<?php
	
$did_reach_end = false;

$changing = 'jmp_to_nop';
$last_jmp_index_changed = 0;
$last_nop_index_changed = 0;

load_instructions();

while ( $did_reach_end == false )
{
	$did_reach_end = did_reach_end($instructions, $opcodes, $operands);
	
	if ( $did_reach_end == false )
	{
		load_instructions();

		if ( $changing == 'jmp_to_nop' && $last_jmp_index_changed == count($jmp_pcs) )
		{
			// no more jmp_to_nop changes to try.
			// try nop_to_jmp changes instead.
			
			$changing == 'nop_to_jmp';
			continue;
		}

		else if ( $changing == 'nop_to_jmp' && $last_jmp_index_changed == count($jmp_pcs) )
		{
			// no more changes to try.
			// unsolvable according to stated rules.

			break;
		}

		if ( $changing == 'jmp_to_nop' )
		{			
			$opcodes[$jmp_pcs[$last_jmp_index_changed] - 1] = 'nop';

			++$last_jmp_index_changed;
		}
		else if ( $changing == 'nop_to_jmp' )
		{			
			$opcodes[$nop_pcs[$last_nop_index_changed] - 1] = 'nop';
			
			++$last_nop_index_changed;
		}	
	}
	else 
	{
		print "reached end with acc = $acc\n";
	}
}

exit;


function load_instructions()
{
	global $instructions, $opcodes, $operands, $jmp_pcs, $nop_pcs, $acc;
	
	$opcodes = [];
	$instructions = [];
	$jmp_pcs = [];
	$nop_pcs = [];
	
	$instructions = preg_split('/\n/', trim(file_get_contents('input.txt')));
	
	$acc = 0;
	
	foreach ( $instructions as $instruction )
	{
		preg_match('/^(.*?)\s+(.*?)$/', $instruction, $matches);
		
		$opcodes[] = $matches[1];
		$operands[] = $matches[2];
		
		if ( $matches[1] == 'jmp' )
		{
			$jmp_pcs[] = count($opcodes);
		}
		else if ( $matches[1] == 'nop' )
		{
			$nop_pcs[] = count($opcodes);
		}
	}
}

function did_reach_end()
{
	global $instructions, $opcodes, $operands, $acc;
	
	$pc = 1;
	$pcs_visited = [];
	$reached_end = false;
	
	for ( ;; )
	{
		if ( in_array($pc, $pcs_visited) )
		{
			// infinite loop detected
			
			break;
		}
		
		if ( $pc > count($instructions) )
		{
			// end of code reached!
			
			$reached_end = true;
			break;
		}
		
		$opcode = $opcodes[$pc - 1];
		$operand = $operands[$pc - 1];
		
		$pcs_visited[] = $pc;
			
		if ( $opcode == 'nop' )
		{
			++$pc;
		}
		else if ( $opcode == 'acc' )
		{
			$acc += $operand;
			++$pc;
		}
		else if ( $opcode == 'jmp' )
		{
			$pc += $operand;
		}
	}
	
	return $reached_end;
}