#include "charset.h"
#include <string.h>

static int is_utf16_le(const unsigned char *in, size_t len)
{
	return len >= 2 && in[0] == 0xFF && in[1] == 0xFE;
}

static int is_utf16_be(const unsigned char *in, size_t len)
{
	return len >= 2 && in[0] == 0xFE && in[1] == 0xFF;
}

static int is_utf8(const unsigned char *in, size_t len)
{
	size_t i;
	int has_multibyte = 0;

	for (i = 0; i < len; i++) {
		if (in[i] < 0x80)
			continue;
		has_multibyte = 1;
		if ((in[i] & 0xE0) == 0xC0) {
			if (i + 1 >= len || (in[i+1] & 0xC0) != 0x80)
				return 0;
			i += 1;
		} else if ((in[i] & 0xF0) == 0xE0) {
			if (i + 2 >= len || (in[i+1] & 0xC0) != 0x80 ||
			    (in[i+2] & 0xC0) != 0x80)
				return 0;
			i += 2;
		} else if ((in[i] & 0xF8) == 0xF0) {
			if (i + 3 >= len || (in[i+1] & 0xC0) != 0x80 ||
			    (in[i+2] & 0xC0) != 0x80 || (in[i+3] & 0xC0) != 0x80)
				return 0;
			i += 3;
		} else {
			return 0;
		}
	}
	return has_multibyte;
}

static int utf8_to_codepoint(const unsigned char *in, size_t remaining,
                             unsigned int *cp)
{
	if (in[0] < 0x80) {
		*cp = in[0];
		return 1;
	} else if ((in[0] & 0xE0) == 0xC0) {
		if (remaining < 2) return 0;
		*cp = ((in[0] & 0x1F) << 6) | (in[1] & 0x3F);
		return 2;
	} else if ((in[0] & 0xF0) == 0xE0) {
		if (remaining < 3) return 0;
		*cp = ((in[0] & 0x0F) << 12) | ((in[1] & 0x3F) << 6) |
		      (in[2] & 0x3F);
		return 3;
	} else if ((in[0] & 0xF8) == 0xF0) {
		if (remaining < 4) return 0;
		*cp = ((in[0] & 0x07) << 18) | ((in[1] & 0x3F) << 12) |
		      ((in[2] & 0x3F) << 6) | (in[3] & 0x3F);
		return 4;
	}
	return 0;
}

static int utf16le_to_iso8859_1(const unsigned char *in, size_t in_len,
                                char *out, size_t out_size)
{
	size_t i, o = 0;

	for (i = 2; i + 1 < in_len && o + 1 < out_size; i += 2) {
		unsigned int cp = in[i] | (in[i+1] << 8);
		out[o++] = cp <= 0xFF ? (char)cp : '?';
	}
	out[o] = '\0';
	return (int)o;
}

static int utf16be_to_iso8859_1(const unsigned char *in, size_t in_len,
                                char *out, size_t out_size)
{
	size_t i, o = 0;

	for (i = 2; i + 1 < in_len && o + 1 < out_size; i += 2) {
		unsigned int cp = (in[i] << 8) | in[i+1];
		out[o++] = cp <= 0xFF ? (char)cp : '?';
	}
	out[o] = '\0';
	return (int)o;
}

static int utf8_to_iso8859_1(const unsigned char *in, size_t in_len,
                             char *out, size_t out_size)
{
	size_t i = 0, o = 0;

	while (i < in_len && o + 1 < out_size) {
		unsigned int cp;
		int consumed = utf8_to_codepoint(in + i, in_len - i, &cp);
		if (consumed == 0) {
			out[o++] = '?';
			i++;
			continue;
		}
		out[o++] = cp <= 0xFF ? (char)cp : '?';
		i += consumed;
	}
	out[o] = '\0';
	return (int)o;
}

int to_iso8859_1(const unsigned char *in, size_t in_len,
                 char *out, size_t out_size)
{
	if (!in || !out || out_size == 0)
		return 0;

	if (in_len == 0) {
		out[0] = '\0';
		return 0;
	}

	if (is_utf16_le(in, in_len))
		return utf16le_to_iso8859_1(in, in_len, out, out_size);

	if (is_utf16_be(in, in_len))
		return utf16be_to_iso8859_1(in, in_len, out, out_size);

	if (is_utf8(in, in_len))
		return utf8_to_iso8859_1(in, in_len, out, out_size);

	/* Already ISO-8859-1 / ASCII — passthrough */
	size_t copy = in_len < out_size - 1 ? in_len : out_size - 1;
	memcpy(out, in, copy);
	out[copy] = '\0';
	return (int)copy;
}