwd_path[last->seperator + 1 + i];
                dest_i += 1;
            }
            for (size_t i = 0; i < 3; i += 1) {
                short_path[dest_i] = '.';
                dest_i += 1;
            }
            for (size_t i = 0; i < 8; i += 1) {
                short_path[dest_i] = cwd_path[last->seperator + 1 + last->length - 8 + i];
                dest_i += 1;
            }
        } else {
            for (size_t i = 0; i < last->length; i += 1) {
                short_path[dest_i] = cwd_path[last->seperator + 1 + i];
                dest_i += 1;
            }
        }
    }
    short_path[dest_i] = '\0';

    fputs(short_path, stdout);
}