'use strict';

// ------------------------------------------------------------------
// Definitions
// ------------------------------------------------------------------

// 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}
// Can have offset property
const add = Symbol('add');
// > → {type: moveHead, value: 1}
const moveHead = Symbol('moveHead');
// . → {type: writeByte}
// Can have offset property
const writeByte = Symbol('writeByte');
// , → {type: readByte}
// Can have offset property
const readByte = Symbol('readByte');
// [-] → {type: loop, contents: [{type: add, value: -1}]}
// Can have isBalanced property
const loop = Symbol('loop');

// [-] → {type: clear}
// Can have offset property
const clear = Symbol('clear');

// TODO: Add extensions from Eldis

// ------------------------------------------------------------------
// Parsing
// ------------------------------------------------------------------

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;
					}
				}

				// Only add the command is value is not 0
				if(value != 0) {
					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;
					}
				}

				// Only add the command is value is not 0
				if(value != 0) {
					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/offsetCommandObjects]) <io>
function prettyPrint(parsed) {
	// ([commandObjects/offsetCommandObjects], string) <io>
	function printIndented(parsed, indent = '') {
		for(let command of parsed) {
			let line = indent;
			if(command.type == add) {
				line += `add ${command.value}`;
				if('offset' in command) {
					line += ` (${command.offset})`;
				}
				console.log(line);
			} else if(command.type == moveHead) {
				line += `moveHead ${command.value}`;
				console.log(line);
			} else if(command.type == writeByte) {
				line += 'writeByte';
				if('offset' in command) {
					line += ` (${command.offset})`;
				}
				console.log(line);
			} else if(command.type == readByte) {
				line += 'readByte';
				if('offset' in command) {
					line += ` (${command.offset})`;
				}
				console.log(line);
			} else if(command.type == loop) {
				line += 'loop';
				if('isBalanced' in command) {
					line += ` (balanced: ${command.isBalanced})`;
				}
				console.log(line);
				printIndented(command.contents, indent + '  ');
			} else if(command.type == clear) {
				line += 'clear';
				if('offset' in command) {
					line += ` (${command.offset})`;
				}
				console.log(line);
			} else {
				line += `unknown ${command.type}`;
				console.log(line);
			}
		}
	}
	printIndented(parsed);
}

// ------------------------------------------------------------------
// Optimization passes
// ------------------------------------------------------------------

class UnknownIRError extends Error {}

// ([commandObjects]) → [commandObjects]
function joinAdjacentOps(parsed) {
	// ([commandObjects], commandType) → [commandObjects]
	function worker(parsed, type) {
		let optimized = [];

		let prevOfType = false, value = 0;
		for(let command of parsed) {
			if(prevOfType && command.type == type) {
				// Update value, don't add to optimized yet
				value += command.value;
			} else if(!prevOfType && command.type == type) {
				// Start of a possible run of commands
				prevOfType = true;
				value = command.value;
			} else if(prevOfType && command.type != type) {
				// A run has ended, add it to optimized
				// However, skip it if value is 0
				if(value != 0) {
					optimized.push({type: type, value: value});
				}
				// Also add the command for this round
				optimized.push(command);
				prevOfType = false;
			} else {
				optimized.push(command);
				prevOfType = false;
			}
		}
		// Did we end with a command of given type
		if(prevOfType) {
			// Yes, add it to optimized (unless value is 0)
			if(value != 0) {
				optimized.push({type: type, value: value});
			}
		}

		return optimized;
	}

	return worker(worker(parsed, moveHead), add);
}

// ([commandObjects]) → [commandObjects]
function transformClearLoops(parsed) {
	let optimized = [];

	for(let command of parsed) {
		// Only match loops like [-] or [+]
		let isClearLoop = command.type == loop &&
			command.contents.length == 1 &&
			command.contents[0].type == add &&
			(command.contents[0].value == 1 ||
			command.contents[0].value == -1);
		if(isClearLoop) {
			optimized.push({type: clear});
		} else if(command.type == loop) {
			// Run for inner loops
			optimized.push({type: loop,
				contents: transformClearLoops(command.contents)});
		} else {
			optimized.push(command);
		}
	}

	return optimized;
}

// ([commandObjects]) → [offsetCommandObjects]
function addOffsetProperties(parsed) {
	// ([commandObjects]) → {offsetted: [offsetCommandObjects], isBalanced: bool}
	function worker(parsed) {
		let offsetted = [];
		let isBalanced = true;
		let headChange = 0;

		let offset = 0;
		for(let command of parsed) {
			if(command.type == add) {
				offsetted.push({type: add,
					value: command.value,
					offset: offset});
			} else if(command.type == moveHead) {
				offset += command.value;
			} else if(command.type == writeByte) {
				offsetted.push({type: writeByte,
					offset: offset});
			} else if(command.type == readByte) {
				offsetted.push({type: readByte,
					offset: offset});
			} else if(command.type == clear) {
				offsetted.push({type: clear,
					offset: offset});
			} else if(command.type == loop) {
				// A loop should be self-contained
				// If offset is not 0, add a moveHead
				if(offset != 0) {
					offsetted.push({type: moveHead,
						value: offset});
					// Mark we've moved the head
					headChange += offset;
				}
				offset = 0;

				// Run optimization on the loop
				let result = worker(command.contents);
				// We're only balanced if our loops are
				isBalanced = isBalanced && result.isBalanced;
				offsetted.push({type: loop,
					contents: result.offsetted,
					isBalanced: result.isBalanced});
				// headChange's value becomes invalid if the
				// loop is not balanced. However, we only
				// care about its value when figuring out
				// our isBalanced, which will be forced to
				// false if any inner loop is not balanced
			} else {
				throw new UnknownIRError();
			}
		}

		// We need to move the tape head in the end anyways, so
		// generate moveHead is offseet is not 0
		if(offset != 0) {
			offsetted.push({type: moveHead,
				value: offset});
		}

		// We're only balanced if relative to start of the loop ends
		// up as 0
		isBalanced = isBalanced && offset + headChange == 0;

		return {offsetted, isBalanced};
	}

	return worker(parsed).offsetted;
}

// TODO: Optimization pass to turn copy loops into copy commands

// ([commandObjects]) → [offsetCommandObjects]
function optimize(parsed) {
	const optimizations = [
		joinAdjacentOps,
		transformClearLoops,
		addOffsetProperties
	]
	return optimizations.reduce((IR, optimization) =>
		optimization(IR), parsed);
}

// ------------------------------------------------------------------
// Virtual machine
// ------------------------------------------------------------------

// TODO: Implement a brainfuck VM for running the optimized programs