JS1k: The JavaScript code golfing competition

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// Program variables.
var keys = [];
var size = 600;
var player_x = 300;
var player_y = 550;
var frame = 0;
var bullets = [];
var bots = [];
var game_on = 1;
var invincible = 0;
var game_over_text = "";
var i;
var j;
var black = '#000';
var white = '#fff';

c.textAlign = "center";
c.font = '20px Arial';

// Set keyboard handlers.
onkeydown = function(e)
{
    keys[e.which] = 1;
    //console.log(e.which);
};

onkeyup = function(e)
{
    keys[e.which] = 0;
};

// Initialize charge bots
for (i = 0; i < 10; i++)
{
    bots.push({
        x: Math.random() * size,
        y: Math.random() * size,
        health: 100,
        charge: Math.random() < 0.5 ? black : white
    });
}

setInterval(function()
{
    frame++;

    if (keys[73])
    {
        invincible = !invincible;
    }
    
    if (game_on)
    {
        var player_speed = keys[16] ? 2 : 4;
        
        // Move up.
        if (keys[38])
        {
            player_y -= player_speed;
            player_y = Math.max(player_y, 0);
        }

        // Move down.
        if (keys[40])
        {
            player_y += player_speed;
            player_y = Math.min(player_y, size - 1);
        }

        // Move left.
        if (keys[37])
        {
            player_x -= player_speed;
            player_x = Math.max(player_x, 0);
        }

        // Move right.
        if (keys[39])
        {
            player_x += player_speed;
            player_x = Math.min(player_x, size - 1);
        }

        // Clear the screen. Prevent clearing on
        // game over for a neat effect.
        c.fillStyle = '#777';
        c.fillRect(0, 0, size, size);

        // Check charge bot and laser collision.
        if (keys[90])
        {
            var count = 0;
            for (i in bots)
            {
                if (bots[i].y - 33 < player_y &&
                    bots[i].x - 35 < player_x &&
                    bots[i].x + 35 > player_x)
                {
                    bots[i].health -= 1;
                }
                
                count++;

                // Prune bots who lose all of their health.
                if (bots[i].health < 0)
                {
                    count--;
                    delete bots[i];
                }
            }

            if (!count)
            {
                game_over_text = 'VICTORY ACHIEVED';
                game_on = 0;
            }
        }
    }

    // Spawn bullets.
    for (i = 0; i < 5; i++)
    {
        bullets.push({
            x: 300,
            y: 100,
            dx: Math.random() * 4 - 2,
            dy: Math.random() * 4 - 2,
            charge: Math.random() < .5 ? black : white
        });
    }
    
    // Prune old bullets.
    for (i = bullets.length; i > 2000; i--)
    {
        bullets.shift();
    }

    // Move and draw bullets.
    for (i in bullets)
    {
        // Attract and repel bullets about the charge bots.
        for (j in bots)
        {
            var dist_x = bots[j].x - bullets[i].x;
            var dist_y = bots[j].y - bullets[i].y;
            var dist = dist_x * dist_x + dist_y * dist_y;
            
            if (bots[j].charge == bullets[i].charge)
            {
                bullets[i].dx -= dist_x / dist;
                bullets[i].dy -= dist_y / dist;
            }
            else
            {
                bullets[i].dx += dist_x / dist;
                bullets[i].dy += dist_y / dist;
            }
        }

        // Draw the bullet.
        c.fillStyle = bullets[i].charge;
        c.beginPath();
        c.arc(bullets[i].x += bullets[i].dx,
              bullets[i].y += bullets[i].dy,
              10, 0, 7);
        c.fill();
    }

    // Draw charge bots.
    for (i in bots)
    {
        c.lineWidth = 3;
        c.strokeStyle = bots[i].charge;

        // Draw the health ring.
        c.beginPath();
        c.arc(bots[i].x,
              bots[i].y,
              30,
              0,
              2 * Math.PI * bots[i].health / 100);
        c.stroke();

        c.lineWidth = 1;

        // Draw the wire under the health ring.
        c.beginPath();
        c.arc(bots[i].x,
              bots[i].y,
              30,
              0,
              7);
        c.stroke();

        // Draw the core.
        c.fillStyle = c.strokeStyle;
        c.beginPath();
        c.arc(bots[i].x,
              bots[i].y,
              10,
              0,
              7);
        c.fill();
    }

    // Draw player laser.
    if (game_on && keys[90] && frame % 2 == 0)
    {
        c.fillStyle = white;
        c.fillRect(player_x - 6, player_y - 4, 11, -999);
    }
    
    // Check player and entity collision by looking at the pixel
    // color at the player.
    if (!invincible && game_on && c.getImageData(player_x, player_y, 1, 1).data[0] != 0x77)
    {
        game_over_text = 'YOU DIED';
        game_on = 0;
    }

    // Draw player.
    c.fillStyle = black;
    c.fillRect(player_x - 3, player_y - 3, 5, 5);

    // Draw game over text.
    if (!game_on)
    {
        c.globalAlpha = .5;
        c.fillRect(0, 283, 600, 19);
        c.fillStyle = white;
        c.globalAlpha = 1;
        c.fillText(game_over_text, 300, 300);
    }
}, 33);