/**
 * Bootstrap Gibble Lisp ("Prebirth")
 *
 *  Copyright (C) 2017 Mike Gerwitz
 *
 *  This file is part of Gibble.
 *
 *  Gibble is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU Affero General Public License as
 *  published by the Free Software Foundation, either version 3 of the
 *  License, or (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU Affero General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * THIS IS TEMPORARY CODE that will be REWRITTEN IN GIBBLE LISP ITSELF after
 * a very basic bootstrap is complete.  It is retained as an important
 * artifact for those who wish to build Gibble from scratch without using
 * another version of Gibble itself.  This is called "self-hosting".
 *
 * Rather than producing a sophisticated self-hosting language, this
 * language will be a terribly incomplete and inadequate version of what
 * will ultimately become a formidable and competent language.
 *
 * I refer to this entire complication process as "Prebirth".¹ The "Birth"
 * of Gibble is the act of reimplementing this Prebirth in a Prebirth
 * version of Gibble Lisp itself.  It's the chicken-and-egg paradox, without
 * the paradox.²
 *
 * Gibble Lisp is _not_ the most primitive language that will be understood
 * by the system---it is too high-level.  After Birth, the language can
 * devolve into something more powerful and workable.
 *
 * Some minor terminology:
 *   - AST: Abstract Syntax Tree, a processed form of the CST.
 *   - CST: Concrete Syntax Tree, a 1-1 conversion of source input to
 *          tokens.
 *   - token: an object produced by the lexer that represents a portion of
 *            the input language
 *   - lexer: sometimes called a ``tokenizer''---produces tokens by applying
 *            the grammar to a string of input.
 *   - grammar: a definition of the language (syntax).
 *   - lexeme: the portion of the original source string associated with a
 *             given token.
 *   - LL(0): Left-to-right, Leftmost derivation, 0 tokens lookahead
 *   - sexp: symbolic expression, (involving (lots (of (((parentheses))))))
 *
 * Excited?  Great!  My extemporaneous rambling is costing me more time than
 * I spent making this damn thing!  (No, really, it is.)
 */

'use strict';


/**
 * A very rudimentary (and extremely permissive) LL(0) Lisp parser
 *
 * This provides just enough to get by.  It transforms lists into nested
 * arrays of tokens with some very basic error checking (e.g. for proper
 * nesting).  This is not a general-purpose lisp parser.
 */
class Parser
{
    /**
     * Produce an AST from the given string SRC of sexps
     *
     * This is essentially the CST with whitespace removed.  It first
     * invokes the lexer to produce a token string from the input
     * sexps SRC.  From this, it verifies only proper nesting (that SRC does
     * not close sexps too early and that EOF isn't reached before all sexps
     * are closed) and produces an AST that is an isomorphism of the
     * original sexps.
     *
     * @param {string} src input Lisp
     *
     * @throws {SyntaxError} on improper sexp nesting
     *
     * @return {Array} primitive abstract syntax tree of SRC
     */
    parseLisp( src )
    {
        // token string from lexing
        const toks = this._lex( src );

        // perform a leftmost reduction on the token string
        const [ depth, ast ] = toks.reduce( ( result, token ) =>
        {
            const [ depth, xs, stack ] = result;
            const { type, pos }        = token;

            // there are very few token types to deal with (again, this is
            // a very simple bootstrap lisp)
            switch ( type )
            {
                // closing parenthesis (end of sexp)
                case 'close':
                    if ( depth === 0 ) {
                        this._error(
                            src, pos, `Unexpected closing parenthesis`
                        );
                    }

                    // the sexp is complete; add to the AST, reduce depth
                    const top = stack.pop();
                    top.push( xs );

                    return [ ( depth - 1 ), top, stack ];

                // opening parenthesis (start of sexp)
                case 'open':
                    stack.push( xs );
                    return [ ( depth + 1 ), [], stack ];

                // symbol or primitive; just copy the token in place
                case 'string':
                case 'symbol':
                    xs.push( token );
                    return [ depth, xs, stack ];

                // should never happen unless there's a bug in the tokenizer
                // or we forget a token type above
                default:
                    this._error( src, pos, `Unexpected token '${type}'` );
            }
        }, [ 0, [], [] ] );

        // if we terminate at a non-zero depth, that means there
        // are still open sexps
        if ( depth > 0 ) {
            throw SyntaxError(
                `Unexpected end of input at depth ${depth}`
            );
        }

        // the result is a set of tokens organized into ES arrays
        // isomorphic to the original sexp structure (the same structure)
        return ast;
    }


    /**
     * Throw a SyntaxError with a window of surrounding source code
     *
     * The "window" is simply ten characters to the left and right of the
     * first character of the source input SRC that resulted in the error.
     * It's a little more than useless.
     *
     * @param {string} src source code (sexps)
     * @param {number} pos position of error
     * @param {string} msg error message
     *
     * @throws {SyntaxError}}
     *
     * @return {undefined}
     */
    _error( src, pos, msg )
    {
        const window = src.substr( pos - 10, pos + 10 )
              .replace( "\n", " " );

        throw new SyntaxError( `${msg}: '${window}'` );
    }


    /**
     * Convert source input into a string of tokens
     *
     * This is the lexer.  Whitespace is ignored.  The grammar consists of
     * simple s-expressions.
     *
     * This function is mutually recursive with `#_token'.  It expects that
     * the source SRC will be left-truncated as input is
     * processed.  POS exists for producing metadata for error
     * reporting---it has no impact on parsing.
     *
     * @param {string} src source code
     * @param {number} pos position (character offset) in source
     *
     * @return {Array} string of tokens
     */
    _lex( src, pos = 0 )
    {
        // ignore whitespace, if any
        const ws   = src.match( /^\s+/ ) || [ "" ];
        const trim = src.substr( ws[ 0 ].length );

        // adjust position to account for any removed whitespace
        pos += ws[ 0 ].length;

        // EOF and we're done
        if ( trim === '' ) {
            return [];
        }

        // left and right parenthesis are handled in the same manner: they
        // produce distinct tokens with single-character lexemes
        if ( trim[ 0 ] === '(' ) {
            return this._token( 'open', '(', trim, pos );
        }
        if ( trim[ 0 ] === ')' ) {
            return this._token( 'close', ')', trim, pos );
        }

        // strings are delimited by opening and closing ASCII double quotes,
        // which can be escaped with a backslash
        if ( trim[ 0 ] === '"' ) {
            const str = trim.match( /^"(|.*?[^\\])"/ );
            if ( !str ) {
                this._error( src, pos, "missing closing string delimiter" );
            }

            // a string token consists of the entire string including quotes
            // as its lexeme, but its value will be the value of the string
            // without quotes due to the `str' match group (see `#_token')
            return this._token( 'string', str, trim, pos );
        }

        // anything else is considered a symbol up until whitespace or any
        // of the aforementioned delimiters
        const symbol = trim.match( /^[^\s()"]+/ );
        return this._token( 'symbol', symbol, trim, pos );
    }


    /**
     * Produce a token and recurse
     *
     * The token will be concatenated with the result of the mutually
     * recursive method `_lex'.
     *
     * For the record: I'm not fond of mutual recursion from a clarity
     * standpoint, but this is how the abstraction evolved to de-duplicate
     * code, and I don't much feel like refactoring it.
     *
     * @param {string}       type  token type
     * @param {string|Array} match lexeme match
     * @param {string}       src   source code string, left-truncated
     * @param {number}       pos   offset relative to original src
     *
     * @return {Array} string of tokens
     */
    _token( type, match, src, pos )
    {
        const parts = ( Array.isArray( match ) )
            ? match
            : [ match ];

        // the value is the first group of the match (indicating what we
        // are actually interested in), and the lexeme is the full match,
        // which might include, for example, string delimiters
        const [ lexeme, value ] = parts;

        const token = {
            type:   type,
            lexeme: lexeme,
            value:  value || lexeme,
            pos:    pos
        };

        // continue producing tokens by recursing, left-truncating the
        // source string to discard what we have already processed
        return [ token ].concat(
            this._lex(
                src.substr( lexeme.length ),
                ( pos + lexeme.length )
            )
        );
    }
};



/**
 * Dumb compiler to transform AST into ECMAScript
 *
 * This is a really dumb code generator: it takes the AST and essentially
 * transforms it 1:1 wherever possible into the target language.
 *
 * This is nothing like what we actually want the _ultimate_ compiler to do
 * after Birth, but it gets us to a point where we can self-host on a basic
 * Prebirth language and evolve from there.
 *
 * The code generation can be pretty much summed up by the last line of
 * `Compiler#_cdfn'.
 */
class Compiler
{
    /**
     * Compile AST into ECMAScript
     *
     * Every block is mapped 1:1 to a function in ECMAScript.  So, we just
     * map all root children (which are expected to be block definitions) to
     * functions.
     *
     * @param {Array} tree root of tree containing top-level block definitions
     */
    compile( tree )
    {
        // map every definition to a ES function definition and delimit them
        // (for readability) by two newlines
        return tree.map( this._cdfn.bind( this ) )
            .join( "\n\n" ) + "\n";
    }


    /**
     * Compile block definition into a ES function definition
     *
     * This will fail if the given token is not a `define-block'.
     *
     * @param {Object} t token
     *
     * @return {string} compiled block definition
     */
    _cdfn( t )
    {
        this.assertApply( t, 'define-block' );

        // e.g. (define-block <foo> ((input ...)) body)
        const [ , { value: name }, desc, ...body ] = t;

        const id     = this._idFromName( name );
        const bodyjs = this._bodyToEs( body );

        // this is the final format---each block becomes its own function
        // definition
        return `function ${id}()\n{\n${bodyjs}\n};`;
    }


    /**
     * Generate ECMAScript-friendly name from the given id
     *
     * @param {string} name source name
     *
     * @return {string} ES-friendly identifier
     */
    _idFromName( name )
    {
        return name.replace( /[^a-zA-Z0-9_]/g, '$' );
    }


    /**
     * Compile body s-expressions into ECMAScript
     *
     * This produces a 1:1 mapping of BODY s-expressions to ES statements,
     * recursively.  The heavy lifting is done by `#_sexpToEs'.
     *
     * @param {Array} body s-expressions representing block body
     *
     * @return {string} compiled BODY
     */
    _bodyToEs( body )
    {
        // the body must be an array of expressions (this should always be
        // the case unless we have a bug in the compiler)
        if ( !Array.isArray( body ) ) {
            throw Error( "body must be an Array" );
        }

        // process each s-expression in BODY
        const js = body.map( this._sexpToEs.bind( this ) );

        // the result (that is, an array of compiled s-expressions) is
        // joined semicolon-delimited, with a `return' statement preceding
        // the final expression
        return js.reduce( ( result, s, i ) =>
        {
            const ret = ( i === ( js.length - 1 ) ) ? "return " : "";
            return result + "    " + ret + s + ";";
        }, "" );
    }


    /**
     * Convert s-expression or scalar into ECMAScript
     *
     * T may be either an array of tokens or a primitive token (e.g. string,
     * symbol).  This method is applied recursively to T as needed if T is
     * an array.
     *
     * @param {Array|Object} t tokens representing s-expressions/scalars
     *
     * @return {string} compiled s-expression/scalar
     */
    _sexpToEs( t )
    {
        // just output symbols as identifiers as-is for now
        if ( !Array.isArray( t ) ) {
            switch ( t.type )
            {
                // strings are output as-is (note that we don't escape
                // double quotes, because the method of escaping them is the
                // same in Scheme as it is in ECMAScript---a backslash)
                case 'string':
                    return `"${t.value}"`;

                // symbols have the same concerns as block definitions: the
                // identifiers generated need to be ES-friendly
                case 'symbol':
                    return this._idFromName( t.value );

                default:
                    throw Error( "Cannot compile unknown token `${t.type}'" );
            }
        }

        // only support block form for now, and assume that `fn' is a
        // string value (in the future, this doesn't have to be the
        // case---fn should be able to be an arbitrary sexp)
        const [ { value: fn }, argmap ] = t;

        if ( !this._isBlockForm( t ) ) {
            throw Error( `\`${fn}' application is not in block form`)
        }

        // convert all remaining symbols (after the symbol representing the
        // function application) into arguments by parsing their sexps or
        // scalar values; we're not going to worry about mapping them for
        // now; they will be compiled in the order in which they appear
        const idfn   = this._idFromName( fn );
        const args   = argmap.map( ([ , v ]) => this._sexpToEs( v ) );
        const argstr = args.join( ", " );

        // make the dangerous assumption that arguments are ordered
        // for now
        return `${idfn}(${argstr})`;
    }


    /**
     * Determine whether T represents a block form
     *
     * Block form is an application of a block, which has a certain
     * syntax.  Specifically: `(<block> ((key value) ...))'.
     *
     * @param {*} t hopefully a token list
     *
     * @return {boolean} whether T represents a block form
     */
    _isBlockForm( t )
    {
        // the first symbol is the function name, second is an sexp
        // containing each of the key/value argument mappings
        const [ fn, argmap ] = t;

        // enforce block id convention (at least for now)
        const isblockid = /^<[^>]+>$/.test( fn.value );

        return (
            Array.isArray( t )
                && isblockid
                && Array.isArray( argmap )
        );
    }


    /**
     * Determine whether T is an application of a symbol NAME, or error
     *
     * @param {*}      t    hopefully a token or token list
     * @param {string} name block name to assert against
     */
    assertApply( t, name )
    {
        // an application must be an s-expression
        if ( !Array.isArray( t ) ) {
            throw Error(
                `\`${name}' application expected, found symbol \`${t.value}'`
            );
        }

        // if there's a match, we can stop here
        if ( t[ 0 ].value === name ) {
            return;
        }

        // otherwise, provide an informative error of what we found and what
        // we should have found
        throw Error(
            `\`${name}' expected, found \`${t[ 0 ].value}'`
        );
    }
}



/*
 * Prebirth was originally intended to be run via the command line using
 * Node.js.  But it doesn't have to be.  If you want, feel free to run it in
 * your web browser; you'll just have to instantiate your own objects.
 */
( function ()
{
    if ( typeof process === 'undefined' )
    {
        return;
    }

    const p = new Parser();
    const c = new Compiler();

    const src  = require( 'fs' ).readFileSync( '/dev/stdin' ).toString();
    const tree = p.parseLisp( src );

    process.stdout.write( c.compile( tree ) );
} )();



/*
 * Now that we have output, the next step is the hard part: rewriting this
 * file in Prebirth Lisp.  As I mentioned, this process is called
 * "Birth".  It's at this point that we have to decide on basic
 * abstractions---we are starting from scratch.  The initial implementation
 * is therefore unlikely to be as concise and elegant as Prebirth
 * itself---it will be refactored.
 *
 * Here is an example Hello, World!:
 *
 *   (define-block <hello-world>
 *     ()
 *     (<js:console> ((message "Hello, world!"))))
 *
 *
 * ¹ This term should invoke visuals of an abstract being entering existence
 *   in some strange nonlinear-time² kind of way.  If you thought of
 *   something less pleasant, well, I'm sorry you went through that.
 *
 * ² Because we're dealing with nonlinear time!¹  This would be some bizarre
 *   recursive footnote crap if it weren't for that.²
 */