/* Copyright (C) 2020  C. McEnroe <june@causal.agency>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

#include <err.h>
#include <errno.h>
#include <iconv.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include <sysexits.h>

#include "archive.h"

struct Buffer {
	size_t cap;
	size_t len;
	char *ptr;
};

static struct Buffer bufferAlloc(size_t cap) {
	struct Buffer buf = {
		.cap = cap,
		.len = 0,
		.ptr = malloc(cap),
	};
	if (!buf.ptr) err(EX_OSERR, "malloc");
	return buf;
}

static char *bufferDest(struct Buffer *buf, size_t len) {
	if (buf->len + len > buf->cap) {
		buf->cap *= 2;
		buf->ptr = realloc(buf->ptr, buf->cap);
		if (!buf->ptr) err(EX_OSERR, "realloc");
	}
	char *dest = &buf->ptr[buf->len];
	buf->len += len;
	return dest;
}

static void bufferCopy(struct Buffer *buf, const char *src, size_t len) {
	char *dst = bufferDest(buf, len);
	memcpy(dst, src, len);
}

static char *bufferString(struct Buffer *buf) {
	*bufferDest(buf, 1) = '\0';
	return buf->ptr;
}

static void convertCharset(
	struct Buffer *dst, const char *charset, const char *src, size_t len
) {
	iconv_t conv = iconv_open("utf-8", charset);
	if (conv == (iconv_t)-1) {
		warn("cannot convert from %s to utf-8", charset);
		return;
	}

	for (size_t pad = 0; len; ++pad) {
		char *ptr = bufferDest(dst, len + pad);
		size_t cap = dst->cap - (ptr - dst->ptr);
		size_t n = iconv(conv, (char **)&src, &len, &ptr, &cap);
		if (n == (size_t)-1 && errno != E2BIG) {
			warn("iconv");
			break;
		}
		dst->len = dst->cap - cap;
	}

	iconv_close(conv);
}

static void decodeQ(struct Buffer *dst, const char *src, size_t len) {
	while (len) {
		if (*src == '=') {
			if (len < 3) break;
			*bufferDest(dst, 1) = strtoul(&src[1], NULL, 16);
			len -= 3;
			src += 3;
		} else {
			*bufferDest(dst, 1) = (*src == '_' ? ' ' : *src);
			len--;
			src++;
		}
	}
}

static void decodeEncoding(
	struct Buffer *dst, const char *encoding, const char *src, size_t len
) {
	if (!strcasecmp(encoding, "Q")) {
		decodeQ(dst, src, len);
	} else {
		// TODO
		bufferCopy(dst, src, len);
	}
}

static void decode(
	struct Buffer *dst, const char *encoding, const char *charset,
	const char *src, size_t len
) {
	if (
		!charset ||
		!strcasecmp(charset, "us-ascii") ||
		!strcasecmp(charset, "utf-8")
	) {
		decodeEncoding(dst, encoding, src, len);
	} else {
		// TODO: Avoid copying if encoding is 8bit.
		struct Buffer decoded = bufferAlloc(len);
		decodeEncoding(&decoded, encoding, src, len);
		convertCharset(dst, charset, decoded.ptr, decoded.len);
		free(decoded.ptr);
	}
}

static void decodeWord(struct Buffer *dst, const char *src, size_t len) {
	struct Buffer word = bufferAlloc(len + 1);
	bufferCopy(&word, src, len);

	char *ptr = bufferString(&word);
	strsep(&ptr, "?");
	char *charset = strsep(&ptr, "?");
	char *encoding = strsep(&ptr, "?");
	char *encoded = strsep(&ptr, "?");

	if (charset && encoding && encoded && ptr && *ptr == '=') {
		decode(dst, encoding, charset, encoded, strlen(encoded));
	} else {
		bufferCopy(dst, src, len);
	}

	free(word.ptr);
}

char *decodeHeader(const char *header) {
	struct Buffer buf = bufferAlloc(strlen(header) + 1);
	while (*header) {
		size_t len = strcspn(header, " ");
		if (!strncmp(header, "=?", 2)) {
			decodeWord(&buf, header, len);
		} else {
			if (header[len]) len++;
			bufferCopy(&buf, header, len);
		}
		header += len;
	}
	return bufferString(&buf);
}

int decodeContent(
	FILE *file, EscapeFn *escape,
	const struct BodyPart *part, const char *content
) {
	if (!escape) escape = escapeNull;
	// TODO
	return escape(file, content);
}