force_inline B32 token_is_assign(Token_Kind token){return token >= TK_FirstAssign && token <= TK_LastAssign;}
force_inline B32 token_is_assign(Token *token){return token_is_assign(token->kind);}
force_inline B32 token_is_compare(Token_Kind token){return token >= TK_FirstCompare && token <= TK_LastCompare;}
force_inline B32 token_is_compare(Token *token){return token_is_compare(token->kind);}

CORE_Static U8
lexc(Lex_Stream *s){
  return s->stream.str[s->iter];
}

CORE_Static U8
lexci(Lex_Stream *s, S32 i){
  return s->stream.str[s->iter+i];
}

CORE_Static U8 *
lexcp(Lex_Stream *s){
  return s->stream.str + s->iter;
}

CORE_Static B32
lex_is_whitespace(U8 c){
  B32 result = c == ' ' || c == '\r';
  return result;
}

CORE_Static B32
lex_is_alphabetic(U8 c){
  B32 result = (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
  return result;
}

CORE_Static B32
lex_is_numeric(U8 c){
  B32 result = c >= '0' && c <= '9';
  return result;
}

CORE_Static B32
lex_is_numeric_base16(U8 c){
  B32 result = (c >= '0' && c <= '9') || (c >= 'A' && c <= 'F') ||
               (c >= 'a' && c <= 'f');
  return result;
}

CORE_Static B32
lex_is_alphanumeric(U8 c){
  B32 result = lex_is_numeric(c) || lex_is_alphabetic(c);
  return result;
}

CORE_Static void
lex_set_len(Lex_Stream *s, Token *token){
  assert(lexcp(s) >= token->str);
  token->len = lexcp(s) - token->str;
}

CORE_Static void
lex_set_keywords(Core_Ctx *lexer, Array<String> keywords){
  Intern_String keyword = {};
  For(keywords){
    keyword = intern_string(&lexer->interns, it);
    if(&it == keywords.begin())
      lexer->interns.first_keyword = keyword.str;
  }
  lexer->interns.last_keyword = keyword.str;
}

CORE_Static B32
lex_is_keyword(Intern_Table *lexer, Intern_String keyword){
  B32 result = keyword.str >= lexer->first_keyword && keyword.str <= lexer->last_keyword;
  return result;
}

CORE_Static void
token_error(Token *t, String error_val){
  t->kind = TK_Error;
  t->error_val = error_val;
}

CORE_Static void
lex_parse_u64(Core_Ctx *lexer, Token *t, S64 base){
  Scratch_Scope _scope(lexer->scratch);
  Set_BigInt_Arena(lexer->scratch);

  t->kind = TK_Integer;
  BigInt m = bigint_u64(1);
  BigInt base_mul = bigint_u64(base);
  BigInt result = bigint_u64(0);

  for(S64 i = t->len - 1; i >= 0; --i){
    U64 value = t->str[i];
    if(t->str[i] >= 'a') value = value - 'a' + 10;
    else if(t->str[i] >= 'A') value = value - 'A' + 10;
    else value -= '0';

    BigInt val = bigint_u64(value);
    BigInt new_val = bigint_mul(&val, &m);
    result = bigint_add(&result, &new_val);
    m = bigint_mul(&m, &base_mul);
  }

  t->int_val = bigint_copy(lexer->perm, &result);
}

CORE_Static void
lex_parse_f64(Token *t){
  t->kind = TK_Float;
  char buffer[128];
  S64 len = clamp_top((int)t->len, 126);
  memory_copy(buffer, t->str, len);
  buffer[len] = 0;
  t->f64_val = strtod(buffer, 0);
}

CORE_Static void
lex_advance(Lex_Stream *s){
  if(s->iter >= s->stream.len){
    return;
  }
  else if(lexc(s) == '\n'){
    s->iter++;
    s->line++;
    s->line_begin = lexcp(s);
  }
  else{
    s->iter++;
  }
}

CORE_Static void
lex_parse_string(Lex_Stream *s, Token *t, U8 c){
  for(;;){
    if(lexc(s) == '\\') lex_advance(s);
    else if(lexc(s) == c) break;
    else if(lexc(s) == 0){
      token_error(t, "Unterminated string, reached end of file"_s);
      break;
    }
    lex_advance(s);
  }
  if(t->kind != TK_Error){
    lex_advance(s);
    lex_set_len(s,t);
  }
}

CORE_Static void
lex_parse_ident(Intern_Table *table, Lex_Stream *s, Token *t){
  while(lex_is_alphanumeric(lexc(s)) || lexc(s) == '_')
    lex_advance(s);
  lex_set_len(s,t);
}

#define CASE2(op, OpName, Assign)                                                        \
  case op:                                                                               \
    if (lexc(s) == '=') {                                                                \
      lex_advance(s);                                                                    \
      t.kind = Assign;                                                                   \
    } else {                                                                             \
      t.kind = OpName;                                                                   \
    }                                                                                    \
    break
#define CASE3(op, OpName, Assign, Incr)                                                  \
  case op:                                                                               \
    if (lexc(s) == '=') {                                                                \
      lex_advance(s);                                                                    \
      t.kind = Assign;                                                                   \
    } else if (lexc(s) == op) {                                                          \
      lex_advance(s);                                                                    \
      t.kind = Incr;                                                                     \
    } else {                                                                             \
      t.kind = OpName;                                                                   \
    }                                                                                    \
    break

CORE_Static Token
token_make(Core_Ctx *lexer, U8 *str, Intern_String file, int line, U8 *line_begin){
  Token t = {};
  t.str   = str;
  t.file  = file;
  t.line  = line;
  t.line_begin = line_begin;
  t.di    = lexer->token_debug_ids++;
  return t;
}

CORE_Static Token
token_make(Core_Ctx *lexer){
  return token_make(lexer, lexcp(&lexer->stream), lexer->stream.file, lexer->stream.line, lexer->stream.line_begin);
}

CORE_Static Token *
lex_last_indent_token(Lex_Stream *s){
  if (s->indent_stack.len > 0) {
    return *s->indent_stack.last();
  }
  return &pctx->same_scope_token;
}

CORE_Static B32
lex_is_scope(Token *t){
  B32 result = t->kind == OPEN_SCOPE || t->kind == CLOSE_SCOPE || t->kind == SAME_SCOPE;
  return result;
}

CORE_Static void
lex_add_token(Core_Ctx *ctx, Token *token) {
  Token *top = (Token *)arena_push_size(ctx->stage_arena, sizeof(Token));
  *top = *token;
  ctx->tokens.len += 1;
  ctx->tokens.data = (Token *)ctx->stage_arena->memory.data;
}

CORE_Static void
lex_unwind_indent_stack(Core_Ctx *ctx, Token *t, Lex_Stream *s){
  for(S64 i = s->indent_stack.len-1; i >= 0; i-=1){
    auto it = s->indent_stack.data[i];
    assert(lex_is_scope(it));
    if(it->indent == t->indent){
      t->kind = SAME_SCOPE;
      lex_add_token(ctx, t);
      break;
    }
    else if(it->indent < t->indent){
      token_error(t, "Bad indentation"_s);
      lex_add_token(ctx, t);
      break;
    }
    else{
      s->indent_stack.pop();
      t->kind = CLOSE_SCOPE;
      lex_add_token(ctx, t);
    }
  }
}

CORE_Static void
lex__stream(Core_Ctx *lexer){
  Intern_Table *table = &lexer->interns;
  Lex_Stream *s       = &lexer->stream;

  B32 beginning = true;
  for(;;){
    if(lexc(s) == 0 || s->iter >= s->stream.len){
      end_of_stream:
      Token t = token_make(lexer);
      lex_unwind_indent_stack(lexer, &t, s);
      break;
    }

    // @note: the lexer is going to be a 2 stage process
    // first we tokenize the indentation and then proceed to tokenize
    // the good stuff

    // for blocks of stmts we parse till we cant find another new line
    // of same scope.
    // parse_decl doesn't require preceding new line
    //
    // in that way new lines act as commas in CORE_Static params
    // seeing a comma means that there is a next thing to parse
    // and it's easy to parse stuff using a do while loop

    // @note: first handle indentation
    // mostly we want to merge multiple new lines
    // but for down scopes we want to emit 2 new lines
    // that will ease out parsing, one token to break out
    // from a block parsing, second to allow continuation of surrounding scope
    Token t = token_make(lexer);
    B32 should_emit = beginning;
    for(;;){
      switch(lexc(s)){
        case   0 : goto end_of_stream; break;
        case '\t': case ' ': lex_advance(s); t.indent++;                        break;
        case '\r':           lex_advance(s);                                    break;
        case '/': {
          if(lexci(s,1) == '/'){
            lex_advance(s); lex_advance(s);
            t.kind = TK_Comment;
            for(;;){
              if(lexc(s) == '\n' || lexc(s) == 0) break;
              lex_advance(s);
            }
          }
          else if(lexci(s,1) == '*'){
            lex_advance(s); lex_advance(s);
            t.kind = TK_Comment;
            for(;;){
              if(lexc(s) == '*' && lexci(s,1) == '/'){
                lex_advance(s); lex_advance(s);
                break;
              }
              else if(lexc(s) == 0){
                token_error(&t, "Unterminated block comment"_s);
                break;
              }
              lex_advance(s);
            }
          }
          else goto indent_loop_break;
        } break;

        // @todo: add [;;] operator which adds new scope
        // @todo: also need some way to detect indentation so that
        // first of all we can check for consistency and second of
        // all because we would know by how much to indent
        // @todo: after detecting indentation 2 spaces would become 1 indent value
        case ';' : {
          Token semi  = token_make(lexer);
          Token *last = lex_last_indent_token(s);
          semi.indent = last->indent;
          lex_advance(s);
          if(lexc(s) == ';'){
            lex_advance(s);
            semi.kind = OPEN_SCOPE;
            semi.indent = last->indent + 2; // @todo: proper detection of indentation
            lex_add_token(lexer, &semi);
            s->indent_stack.add(lexer->tokens.last());
          } else{
            semi.kind   = SAME_SCOPE;
            lex_add_token(lexer, &semi);
          }
        } break;

        case '\n':{
          lex_advance(s);
          should_emit = true;
          t = token_make(lexer);
        } break;

        default:{
          if(s->inside_brace_paren) should_emit = false;
          if(should_emit){
            Token *last = lex_last_indent_token(s);
            if(t.indent > last->indent){
              t.kind = OPEN_SCOPE;
              lex_add_token(lexer, &t);
              s->indent_stack.add(lexer->tokens.last());
            }

            else if(t.indent < last->indent){
              lex_unwind_indent_stack(lexer, &t, s);
            }
            else {
              t.kind = SAME_SCOPE;
              lex_add_token(lexer, &t);
            }
          }

          goto indent_loop_break;
        }
      }
    } indent_loop_break:
    beginning = false;

    // @note: handle the indented token
    t = token_make(lexer);
    lex_advance(s);
    switch(*t.str){
      case  0 : goto end_of_stream; break;
      case '@': t.kind = TK_At; break;
      case '(': s->inside_brace_paren++; t.kind = TK_OpenParen; break;
      case ')': s->inside_brace_paren--; t.kind = TK_CloseParen; break;
      case '{': s->inside_brace_paren++; t.kind = TK_OpenBrace; break;
      case '}': s->inside_brace_paren--; t.kind = TK_CloseBrace; break;
      case '[': s->inside_brace_paren++; t.kind = TK_OpenBracket; break;
      case ']': s->inside_brace_paren--; t.kind = TK_CloseBracket; break;
      case ',': t.kind = TK_Comma; break;
      case '~': t.kind = TK_Neg; break;
      case '?': t.kind = TK_Question; break;
      case '^': t.kind = TK_BitXor; break;
      CASE2('!', TK_Not, TK_NotEquals);
      CASE2('=', TK_Assign, TK_Equals);
      CASE2('*', TK_Mul, TK_MulAssign);
      CASE2('%', TK_Mod, TK_ModAssign);
      CASE3('+', TK_Add, TK_AddAssign, TK_Increment);
      CASE3('&', TK_BitAnd, TK_AndAssign, TK_And);
      CASE3('|', TK_BitOr, TK_OrAssign, TK_Or);

      case '#': {
        t.kind = TK_Pound;
        lex_parse_ident(table, s, &t);
        t.str += 1;
        t.len -= 1;
        t.intern_val = intern_string(table, t.string);
        if(t.len == 0) token_error(&t, "Macro token without content"_s);
      }break;

      case '.': {
        if(lexc(s) == '.'){
          lex_advance(s);
          if(lexci(s,1) == '.') {
            lex_advance(s);
            t.kind = TK_ThreeDots;
          }
          else t.kind = TK_TwoDots;
        }
        else t.kind = TK_Dot;
      } break;

      case '\'':{
        assert(s->stream.len >= s->iter);
        UTF32_Result decode = utf8_to_utf32(lexcp(s), s->stream.len - s->iter);
        if(!decode.error){
          for(S32 i = 0; i < decode.advance; i++) lex_advance(s);
          t.unicode = decode.out_str;
          t.kind = TK_UnicodeLit;
          if(lexc(s) == '\''){
            lex_advance(s);
          }
          else{
            token_error(&t, "Unclosed unicode literal"_s);
          }
        }
        else{
          token_error(&t, "Invalid UTF8 sequence in unicode literal"_s);
        }
      } break;

      case '<': {
        if (lexc(s) == '<') {
          lex_advance(s);
          if (lexc(s) == '=') {
            lex_advance(s);
            t.kind = TK_LeftShiftAssign;
          }
          else {
            t.kind = TK_LeftShift;
          }
        }
        else if (lexc(s) == '=') {
          lex_advance(s);
          t.kind = TK_LesserThenOrEqual;
        }
        else {
          t.kind = TK_LesserThen;
        }
      } break;

      case '>': {
        if (lexc(s) == '>') {
          lex_advance(s);
          if (lexc(s) == '=') {
            lex_advance(s);
            t.kind = TK_RightShiftAssign;
          }
          else {
            t.kind = TK_RightShift;
          }
        }
        else if (lexc(s) == '=') {
          lex_advance(s);
          t.kind = TK_GreaterThenOrEqual;
        }
        else {
          t.kind = TK_GreaterThen;
        }
      } break;

      case ':': {
        if (lexc(s) == ':') {
          lex_advance(s);
          t.kind = TK_DoubleColon;
        }
        else if(lexc(s) == '='){
          lex_advance(s);
          t.kind = TK_ColonAssign;
        }
        else {
          t.kind = TK_Colon;
        }
      } break;

      case '-':{
        if (lexc(s) == '=') {
          lex_advance(s);
          t.kind = TK_SubAssign;
        }
        else if (lexc(s) == '-') {
          lex_advance(s);
          t.kind = TK_Decrement;
        }
        else if (lexc(s) == '>') {
          lex_advance(s);
          t.kind = TK_Arrow;
        }
        else {
          t.kind = TK_Sub;
        }
      } break;

      case '"': {
        t.kind = TK_StringLit;
        lex_parse_string(s,&t,'"');
        if(t.kind != TK_Error){
          t.str += 1;
          t.len -= 2;
        }
        t.intern_val = intern_string(table, t.string);
      } break;

      case '/': {
        if(lexc(s) == '='){
          t.kind = TK_DivAssign;
          lex_advance(s);
        }
        else {
          t.kind = TK_Div;
        }
      } break;

      case '0':{
        if(lexc(s) == 'x'){
          lex_advance(s);
          while(lex_is_numeric_base16(lexc(s)))
            lex_advance(s);
          lex_set_len(s, &t);
          t.str += 2;
          t.len -= 2;
          if(t.len == 0)
            token_error(&t, "Hex constant doesn't have value"_s);
          else
            lex_parse_u64(lexer, &t, 16);
          break;
        }
        else if(lexc(s) == 'b'){
          lex_advance(s);
          while(lexc(s) == '0' || lexc(s) == '1')
            lex_advance(s);
          lex_set_len(s, &t);
          t.str += 2;
          t.len -= 2;
          if(t.len == 0)
            token_error(&t, "Hex constant doesn't have value"_s);
          else
            lex_parse_u64(lexer, &t, 2);
          break;
        }

      }
      case '1':case '2':case '3':case '4':
      case '5':case '6':case '7':case '8':case '9':{
        B32 found_dot = false;
        for(;;){
          if(lex_is_numeric(lexc(s)))
            ;
          else if(lexc(s) == '.'){
            if(found_dot){
              token_error(&t, "Multiple '.' in float literal"_s);
              goto end_of_switch;
            }
            found_dot = true;
          }
          else break;

          lex_advance(s);
        }
        lex_set_len(s, &t);
        if(found_dot) lex_parse_f64(&t);
        else          lex_parse_u64(lexer, &t, 10);

      } break;

      case 'A':case 'a':case 'M':case 'm':case 'B':
      case 'b':case 'N':case 'n':case 'C':case 'c':case 'O':
      case 'o':case 'D':case 'd':case 'P':case 'p':case 'E':
      case 'e':case 'Q':case 'q':case 'F':case 'f':case 'R':
      case 'r':case 'G':case 'g':case 'S':case 's':case 'H':
      case 'h':case 'T':case 't':case 'I':case 'i':case 'U':
      case 'u':case 'J':case 'j':case 'V':case 'v':case 'K':
      case 'k':case 'W':case 'w':case 'L':case 'X':case 'l':
      case 'x':case 'Z':case 'z':case 'Y':case 'y':case '_': {
        t.kind = TK_Identifier;
        lex_parse_ident(table, s, &t);
        t.intern_val = intern_string(table, t.string);
        if(lex_is_keyword(table, t.intern_val)){
          t.kind = TK_Keyword;
        }
      } break;

      default: {
        token_error(&t, "Unknown token"_s);
      }
    }end_of_switch:

    if(t.len==0)
      lex_set_len(s,&t);

    lex_add_token(lexer, &t);
  }
#undef CASE2
#undef CASE3
}

CORE_Static void
lex_restream(Core_Ctx *lexer, String istream, String file){
  lexer->stream = {};
  lexer->stream.stream = istream;
  lexer->stream.line_begin = istream.str;
  lexer->stream.file = lexer->intern(file);

  Scratch_Scope _scope(lexer->scratch);
  lexer->stream.indent_stack.allocator = lexer->scratch;
  lexer->stream.indent_stack.add(&lexer->same_scope_token);
  lex__stream(lexer);
}

//-----------------------------------------------------------------------------
// Token metadata
//-----------------------------------------------------------------------------

CORE_Static const char *
name(Token_Kind kind){
  switch(kind){
    case TK_End: return "End of stream";
/*#
import meta
for i in meta.token_kinds:
  if i[1] != "SPECIAL":
    print("case TK_" + i[0] + f": return \"{i[1]}\";")
*/
case TK_Mul: return "*";
case TK_Div: return "/";
case TK_Mod: return "%";
case TK_LeftShift: return "<<";
case TK_RightShift: return ">>";
case TK_Add: return "+";
case TK_Sub: return "-";
case TK_Equals: return "==";
case TK_LesserThenOrEqual: return "<=";
case TK_GreaterThenOrEqual: return ">=";
case TK_LesserThen: return "<";
case TK_GreaterThen: return ">";
case TK_NotEquals: return "!=";
case TK_BitAnd: return "&";
case TK_BitOr: return "|";
case TK_BitXor: return "^";
case TK_And: return "&&";
case TK_Or: return "||";
case TK_Neg: return "~";
case TK_Not: return "!";
case TK_Decrement: return "--";
case TK_Increment: return "++";
case TK_PostDecrement: return "--";
case TK_PostIncrement: return "++";
case TK_Assign: return "=";
case TK_ColonAssign: return ":=";
case TK_DivAssign: return "/=";
case TK_MulAssign: return "*=";
case TK_ModAssign: return "%=";
case TK_SubAssign: return "-=";
case TK_AddAssign: return "+=";
case TK_AndAssign: return "&=";
case TK_OrAssign: return "|=";
case TK_XorAssign: return "^=";
case TK_LeftShiftAssign: return "<<=";
case TK_RightShiftAssign: return ">>=";
case TK_OpenParen: return "(";
case TK_CloseParen: return ")";
case TK_OpenBrace: return "{";
case TK_CloseBrace: return "}";
case TK_OpenBracket: return "[";
case TK_CloseBracket: return "]";
case TK_Comma: return ",";
case TK_Pound: return "#";
case TK_Question: return "?";
case TK_ThreeDots: return "...";
case TK_Semicolon: return ";";
case TK_Dot: return ".";
case TK_TwoDots: return "..";
case TK_NewLine: return "[NewLine]";
case TK_Colon: return ":";
case TK_DoubleColon: return "::";
case TK_At: return "@";
case TK_Arrow: return "->";
case TK_ExprSizeof: return "[SizeOf]";
case TK_DocComment: return "[///]";
case TK_Comment: return "//";
case TK_Identifier: return "[Ident]";
case TK_UnicodeLit: return "[Unicode]";
case TK_StringLit: return "[String]";
case TK_Error: return "[Error]";
case TK_Float: return "[Float]";
case TK_Integer: return "[Int]";
case TK_Keyword: return "[Keyword]";
/*END*/
    case CLOSE_SCOPE: return "Close_Scope";
    case OPEN_SCOPE: return "Open_Scope";
    case SAME_SCOPE: return "Same_Scope";
    default: invalid_codepath; return "<Undefined>";
  }
}