#define _GNU_SOURCE
#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifndef BUFSIZE
#define BUFSIZE 1048576
#endif

#define STATE_HEADERS 0
#define STATE_BODY 1  // Body with a Content-Length header
#define STATE_CHUNK_LINE 2
#define STATE_CHUNK_CONTENTS 3

#ifdef DEBUG
#define DEBUG_PRINTF(...) do { fprintf(stderr, __VA_ARGS__); } while (false)
#else
#define DEBUG_PRINTF(...) do {} while (false)
#endif

int main(int argc, char* argv[]) {
	//TODO --meta or a similar way to get something like that?

	// Read stdin, decode HTTP responses, dump all bodies to stdout.
	// One LF is inserted at the end of each response to ensure that a new response always begins on a new line.
	// Headers and chunk lines must fit into BUFSIZE.
	// Does not fully comply with the HTTP spec. For example, headers must be capitalised canonically, and continuation lines are unsupported.
	char buf[2 * BUFSIZE];
	size_t n;
	int state = STATE_HEADERS;
	char* bufp;
	char* m0;
	char* m1;
	char* eoh;
	size_t nscan;
	size_t bytes_read;
	size_t length;

	while ((n = fread(buf, 1, BUFSIZE, stdin)) > 0) {
		bufp = buf;
checkstate:
		DEBUG_PRINTF("Have %zu bytes of buffer (at %p)\n", n, (void*)bufp);
		DEBUG_PRINTF("Beginning of buffer: ");
		for (int i = 0; i < 64; ++i) DEBUG_PRINTF(isprint(*(bufp + i)) ? "%c" : "\\x%02x", *(bufp + i) & 0xFF);
		DEBUG_PRINTF("\n");
		if (n == 0) {
			break;
		}
		DEBUG_PRINTF("State: %d\n", state);
		if (state == STATE_HEADERS) {
			if (n < 9) {
				fprintf(stderr, "Error: too little data before HTTP headers\n");
				return 1;
			}
			if (memcmp(bufp, "HTTP/1.1 ", 9) == 0) {
				// Got some headers; find transfer encoding, content length, and end of headers
				eoh = memmem(bufp, n, "\r\n\r\n", 4);
				if (!eoh) {
					fprintf(stderr, "Error: end of headers not found\n");
					return 1;
				}
				eoh += 4;
				DEBUG_PRINTF("Response body begins at %p (offset %zu)\n", (void*)eoh, eoh - bufp);

				m0 = memmem(bufp, n, "\r\nContent-Length:", 17);
				if (m0 && m0 < eoh) {
					DEBUG_PRINTF("Found Content-Length header at %p (offset %zu)\n", (void*)(m0 + 2), m0 + 2 - bufp);
					m1 = memmem(m0 + 1, n - (m0 + 1 - bufp), "\r\n", 2);
					if (!m1) {
						fprintf(stderr, "Error: CRLF after Content-Length missing\n");
						return 1;
					}
					m0 += 17;
					while (m0 < bufp + n && (*m0 == ' ' || *m0 == '\t')) ++m0;
					if (!sscanf(m0, "%zu%n", &length, &nscan)) {
						fprintf(stderr, "Error: invalid Content-Length\n");
						return 1;
					}
					if (nscan > n - (m0 - bufp)) {
						fprintf(stderr, "Error: buffer overread\n");
						return 1;
					}
					m0 += nscan;
					while (m0 < bufp + n && (*m0 == ' ' || *m0 == '\t')) ++m0;
					if (m0 != m1) {
						fprintf(stderr, "Error: invalid Content-Length (noise before EOL)\n");
						return 1;
					}
					DEBUG_PRINTF("Content length: %zu\n", length);

					state = STATE_BODY;
				} else {
					m0 = memmem(bufp, n, "\r\nTransfer-Encoding:", 20);
					if (!m0 || m0 >= eoh) {
						fprintf(stderr, "Error: Content-Length and Transfer-Encoding missing\n");
						return 1;
					}
					DEBUG_PRINTF("Found Transfer-Encoding header at %p (offset %zu)\n", (void*)(m0 + 2), m0 + 2 - bufp);
					m1 = memmem(m0 + 1, n - (m0 + 1 - bufp), "\r\n", 2);
					if (!m1 || m1 >= eoh - 2) {
						fprintf(stderr, "Error: CRLF after Transfer-Encoding missing\n");
						return 1;
					}
					m0 += 20;
					while (m0 < bufp + n && (*m0 == ' ' || *m0 == '\t')) ++m0;
					if (memcmp(m0, "chunked", 7) != 0) {
						fprintf(stderr, "Error: unsupported Transfer-Encoding\n");
						return 1;
					}
					m0 += 7;
					while (m0 < bufp + n && (*m0 == ' ' || *m0 == '\t')) ++m0;
					if (m0 != m1) {
						fprintf(stderr, "Error: unsupported Transfer-Encoding\n");
						return 1;
					}
					DEBUG_PRINTF("Chunked transfer encoding\n");

					state = STATE_CHUNK_LINE;
				}

				DEBUG_PRINTF("Adjusting buffer pointer and n by %zu\n", eoh - bufp);
				n = n - (eoh - bufp);
				bufp = eoh;
				bytes_read = 0;
				goto checkstate;
			} else {
				fprintf(stderr, "Error: expected header line, got something else\n");
				return 1;
			}
		} else if (state == STATE_BODY || state == STATE_CHUNK_CONTENTS) {
			if (length + 2 - bytes_read > n) {
				// Only got part of the body
				DEBUG_PRINTF("Partial body\n");
				DEBUG_PRINTF("Copying %zu bytes to stdout\n", n);
				fwrite(bufp, 1, n, stdout);
				bytes_read += n;
				DEBUG_PRINTF("%zu of %zu bytes from this response written\n", bytes_read, length);
			} else {
				// Remainder of the response is in the buffer. Same logic as above for small records fitting in the buffer with the headers.
				DEBUG_PRINTF("Full body\n");
				DEBUG_PRINTF("Copying %zu bytes to stdout\n", length - bytes_read);
				fwrite(bufp, 1, length - bytes_read, stdout);
				fprintf(stdout, "\n");
				if (state == STATE_CHUNK_CONTENTS && *(bufp + length - bytes_read) == '\r') {
					// Stupid hack to enforce the CRLF
					++length;
				}
				if (memcmp(bufp + length - bytes_read, "\n", 1) != 0) {
					fprintf(stderr, "Error: end of HTTP body not found\n");
					return 1;
				}
				DEBUG_PRINTF("Adjusting buffer pointer and n by %zu\n", length + 1 - bytes_read);
				n = n - (length + 1 - bytes_read);
				bufp = bufp + length + 1 - bytes_read;
				if (n < BUFSIZE) {
					DEBUG_PRINTF("Buffer too small (%zu bytes), moving and refilling\n", n);
					memmove(buf, bufp, n);
					bufp = buf;
					n += fread(buf + n, 1, BUFSIZE, stdin);
				}
				if (state == STATE_BODY) {
					state = STATE_HEADERS;
				} else {
					state = STATE_CHUNK_LINE;
				}
				goto checkstate;
			}
		} else if (state == STATE_CHUNK_LINE) {
			m1 = memmem(bufp, n, "\r\n", 2);
			if (!m1) {
				fprintf(stderr, "Error: chunk line EOL missing\n");
				return 1;
			}
			m0 = bufp;
			while (m0 < bufp + n && (*m0 == ' ' || *m0 == '\t')) ++m0;
			if (!sscanf(m0, "%x%n", &length, &nscan)) {
				fprintf(stderr, "Error: invalid chunk length\n");
				return 1;
			}
			if (nscan > n - (m0 - bufp)) {
				fprintf(stderr, "Error: buffer overread\n");
				return 1;
			}
			m0 += nscan;
			while (m0 < m1 && (*m0 == ' ' || *m0 == '\t')) ++m0;
			if (*m0 != ';' && m0 != m1) {
				fprintf(stderr, "Error: invalid Content-Length (noise before EOL)\n");
				return 1;
			}
			DEBUG_PRINTF("Chunk length: %zu bytes\n", length);

			DEBUG_PRINTF("Adjusting buffer pointer and n by %zu\n", m1 + 2 - bufp);
			n = n - (m1 + 2 - bufp);
			bufp = m1 + 2;

			if (length == 0) {
				// End of response, must be followed by CRLF + LF
				if (n < 3) {
					fprintf(stderr, "Error: buffer exhausted while looking for empty chunk CRLF\n");
					return 1;
				}
				if (*(m1 + 2) != '\r' || *(m1 + 3) != '\n' || *(m1 + 4) != '\n') {
					fprintf(stderr, "Error: end of HTTP body not found\n");
					return 1;
				}
				n -= 3;
				bufp += 3;
				state = STATE_HEADERS;
			} else {
				state = STATE_CHUNK_CONTENTS;
			}

			if (n < BUFSIZE) {
				DEBUG_PRINTF("Buffer too small (%zu bytes), moving and refilling\n", n);
				memmove(buf, bufp, n);
				bufp = buf;
				n += fread(buf + n, 1, BUFSIZE, stdin);
			}
			goto checkstate;
		}
	}
	if (state != STATE_HEADERS) {
		fprintf(stderr, "Error: incomplete body at the end of input\n");
		return 1;
	}
}