/*  lifted from /sys/src/cmd/ip/ppp/mppc.c (RFC 2118) */
#include <u.h>
#include <libc.h>
#include <draw.h>
#include "dat.h"
#include "fns.h"

#define DBG if(0)
//#define DBG

enum
{
	HistorySize		= 8*1024,

	Preset=			0x80,	/* reset history */
	Pfront=			0x40,	/* move packet to front of history */
	Pcompress=		0x20,	/* packet is compressed */
	Pbulk64=			0x01,	/* RPD5 bulk compression (64K history) */
};

enum {
	Lit7,		/* seven bit literal */
	Lit8,		/* eight bit literal */
	Off6,		/* six bit offset */
	Off8,		/* eight bit offset */
	Off13,		/* thirteen bit offset */
};

/* decode first four bits */
static int decode[16] = {
	Lit7,	
	Lit7,
	Lit7,
	Lit7,
	Lit7,
	Lit7,
	Lit7,
	Lit7,
	Lit8,
	Lit8,	
	Lit8,
	Lit8,	
	Off13,
	Off13,
	Off8,
	Off6,
};

typedef struct Uncstate Uncstate;
struct Uncstate
{
	uchar	his[HistorySize];
	int	indx;		/* current indx in history */
	int	size;		/* current history size */
};

static Uncstate uncstate;


#define NEXTBYTE	sreg = (sreg<<8) | *p++; n--; bits += 8
uchar*
uncomp(uchar* buf, int nbytes, int flags, int* psize)
{
	int n, bits, off, len, ones, t;
	ulong sreg;
	uchar *p, c, *hp, *hs, *he, *hq;
	Uncstate *s;

	s = &uncstate;
	p = buf;
	n = nbytes;

	if(flags&Preset){
		s->indx = 0;
		s->size = 0;
	}
	if(flags&Pfront) {
		s->indx = 0;
DBG		fprint(2, "mppc: front flag set\n"); 
	}
	if(!(flags&Pcompress)){
		*psize = n;
		return buf;
	}

	if(flags&Pbulk64){
		werrstr("bigger history is not supported");
		return nil;
	}

	bits = 0;
	sreg = 0;
	hs = s->his;			/* history start */
	hp = hs+s->indx;		/* write pointer in history */
	he = hs+sizeof(s->his);	/* history end */
	for(;;){
		if(bits<4){
			if(n==0) goto Done;
			NEXTBYTE;
		}
		t = decode[(sreg>>(bits-4))&0xf];
		switch(t){
		default:
			sysfatal("mppc: bad decode!");
		case Lit7:
			bits -= 1;
			if(bits<7){
				if(n==0) goto Done;
				NEXTBYTE;
			}
			c = (sreg>>(bits-7))&0x7f;
			bits -= 7;
			if(hp >= he) goto His;
			*hp++ = c;
			continue;
		case Lit8:
			bits -= 2;
			if(bits<7) {
				if(n==0) goto Eof;
				NEXTBYTE;
			}
			c = 0x80 | ((sreg>>(bits-7))&0x7f);
			bits -= 7;
			if(hp >= he) goto His;
			*hp++ = c;
			continue;
		case Off6:
			bits -= 4;
			if(bits<6){
				if(n==0) goto Eof;
				NEXTBYTE;
			}
			off = (sreg>>(bits-6))&0x3f;
			bits -= 6;
			break;
		case Off8:
			bits -= 4;
			if(bits<8){
				if(n==0) goto Eof;
				NEXTBYTE;
			}
			off = ((sreg>>(bits-8))&0xff)+64;
			bits -= 8;
			break;
		case Off13:
			bits -= 3;
			while(bits<13){
				if(n==0) goto Eof;
				NEXTBYTE;
			}
			off = ((sreg>>(bits-13))&0x1fff)+320;
			bits -= 13;
			break;
		}
		for(ones=0;;ones++) {
			if(bits == 0) {
				if(n==0) goto Eof;
				NEXTBYTE;
			}
			bits--;
			if(!(sreg&(1<<bits)))
				break;
		}
		if(ones>11){
			werrstr("bad length %d\n", ones);
			return nil;
		}
		if(ones == 0) {
			len = 3;
		} else {
			ones++;
			while(bits<ones) {
				if(n==0) goto Eof;
				NEXTBYTE;
			}
			len = (1<<ones) | ((sreg>>(bits-ones))&((1<<ones)-1));
			bits -= ones;
		}

		hq = hp-off;
		if(hq < hs) {
			hq += sizeof(s->his);
			if(hq-hs+len > s->size)
				goto His;
		}
		if(hp+len > he)
			goto His;
		while(len) {
			*hp++ = *hq++;
			len--;
		}
	}

Done:
	hq = hs+s->indx;
	len = hp-hq;
DBG fprint(2, "mppc: len %d bits = %d n=%d\n", len, bits, n);

	s->indx += len;
	if(s->indx > s->size)
		s->size = s->indx;

	*psize = len;
	return hq;

Eof:
	werrstr("unexpected end of data");
	return nil;
His:
	werrstr("bad history reference");
	return nil;
}