{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "059837a0",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "cuda\n"
     ]
    }
   ],
   "source": [
    "import torch\n",
    "import torch.nn as nn\n",
    "from torch.nn import functional as F\n",
    "device = torch.device(\"cuda\" if torch.cuda.is_available() else \"cpu\")\n",
    "print(device)\n",
    "block_size = 128\n",
    "batch_size = 32\n",
    "max_iters = 4000\n",
    "learning_rate = 3e-4\n",
    "eval_every = 500\n",
    "n_embd = 384\n",
    "n_head = 8\n",
    "n_layer = 8\n",
    "dropout = 0.2"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "1fdc69a7",
   "metadata": {},
   "outputs": [],
   "source": [
    "with open(\"shakespeare.txt\") as f:\n",
    "    text = f.read()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "0c09eeb0",
   "metadata": {},
   "outputs": [],
   "source": [
    "chars = sorted(set(text))\n",
    "vocab_size = len(chars)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "id": "a278e7b9",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Vocab size: 101\n",
      "Text length: 5357910\n"
     ]
    }
   ],
   "source": [
    "print(f\"Vocab size: {vocab_size}\")\n",
    "print(f\"Text length: {len(text)}\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "id": "2a540d96",
   "metadata": {},
   "outputs": [],
   "source": [
    "string_to_int = {ch: i for i, ch in enumerate(chars)}\n",
    "int_to_string = {i: ch for i, ch in enumerate(chars)}\n",
    "\n",
    "encode = lambda s: [string_to_int[ch] for ch in s]\n",
    "decode = lambda x: ''.join([int_to_string[i] for i in x])\n",
    "\n",
    "data = torch.tensor(encode(text), dtype=torch.long, device=device)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "id": "c7c8e4aa",
   "metadata": {},
   "outputs": [],
   "source": [
    "n = int(0.8 * len(data))\n",
    "train_data = data[:n]\n",
    "val_data = data[n:]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "id": "54d80f45",
   "metadata": {},
   "outputs": [],
   "source": [
    "def get_batch(split):\n",
    "    data = train_data if split == 'train' else val_data\n",
    "    ix = torch.randint(len(data) - block_size, (batch_size,))\n",
    "    x = torch.stack([data[i:i+block_size] for i in ix])\n",
    "    y = torch.stack([data[i+1:i+block_size+1] for i in ix])\n",
    "    x, y = x.to(device), y.to(device)\n",
    "    return x, y"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "id": "618df2dc",
   "metadata": {},
   "outputs": [],
   "source": [
    "@torch.no_grad()\n",
    "def estimate_loss():\n",
    "    out = {}\n",
    "    model.eval()\n",
    "    for split in ['train', 'val']:\n",
    "        losses = torch.zeros(eval_every)\n",
    "        for k in range(eval_every):\n",
    "            X, Y = get_batch(split)\n",
    "            logits, loss = model(X, Y)\n",
    "            losses[k] = loss.item()\n",
    "        out[split] = losses.mean()\n",
    "    model.train()\n",
    "    return out"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "id": "d0a21928",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "step: 0, train loss: 4.615, val loss: 4.613\n",
      "step: 500, train loss: 1.923, val loss: 1.961\n",
      "step: 1000, train loss: 1.662, val loss: 1.753\n",
      "step: 1500, train loss: 1.531, val loss: 1.655\n",
      "step: 2000, train loss: 1.453, val loss: 1.608\n",
      "step: 2500, train loss: 1.398, val loss: 1.567\n",
      "step: 3000, train loss: 1.365, val loss: 1.543\n",
      "step: 3500, train loss: 1.340, val loss: 1.529\n",
      "1.3418211936950684\n"
     ]
    }
   ],
   "source": [
    "\n",
    "class Head(nn.Module):\n",
    "    \"\"\" one head of self-attention \"\"\"\n",
    "\n",
    "    def __init__(self, head_size):\n",
    "        super().__init__()\n",
    "        self.key = nn.Linear(n_embd, head_size, bias=False)\n",
    "        self.query = nn.Linear(n_embd, head_size, bias=False)\n",
    "        self.value = nn.Linear(n_embd, head_size, bias=False)\n",
    "        self.register_buffer('tril', torch.tril(torch.ones(block_size, block_size)))\n",
    "\n",
    "        self.dropout = nn.Dropout(dropout)\n",
    "\n",
    "    def forward(self, x):\n",
    "        # input of size (batch, time-step, channels)\n",
    "        # output of size (batch, time-step, head size)\n",
    "        B,T,C = x.shape\n",
    "        k = self.key(x)   # (B,T,hs)\n",
    "        q = self.query(x) # (B,T,hs)\n",
    "        # compute attention scores (\"affinities\")\n",
    "        wei = q @ k.transpose(-2,-1) * k.shape[-1]**-0.5 # (B, T, hs) @ (B, hs, T) -> (B, T, T)\n",
    "        wei = wei.masked_fill(self.tril[:T, :T] == 0, float('-inf')) # (B, T, T)\n",
    "        wei = F.softmax(wei, dim=-1) # (B, T, T)\n",
    "        wei = self.dropout(wei)\n",
    "        # perform the weighted aggregation of the values\n",
    "        v = self.value(x) # (B,T,hs)\n",
    "        out = wei @ v # (B, T, T) @ (B, T, hs) -> (B, T, hs)\n",
    "        return out\n",
    "\n",
    "# [1, 0, 0]\n",
    "# [1, 0.6, 0]\n",
    "# [1, 0.6, 0.4]\n",
    "class MultiHeadAttention(nn.Module):\n",
    "    \"\"\" multiple heads of self-attention in parallel \"\"\"\n",
    "\n",
    "    def __init__(self, num_heads, head_size):\n",
    "        super().__init__()\n",
    "        self.heads = nn.ModuleList([Head(head_size) for _ in range(num_heads)])\n",
    "        self.proj = nn.Linear(head_size * num_heads, n_embd)\n",
    "        self.dropout = nn.Dropout(dropout)\n",
    "\n",
    "    def forward(self, x):\n",
    "        out = torch.cat([h(x) for h in self.heads], dim=-1) # (B, T, F) -> (B, T, [h1, h1, h1, h1, h2, h2, h2, h2, h3, h3, h3, h3])\n",
    "        out = self.dropout(self.proj(out))\n",
    "        return out\n",
    "    \n",
    "\n",
    "class FeedFoward(nn.Module):\n",
    "    \"\"\" a simple linear layer followed by a non-linearity \"\"\"\n",
    "\n",
    "    def __init__(self, n_embd):\n",
    "        super().__init__()\n",
    "        self.net = nn.Sequential(\n",
    "            nn.Linear(n_embd, 4 * n_embd),\n",
    "            nn.ReLU(),\n",
    "            nn.Linear(4 * n_embd, n_embd),\n",
    "            nn.Dropout(dropout),\n",
    "        )\n",
    "\n",
    "    def forward(self, x):\n",
    "        return self.net(x)\n",
    "    \n",
    "class Block(nn.Module):\n",
    "    \"\"\" Transformer block: communication followed by computation \"\"\"\n",
    "\n",
    "    def __init__(self, n_embd, n_head):\n",
    "        # n_embd: embedding dimension, n_head: the number of heads we'd like\n",
    "        super().__init__()\n",
    "        head_size = n_embd // n_head\n",
    "        self.sa = MultiHeadAttention(n_head, head_size)\n",
    "        self.ffwd = FeedFoward(n_embd)\n",
    "        self.ln1 = nn.LayerNorm(n_embd)\n",
    "        self.ln2 = nn.LayerNorm(n_embd)\n",
    "\n",
    "    def forward(self, x):\n",
    "        y = self.sa(x)\n",
    "        x = self.ln1(x + y)\n",
    "        y = self.ffwd(x)\n",
    "        x = self.ln2(x + y)\n",
    "        return x\n",
    "    \n",
    "class GPTLanguageModel(nn.Module):\n",
    "    def __init__(self, vocab_size):\n",
    "        super().__init__()\n",
    "        self.token_embedding_table = nn.Embedding(vocab_size, n_embd)\n",
    "        self.position_embedding_table = nn.Embedding(block_size, n_embd)\n",
    "        self.blocks = nn.Sequential(*[Block(n_embd, n_head=n_head) for _ in range(n_layer)])\n",
    "        self.ln_f = nn.LayerNorm(n_embd) # final layer norm\n",
    "        self.lm_head = nn.Linear(n_embd, vocab_size)\n",
    "        \n",
    "        \n",
    "        self.apply(self._init_weights)\n",
    "\n",
    "    def _init_weights(self, module):\n",
    "        if isinstance(module, nn.Linear):\n",
    "            torch.nn.init.normal_(module.weight, mean=0.0, std=0.02)\n",
    "            if module.bias is not None:\n",
    "                torch.nn.init.zeros_(module.bias)\n",
    "        elif isinstance(module, nn.Embedding):\n",
    "            torch.nn.init.normal_(module.weight, mean=0.0, std=0.02)\n",
    "\n",
    "    def forward(self, index, targets=None):\n",
    "        B, T = index.shape\n",
    "        \n",
    "        \n",
    "        # idx and targets are both (B,T) tensor of integers\n",
    "        tok_emb = self.token_embedding_table(index) # (B,T,C)\n",
    "        pos_emb = self.position_embedding_table(torch.arange(T, device=device)) # (T,C)\n",
    "        x = tok_emb + pos_emb # (B,T,C)\n",
    "        x = self.blocks(x) # (B,T,C)\n",
    "        x = self.ln_f(x) # (B,T,C)\n",
    "        logits = self.lm_head(x) # (B,T,vocab_size)\n",
    "        \n",
    "        if targets is None:\n",
    "            loss = None\n",
    "        else:\n",
    "            B, T, C = logits.shape\n",
    "            logits = logits.view(B*T, C) # reshape to what torch.cross_entropy expects\n",
    "            targets = targets.view(B*T)\n",
    "            loss = F.cross_entropy(logits, targets) \n",
    "        return logits, loss\n",
    "    \n",
    "    def generate(self, index, max_new_tokens):\n",
    "        # index is (B, T) array of indices in the current context\n",
    "        for _ in range(max_new_tokens):\n",
    "            # crop idx to the last block_size tokens\n",
    "            index_cond = index[:, -block_size:]\n",
    "            # get the predictions\n",
    "            logits, loss = self.forward(index_cond)\n",
    "            # focus only on the last time step\n",
    "            logits = logits[:, -1, :] # becomes (B, C)\n",
    "            # apply softmax to get probabilities\n",
    "            probs = F.softmax(logits, dim=-1) # (B, C)\n",
    "            # sample from the distribution\n",
    "            index_next = torch.multinomial(probs, num_samples=1) # (B, 1)\n",
    "            # append sampled index to the running sequence\n",
    "            index = torch.cat((index, index_next), dim=1) # (B, T+1)\n",
    "        return index\n",
    "\n",
    "model = GPTLanguageModel(vocab_size).to(device)\n",
    "\n",
    "# create a PyTorch optimizer\n",
    "optimizer = torch.optim.AdamW(model.parameters(), lr=learning_rate)\n",
    "\n",
    "for iter in range(max_iters):\n",
    "    if iter % eval_every == 0:\n",
    "        losses = estimate_loss()\n",
    "        print(f\"step: {iter}, train loss: {losses['train']:.3f}, val loss: {losses['val']:.3f}\")\n",
    "\n",
    "    # sample a batch of data\n",
    "    xb, yb = get_batch('train')\n",
    "\n",
    "    # evaluate the loss\n",
    "    logits, loss = model.forward(xb, yb)\n",
    "    optimizer.zero_grad(set_to_none=True)\n",
    "    loss.backward()\n",
    "    optimizer.step()\n",
    "print(loss.item())"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "id": "99a66247",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "\t them part it. The leison drows\n",
      "Let them napposes them.\n",
      "\n",
      "SUFFUE.\n",
      "Yea, erow now, he was near angless.\n"
     ]
    }
   ],
   "source": [
    "\n",
    "context = torch.zeros((1,1), dtype=torch.long, device=device)\n",
    "generated_chars = decode(model.generate(context, max_new_tokens=100)[0].tolist())\n",
    "print(generated_chars)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "id": "c2b03115",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "To be or not to be, my lord; and at Worces and will.\n",
      "\n",
      "FRIAR L JOHN.\n",
      "My dory Gold Catesby say the King vow you are.\n",
      "\n",
      "ENT\n"
     ]
    }
   ],
   "source": [
    "\n",
    "prompt = 'To be or not to be,'\n",
    "context = torch.tensor(encode(prompt), dtype=torch.long, device=device)\n",
    "generated_chars = decode(model.generate(context.unsqueeze(0), max_new_tokens=100)[0].tolist())\n",
    "print(generated_chars)"
   ]
  }
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