export type Token = (
    // Literals
    | { kind: 'int', value: number }
    | { kind: 'float', value: number }
    | { kind: 'string', value: string }
    | { kind: 'ident', value: string }
    | { kind: 'type-ident', value: string }

    // Brackets
    | { kind: 'open-paren' }
    | { kind: 'close-paren' }
    | { kind: 'open-brace' }
    | { kind: 'close-brace' }
    | { kind: 'open-bracket' }
    | { kind: 'close-bracket' }

    // Comparison
    | { kind: 'equals' }
    | { kind: 'equals-equals' }
    | { kind: 'not-equals' }
    | { kind: 'greater-than' }
    | { kind: 'greater-equals' }
    | { kind: 'less-than' }
    | { kind: 'less-equals' }

    // Symbols
    | { kind: 'colon' }
    | { kind: 'colon-equals' }
    | { kind: 'semicolon' }
    | { kind: 'backslash' }
    | { kind: 'pipe' }
    | { kind: 'tilde' }
    | { kind: 'comma' }
    | { kind: 'ampersand' }
    | { kind: 'underscore' }
    | { kind: 'dot' }
    | { kind: 'dot-dot-dot' }
    | { kind: 'at' }

    // Anithmetic
    | { kind: 'plus' }
    | { kind: 'minus' }
    | { kind: 'star' }
    | { kind: 'slash' }
    | { kind: 'percent' }
    | { kind: 'caret' }

    | { kind: 'eof' }
) & {
    line: number;
    column: number;
    start: number; // char offset in source
}

export function tokenize(source: string): Token[] {
    const tokens: Token[] = [];

    let i = 0;
    let line = 1;
    let column = 1;

    function advance() {
        if (source[i] === '\n') {
            line++;
            column = 1
        } else {
            column++;
        }
        i++;
    }

    while (i < source.length) {
        const char = source[i];
        const start = i;
        const startLine = line;
        const startColumn = column;

        // Whitespace
        if (/\s/.test(char)) {
            advance();
            continue;
        }

        // Comments
        if (char === '#') {
            while (i < source.length && source[i] !== '\n') {
                advance();
            }
            continue;
        }

        // Numbers
        if (/[0-9]/.test(char)) { // have to start with a digit (?)
            let num = '';
            let hasDot = false;

            while (i < source.length && /[0-9.]/.test(source[i])) {
                if (source[i] === '.') {
                    if (hasDot) break;
                    hasDot = true;
                }
                num += source[i];
                advance();
            }

            tokens.push(hasDot
                ? { kind: 'float', value: parseFloat(num), line: startLine, column: startColumn, start }
                : { kind: 'int', value: parseInt(num), line: startLine, column: startColumn, start });

            continue;
        }

        // Idents & Wildcard
        if (/[A-Za-z_]/.test(char)) {
            let str = '';
            while (i < source.length && /[A-Za-z0-9_!-]/.test(source[i])) {
                str += source[i];
                advance();
            }

            if (str === '_') {
                // Wildcards
                tokens.push({ kind: 'underscore', line: startLine, column: startColumn, start });
            } else {
                const isType = /[A-Z]/.test(str[0]);

                tokens.push(isType
                    ? { kind: 'type-ident', value: str, line: startLine, column: startColumn, start }
                    : { kind: 'ident', value: str, line: startLine, column: startColumn, start });
            }

            continue;
        }

        // Strings
        if (char === '"') {
            advance();
            let str = '';

            while (i < source.length && source[i] !== '"') {
                if (source[i] === '\\') {
                    advance();

                    if (i >= source.length) {
                        throw new Error('Unterminated string');
                    }

                    switch(source[i]) {
                        case 'n': str += '\n'; break;
                        case '"': str += '"'; break;
                        case '\\': str += '\\'; break;
                        case 't': str += '\t'; break;
                    }
                } else {
                    str += source[i];
                }

                advance();
            }

            if (i >= source.length) {
                throw new Error('Unterminated string');
            }

            tokens.push({ kind: 'string', value: str, line: startLine, column: startColumn, start });

            advance();

            continue;
        }

        switch (char) {
            case '>': {
                if (source[i + 1] === '=') {
                    tokens.push({ kind: 'greater-equals', line: startLine, column: startColumn, start });
                    advance();
                } else {
                    tokens.push({ kind: 'greater-than', line: startLine, column: startColumn, start });
                }
                break;
            }
            case '<': {
                if (source[i + 1] === '=') {
                    tokens.push({ kind: 'less-equals', line: startLine, column: startColumn, start });
                    advance();
                } else {
                    tokens.push({ kind: 'less-than', line: startLine, column: startColumn, start });
                }
                break;
            }
            case '=': {
                if (source[i + 1] === '=') {
                    tokens.push({ kind: 'equals-equals', line: startLine, column: startColumn, start });
                    advance();
                } else {
                    tokens.push({ kind: 'equals', line: startLine, column: startColumn, start });
                }
                break;
            }
            case '!': {
                if (source[i + 1] === '=') {
                    tokens.push({ kind: 'not-equals', line: startLine, column: startColumn, start });
                    advance();
                } else {
                    throw new Error(`Unexpected character: ${char}`)
                }
                break;
            }
            case '.': {
                if (source[i + 1] === '.' && source[i + 2] === '.') {
                    tokens.push({ kind: 'dot-dot-dot', line: startLine, column: startColumn, start })
                    i += 2;
                } else {
                    tokens.push({ kind: 'dot', line: startLine, column: startColumn, start });
                }
                break;
            }
            case ':': {
                if (source[i + 1] === '=') {
                    tokens.push({ kind: 'colon-equals', line: startLine, column: startColumn, start });
                    advance();
                } else {
                    tokens.push({ kind: 'colon', line: startLine, column: startColumn, start });
                }
                break;
            }
            case ';': tokens.push({ kind: 'semicolon', line: startLine, column: startColumn, start }); break;
            case '\\': tokens.push({ kind: 'backslash', line: startLine, column: startColumn, start }); break;
            case '~': tokens.push({ kind: 'tilde', line: startLine, column: startColumn, start }); break;
            case '|': tokens.push({ kind: 'pipe', line: startLine, column: startColumn, start }); break;
            case ',': tokens.push({ kind: 'comma', line: startLine, column: startColumn, start }); break;
            case '&': tokens.push({ kind: 'ampersand', line: startLine, column: startColumn, start }); break;
            case '@': tokens.push({ kind: 'at', line: startLine, column: startColumn, start }); break;

            // Arithmetic
            case '+': tokens.push({ kind: 'plus', line: startLine, column: startColumn, start }); break;
            case '-': tokens.push({ kind: 'minus', line: startLine, column: startColumn, start }); break;
            case '*': tokens.push({ kind: 'star', line: startLine, column: startColumn, start }); break;
            case '/': tokens.push({ kind: 'slash', line: startLine, column: startColumn, start }); break;
            case '^': tokens.push({ kind: 'caret', line: startLine, column: startColumn, start }); break;
            case '%': tokens.push({ kind: 'percent', line: startLine, column: startColumn, start }); break;

            // Brackets
            case '(': tokens.push({ kind: 'open-paren', line: startLine, column: startColumn, start }); break;
            case ')': tokens.push({ kind: 'close-paren', line: startLine, column: startColumn, start }); break;
            case '{': tokens.push({ kind: 'open-brace', line: startLine, column: startColumn, start }); break;
            case '}': tokens.push({ kind: 'close-brace', line: startLine, column: startColumn, start }); break;
            case '[': tokens.push({ kind: 'open-bracket', line: startLine, column: startColumn, start }); break;
            case ']': tokens.push({ kind: 'close-bracket', line: startLine, column: startColumn, start }); break;
        }

        advance();
    }

    return tokens;
}