#define _POSIX_C_SOURCE 200809L

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

static char *alloc_cwd(void)
{
    size_t len = 1;
    char *p = NULL;

    for (;;)
    {
        char *const pp = realloc(p, len);

        if (!pp)
        {
            fprintf(stderr, "%s: realloc(3): %s\n", __func__, strerror(errno));
            break;
        }

        p = pp;

        if (!getcwd(pp, len))
        {
            if (errno != ERANGE)
            {
                fprintf(stderr, "%s: getcwd(3): %s\n",
                    __func__, strerror(errno));
                break;
            }
            else
                len++;
        }
        else
            return p;
    }

    free(p);
    return NULL;
}

static char *get_parent(char *p)
{
    char *last;
    size_t len;

    if (!p || !(last = strrchr(p, '/')) || strlen(last) < strlen("/a"))
    {
        fprintf(stderr, "%s: parent folder expected\n", __func__);
        return NULL;
    }

    len = last == p ? strlen("/") : last - p;

    if (!(p = realloc(p, len + 1)))
    {
        fprintf(stderr, "%s: realloc(3): %s\n", __func__, strerror(errno));
        return NULL;
    }

    p[len] = '\0';
    return p;
}

static char *get_component(const char **const path)
{
    char *ret;

    while (**path == '/')
        (*path)++;

    const char *const next = strchr(*path, '/');
    const size_t len = next ? next - *path : strlen(*path);

    if (!(ret = malloc(len + 1)))
    {
        fprintf(stderr, "%s: malloc(3): %s\n", __func__, strerror(errno));
        return NULL;
    }

    memcpy(ret, *path, len);
    ret[len] = '\0';
    *path += len;
    return ret;
}

static char *resolve_component(const char **const path, char *p)
{
    char *const c = get_component(path);

    if (!*c)
    {
        if (!p && !(p = strdup("/")))
        {
            fprintf(stderr, "%s: strdup(3): %s\n", __func__, strerror(errno));
            goto end;
        }

        goto end;
    }
    else if (!strcmp(c, "..") && !(p = get_parent(p)))
    {
        fprintf(stderr, "%s: get_parent failed\n", __func__);
        goto end;
    }
    else if (strcmp(c, "."))
    {
        const size_t len = p ? strlen(p) : 0, clen = strlen(c);

        if (!(p = realloc(p, len + strlen("/") + clen + 1)))
        {
            fprintf(stderr, "%s: realloc(3): %s\n",
                __func__, strerror(errno));
            goto end;
        }
        else
        {
            p[len] = '/';
            memcpy(&p[len + 1], c, clen);
            p[len + 1 + clen] = '\0';
        }
    }

end:
    free(c);
    return p;
}

static char *resolve(const char *path, char *p)
{
    while (*path)
        if (!(p = resolve_component(&path, p)))
        {
            fprintf(stderr, "%s: resolve_component failed\n", __func__);
            return NULL;
        }

    return p;
}

char *crealpath(const char *const path)
{
    char *p = NULL, *ret = NULL;

    if (!*path)
    {
        fprintf(stderr, "%s: expected non-empty path\n", __func__);
        goto end;
    }
    else if (*path != '/' && !(p = alloc_cwd()))
    {
        fprintf(stderr, "%s: alloc_cwd failed\n", __func__);
        goto end;
    }
    else if (!(p = resolve(path, p)))
    {
        fprintf(stderr, "%s: resolve failed\n", __func__);
        goto end;
    }

    ret = p;

end:
    if (!ret)
        free(p);

    return ret;
}