/*
 * Equi platform reference implementation
 *
 * Created in 2022 by Luxferre, released into public domain
 *
 * See equi.md file for the specification and manual
 *
 * @license Unlicense <https://unlicense.org>
 * @author Luxferre
 */

/* Standard includes with size optimizations for TCC */

#ifdef __TINYC__
#include <tcclib.h>
#define stderr 2
#else
#include <stdlib.h>
#include <stdio.h>
#endif

/*
 * Non-standard includes
 * To save space, we emulate necessary conio functions on non-6502 systems, not vice versa
 */

#ifdef __CC65__
#include <conio.h>
#else

#define cgetc() (getchar())
#define cputc(c) (putchar(c))
#endif

/* Definitions section */

#define EQUI_VER "0.0.1"

#define cerr(s) (fputs(s, stderr))
#define ushort unsigned short /* basic 16-bit integer */
#define uchar unsigned char /* basic 8-bit integer */
#define WS sizeof(ushort) /* Equi word size in bytes */
#define BS 8u /* Backspace character code */
#define CR 13u /* Character return code */
#define LF 10u /* Line feed code */

/* Configuration section (constants overridable at compile-time) */

/* Main and return stack size in bytes */
#ifndef STACK_SIZE
#define STACK_SIZE 256u
#endif

/* Literal stack size in bytes */
#ifndef LIT_STACK_SIZE
#define LIT_STACK_SIZE 32u
#endif

/* Command buffer size in bytes */
#ifndef GPD_AREA_SIZE
#define GPD_AREA_SIZE 6400u
#endif

/* Command buffer size in bytes */
#ifndef CMD_BUF_SIZE
#define CMD_BUF_SIZE 15600u
#endif

/* Maximum amount of CLT entries */
#ifndef CLT_ENTRIES_MAX
#define CLT_ENTRIES_MAX 512u
#endif

/* Some necessary constants and offsets derived from the above values */
#define STACK_SIZE_WORDS (STACK_SIZE / WS) /* Main and return stack size in words */

/*
 * Structures that describe Equi machine RAM using the above configuration
 */

struct CLTEntry {
  ushort nhash; /* compiled word name hash */
  ushort loc;   /* compiled word location */
};

struct EquiRAM { 
  ushort main_stack[STACK_SIZE_WORDS];
  ushort return_stack[STACK_SIZE_WORDS];
  uchar literal_stack[LIT_STACK_SIZE];
  ushort pc; /* program counter */
  ushort msp; /* main stack pointer */
  ushort rsp; /* return stack pointer */
  uchar lsp; /* literal stack pointer */
  ushort cbp; /* compilation buffer pointer */
  ushort cltp; /* compilation lookup table pointer */
  unsigned int II:1; /* instruction ignore mode */
  unsigned int CM:1; /* compilation mode */
  unsigned int IM:1; /* interpretation mode */
  ushort gpd_start;
  ushort cmd_start;
  ushort cmd_size;
  ushort ibp; /* input buffer pointer */
  struct CLTEntry clt[CLT_ENTRIES_MAX]; /* compilation lookup table */
  uchar gpd[GPD_AREA_SIZE];
  uchar cmdbuf[CMD_BUF_SIZE];
};

/* Before running the main code, instantiate the machine RAM */
static struct EquiRAM ram;

/* Also create an alternative view of the same RAM area for direct offset-based access */
static uchar* flatram = (uchar *)&ram;

/* Error reporting codes */
enum EquiErrors {
  SUCCESS=0,
  STACK_OVERFLOW,
  STACK_UNDERFLOW,
  DIV_BY_ZERO,
  CLT_OVERFLOW,
  CMD_OVERFLOW,
  INVALID_INSTRUCTION,
  INVALID_WORD,
  PORT_IO_ERROR
};

/* Error reporting method */
void trapout(errcode) {
  if(errcode > 0) {
    fprintf(stderr, "\nError %d at 0x%x (instruction %c): ", errcode, ram.pc, ram.cmdbuf[ram.pc]);
    switch(errcode) {
      case STACK_OVERFLOW:
        cerr("Stack overflow\n");
        break;
      case STACK_UNDERFLOW:
        cerr("Stack underflow\n");
        break;
      case DIV_BY_ZERO:
        cerr("Division by zero\n");
        break;
      case CLT_OVERFLOW:
        cerr("Compilation lookup table full\n");
        break;
      case CMD_OVERFLOW:
        cerr("Command buffer full\n");
        break;
      case INVALID_INSTRUCTION:
        cerr("Invalid instruction\n");
        break;
      case INVALID_WORD:
        cerr("Word not found in CLT\n");
        break;
      case PORT_IO_ERROR:
        cerr("Port I/O error\n");
        break;
    }
    exit(errcode);
  }
}

/* Equi instructions enum */

enum EquiInstructions {
  INS_IISTART='(',
  INS_IIEND=')',
  INS_CMSTART=':',
  INS_CMEND=';',
  INS_LITINT='#',
  INS_LITSTR='"',
  INS_LITCALL='\'',
  INS_RET='R',
  INS_M2R=']', /* main -> return */
  INS_R2M='[', /* return -> main */
  INS_LOAD='L',
  INS_STORE='S',
  INS_STOREBYTE='W',
  INS_DROP='!',
  INS_DUP='$',
  INS_SWAP='%',
  INS_ROT='@',
  INS_OVER='\\',
  INS_JUMP='J',
  INS_IF='I',
  INS_EXPOINT='X', /* locate execution point */
  INS_GPDSTART='G', /* locate GPD area start */
  INS_GT='>',
  INS_LT='<',
  INS_EQ='=',
  INS_ADD='+',
  INS_SUB='-',
  INS_MUL='*',
  INS_DIV='/',
  INS_NEG='N',
  INS_SHIFT='T',
  INS_NOT='~',
  INS_AND='&',
  INS_OR='|',
  INS_XOR='^',
  INS_COUT='.', /* character output */
  INS_NHOUT='H', /* numeric hex output */
  INS_NBKIN=',', /* non-blocking key input */
  INS_BKIN='?', /* blocking key input */
  INS_PORTIO='P',
  INS_PERSIST_WRITE='}',
  INS_PERSIST_READ='{',
  INS_QUIT='Q'
};

/* push a value onto the main stack */
void pushMain(ushort val) {
  ram.main_stack[ram.msp++] = val;
  if(ram.msp >= STACK_SIZE_WORDS)
    trapout(STACK_OVERFLOW);
}

/* push a value onto the return stack */
void pushRet(ushort val) {
  ram.return_stack[ram.rsp++] = val;
  if(ram.rsp >= STACK_SIZE_WORDS)
    trapout(STACK_OVERFLOW);
}

/* pop a value from the main stack */
ushort popMain() {
  if(ram.msp == 0) {
    trapout(STACK_UNDERFLOW);
    return 0;
  }
  else
    return ram.main_stack[--ram.msp];
}

/* pop a value from the return stack */
ushort popRet() {
  if(ram.rsp == 0) {
    trapout(STACK_UNDERFLOW);
    return 0;
  }
  else
    return ram.return_stack[--ram.rsp];
}

/* push a character to the literal stack */
void pushLit(uchar c) {
  ram.literal_stack[ram.lsp++] = c;
  if(ram.lsp >= LIT_STACK_SIZE)
    trapout(STACK_OVERFLOW);
}

/* pop a character from the literal stack */
uchar popLit() {
  if(ram.lsp == 0) {
    trapout(STACK_UNDERFLOW);
    return 0;
  }
  else
    return ram.literal_stack[--ram.lsp];
}

/* convert ASCII code of a hex digit to the digit value */
uchar a2d(uchar a) {
  return (a < 0x3aU) ? (a - 0x30U) : (a - 55U);
}

/* shape 2-byte vlaue on the main stack from  up to 4 values of the literal stack */
void pushLitVal() {
  uchar p[4U] = {0,0,0,0}, i, thr = 4U;
  if(ram.lsp < 4U) thr = ram.lsp;
  for(i=0;i<thr;++i)
    p[3-i] = a2d(popLit());
  pushMain((p[0]<<12U) | (p[1]<<8U) | (p[2]<<4U) | p[3]);
  ram.lsp = 0; /* clear the literal stack */
}

/* CCITT CRC16 helper */

ushort crc16(const uchar* data_p, uchar length) {
  uchar x;
  ushort crc = 0xFFFFu;
  while(length--) {
    x = crc>>8U ^ *data_p++;
    x ^= x>>4U;
    crc = (crc << 8U) ^ ((ushort)(x << 12U)) ^ ((ushort)(x << 5U)) ^ ((ushort)x);
  }
  return crc;
}

/* persistent operation handler */

ushort persistOp(maddr, dataLen, adl, adh, isWrite) {
  return 0;
}

/* Main interpreter loop */

void equi_main_loop() {
  uchar instr, i;
  ushort lhash, pbuf, pbuf2;
  /* reset all stacks before running and reinit CLT */
  ram.msp = ram.rsp = ram.lsp = ram.cltp = 0;
  /* reset pc */
  ram.pc = 65535U;
  while(1) { /* iterate over the instructions in the command buffer */
    instr = ram.cmdbuf[++ram.pc];
    /* silently exit on zero or FF */
    if(instr == 0 || instr == 0xFFu) break;
    /* first, check for II mode */
    if(ram.II) {
      if(instr == INS_IIEND)
        ram.II = 0; /* unset instruction ignore mode flag */
      continue;
    }
    /* then, check for compilation mode */
    if(ram.CM) {
      if(instr == INS_CMEND) { /* trigger word compilation logic as per the spec */
        if(ram.lsp < 1)
          trapout(STACK_UNDERFLOW);
        ram.cmdbuf[ram.pc] = INS_RET; /* in-place patch this instruction to R */
        /* hash and save compiled word */
        ram.clt[ram.cltp].nhash = crc16(&ram.literal_stack[0], ram.lsp);
        ram.lsp = 0; /* clear the literal stack */
        ram.clt[ram.cltp].loc = ram.cbp; /* stored the compiled code location from the most recent CBP value */
        ++ram.cltp; /* increase the word */
        if(ram.cltp == CLT_ENTRIES_MAX)
          trapout(CLT_OVERFLOW);
        ram.CM = 0; /* unset compilation mode flag */
      }
      continue;
    }
    /* other than that, continue with interpretation mode */
    /* first, detect lowercase, underscore, digit and A to F */
    if((instr >= '0' && instr <= '9') || (instr >= 'A' && instr <= 'F') || instr == '_' || (instr >= 'a' && instr <= 'z')) {
      pushLit(instr);
      continue;
    }
    /* then trigger literal auto-push if applicable */
    if(ram.lsp > 0 && instr != INS_LITSTR && instr != INS_LITCALL && instr != INS_LITINT && instr != INS_CMSTART)
      pushLitVal();
    switch(instr) { /* then perform all main interpretation logic */
      case INS_IISTART: /* instruction ignore start */
        ram.II = 1; /* raise II flag */
        break;
      case INS_CMSTART: /* compilation start */
        ram.cbp = ram.pc + 1U; /* save CBP */
        ram.CM = 1U; /* raise CM flag */
        break;
      case CR:
      case LF:
      case '\t':
      case ' ': /* all nops in interpretation mode */
        break;
      case INS_QUIT: /* gracefully quit the interpretation mode */
        goto brx;
      case INS_LITINT: /* literal stack -> main stack as short */
        pushLitVal();
        break;
      case INS_LITSTR: /* literal stack -> each char at main stack as short */
        while(ram.lsp)
          pushMain((ushort)popLit());
        ram.lsp = 0;
        break;
      case INS_LITCALL: /* call the saved word from the literal */
        if(ram.lsp < 1)
          trapout(STACK_UNDERFLOW);
        lhash = crc16(&ram.literal_stack[0], ram.lsp);
        for(pbuf=0;pbuf<CLT_ENTRIES_MAX;++pbuf)
          if(ram.clt[pbuf].nhash == lhash) {
            pushRet(ram.pc); /* first, save the PC into the return stack */
            ram.pc = ram.clt[pbuf].loc; /* then jump to the word location */
            break;
          }
        if(pbuf >= CLT_ENTRIES_MAX)
          trapout(INVALID_WORD);
        ram.lsp = 0; /* clear the literal stack */
        break;
      case INS_RET: /* jump to the instruction at ret stack */
        ram.pc = popRet();
        break;
      case INS_M2R: /* main -> ret stack */
        pushRet(popMain());
        break;
      case INS_R2M: /* ret -> main stack */
        pushMain(popRet());
        break;
      case INS_LOAD: /* mem -> main stack */
        pbuf = popMain();
        pushMain((flatram[pbuf] << 8)|flatram[pbuf+1U]);
        break;
      case INS_STORE: /* main stack -> mem (word) */
        pbuf = popMain();
        pbuf2 = popMain();
        flatram[pbuf] = pbuf2 >> 8U;
        flatram[pbuf + 1U] = pbuf2 & 255U;
        break;
      case INS_STOREBYTE: /* main stack -> mem (byte) */
        pbuf = popMain();
        flatram[pbuf] = popMain() & 255U;
        break;
      case INS_DROP: /* ( a -- ) */
        pbuf = popMain();
        break;
      case INS_DUP: /* ( a -- a a )	*/
        if(ram.msp < 1)
          trapout(STACK_UNDERFLOW);
        pushMain(ram.main_stack[ram.msp-1]);
        break;
      case INS_SWAP: /* ( a b -- b a ) */
        if(ram.msp < 2)
          trapout(STACK_UNDERFLOW);
        pbuf = ram.main_stack[ram.msp-2];
        ram.main_stack[ram.msp-2] = ram.main_stack[ram.msp-1];
        ram.main_stack[ram.msp-1] = pbuf;
        break;
      case INS_ROT: /* ( a b c -- b c a ) */
        if(ram.msp < 3)
          trapout(STACK_UNDERFLOW);
        pbuf = ram.main_stack[ram.msp-3];
        pbuf2 = ram.main_stack[ram.msp-1];
        ram.main_stack[ram.msp-3] = ram.main_stack[ram.msp-2]; 
        ram.main_stack[ram.msp-2] = pbuf2;
        ram.main_stack[ram.msp-1] = pbuf;
        break;
      case INS_OVER: /* ( a b -- a b a ) */
        if(ram.msp < 2)
          trapout(STACK_UNDERFLOW);
        pushMain(ram.main_stack[ram.msp-2]);
        break;
      case INS_JUMP: /* unconditional signed jump: ( rel -- ) */
        ram.pc = (ushort) (ram.pc + (signed short) popMain());
        break;
      case INS_IF: /* conditional signed jump if cond is not zero: ( cond rel -- ) */
        pbuf = popMain(); /* reladdr */
        pbuf2 = popMain(); /* cond */
        if(pbuf2 != 0)
          ram.pc = (ushort) (ram.pc + (signed short) pbuf);
        break;
      case INS_EXPOINT: /* Locate execution point */
        pushMain(ram.pc + 1);
        break;
      case INS_GPDSTART: /* Locate GPD area start */
        pushMain(ram.gpd_start);
        break;
      case INS_GT: /* ( a b -- a>b ) */
        pushMain((popMain() < popMain()) ? 1u : 0u);
        break;
      case INS_LT: /* ( a b -- a<b ) */
        pushMain((popMain() > popMain()) ? 1u : 0u);
        break;
      case INS_EQ: /* ( a b -- a==b ) */
        pushMain((popMain() == popMain()) ? 1u : 0u);
        break;
      case INS_ADD: /* ( a b -- a+b ) */
        pushMain((popMain() + popMain())&65535U);
        break;
      case INS_SUB: /* ( a b -- a-b ) */
        pushMain((-popMain() + popMain())&65535U);
        break;
      case INS_MUL: /* ( a b -- a*b )	*/
        pushMain((ushort)(popMain() * popMain())&65535U);
        break;
      case INS_DIV: /* ( a b -- a/b rem ) */
        pbuf2 = popMain();
        pbuf = popMain();
        pushMain((ushort) pbuf/pbuf2);
        pushMain((ushort) pbuf%pbuf2);
        break;
      case INS_NEG: /* ( a -- -a ) */
        pushMain((ushort)-popMain());
        break;
      case INS_SHIFT: /* ( a XY -- [a >> X] << Y ) */
        pbuf = popMain();
        pushMain((ushort)(popMain() >> (pbuf&15U)) << (pbuf>>4U));
        break;
      case INS_NOT: /* ( a -- ~a ) */
        pushMain((ushort)(~popMain()&65535U));
        break;
      case INS_AND: /* ( a b -- a&b ) */
        pushMain(popMain() & popMain());
        break;
      case INS_OR: /* ( a b -- a|b ) */
        pushMain(popMain() | popMain());
        break;
      case INS_XOR: /* ( a b -- a^b ) */
        pushMain(popMain() ^ popMain());
        break;
      case INS_COUT: /* output a character, no Unicode support for now */
        cputc((uchar)(popMain()&255U));
        break;
      case INS_NHOUT: /* output the hex number from the stack top */
        printf("%04X", popMain());
        break;
      case INS_NBKIN: /* non-blocking key input - in this implementation it's equal to: */
      case INS_BKIN: /* blocking key input (again, no Unicode support yet, assuming a single byte incoming to the 16-bit value on the stack) */
        pushMain((ushort)cgetc());
        break;
      case INS_PORTIO: /* ( p1 p2 port -- r1 r2 status ) - simulate port I/O according to the spec */
        pbuf = popMain(); /* port */
        pbuf2 = popMain(); /* p2 */
        fprintf(stderr, "[PORTIO ] Call to port 0x%X with P1=%X and P2=%X, returning status=0, R1=0, R2=0\n", pbuf, pbuf2, popMain());
        pushMain(0u);
        pushMain(0u);
        pushMain(0u);
        break;
      case INS_PERSIST_READ: /* ( adh adl len maddr -- status) */
        pushMain(persistOp(popMain(), popMain(), popMain(), popMain(), 0));
        break;
      case INS_PERSIST_WRITE: /* ( adh adl len maddr -- status) */
        pushMain(persistOp(popMain(), popMain(), popMain(), popMain(), 1));
        break;
      default: /* all characters not processed before are invalid instructions */
        trapout(INVALID_INSTRUCTION);
        goto brx;
    }
    if(ram.msp >= STACK_SIZE_WORDS || ram.rsp >= STACK_SIZE_WORDS) /* check for stack overflow after any operation */
      trapout(STACK_OVERFLOW);
    continue;
    brx: break;
  }
  /* unset interpretation mode flag and exit */
  ram.IM = 0;
  ram.pc = ram.ibp = 65535U;
  /* clear all stacks and CLT */
  ram.msp = ram.rsp = ram.lsp = ram.cltp = 0;
}

/* Equi VM entry point */

int main(int argc, char* argv[]) {
  /* _attempt_ to disable buffering for char input/output */
#if !defined __CC65__ && !defined __TINYC__
  setvbuf(stdin, NULL, _IONBF, 0);
  setvbuf(stdout, NULL, _IONBF, 0);
#endif
  uchar instr;
  /* initialize the RAM in the most standard way */
  ram.gpd_start = (uchar *)&ram.gpd - (uchar *)&ram.main_stack;
  ram.cmd_start = (uchar *)&ram.cmdbuf - (uchar *)&ram.main_stack;
  ram.cmd_size = CMD_BUF_SIZE;
  ram.II = ram.CM = ram.IM = 0; /* reset all flags */
  /* Start both execution and input buffering from the start of command buffer (-1 because we use prefix increment) */
  ram.pc = ram.ibp = 65535U;

  printf("Welcome to Equi v" EQUI_VER " by Luxferre, 2022\n\nCLT: 0x%04X (%d bytes)\nGPD: 0x%04X (%d bytes)\nCommand buffer: 0x%04X (%d bytes)\nEqui ready\n\n",
      (unsigned int) ((uchar *)&ram.clt - (uchar *)&ram.main_stack),
      (unsigned int) ((uchar *)&ram.gpd - (uchar *)&ram.clt),
      ram.gpd_start,
      ram.cmd_start - ram.gpd_start,
      ram.cmd_start, ram.cmd_size);

  cputc('>');
  cputc(' ');

  while(1) { /* Now, we're in the command mode loop */
    instr = ram.cmdbuf[++ram.ibp] = cgetc(); /* Fetch the next instruction from the keyboard */
    if(instr == 0xFFU || instr == 0U) /* exit */
      break;
    else if(instr == BS && ram.ibp > ram.cmd_start) { /* process the backspace */
#ifdef __CC65__
      cputc(instr); /* echo it */
#endif
      --ram.ibp; 
    } else if(instr == INS_IISTART) { /* process II just to avoid quitting in command mode */
#ifdef __CC65__
      cputc(instr); /* echo it */
#endif
      ram.II = 1;
    } else if(instr == INS_IIEND) { /* process II just to avoid quitting in command mode */
#ifdef __CC65__
      cputc(instr); /* echo it */
#endif
      ram.II = 0;
    } else if(instr == CR || instr == LF) { /* process carriage return or linefeed */
      cputc(CR); /* echo it */
      cputc(LF); /* echo it */
      ram.cmdbuf[ram.ibp] = 0; /* replace itself with 0 */
      ram.IM = 1; /* set the mandatory interpretation mode flag */
      equi_main_loop(); /* and run the interpreter loop */
      cputc(CR); /* echo it */
      cputc(LF); /* echo it */
      cputc('>');
      cputc(' ');
    }
#ifdef __CC65__
    else cputc(instr); /* echo it */
#endif
  } /* command mode loop end */
  return 0;
}