gir/gir.js

'use strict';

// Use symbols for the names of the instructions
// Unsure if this helps in any way over strings, but I feel it neater

// +++++ → {type: add, value: 5}
const add = Symbol('add');
// > → {type: moveHead, value: 1}
const moveHead = Symbol('moveHead');
// . → {type: writeByte}
const writeByte = Symbol('writeByte');
// , → {type: readByte}
const readByte = Symbol('readByte');
// [-] → {type: loop, contents: [{type: add, value: -1}]}
const loop = Symbol('loop');

// TODO: Add extensions from Eldis

class ParsingError extends Error {}

// (string) → [commandObjects]
// May throw
function parse(program) {
	// (string, int, bool) → {parsed: [commandObjects], lastIndex: int}
	// index is the index of the next character to consume
	// inLoop tells whether we're parsing a loop or the top level
	// lastIndex is the last index the function consumed
	function constructTree(program, index = 0, inLoop = false) {
		let commands = [];

		// Move this out of the loop body since we need to return
		// the index of the last character we parsed
		let i = index;
		for(;;) {
			if(i >= program.length) {
				// If we're parsing a loop, we have a
				// missing ]
				// If we're parsing the top level, this is
				// where we should exit
				if(inLoop) {
					throw new ParsingError('Missing ]');
				} else {
					break;
				}
				i++;

			} else if(program[i] == ']') {
				// If we're parsing a loop, this is where we
				// should exit
				// If we're parsing the top level, we have a
				// missing [
				if(inLoop) {
					break;
				} else {
					throw new ParsingError('Missing [');
				}
				i++;

			} else if(program[i] == '+' || program[i] == '-') {
				// Fold a run of +s and -s into one node
				let value = 0;
				for(; i < program.length; i++) {
					if(program[i] == '+') {
						value++;
					} else if(program[i] == '-') {
						value--;
					} else {
						// Reached end of the run
						break;
					}
				}

				commands.push({type: add, value: value});
				// i is not incremented, since it already
				// points to a location containig a char we
				// have not yet handled

			} else if(program[i] == '<' || program[i] == '>') {
				// Fold a run of <s and >s into one node
				let value = 0;
				for(; i < program.length; i++) {
					if(program[i] == '>') {
						value++;
					} else if(program[i] == '<') {
						value--;
					} else {
						// Reached end of the run
						break;
					}
				}

				commands.push({type: moveHead, value: value});
				// see +/- for why we don't increment i

			} else if(program[i] == '.') {
				commands.push({type: writeByte});
				i++;

			} else if(program[i] == ',') {
				commands.push({type: readByte});
				i++;

			} else if(program[i] == '[') {
				// Parse a loop. This is done by calling the
				// same parser function recursively
				// Due to this the loop appears as one node
				// in the parsed result
				let {parsed, lastIndex} = constructTree(
					program, // Same program data
					i + 1, // Start at the next char
					true // We're parsing a loop
				);
				commands.push({type: loop, contents: parsed});
				// Since lastIndex was consumed by the inner
				// function, we don't want to consume it a
				// second time
				i = lastIndex + 1;
			} else {
				// All others characters are comments,
				// ignore them
				i++;
			}
		}

		return {parsed: commands, lastIndex: i};
	}

	// We only care about the parsed contents, since under normal
	// operarion we only get out of the loop if we've reached the end
	// of the program
	let {parsed} = constructTree(program);
	return parsed;
}

// ([commandObjects]) <io>
function prettyPrint(parsed) {
	// ([commandObjects], string) <io>
	function printIndented(parsed, indent = '') {
		for(let command of parsed) {
			let line = indent;
			if(command.type == add) {
				line += `add ${command.value}`;
				console.log(line);
			} else if(command.type == moveHead) {
				line += `moveHead ${command.value}`;
				console.log(line);
			} else if(command.type == writeByte) {
				line += 'writeByte';
				console.log(line);
			} else if(command.type == readByte) {
				line += 'readByte';
				console.log(line);
			} else if(command.type == loop) {
				line += 'loop';
				console.log(line);
				printIndented(command.contents, indent + '  ');
			} else {
				line += `unknown ${command.type}`;
				console.log(line);
			}
		}
	}
	printIndented(parsed);
}