/*! \file tcobsv1Decode.c
\author Thomas.Hoehenleitner [at] seerose.net
\details See ./TCOBSv1Specification.md.
*******************************************************************************/

#include <stdint.h>
#include <stddef.h>
#include <string.h> // memcpy
#include "tcobs.h"
#include "tcobsv1Internal.h"

static int sigilAndOffset(uint8_t* sigil, uint8_t b);
static uint8_t repeatByte(int offset, uint8_t* in, int len);

int TCOBSDecode(void* __restrict output, size_t max, const void* __restrict input, size_t length) {
	uint8_t* in = (uint8_t*)input;
	int ilen = (int)length; // remaining input length
	uint8_t* out = (uint8_t*)output;
	int olen = 0; // output length
	while (ilen > 0) {
		uint8_t next = in[ilen - 1]; // get next sigil byte (starting from the end)
		uint8_t sigil;
		uint8_t r;
		int offset = sigilAndOffset(&sigil, next);
		if (offset + 1 > ilen) {
			return INPUT_DATA_CORRUPTED - __LINE__;
		}
		ilen -= 1; // remove sigil byte from buffer
		switch (sigil) {
		case N:
			goto copyBytes;

		case Z3:
			olen += 1;
			out[max - olen] = 0;
			// fallthrough
		case Z2:
			olen += 1;
			out[max - olen] = 0;
			// fallthrough
		case Z1:
			olen += 1;
			out[max - olen] = 0;
			goto copyBytes;

		case F4:
			olen += 1;
			out[max - olen] = 0xFF;
			// fallthrough
		case F3:
			olen += 1;
			out[max - olen] = 0xFF;
			// fallthrough
		case F2:
			olen += 1;
			out[max - olen] = 0xFF;
			olen += 1;
			out[max - olen] = 0xFF;
			goto copyBytes;

		case R4:
			olen += 1;
			out[max - olen] = repeatByte(offset, in, ilen);
			// fallthrough
		case R3:
			olen += 1;
			out[max - olen] = repeatByte(offset, in, ilen);
			// fallthrough
		case R2:
			r = repeatByte(offset, in, ilen);
			olen += 1;
			out[max - olen] = r;
			olen += 1;
			out[max - olen] = r;
			goto copyBytes;

		default:
			return INPUT_DATA_CORRUPTED - __LINE__;
		}

	copyBytes: {
		uint8_t* to = out + max - olen - offset; // to := len(d) - n - offset
		uint8_t* from = in + ilen - offset;      // from := len(in) - offset // sigil byte is already removed
		if (to < out) {
			return OUT_BUFFER_TOO_SMALL - __LINE__;
		}
		memcpy(to, from, offset); // n += copy(d[to:], in[from:])
		olen += offset;
		ilen -= offset; // in = in[:len(in)-offset] // remove copied bytes
		continue;
	}
	}
	return olen;
}

// sigilAndOffset interprets b as sigil byte with offset, fills sigil and returns offset.
// For details see TCOBSv1Specification.md.
static int sigilAndOffset(uint8_t* sigil, uint8_t b) {
	int offset;
	*sigil = b & 0xE0; // 0x11100000
	if (*sigil == 0) {
		*sigil = b & 0xF8; // 0x11111000
		offset = 7 & b;    // 0x00000111
	} else {
		offset = 0x1F & b; // 0x00011111
	}
	return offset;
}

// repeatByte returns the value to repeat
static uint8_t repeatByte(int offset, uint8_t* in, int len) {
	if (offset == 0) {      // left byte of Ri is a sigil byte (probably N)
		return in[len - 2]; // a buffer cannot start with Ri
	} else {
		return in[len - 1];
	}
}