lexer.cpp

#include "lexer.h"

const char number = '8';

// make a Token_stream that reads from istream
Token_stream::Token_stream(std::istream& is)
        :is(is), full(false), buffer{0} {
}

// get a Token
Token Token_stream::get() {
    if (full) {     // do we already have a Token ready?
        // remove Token from buffer
        full = false;
        return buffer;
    }

    char ch;
    is >> ch;   // note that >> skips whitespace (space, newline, tab, etc.)

    switch (ch) {
        case ';':   // for "print"
        case 'q':   // for "quit"
        case '(': case ')': case '+': case '-': case '*': case '/':
            return Token{ch};   // let each character represent itself
        case '.':
        case '0': case '1': case '2': case '3': case '4':
        case '5': case '6': case '7': case '8': case '9': {
            is.putback(ch);         // put digit back into the input stream
            double val;
            is >> val;              // read a floating-point number
            return Token{number, val};
        }
        default:
            throw Lexer_error("Bad token");
    }
}

// put a Token back
void Token_stream::putback(Token t) {
    if (full)
        throw Lexer_error("putback() into a full buffer");
    buffer = t;     // copy t to buffer
    full = true;    // buffer is now full
}

// discard Token in buffer
void Token_stream::ignore() {
    full = false;
}