#include <assert.h>
#include <stdio.h>
#include <stdbool.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <curl/curl.h>

#define VIRTUAL_BUFFER_SIZE 8192
#define CURL_RECVBUF_SIZE   1024

/*
 * Global variables
 */
CURLM *curl_multi_handle;
CURL *curl;
bool paused;
size_t total_length;
size_t buffered;
size_t downloaded;

static size_t
http_header(void *buffer, size_t size, size_t nmemb, void *userp)
{
	const char content_length[] = "content-length:";
	size_t n = strlen(content_length);
	if (strncasecmp((char *)buffer, content_length, n) == 0)
		total_length = strtoul(buffer + n + 1, NULL, 10);
	return nmemb;
}

static size_t
http_body(void *buffer, size_t size, size_t nmemb, void *userp)
{
	if (buffered + nmemb > VIRTUAL_BUFFER_SIZE) {
		/* Not enough space in the "buffer" => pause transfer */
		fprintf(stderr, "Pause (buffered=%zu)\n", buffered);
		paused = true;
		return CURL_WRITEFUNC_PAUSE;
	}
	buffered += nmemb;
	return nmemb;
}

#define MIN(a,b) ((a) < (b) ? (a) : (b))

int
main(int argc, char **argv)
{
	const char *url = "https://ftp.sunet.se/mirror/archlinux/archive/floppy/20060104/boot.img";
	int request_size = 32 << 10;
	struct CURLMsg *msg;
	int running_handles;
	int msgs_left;
	char range[128];

	if (argc > 1)
		url = argv[1];

	curl_global_init(CURL_GLOBAL_DEFAULT);
	curl_multi_handle = curl_multi_init();
	curl = curl_easy_init();

	curl_easy_setopt(curl, CURLOPT_URL, url);
	curl_easy_setopt(curl, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
	curl_easy_setopt(curl, CURLOPT_BUFFERSIZE, CURL_RECVBUF_SIZE);
	curl_easy_setopt(curl, CURLOPT_FAILONERROR, 1L);
	snprintf(range, sizeof range, "%d-%d", 0, request_size - 1);
	curl_easy_setopt(curl, CURLOPT_RANGE, range);
	curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, http_header);
	curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, http_body);
	curl_easy_setopt(curl, CURLOPT_VERBOSE, 1);

	curl_multi_add_handle(curl_multi_handle, curl);

	total_length = request_size;

	curl_multi_perform(curl_multi_handle, &running_handles);

	while (running_handles > 0) {
		/* Resume if paused now that we have space */
		if (paused && buffered < VIRTUAL_BUFFER_SIZE/2) {
			fprintf(stderr, "Continue (buffered=%zu)\n", buffered);
			curl_easy_pause(curl, CURLPAUSE_CONT);
			paused = false;
		}

		curl_multi_perform(curl_multi_handle, &running_handles);

		/* Consume some data from the "buffer" */
		if (buffered > 0) {
			size_t n = MIN(buffered, 1024);
			buffered -= n;
			downloaded += n;
		}

		usleep(10*1000);

		{
			static size_t last_buffered = ~0, last_downloaded = ~0;
			if (last_buffered != buffered || last_downloaded != downloaded) {
				printf("run=%d buffered=%zu progress=%zu\n", running_handles, buffered, downloaded);
				last_buffered = buffered;
				last_downloaded = downloaded;
			}
		}
	}

	msg = curl_multi_info_read(curl_multi_handle, &msgs_left);
	fprintf(stderr, "Connection finished: %s\n", curl_easy_strerror(msg->data.result));

	downloaded += buffered;
	printf("Total downloaded: %zu\n", downloaded);
	curl_multi_remove_handle(curl_multi_handle, curl);
	curl_easy_cleanup(curl);
	curl_multi_cleanup(curl_multi_handle);

	return 0;
}